A Filtering Scheme for Reducing Harmonics Penetration …apic/uploads/Research/sample14.pdf · low...

1

Abstract--This paper presents a novel scheme to reduce har-

monics penetration into transmission systems. The scheme utilizes

the tertiary winding of the substation transformer to construct a

low impedance path to trap harmonics at tuned frequencies. This

is achieved by inserting capacitors and inductors into the delta

loop of the tertiary winding for zero-sequence harmonics filtering

and by connecting shunt capacitors and inductors to the tertiary

winding for non-zero-sequence harmonics filtering. The proposed

topology is capable of trapping two zero-sequence harmonics and

three non-zero-sequence harmonics simultaneously. Design pro-

cedure is given in details. Simulations conducted on a detailed

modeled distribution system have demonstrated the effectiveness

of the tertiary winding. In addition, economic analysis further

confirms that the proposed tertiary winding is a much more cost-

effective solution compared to medium-voltage LC filter package.

Index Terms—Power quality, harmonics, filter.

I. INTRODUCTION

ARMONIC distortion is one of the main power quality

concerns for utility companies. Traditional approaches

for managing harmonic distortions are to limit the harmonic

currents injected by customers into the power supply systems.

An example is the IEEE Std. 519 [1]. The approach has been

very effective in mitigating the harmonic distortions caused by

large, concentrated harmonic sources such as industry facilities

with variable frequency drives. With the wide spread adoption

of standards such as the IEEE Std. 519, harmonics generated

by large industries and commercial facilities are no longer a

major concern to utility companies.

In recent years, the proliferation of energy efficient but

harmonic-producing home appliances and consumer electronic

devices has resulted in another type of harmonic distortions in

power distribution systems. These new harmonic sources have

comparable sizes and are distributed all over a network. Alt-

hough they produce insignificant amount harmonic currents

individually, the collective effect of a large number of such

loads can be substantial. Several power quality concerns have

been identified due to such distributed harmonic sources. One

of them is that residential feeders have become significant

harmonic sources, injecting unnegligible harmonic currents

into power transmission systems. The consequences could be

the overloading of transmission capacitors and transmission

level harmonic resonances etc [2].

This work was supported by the Natural Sciences and Engineering Re-

search Council of Canada and Alberta Power Industry Consortium.

The authors are with the department of Electrical and Computer Engineer-

ing, University of Alberta, Edmonton, AB T6G 2V4, Canada (email:

[email protected]).

Two types of solutions have been proposed to address this

emerging harmonic issue so far. One solution is to reduce the

harmonic emission from individual electronic devices. IEC

61000 3-2 has been established for this purpose [3]. The sec-

ond solution is to reduce the harmonic distortions in power

distribution systems through, for example, medium and low

voltage filters [4]. Both solutions can reduce the amount of

harmonic currents injected into the transmission systems.

The penetration of harmonics from distribution systems into

transmission systems is a sensitive issue since transmission and

distribution systems often have different owners. Both types of

companies will, therefore, benefit from a solution that is dedi-

cated to preventing distribution system harmonics from pene-

trating into the transmission systems. Distribution system own-

ers may use the solution to meet the interconnection require-

ments of the transmission system regardless the harmonic dis-

tortion levels inside the distribution systems. The transmission

owner can easily verify the effectiveness of the solution in

meeting its requirements.

The objective of this paper is to investigate schemes that

can reduce the harmonic injection from distribution systems

into transmission systems. A novel filtering scheme, called the

tertiary winding filter, is proposed and its advantages and ef-

fectiveness are demonstrated through simulation studies. Eco-

nomic analysis further confirms that the proposed filter scheme

is a cost-effective solution.

This paper is organized as follows: Section II reviews the

techniques that are applicable to reduce harmonic injections

into transmission systems. Section III presents the proposed

tertiary winding filter and explains its principles. The design

method for the proposed tertiary winding filter is given in Sec-

tion IV. Section V demonstrates the effectiveness of the pro-

posed scheme through computer simulation studies and inves-

tigates the overall system performance when a few tertiary

winding filters are installed. Economic comparison is conduct-

ed in Section VI.

II. REVIEW OF APPLICABLE HARMONIC MITIGATION SCHEMES

Although little research work has been done in the area of

preventing harmonics from entering transmission systems,

techniques that can be adopted for the purpose do exist. These

techniques can be classified into two types. One is to prevent

or reduce the injection of zero sequence harmonics into the

transmission system. The other type is to deal with the non-

zero sequence harmonics.

J. W. Hagge, Senior Member, IEEE, and L. L. Grigsby, Fellow, IEEE

A Filtering Scheme for Reducing Harmonics

Penetration into Transmission Systems (V1.0)

H

2

A. Zero Sequence Harmonic Mitigation Techniques

Zero Sequence (ZS) harmonics are those harmonic compo-

nents whose are in zero sequence according to the definition of

symmetrical components. ZS components are dominant in the

3rd, 9th, 15th and other triple-order harmonics. Single phase

non-linear loads such as energy efficient home appliances are

the most significant source of ZS harmonics. It is quite com-

mon to observe high levels of ZS harmonics in a distribution

substation feeding residential loads these days.

1) Transformer Connection

For multi-grounded neutral (MGN) configuration widely

adopted in North America distribution systems, the secondary

side windings of the substation transformer are connected as

grounded Wye, while the primary side windings' connection

may vary according to corresponding requirements of the

transmission systems. The simplest way to prevent ZS harmon-

ics' propagation into transmission systems is to modify the

primary side windings' of substation transformer connection as

ungrounded Wye as shown in Fig. 1. For such a connection,

there is no pathway for ZS harmonic currents to pass through

from the secondary side to the primary side of the substation

transformer. Thus no ZS harmonics could propagate from dis-

tribution systems to transmission systems.

H XDistribution

Feeders and Loads

Transmission

System

MV BusHV Bus Substation

Transformer

Grounding

Transformer

Fig. 1 Substation transformer connection that can prevent ZS harmonics'

propagation into transmission systems.

For cases where the primary side must be grounded, a

grounding transformer may be installed at the primary bus as

shown in Fig. 1 with the dotted connection. The most common

grounding transformer is the unloaded Yg/Delta transformer

with Yg side connected to the primary bus.

2) Shunt Passive ZS Filter

A shunt filter installed at the secondary side of the substa-

tion transformer can also reduce the injection of distribution

system harmonics into the transmission system.

Shunt passive ZS filters create shunt low ZS impedance to

trap the ZS harmonics. They have various topologies (as

shown in Fig. 2) and can be broadly classified into two types.

The first type is the LC ZS filter which only consists of ca-

pacitors and inductors. It has positive/negative sequence im-

pedance so it affects the flow of non-ZS harmonics. A repre-

sentative example of such filters is the star-connected capaci-

tors grounded through an inductor (Fig. 2(a)) which is tuned to

create a low ZS impedance. The main attractive characteristic

of this topology is that by adding a three-phase inductor (Fig. 2

(b)), the capacitors can be tuned to filter positive and negative

sequence harmonics as well. The downside of this filter is that

it can affect the fundamental frequency power flow and can

lead to positive or negative sequence resonances at other fre-

quencies.

The second type of ZS filter is the transformer based ZS fil-

ter. It is developed based on the concept of grounding trans-

former. Such a filter behaves as open circuit at positive and

negative sequences so it has no impact on normal power sys-

tem operation and on non-zero-sequence harmonics. One of

the examples is the zig-zag transformer based filter (Fig. 2(c)).

A drawback of this filter is that it needs a nonstandard trans-

former. Another example consists of a Yg/delta transformer

with tuned capacitors and inductors inserted into the delta loop

(Fig. 2 (d) (e)). By proper selection of the capacitors and in-

ductors, a ZS impedance as low as the transformer’s resistance

could be achieved at desired harmonic frequencies. This leads

to attractive ZS filter topologies without using non-standard

transformers.

A B C

A B C

(a) LC Fitler 1 (b) LC Filter 2

A B C

A

B

C

A

B

C

(c) Zig-zag Trans-

former

(d) Transformer-based

Single Tuned Filter

(e) Transformer-based

Double Tuned Filter

Fig. 2 Topologies of ZS shunt filters.

It should be noted that the ZS filter can be installed at the

either side of the substation transformer theoretically. Howev-

er, because of high voltage, the primary side scheme is uneco-

nomic and hard to implement. The practical implementation,

therefore, is to install the ZS filter at the secondary side (as

shown in Fig. 3).

H XDistribution

Feeders and Loads

Transmission

System

MV BusHV Bus Substation

Transformer

ZS

Filter

Fig. 3 ZS filter installation location.

B. Non Zero Sequence Harmonic Mitigation Technique

Non Zero Sequence (non-ZS) harmonics are those harmon-

ic components whose are in positive sequence or negative se-

quence according the definition of symmetrical components.

Non-ZS components are dominant in the 5th, 7th, 11th and

other non-triple order harmonics. All the non-linear loads in-

cluding the single phase non-linear load, the two phase non-

3

linear load and the three phase non-linear loads generate non-

ZS components. This results in that the non-ZS harmonic dis-

tortion level is usually more serious than the ZS harmonic dis-

tortion level in the system.

Considering the connection types of the substation trans-

former have no impacts on the non-ZS harmonic currents’ flow

path, the only effective way to prevent the non-ZS harmonics'

propagation into the transmission system is by installing filters.

Active filters sound attractive to be applied for the MV ap-

plication due to its great flexibility. In terms of cost and relia-

bility, however, they still could not compete with the passive

filters of similar filtering performance. Similar to that in ZS

harmonics mitigation techniques, passive filters are also play-

ing a leading role in non-ZS harmonics mitigation techniques.

Among all passive filters, shunt passive filters which are

designed to be connected in parallel with the load are the most

widely used in the MV level because of their lower installation

and maintenance cost and higher operation reliability. Shunt

passive filters could be further categorized as LC non-ZS fil-

ter, LC ZS filter and transformer based ZS filter. While LC ZS

filter and transformer based ZS filter have been discussed in

the former section, Fig. 4 gives the topologies of some com-

monly used LC non-ZS filter. It should be noted that these

topologies could be used for the LC ZS filter as well, as long

as the three phase filter branches are connected in delta or

grounded Wye.

L

C

L

R

2C

1C

L

R

2C

1C

(a)LC Filter 3 (b)LC Filter 4 (c)LC Filter 5

Fig. 4 Topologies of LC non-ZS filters.

As shown in Fig. 5, similar to the ZS filter installation loca-

tion, the practical scheme to trap non-ZS harmonics also install

the filter at the secondary side of the substation transformer,

instead of the primary side considering the high cost and im-

plementation difficulty of high voltage filter.

H XDistribution

Feeders and Loads

Transmission

System

MV BusHV Bus

Filter

Substation

Transformer

Fig. 5 Non-ZS filter installation location.

C. Combined Filter

Depending on the requirements of transmission companies,

both types of filters may be required at a substation. It is pos-

sible to combine the two types of filters into one unit for cost

savings. One possible solution is the combined filter shown in

Fig. 6.

Combined Filter

A

B

C

Fig. 6 Topology of the combined filter.

The generic theory of the combined filter is based on the

Yg/delta transformer and the tuned filter. It utilizes the leakage

inductance of the transformer to create a low impedance path

to trap harmonic current at tuned frequencies. This is achieved

by inserting capacitors and inductors into the delta loop of

secondary windings for ZS harmonic filtering and by connect-

ing shunt capacitor and inductors to the secondary winding for

non-ZS sequence harmonic filtering. Similar to other tuned

filters, at tuned frequencies, the combined filter’s impedance

will only contain the resistive component (i.e. the transformer

short circuit resistance) since the reactive component of the

combined filter will be canceled out by the tuning capacitor. If

this impedance is lower than the upstream equivalent harmonic

impedance (i.e. the transmission system equivalent harmonic

impedance plus the substation transformer equivalent harmon-

ic impedance), harmonics originated from the distribution sys-

tem will be trapped by the combined filter.

This filter is very compact and could reduce costs by using

low voltage LC components. However, as mentioned above,

for this filter to effectively reduce the harmonics penetrating

into transmission systems, the transformer short circuit re-

sistance should be less than the upstream equivalent harmonic

impedance, which means a large transformer comparable to

the substation transformer is needed.

D. Summary

Current available techniques for preventing harmonics

propagation into transmission systems are summarized as

shown in Fig. 7.

Fig. 7 Applicable techniques for preventing harmonics' propagation into

transmission systems.

To prevent the harmonics from distribution systems (which

usually has a wide spectrum) from entering transmission sys-

tems, both Zero Sequence harmonic mitigation techniques and

4

non-Zero Sequence harmonic mitigation techniques should be

used.

Among all combinations, the most compact one is to install

a combined filter at the secondary side of the substation trans-

former. However, this scheme requires a large transformer to

work effectively. And other schemes with transformer based

filters also have the same problem. Considering the high cost

of a large transformer, all these schemes are not optimal op-

tions.

Compared to the scheme of the combination of the LC ZS

filter and the LC non-ZS filter, the scheme with ungrounded

primary windings and LC non-ZS filter is of less cost because

of the absence of the LC ZS filter. Nevertheless, for the cases

that the grounding at the transmission side is a must, expensive

high voltage grounding transformer should be incorporated

into this scheme (as shown in Fig. 1) and this will make this

scheme loose its cost advantage.

Actually, the primary side winding connections of the sub-

station transformer is usually fixed and could not be changed

arbitrarily which further render the applicability of the un-

grounding scheme. It should be further noted that three wind-

ing transformers are widely used by utility companies as dis-

tribution substation transformer and their common connection

is Yg/yg/delta with no loads served by the delta connected

tertiary windings.

The above situation naturally leads to the research on other

feasible passive solutions to prevent harmonics' propagation

into the transmission system for the substation with

Yg/yg/delta configured substation transformer besides the

scheme using the combination of LC ZS filter and LC non-ZS

filter.

III. PROPOSED TERTIARY WINDING FILTER

As reviewed in the previous section, several options are

available to reduce the injection of distribution system har-

monics. Each option has its advantages and limitations. The

transformer based filters have several attractive features. How-

ever, they require a transformer with small short-circuit im-

pedance. In view that many substation transformers have a

tertiary winding, it may be possible to utilize the substation

transformer as a filtering transformer on top of its power

transmission function. The need for a dedicated transformer is

thus eliminated. This reasoning has led us to propose a new

filtering scheme called “Tertiary Winding Filter”.

A. Basic Principle of the Tertiary Winding Filter

Similar to the combined filter, the basic idea of the tertiary

winding filter is to utilize the leakage inductance of the tertiary

winding to create a low impedance path to trap harmonic cur-

rents at tuned frequencies. This is achieved by inserting capac-

itors and inductors into the delta loop of the tertiary winding

for ZS harmonic filtering and by connecting shunt capacitors

and inductors to the tertiary winding for non-ZS harmonic fil-

tering. Topology of the filter is depicted in Fig. 8. Frequency

response of a sample tertiary winding filter tuned to the 3rd

and

9th

ZS harmonics and 5th

, 7th

and 11th

non-ZS harmonics is

shown in Fig. 9.

Transmission System

VSA

VSB

VSC

Zup

Substation Transformer

Bus

#1 #2

C

B

A

N

Distribution

Feeders and Loads

Tertiary Winding Filter

#3

Fig. 8 Topology of the proposed tertiary winding filter.

1 3 5 7 9 11 13 15 17 190

100

200

300

400

500

600

700

800

900

1000

Harmonic Order

Filte

r Im

pe

da

ne

ce

()

Zero sequence

Positive/Negative sequence

Fig. 9 Frequency response of a sample tertiary winding filter.

The system equivalent circuit at these tuned frequencies

seen from the secondary side of the substation transformer is

shown in Fig. 10.

Transmission System


Distribution

Feeders and LoadsBus

/ /0( )eq

DownZ h / /0( )eq

DownI h / /0 ( )eq

UpZ h

_1 0 _1Xfrm XfrmR jh L _ 2 0 _ 2Xfrm XfrmR jh L

_ 3XfrmR

#1 #2

#3

Fig. 10 Equivalent circuit of the tertiary winding filter at tuned frequencies.

Where

_1XfrmR and _1XfrmL represent the resistance and leakage

inductance of the substation transform-

er's primary winding (referred to the

secondary side)

_ 2XfrmR and _ 2XfrmL represent the resistance and leakage

inductance of the substation transform-

er's secondary winding

_ 3XfrmR represent the resistance of the substation

transformer's tertiary winding (referred

to the secondary side)

5

/ /0 ( )eq

UpZ h represents equivalent transmission sys-

tem harmonic impedance seen at the

primary side of the substation trans-

former (but referred to the secondary

side)

/ /0 ( )eq

DownZ h represents equivalent harmonic imped-

ance of distribution feeders and loads

/ /0 ( )eq

DownI h represents equivalent harmonic current

source of distribution feeders and loads

It can be seen that the tertiary winding’s impedance only

contains the resistive component since the reactive component

has been canceled out by the tuning capacitor. As a result, a

low impedance path separates the transmission and distribution

systems. Harmonics originated from the distribution system

will be bypassed by the tertiary winding before it can reach the

transmission system. In addition, typical voltage of a substa-

tion tertiary winding is 4.16kV to 13.8kV. Low voltage LC

components can be used to construct the filter, which results in

cost savings.

B. Equivalent Circuits of Tertiary Winding Filter

As shown in Fig. 8, the tertiary winding filter is composed

of two parts, i.e., the delta connection part (the delta loop) and

the star connection part (the shunt components at the tertiary

side).

Since for ZS harmonics, the star connected shunt compo-

nents at the tertiary side behaves as open circuit, the tertiary

winding filter ZS equivalent circuit only consists of the delta

connection parts (see Fig. 11(a)). For non-ZS harmonics, the

star connected shunt components at the tertiary side behave as

normal loads. Thus the tertiary winding filter non-ZS equiva-

lent circuit consists of both the delta connection part and the

star connection part (see Fig. 11(b)).

Based on the above characteristics of the tertiary filter's

equivalent circuit, the delta connection part is designed to form

a double tuned filter for the ZS harmonics and the delta con-

nection part and the star connection part together are designed

to form a triple tuned filter for the non-ZS harmonics.

Since the delta connection part is essentially the same as the

conventional double tuned filter, its components selection can

use the existing approach directly. The selection of star con-

nection part components can be illustrated intuitively by Fig.

12. As shown in Fig. 12, the delta connection part has a fre-

quency response with two series resonant tuned points (at the

two designed ZS harmonic orders h1 and h2), and one parallel

resonant point (between the two designed ZS harmonic or-

ders). The desired three non-ZS harmonic orders to filter de-

termine the crossing points of the delta connection part fre-

quency response and the star connection part. Thus the rough

shape of the star connection part frequency response is deter-

mined. According to this, the components size of the star con-

nection part could be obtained based on rigorous mathematical

equations described in Section IV.

2

2a C2

2 / aL

2

_ 3 / aXfrmL

2

_ 3 / aXfrmR

Delta connection part2

1a C

2

1 / aL

2

3a / 3C2

4a / 3C

Star connection part

2

33 / aL

2

2a C2

2 / aL

2

_ 3 / aXfrmL

2

_ 3 / aXfrmR

2

1a C

2

1 / aL

Delta connection part

2

43 / aL

(a) Equivalent ZS circuit (b) Equivalent non-ZS circuit

Fig. 11 Equivalent circuit of the tertiary winding filter.

Fig. 12 Frequency response of delta connection part and star connection part

of the tertiary winding filter.

C. Performance Analysis

This subsection is to compare the relative size of the prima-

ry impedance versus tuned tertiary impedance and show that

most harmonics will enter the tertiary path.

According to Fig. 10, for the tertiary winding filter to be ef-

fective, the equivalent impedance of the tertiary winding filter

should be less than the transmission system impedance plus

substation transformer primary side winding's impedance at

corresponding frequencies. This is an easily satisfied condi-

tion. Intuitively, the minimum impedance of the tertiary wind-

ing filter at tuned harmonic orders could be achieved as the

substation transformer tertiary winding resistance referred to

the secondary side by proper selection of the capacitors and

inductors. According to the typical parameters of three wind-

ing transformer, the tertiary winding resistance is comparable

to the primary winding resistance. And for a substation trans-

former, its reactance resistance ratio is usually very high which

means 0 _1 _1Xfrm XfrmL R .

Thus

'

_ 3 _1 0 _1

_1 0 _1 / /0 ( )

Xfrm Xfrm Xfrm

eq

Xfrm Xfrm Up

R R jh L

R jh L Z h

(1)

which means the impedance of the tertiary winding filter is far

smaller than upstream system impedance. In this way, harmon-

ics at tuned frequencies will be trapped into the tertiary side

6

rather than propagating into the transmission system.

A rough estimation of the percentage of the harmonic cur-

rent that will be trapped by the tertiary winding filter could be

obtained by the following equation:

0 _1

'

_ 3 0 _1

100%Xfrm

Xfrm Xfrm

h LRatio

R h L

(2)

where h is the tuned harmonic order.

To show the performance of the tertiary winding filter,

some of the typical sizes of the three winding transformers

used by the utility company are listed in Table I. The percent-

age of the third harmonic current that will be trapped by the

tertiary winding filter is provided in the last column. For high-

er tuned order harmonics, larger percentage will be trapped by

the tertiary winding filter.

TABLE I: TRAPPED RATIO FOR DIFFERENT SIZES OF THREE WINDING

TRANSFORMERS

Transformer Trapped

Ratio

(%)

Rated

Capacitya

(MVA)

Rated

Voltageb

(kV)

Short Circuit

Impedance

(%)

On-Load

Loss

(kW)

20/20/10 144/25/6.3

H-M 10.5

106.3 98.41 H-L 18

M-L 6.5

25/25/8 144/25/13.8

H-M 10.5

125.8 97.67 H-L 18

M-L 6.5

40/40/13.3 144/25/6.3

H-M 10.5

178.5 97.91 H-L 17

M-L 6.5

50/50/16.7 144/25/13.8

H-M 10.5

212.5 95.73 H-L 18

M-L 6.5 a The rated capacity of each winding. b The nominal line-to-line voltage (LL-rms)

IV. TERTIARY WINDING FILTER DESIGN

Similar to the design of the transformer based double tuned

ZS filter [5], the design of the tertiary winding filter is also an

iterative process based on the system harmonic load flow study

and components loading assessment. However the tertiary

winding filter design does not need to select the transformer

size, since the size of the distribution substation transformer is

primarily determined by the loads served by the substation.

The tertiary winding filter design is just the proper selection of

LC components to coordinate the transformer to trap corre-

sponding harmonics into its tertiary windings. The flowchart of

design procedure for the tertiary winding filter is shown as Fig.

13.

A. LC Components' Size Determination

As shown in Fig. 13, the determination of components’ size

consists of two steps. The first step is to determine the delta

connection part components size. As seen from Fig. 11(a), the

delta connection part is actually the same to the conventional

double tuned filter. Therefore, the proper component sizes of

the delta connection part can be derived by using the same

mathematical approach derived for the conventional double

tuned filter. According to [5] [6], the component sizes of the

delta connection part can be determined by the following equa-

tion set (3).

Set

1 _ 3XfrmL L

0, 0

Calculate the delta connection part

components’parameters

Are the delta connection part

components’ parameters reasonable?No

3 _ 3

1

3XfrmL L

Yes

0.1

Calculate the star connection part

components’ parameters

Are the star connection part

components’ parameters’ reasonable?No 0.1

Perform HLF to examine the filter performance in the system

Yes

Can the filter mitigate the problem effectively?

Overloading of the transformer winding (TLL>1)?

Yes

Overloading in capacitors?

No

The Final Filter Design

No

No

Yes

Increase the capacitor rating and Kvar without changing its

capacitance (uF)Yes

Fig. 13 Flowchart of design procedure for the tertiary winding filter.

1

2 2

2

_ 3 1

2 2

2 2

0 _ 3 1

2 2

0 _ 3 1

2

2 ( )

( )

( )( )

( )

1

( )

1

( )

i j

i j i j

i j

Xfrm i j

i j

i

i Xfrm

j

j Xfrm

C CC

C C C CC

C C

L L C CL

C C

Ch L L

Ch L L

(3)

where hi and hj represent the ZS tuned order of the tertiary

winding filter.

Once C1, C2 and L2 are determined, the star connection part

components parameter could be determined by the following

way. The non-ZS impedance of the tertiary winding filter at

harmonic order could be expressed as

2

_ 3 / _ 3

( )( ) 1/ a

( )

eq

Xfrm Xfrm

f hZ h R j

g h

(4)

7

where

2 2

0 1 1 3 _ 3

2 2 2 2

0 2 2 0 2 1

2 2

3 4 0 4 3 4

2 2 2 2

1 0 2 2 0 4 3

( ) 1 ( )

(1 )

( )

3 (1 )(1 )

Xfrmf h h C L L L

h L C h L C

C C h L C C

C h L C h L C

(5)

2 2 2 2

0 1 0 2 2 3 4 0 4 3 4( ) (1 )( )g h h C h L C C C h L C C (6)

Since the filter is tuned to have three tuned posi-

tive/negative frequencies, then f(h)=0 at corresponding har-

monic orders h1, h2 and h3, i.e.,

1

2

3

( ) 0

( ) 0

( ) 0

f h

f h

f h

(7)

By solving equations set (7), C3, C4 and L4 are determined.

It is important to note that two compensation inductors L1

and L3 are used in the tertiary winding filter, due to the leakage

inductance of the transformer which is usually small may result

in extremely large capacitance needed. By adjusting the com-

pensation ratios α and β (shown in Fig. 13), tertiary winding

filter with capacitors and inductors of achievable parameters

could be obtained and the reactive power compensated by the

filter could also be set to be the required value.

B. Components Loading Assessment

Components are susceptible to failures and even breakdown

if the voltages across them or the currents flowing through

them exceed a certain degree during a certain period of time

[4]. Thus the components loading assessment is an important

part of the filter design.

1) Transformer Loading Assessment

The transformer loading condition is evaluated by the index

TLL (Transformer Loading Level) which is developed in [5].

The TLL is defined as follows:

TLL

TLL rated

PTLL

P

(8)

where PTLL represents the winding total load loss during filter

operation, the winding maximum permissible loading capacity

is represented by PTLL-rated, which is the winding loading loss

under a rated sinusoidal current. Therefore, as long as TLL of

each winding does not exceed 1 pu, the transformer operates

safely without overheating. If the TLL of one winding exceeds

1pu (indicating the winding is overloaded), the compensation

ratio should be adjusted.

2) Capacitor Loading Assessment

For assessing the loading of the capacitors, the equivalent

loading index based on research findings of partial-discharge

caused capacitor aging is used. This equivalent loading index

is described by the following equation.

( ) ( ) ( )p frmsn nn

eq p rms fV K K K (9)

where

pn , rmsn and

fn are coefficients that describe the signifi-

cance of each factor. Their values are

dependent on the type of films used in

the capacitor.

pK , rmsK and

fK are indices describing the waveform

experienced by the capacitor, as follows

*

1

p

p

p

VK

V (10)

*

1

rms

rms

rms

VK

V (11)

2

2

1 1

Nh

p

h

VK h

V

(12)

where

pV is the peak value of the distorted voltage

*

1pV is the peak value of the rated fundamental frequen-

cy voltage

rmsV is the rms value of the distorted voltage

*

1rmsV is the rms value of the rated fundamental frequen-

cy voltage

h is the harmonic order

hV is the h order harmonic voltage.

The physical meaning of this index is that it represents a

normalized composite or equivalent voltage applied to a ca-

pacitor. If the voltage is above one, the capacitor is considered

as overloaded and its life will be shortened. If the value is less

than one, the capacitor is considered as operating within its

design limits.

3) Inductor Loading Assessment

As for the inductor, the current flowing through it should be

less than its current rated rms value. Thus the loading level of

the inductor could be defined as

*

1

rms

eq

rms

II

I (13)

where

rmsI is the rms value of the distorted current

*

1rmsI is the rms value of the rated fundamental frequen-

cy current.

V. SIMULATION STUDIES

Two test systems are selected to verify the proposed tertiary

winding filter and its design procedure as well as to examine

8

its system performance.

A. Simulation Studies on Distribution System

This subsection conducts the simulation studies on test sys-

tem #1 a generic distribution system supplying residential

loads which are evenly distributed along five feeders. Fig. 14

depicts the network configuration of test system #1, in which

each section block consists of three service transformers which

are connected to the secondary system serving 10 residential

houses. Transmission

System

Zup

Substation

Transformer

#1 #2

#3

Primary System

S72

VSC

VSA

VSB

A B C N

Line

S1

Rg

S2

Feeder 1

S72S1 S2

Feeder 2

S72S1 S2

Feeder 5

Section

Fig. 14 Network configuration of test system #1.

1) Tertiary Winding Filter Design Results

The introduced iterative process (Fig. 13) was employed to

determine the final filter design. Table II presents the compo-

nents size of the designed tertiary winding filter.

TABLE II: DESIGNED COMPONENTS SIZE OF THE TERTIARY WINDING FILTER


L1 None

C1 160.43kvar (0.6 kV)a

L2 0.235mH(794A)b

C2 90.24kvar (0.6kV)


L3 0.490mH(794A)

C3 1033.35kvar(4.16kV)

L4 1.702mH(794A)

C4 637.08kvar (4.16kV) a The voltage in the bracket is the capacitor rated voltage. b The current in the bracket is the inductor rated current, the inductor rated

voltage is 6.3kV.

2) Simulation Results

All the developed models are employed in a multiphase

harmonic power flow program to perform the simulation stud-

ies. As the residential loads are time-varying during a day,

time-varying simulation results are obtained.

In order to establish a sound understanding of the proposed

tertiary winding filter performance, the simulation results for

both the case without tertiary winding filter and the case with

tertiary winding filter are shown in Fig. 15 to Fig. 18.

As seen from Fig. 15 to Fig. 18, both the harmonic currents

propagating into the transmission system and the harmonic

voltages at the primary side of the substation transformer have

been greatly reduced, which demonstrates the effectiveness of

the proposed filter.

1 3 5 7 9 11 13 15 17 19 21 230

2

4

6

8

10

Time (h)

%

TDD -- Without Tertiary Winding Filter

Phase A

Phase B

Phase C

1 3 5 7 9 11 13 15 17 19 21 230

2

4

6

8

10

Time (h)

%

TDD -- With Tertiary Winding Filter

Phase A

Phase B

Phase C

(a) (b)

Fig. 15 TDD variation of the currents propagating into the transmission sys-

tem.

(a) (b)

1 3 5 7 9 11 13 150

1

2

3

4

5

Harmonic Order

%

IDD Spectrum -- Without Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

1 3 5 7 9 11 13 150

1

2

3

4

5

Harmonic Order

%

IDD Spectrum -- With Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

Fig. 16 Typical IDD spectrum of the currents propagating into the transmis-

sion system.

(a) (b)

1 3 5 7 9 11 13 15 17 19 21 230

1

2

3

4

5

Time (h)

%

THD -- Without Tertiary Winding Filter

Phase A

Phase B

Phase C

1 3 5 7 9 11 13 15 17 19 21 230

1

2

3

4

5

Time (h)

%

THD -- With Tertiary Winding Filter

Phase A

Phase B

Phase C

Fig. 17 THD variation of the voltages at the primary side of the substation

transformer.

(a) (b)

1 3 5 7 9 11 13 150

0.5

1

1.5

2

2.5

Harmonic Order

%

IHD Spectrum -- Without Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

1 3 5 7 9 11 13 150

0.5

1

1.5

2

2.5

Harmonic Order

%

IHD Spectrum -- With Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

Fig. 18 Typical IHD spectrum of the voltages at the primary side of the sub-

station transformer.

B. Simulation Studies on Transmission System

This subsection conducts several simulation studies on test

system #2 an extension of the IEEE 14 bus transmission sys-

tem proposed in [10] aiming to further examine:

Will the distribution harmonic loads at other bus lead to the

tertiary winding filter overloading?

Is it essential to equip all the buses with distribution har-

monic loads with the tertiary winding filter?

Fig. 19 presents the network configuration of test system

#2.

1) Influence of Distribution Harmonic Loads at Other Bus-

es

In the previous sections, the tertiary winding filter was

thoroughly examined in the distribution system. However in

the tertiary winding filter design, loading assessment was con-

ducted without considering the influence of distribution har-

monic loads at other buses. Will this be an issue for the tertiary

9

winding filter if there are multiple distribution harmonic loads

at other buses in the transmission system? To answer this ques-

tion, eight sets of cases are studied. Each set of cases consists

of two cases: 1) for the first case there is no other distribution

harmonic load at other buses except where the tertiary winding

filter is installed; 2) for the second case all the load buses are

modified as distribution harmonic loads. The detailed descrip-

tion of these cases is follows:

Case

Set Case

Filter

Placement Distribution Harmonic Loads Location

1

i Bus 4 Bus 4

ii Bus 4 Bus 4, Bus 5, Bus 9, Bus 10, Bus 11,

Bus 12, Bus 13, Bus 14

2

i Bus 5 Bus 5



3

i Bus 9 Bus 9



4

i Bus 10 Bus 10



5

i Bus 11 Bus 11



6

i Bus 12 Bus 12



7

i Bus 13 Bus 13



8

i Bus 14 Bus 14



G

G

G

1

2

3

5

6

12

13

11 10

14

9

4

7

8

SVC

Converter

Fig. 19 Network configuration of test system #2

Loading assessment for all eight case sets show that the dis-

tribution harmonic loads at other buses do have influence on

the loading level of the tertiary winding filter. But the influ-

ence is different for the tertiary winding filter at different loca-

tions. According to Case Set 4, 5 and 6, the tertiary winding

filter designed based on the distribution harmonic load infor-

mation at its own bus will be overloaded by distribution har-

monic loads at other buses. Thus if multiple distribution har-

monic loads exist, the transmission system harmonic power

flow should be incorporated into the tertiary winding filter

loading assessment and components of larger size should be

adopted when overloading issues are identified.

2) Installation Density Study

This subsection presents the sensitivity study to assess the

influence of the installation density of the tertiary winding fil-

ter on the overall transmission system harmonic distortion lev-

el. Simulation results for the following cases are compared in

Table IV and Table V.

i. Test system # 2 with the loads at bus 4, bus 5, bus 9, bus

10, bus 11, bus 12, bus 13 and bus 14 all modified as a

distribution harmonic load.

ii. Based on i, install one tertiary winding filter at any one

of the buses with distribution harmonic loads.

iii. Based on i, install one tertiary winding filter at any two

of the buses with distribution harmonic loads respective-

ly.

iv. Based on i, install one tertiary winding filter at any

three of the buses with distribution harmonic loads re-

spectively.

…

ix. Based on i, install one tertiary winding filter at all buses

with distribution harmonic loads respectively.

In Table III, the average voltage THD for 230kV buses,

115kV buses and the overall system is given, while in Table

IV the average current TDD for 230kV lines, 115kV lines,

transmission transformers and the overall system is given. As

shown in these two tables, the more the tertiary winding filter

installed the lower the overall transmission system harmonic

distortion level is in terms of both the bus voltage THD and

transmission equipment TDD.

TABLE III: VOLTAGE DISTORTION LEVEL

Case

Average Voltage THD (%)

230kV

Buses

115kV

Buses Overall System

i 8.45 9.83 9.24

ii 8.26 8.90 8.63

iii 8.10 8.05 8.07

iv 7.82 7.23 7.48

v 7.21 6.31 6.70

vi 6.17 5.26 5.65

vii 4.81 4.12 4.41

viii 3.32 3.02 3.15

ix 1.70 1.94 1.84

TABLE IV: CURRENT DISTORTION LEVEL

Case

Average Current TDD (%)

230kV Lines 115kV Lines Transmission

Transformers

Overall

System

i 6.56 18.88 18.34 14.23

ii 6.20 18.79 17.68 13.92

iii 5.89 18.44 16.96 13.50

iv 5.54 17.90 16.18 12.98

v 5.02 17.15 15.33 12.30

vi 4.28 16.11 14.37 11.39

vii 3.37 14.76 13.31 10.26

viii 2.39 12.92 12.17 8.88

ix 1.35 11.46 10.90 7.62

10

Fig. 20 depicts the average and minimum voltage distortion

level and current distortion level for each filter installation

density.

Fig. 20 Average minimum harmonic distortion level for different cases

Table V further gives the filter placement which results the

minimum harmonic distortion level for case ii ~ case viii and

the corresponding overall system voltage distortion level and

current distortion level with such placement.

TABLE V: OPTIMUM FILTER PLACEMENT FOR DIFFERENT CASES

Case Optimum Filter Placement

Overall System

Harmonic Distortion

Voltage

THD* (%)

Current TDD*

(%)

ii Bus 9 6.70(8.63) 12.14(13.92)

iii Bus 9, Bus 14 5.61(8.07) 10.49(13.50)

iv Bus 9, Bus 10, Bus 14 4.92(7.48) 10.09(12.98)

v Bus 9, Bus 10, Bus 14, Bus 4 3.86(6.70) 8.83(12.30)

vi Bus 9, Bus 10, Bus 14,

Bus 4, Bus 5 2.93(5.65) 8.37(11.39)

vii Bus 9, Bus 10, Bus 14,

Bus 4, Bus 5, Bus 13 2.25(4.41) 8.31(10.26)

viii Bus 9, Bus 10, Bus 14, Bus 4,

Bus 5, Bus 13, Bus 12 2.01(3.15) 7.87(8.88)

*Figure in the bracket is the average value for each case.

As indicated by Fig.20 and Table V, with optimum filter

placement, better harmonic mitigation effects can be achieved.

Another useful information could be extracted from Table V is

the distribution harmonic loads at Bus 9 have the largest im-

pacts on the overall system distortion level since for all the

optimum filter placement one tertiary winding filter should be

placed at Bus 9. Possible explanation for this is that the trans-

fer impedance between Bus 9 and other buses are much larger

than that between other buses. Further frequency scan studies

are needed to confirm this.

VI. COMPARATIVE ECONOMIC ANALYSIS

The cost of the proposed tertiary winding filter has been in-

vestigated using the test system #1. It is also compared with

the cost of the MV LC filter package.

To obtain a meaningful comparison, the MV LC filter

package is designed to achieve the same harmonic distortion

reduction with the tertiary winding filter specified by Table II.

The designed MV LC filter package shown as Fig. 21 consists

of two inductor grounded single tuned LC filters which are

tuned to filter the 3rd , 5th and 9th and 11th harmonics respec-

tively and one directly grounded single tuned LC filter which

is tuned to filter the 7th harmonic. The designed components

size of the MV LC filter package is given in Table VI.

TABLE VI: DESIGNED COMPONENT SIZE OF MV LC FILTER PACKAGE

Filter

Design

Tuned Harmonic

Order Component Size

Filter 1 3, 5

L 56.141mH (208A)

C 1875.28kvar (31.5kV)

L0 33.269mH (416A)

Filter 2 7 L 28.071mH (69A)

C 488.77kvar (15.92kV)

Filter 3 9, 11

L 11.228mH (69A)

C 494.84kvar (15.92kV)

L0 1.848mH (69A) a The voltage in the bracket is the capacitor rated voltage.

b The current in the bracket is the inductor rated current, the

inductor rated voltage is 14.4kV.

For the cost estimation, the data collected from [5] and the

internet are used. Table VII to Table X show the approximate

expenses with filter components and installation.

TABLE VII: COST OF CAPACITORS

Rated

voltage <1kV 1kV~10kV 10kV~25kV 25kV~50kV

Cost $15/kvar $20/kvar $40/kvar $65/kvar

TABLE VIII: COST OF INDUCTORS

Voltage Level

1kV~10kV 10kV~25kV

Rated Current

<100A $1400 $2800

100A~500A $1700 $3400

500A~1000A $1900 $3800

TABLE IX: COST OF OTHER COMPONENTS

1kV~10kV 10kV~25kV

Switching Device $17,000 $34,000

Protection Device $3,500 $7,000

TABLE X: ESTIMATED COST FOR FILTER INSTALLATION

Civil $7,000

Engineering & Design $28,000

Labor Costa $12,000

a Labor cost is based on: 3 persons * 8hour/day *5days *$100/hour

By the above set of information, the overall cost of the ter-

tiary winding filter and MV LC filter package is estimated as

presented in Table XI. As seen from Table XI, the tertiary

winding filter is found be much more economical.

TABLE XI: OVERALL ESTIMATED COST

Tertiary Winding Filter MV LC Filter Package

Components

Cost $149,110 $557,910

Installation

Cost $40,700 $40,700

Total Cost $189,810 $598,610

VII. CONCLUSIONS

This paper presented a novel and effective filter to prevent

harmonic currents propagating into the transmission system.

The design issues of the proposed filters are fully investigated.

The main findings and contributions of this paper could be

summarized as follows.

The proposed tertiary winding filter could trap two ZS har-

monics and three non-ZS harmonics simultaneously. It be-

comes a very desirable method to mitigate harmonic cur-

rents with a wide spectrum.

11

The proposed filter and its design method have been tested

through two comprehensive test systems. The results have

demonstrated the effectiveness of the filter and also reveal

proper placement could achieve the same reduction of the

overall system harmonic distortion level with fewer filters.

Compared to other transformer based filters, the most im-

portant benefit of the proposed tertiary filter is that it utiliz-

es the existing substation transformer which greatly reduces

the filter cost. Furthermore, actually three winding trans-

formers are widely used by utility companies in Canada as

distribution substation transformer and their common con-

nection is Yg/yg/delta with no loads served by the delta

connected tertiary windings. This means the proposed ter-

tiary filter has a bright application prospect.

Economic analysis further confirmed that the proposed ter-

tiary winding filter is a cost effective solution for prevent-

ing harmonics' propagation from the distribution system in-

to the transmission system.

VIII. REFERENCES

[1] "IEEE Recommended Practices and Requirements for Harmonic Con-

trol in Electrical Power Systems," IEEE Std 519-1992, pp. 1-112, 1993.

[2] J. Aririllaga, D. A. Bradley , P. S. Bodger, Power System Harmonics:

John Wiley & Sons Ltd, 1985.

[3] IEC Standard 61000-3-2: Electromagnetic compatibility (EMC) Part 3-

2: Limits - Limits for harmonic current emissions (equipment input cur-

rent ≤16A per phase), 2009.

[4] A. B. Nassif, X. Wilsun, and W. Freitas, "An Investigation on the Selec-

tion of Filter Topologies for Passive Filter Applications," Power Deliv-

ery, IEEE Transactions on, vol. 24, pp. 1710-1718, 2009.

[5] P. Bagheri, "Methods to mitigate harmonics in residential power distri-

bution systems," M. Sc. dissertation, Dept. Elect. Comput. Eng., Univ.

Alberta, Edmonton, AB, Canada, 2013.

[6] P. Bagheri and W. Xu, "A Technique to Mitigate Zero-Sequence Har-

monics in Power Distribution Systems," IEEE Trans. Power Del., vol.

29, pp. 215-223, Feb. 2014.

[7] D. Salles, J. Chen, W. Xu, W. Freitas, and H. E. Mazin, "Assessing the

Collective Harmonic Impact of Modern Residential Loads - Part I

Methodology," IEEE Trans. Power Del., vol. 27, pp. 1937-1946, Oct.

2012.

[8] J. Chen, D. Salles, W. Xu, and W. Freitas, "Assessing the Collective

Harmonic Impact of Modern Residential Loads - Part II: Application,"

IEEE Trans. Power Del., vol. 27, pp. 1947-1955, Oct. 2012.

[9] W. Xu, J. R. Marti, and H. W. Dommel, "A multiphase harmonic load

flow solution technique," Power Systems, IEEE Transactions on, vol. 6,

pp. 174-182, 1991.

[10] R. Abu-Hashim, R. Burch, G. Chang, M. Grady, E. Gunther, M. Halpin,

et al., "Test systems for harmonics modeling and simulation," Power

Delivery, IEEE Transactions on, vol. 14, pp. 579-587, 1999.

Pooya Bagheri obtained the B.Sc. degree in electrical engineering from

Sharif University of Technology, Tehran, Iran, in 2010 and the M.Sc. degree

in electrical engineering from the University of Alberta, Edmonton, AB,

Canada, in 2013. Currently, he is with the Electrical Engineering Department,

Oil & Gas segment, Stantec Consulting Ltd., Edmonton, AB, Canada. His

research interests are power quality and power distribution systems.

Wilsun Xu (M’90-SM’95-F’05) obtained the Ph.D. degree from the Universi-

ty of British Columbia, Vancouver, in 1989. Currently, he is a

NSERC/iCORE Industrial Research Chair Professor at the University of

Alberta. His current main research interests are power quality and power

disturbance analytics.

Tianyu Ding (S'12) obtained the B.Sc. degree in electrical engineering from

Shandong University, Jinan, China, in 2010. Currently, he is pursuing his

Ph.D. degree in Electrical and Computer Engineering at the University of

Alberta, Edmonton, AB, Canada. His main research interest is power quality.

1

Abstract—The widespread adoption of energy efficient but

harmonic-producing loads in residential homes has led to in-

creased injection of harmonic currents into transmission systems.

In response to this situation, a novel filtering scheme that can

reduce the harmonic penetration into transmission systems is

proposed. The scheme utilizes the tertiary winding of a substation

transformer to construct a tuned low impedance path for the

harmonic currents. The proposed scheme is capable of trapping

two zero-sequence harmonics and three non-zero-sequence har-

monics simultaneously. Design procedure has been developed.

Performance of the scheme is evaluated and demonstrated

through simulation studies. In addition, economic analysis reveals

that the scheme is a cost-effective solution in comparison with

other applicable methods.

Index Terms—Power quality, harmonics, passive filter.

I. INTRODUCTION

N recent years, the proliferation of energy efficient but har-

monic-producing home appliances and consumer electronic

devices has resulted in a new type of harmonic distortions in

power distribution systems. These new harmonic sources have

comparable sizes and are distributed all over a network. Alt-

hough they produce insignificant amount harmonic currents

individually, the collective effect of a large number of such

loads can be substantial [1-6]. Several power quality concerns

have been identified due to such distributed harmonic sources

[1-3, 5-9]. One of them is that distribution systems are inject-

ing significant amount of harmonic currents into transmission

systems [2, 6]. The consequences include overloading of

transmission capacitors, tripping of HVDC filters, and in-

creased incidents of transmission harmonic resonances [10-12].

It is common nowadays that transmission and distribution

systems are owned by different owners due to electricity mar-

ket deregulation. The penetration of harmonic currents from

distribution systems into transmission systems is becoming a

sensitive issue. For example, several jurisdictions have im-

posed limits on such harmonics [13-14]. Therefore, both types

of companies will benefit from a solution that can prevent dis-

tribution system harmonics from penetrating into the transmis-

sion systems. Distribution system owners may use the solution

to meet the interconnection requirements regardless the har-

monic distortion levels inside the distribution systems. The

This work was supported by the Natural Sciences and Engineering Re-

search Council of Canada and Alberta Power Industry Consortium.

The authors are with the department of Electrical and Computer Engineer-

ing, University of Alberta, Edmonton, AB T6G 2V4, Canada (email:

[email protected]).

transmission owner can easily recognize the solution taken by

the distribution company and verify its effectiveness.

Motivated by the above considerations, this paper presents

a novel filtering scheme that can reduce the harmonic injection

from distribution systems into transmission systems. The basic

idea is to utilize the tertiary winding of the substation trans-

former to create a tuned low impedance path for the harmon-

ics. The proposed filtering scheme is therefore called “tertiary

winding filter”. In the following sections, subjects such as fil-

ter topology, design method, performance assessment and cost

analysis are presented.

II. REVIEW OF APPLICABLE MITIGATION SCHEMES

Although little research work has been done in the direction

of preventing harmonics from entering transmission systems,

techniques that can be adopted for the purpose do exist. These

techniques can be classified into two types. One is to prevent

or reduce the injection of zero-sequence (ZS) harmonics into

the transmission system. The other type is to deal with the pos-

itive and negative harmonics. These harmonics are collected

termed as non-zero-sequence (NZS) harmonics in this paper.

A. Zero-Sequence Harmonic Mitigation Techniques

ZS components are dominant in the 3rd

, 9th

and other triple-

order harmonics. Single phase non-linear loads such as energy

efficient home appliances are the most significant source of ZS

harmonics. It is quite common to observe high levels of ZS

harmonics in a distribution substation feeding residential loads

these days [2, 5, 9].

1) Transformer Connection

The simplest way to prevent ZS harmonics' propagation in-

to transmission systems is to configure the substation trans-

former’s primary side into delta or ungrounded Wye connec-

tion. For such connections, there are paths for the ZS harmonic

currents to flow into the primary side. For cases where the

primary side must be grounded, a grounding transformer [15]

may be installed at the primary bus to serve as the grounding

point.

2) Passive Zero-Sequence Filter

Shunt passive ZS filters create low ZS impedance to trap

the ZS harmonics. They have various topologies and can be

broadly classified into two types. The first type is the LC filter.

A representative example of such filters is the star-connected

capacitors grounded through an inductor [16]. This filter has

positive/negative sequence impedance so it affects the flow of

NZS harmonics. The second type is the transformer based ZS

Tianyu Ding, Student Member, IEEE, and Wilsun Xu, Fellow, IEEE

A Filtering Scheme to Reduce the Penetration of

Harmonics into Transmission Systems (Final)

I

2

filter which is derived from the grounding transformer. This

type has no impact on the normal power system operation and

on NZS harmonics since they behave as open circuit at posi-

tive and negative sequences. Examples include the zig-zag

transformer based filter [17] and the Yg/delta transformer

based single tuned ZS filter [18] and double tuned ZS filter

[19].

B. Non-Zero-Sequence Harmonic Mitigation Technique

NZS components are dominant in the 5th

, 7th

and other non-

triple order harmonics. All non-linear loads generate NZS

harmonics. As a result, NZS harmonic distortion level is usual-

ly higher than the ZS harmonic distortion level. At present, the

only known method to mitigate NZS harmonic in medium-

voltage (MV) or high-voltage (HV) systems is the passive

shunt filters. Commonly used LC NZS filters include the tuned

filter, damped filter and the C type filter [20-22]. These filters

can be installed in the secondary side of the substation, creat-

ing a low impedance path to trap the harmonics originated

from the distribution systems.

C. Summary

It can be seen that the options to prevent the harmonics

from entering transmission systems are limited. Some of them

are adopted from the methods developed for other applica-

tions. It is, therefore, worthwhile to investigate solutions that

are dedicated to addressing the problem and, thus, can take

advantages of the specific characteristics of the substation con-

figurations.

III. PROPOSED TERTIARY WINDING FILTER

In view that many substation transformers have a tertiary

winding [23], it may be possible to utilize the substation trans-

former as a filtering transformer on top of its power transmis-

sion function. The need for a dedicated filtering transformer is

thus eliminated. This reasoning has led us to propose a new

filtering scheme called “Tertiary Winding Filter”.

A. Basic Principle of the Tertiary Winding Filter

The basic idea of the tertiary winding filter is to utilize the

leakage inductance of the tertiary winding to create a low im-

pedance path to trap harmonic currents at tuned frequencies.

This is achieved by inserting capacitors and inductors into the

delta loop of the tertiary winding for ZS harmonic filtering and

by connecting shunt capacitors and inductors to the tertiary

winding for NZS harmonic filtering. Topology of the filter is

depicted in Fig. 1.

The system equivalent circuit at these tuned frequencies

seen from the secondary side of the substation transformer is

shown in Fig. 2.

In Fig.2, _1XfrmR and _1XfrmL represent the leakage re-

sistance and leakage inductance of the substation transformer's

primary winding (referred to the secondary side), _ 2XfrmR and

_ 2XfrmL represent the leakage resistance and leakage induct-

ance of the substation transformer's secondary winding,

_ 3XfrmR represents the leakage resistance of the substation

transformer's tertiary winding (referred to the secondary side),

/ /0 ( )eq

UpZ h represents equivalent transmission system harmon-

ic impedance seen at the primary side of the substation trans-

former (but referred to the secondary side), / /0 ( )eq

DownZ h and

/ /0 ( )eq

DownI h represent equivalent harmonic impedance and

harmonic current source of distribution feeders and loads.

Transmission System


#1 #2

C

B

A

N

Distribution

Feeders and Loads

Tertiary Winding Filter

#3

SAV

SBV

SCV

up

Z

Bus

Fig. 1 Topology of the proposed tertiary winding filter.

Transmission System


Distribution Feeders and Loads

Bus

/ /0( )eq

DownZ h / /0( )eq

DownI h / /0 ( )eq

UpZ h

_1 0 _1Xfrm XfrmR jh L _ 2 0 _ 2Xfrm XfrmR jh L

_ 3XfrmR

#1 #2

#3

Fig. 2 Equivalent circuit of the tertiary winding filter at tuned frequencies.

It can be seen that the tertiary winding’s impedance only

contains the resistive component, i.e. the leakage resistance of

the tertiary winding, since the reactive component has been

canceled out by the tuning capacitors. As a result, a low im-

pedance path separates the transmission and distribution sys-

tems. Harmonics originated from the distribution system will

be bypassed by the tertiary winding before it can reach the

transmission system. In addition, typical voltage of a substa-

tion transformer tertiary winding is 4.16kV to 13.8kV. Low

voltage LC components can be used to construct the filter,

which results in cost savings.

B. Equivalent Circuits of Tertiary Winding Filter

As shown in Fig. 1, the tertiary winding filter is composed

of two parts, i.e., the delta connection part (the delta loop) and

the star connection part (the shunt components at the tertiary

side).

Since for ZS harmonics, the star connected shunt compo-

nents at the tertiary side behaves as open circuit, the tertiary

winding filter ZS equivalent circuit only consists of the delta

connection parts (see Fig. 3(a)). For NZS harmonics, the star

3

connected shunt components at the tertiary side behave as

normal loads. Thus the tertiary winding filter NZS equivalent

circuit consists of both the delta connection part and the star

connection part (see Fig. 3(b)). It should be noted that in Fig.

3, a represents the tertiary winding to secondary winding turn

ratio.

2

2a C2

2 / aL

2

_ 3 / aXfrmL

2

_ 3 / aXfrmR

Delta connection part2

1a C

2

1 / aL

2

3a / 3C2

4a / 3C


2

33 / aL

2

2a C2

2 / aL

2

_ 3 / aXfrmL

2

_ 3 / aXfrmR

2

1a C

2

1 / aL


2

43 / aL

(a) Equivalent ZS circuit (b) Equivalent NZS circuit

Fig. 3 Equivalent circuit of the tertiary winding filter.

Based on the above characteristics of the tertiary winding

filter's equivalent circuit, the delta connection part is designed

to form a double tuned filter for the ZS harmonics and the del-

ta connection part and the star connection part together are

designed to form a triple tuned filter for the NZS harmonics.

C. Feasibility Analysis

This subsection is to compare the relative size of the prima-

ry impedance versus tuned tertiary impedance and to show that

most harmonics will enter the tertiary path.

According to Fig. 2, for the tertiary winding filter to be ef-

fective, the equivalent impedance of the tertiary winding filter

should be less than the transmission system impedance plus

substation transformer primary side winding's impedance at

corresponding frequencies. This is an easily satisfied condi-

tion. Intuitively, the minimum impedance of the tertiary wind-

ing filter at tuned harmonic orders could be achieved as the

substation transformer tertiary winding resistance referred to

the secondary side by proper selection of the capacitors and

inductors. According to the typical parameters of three wind-

ing transformer, the tertiary winding resistance is comparable

to the primary winding resistance. And for a substation trans-

former, its reactance resistance ratio is usually very high [24]

which means 0 _1 _1Xfrm XfrmL R .

Thus

'

_ 3 _1 0 _1

_1 0 _1 / /0 ( )

Xfrm Xfrm Xfrm

eq

Xfrm Xfrm Up

R R jh L

R jh L Z h

(1)

which means the impedance of the tertiary winding filter is far

smaller than upstream system impedance. In this way, harmon-

ics at tuned frequencies will be trapped into the tertiary side

rather than propagating into the transmission system.

A rough estimation of the percentage of the harmonic cur-

rent that will be trapped by the tertiary winding filter could be

obtained by the following equation:

0 _1

'

_ 3 0 _1

100%Xfrm

Xfrm Xfrm

h LRatio

R h L

(2)

where h is the tuned harmonic order.

To show the performance of the tertiary winding filter,

some of the typical sizes of the three winding transformers

used by the utility company are listed in Table I. The percent-

age of the third harmonic current that will be trapped by the

tertiary winding filter is provided in the last column. For high-

er tuned order harmonics, larger percentage will be trapped by

the tertiary winding filter.

TABLE I: TRAPPED RATIO FOR DIFFERENT THREE WINDING TRANSFORMERS

Transformer Parameter Trapped

Ratio

(%)

Rated

Capacitya

(MVA)

Rated

Voltageb

(kV)

Short Circuit

Impedance

(%)

On-Load

Loss

(kW)

20/20/10 144/25/6.3

H-M 10.5

106.3 98.41 H-L 18

M-L 6.5

25/25/8 144/25/13.8

H-M 10.5

125.8 97.67 H-L 18

M-L 6.5

32/32/16 144/25/4.16

H-M 10.5

148.8 98.55 H-L 17

M-L 6.5

40/40/13.3 144/25/6.3

H-M 10.5

178.5 97.91 H-L 17

M-L 6.5

50/50/16.7 144/25/13.8

H-M 10.5

212.5 98.11 H-L 18

M-L 6.5 a The rated capacity of each winding. b The nominal line-to-line voltage (LL-rms)

IV. TERTIARY WINDING FILTER DESIGN

The design of the tertiary winding filter is an iterative pro-

cess based on the system harmonic load flow study and com-

ponents loading assessment. The design objective is to deter-

mine proper LC component parameters based on the trans-

former parameters. The flowchart of design procedure is

shown as Fig. 4.

A. LC Components' Size Determination

As shown in Fig. 4, the determination of components’ size

consists of two steps. The first step is to determine the delta

connection part components size. As seen from Fig. 3(a), the

delta connection part is actually the same to the double tuned

filter. Based on analytical derivations, the component parame-

ters of the delta connection part can be determined by the

equation set (3).

4

Set

1 _ 3XfrmL L

0, 0

Size delta connection part components

Increase the

capacitor rated

voltage and kvar

Size star connection part components

Final Filter Design

Increase the

inductor rated

current and kvar

Overloading in

inductors?

Are the parameters

reasonable?

Overloading in capacitors?

Yes

No

Yes

3 _ 3 / 3XfrmL L

Are the parameters

reasonable?

Overloading in transformer windings?

No

No

Yes

No

Yes

0.1

No0.1

Yes

Fig. 4 Flowchart of design procedure for the tertiary winding filter.

1

2 2

2

_ 3 1

2 2

2 2

0 _ 3 1

2 2

0 _ 3 1

2

2 ( )

( )

( )( )

( )

1

( )

1

( )

i j

i j i j

i j

Xfrm i j

i j

i

i Xfrm

j

j Xfrm

C CC

C C C CC

C C

L L C CL

C C

Ch L L

Ch L L

(3)

where hi and hj represent the ZS tuned order of the tertiary

winding filter.

Once C1, C2 and L2 are determined, the star connection part

components parameter could be determined by the following

way. The NZS impedance of the tertiary winding filter at har-

monic order could be expressed as

_ 3 / _ 32

1 ( )( )

( )a

eq

Xfrm Xfrm

f hZ h R j

g h

(4)

where

2 2

0 1 1 3 _ 3

2 2 2 2

0 2 2 0 2 1

2 2

3 4 0 4 3 4

2 2 2 2

1 0 2 2 0 4 3

( ) 1 ( )

(1 )

( )

3 (1 )(1 )

Xfrmf h h C L L L

h L C h L C

C C h L C C

C h L C h L C

(5)

2 2 2 2

0 1 0 2 2 3 4 0 4 3 4( ) (1 )( )g h h C h L C C C h L C C (6)

Since the filter is tuned to three NZS frequencies, then

f(h)=0 at corresponding harmonic orders h1, h2 and h3, i.e.,

1 2 3( ) 0, , ,f h h h h h (7)

By solving equations set (7), C3, C4 and L4 are determined.

It is important to note that two compensation inductors L1

and L3 are used in the tertiary winding filter, due to the leakage

inductance of the transformer which is usually small may result

in extremely large capacitance needed. By adjusting the com-

pensation ratios α and β (shown in Fig. 4), tertiary winding

filter with capacitors and inductors of achievable parameters

could be obtained and the reactive power compensated by the

filter could also be set to be the required value.

B. Components Loading Assessment

Components are susceptible to failures and even breakdown

if the voltages across them or the currents flowing through

them exceed a certain degree during a certain period of time

[22]. Thus the components loading assessment is an important

part of the filter design. In the tertiary winding filter design,

the transformer loading condition is evaluated by the index

TLL (Transformer Loading Level) which is developed in [19].

For assessing the loading of the capacitors, the equivalent

loading index based on research findings of partial-discharge

caused capacitor aging is used [25]. As for the inductor, its

loading level is evaluated by the current flowing through it

normalized by its current rated rms value [20]. As long as the

components overloading is identified, corresponding adjust-

ments should be made as shown in Fig. 4.

V. SIMULATION STUDIES

Two test systems are selected to verify the proposed tertiary

winding filter and its design procedure as well as to examine

its system performance.

A. Simulation Studies on Distribution System

This subsection conducts the simulation studies on test sys-

tem #1 a generic distribution system supplying residential

loads which are evenly distributed along five feeders mainly in

support of the effectiveness of the proposed tertiary winding

filter and its design procedure. Table II gives the main parame-

ters of test system #1. Fig. 5 depicts its network configuration,

in which each section block consists of three service trans-

formers which connects secondary systems serving 10 residen-

tial houses.

5

Primary System

S72

A B C N

Line

S1

Rg

Feeder 1

S72S1

Feeder 5

Section

Transmission System


#1 #2

#3

SAV

SBV

SCV

up

Z


TABLE II: MAIN PARAMETERS OF TEST SYSTEM #1

System Parameters Values

Transmission

System

Voltage level (LL-rms) 144 kV

Equivalent impedance Z+/-=10.18+j32.95 Ω

Z0=2.42+j33.63 Ω

Substation

Transformer

Rated Capacity 20MVA/20MVA/10MVA

Rated Voltage (LL-rms) 144kV/25kV/6.3kV

Connection Type Yg/yg/delta

Short Circuit Impedance

H-M 10.5%

H-L 18%

M-L 6.5%

On-Load Loss 106.3kW

Main Trunk

Number of Feeders 5

Power Line Type Overhead line

# of Sections per Feeder 72

Length of Each Section 0.12km

Grounding Span 100m

Grounding Resistance (Rg) 15 Ω

1) Tertiary Winding Filter Design Results

The introduced iterative process (Fig. 4) was employed to

determine the final filter design. Table III presents the compo-

nent size of the designed tertiary winding filter.

TABLE III: DESIGNED COMPONENTS SIZE OF THE TERTIARY WINDING FILTER

Component Parameters Values


L1 None

C1 160.43kvar (0.6 kV)a

L2 0.235mH(794A)b

C2 90.24kvar (0.6kV)


L3 0.490mH(794A)

C3 1033.35kvar(4.16kV)

L4 1.702mH(794A)

C4 637.08kvar (4.16kV) a The voltage in the bracket is the capacitor rated voltage. b The current in the bracket is the inductor rated current, the inductor rated

voltage is 6.3kV.

2) Simulation Results

All the developed models are employed in a multiphase

harmonic power flow program [4] to perform the simulation

studies. As the residential loads are time-varying during a day,

time-varying simulation results are obtained.

In order to establish a sound understanding of the proposed

tertiary winding filter performance, the simulation results for

both the case without tertiary winding filter and the case with

tertiary winding filter are shown in Fig. 6 to Fig. 9.

As seen from Fig. 6 to Fig. 9, both the harmonic currents

propagating into the transmission system and the harmonic

voltages at the primary side of the substation transformer have

been greatly reduced when the tertiary winding filter is in-

stalled, which demonstrates the effectiveness of the tertiary

winding filter.

1 3 5 7 9 11 13 15 17 19 21 230

2

4

6

8

10

Time (h)

%

TDD -- Without Tertiary Winding Filter

Phase A

Phase B

Phase C

1 3 5 7 9 11 13 15 17 19 21 230

2

4

6

8

10

Time (h)

%

TDD -- With Tertiary Winding Filter

Phase A

Phase B

Phase C

(a) (b)

Fig. 6 TDD variation of currents propagating into transmission system.

1 3 5 7 9 11 13 150

1

2

3

4

5

Harmonic Order

%

IDD Spectrum -- Without Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

1 3 5 7 9 11 13 150

1

2

3

4

5

Harmonic Order

%

IDD Spectrum -- With Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

(a) (b)

Fig. 7 Typical IDD spectra of currents propagating into transmission system.

1 3 5 7 9 11 13 15 17 19 21 230

1

2

3

4

5

Time (h)

%

THD -- Without Tertiary Winding Filter

Phase A

Phase B

Phase C

1 3 5 7 9 11 13 15 17 19 21 230

1

2

3

4

5

Time (h)

%

THD -- With Tertiary Winding Filter

Phase A

Phase B

Phase C

(a) (b) Fig. 8 THD variation of voltages at primary side of substation transformer.

1 3 5 7 9 11 13 150

0.5

1

1.5

2

2.5

Harmonic Order

%

IHD Spectrum -- Without Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

1 3 5 7 9 11 13 150

0.5

1

1.5

2

2.5

Harmonic Order

%

IHD Spectrum -- With Tertiary Winding Filter

Positive sequence

Negative sequence

Zero sequence

(a) (b) Fig. 9 Typical IHD spectra of voltages at primary side of substation trans-

former.

B. Simulation Studies on Transmission System

This subsection conducts several simulation studies on test

system #2 (see Fig. 10) an extension of the IEEE 14 bus

transmission system proposed in [26] aiming to further exam-

ine:

Will the distribution harmonic loads at other bus lead to the

tertiary winding filter overloading?

Is it essential to equip all the buses with distribution har-

monic loads with the tertiary winding filter?

6

G

1

2

3

5

6

12

13

11 10

14

9

47

8

SVC

Converter

G

G


The major difference of test system #2 from IEEE 14 bus

transmission system includes:

All components in the system are modeled in phase domain.

The loads at Bus 4, Bus 5, Bus 9, Bus 10, Bus 11, Bus 12,

Bus 13 and Bus 14 are selected to be modified as distribu-

tion system loads as test system #1. To reduce the complex-

ity, the aggregated distribution system load model (see Fig.

11) is adopted. It should be noted that the total fundamental

frequency loads at Bus 4, Bus 5, Bus 9, Bus 10, Bus 11,

Bus 12, Bus 13 and Bus 14 keep the same with that in the

IEEE 14 bus transmission system, while their harmonic

characteristic parameters which include the harmonic cur-

rent spectrum of each phase and harmonic impedance ma-

trix are derived from test system #1 at peak load instant.

To simplify the analysis, the harmonic currents injected by

HVDC and SVC are neglected.

Substation Transformer MV Bus

#1 #2

C

B

#3

A

HV BusA AP jQ

B BP jQ

C CP jQC

B

A

(a) Fundamental frequency model

#1 #2

C

B

#3

A

C

B

A

3 3{ }Z h

( )AI h ( )BI h ( )CI h


MV BusHV Bus

(b) Harmonic frequency model

Fig. 11 Aggregated distribution system load model.

1) Influence of Distribution Harmonic Loads at Other Bus-

es

In the previous sections, the tertiary winding filter was

thoroughly examined in the distribution system. However in

the tertiary winding filter design, loading assessment was con-

ducted without considering the influence of distribution har-

monic loads at other buses. Will this be an issue for the tertiary

winding filter if there are multiple distribution harmonic loads

at other buses in the transmission system? To answer this ques-

tion, eight sets of cases are studied. Each set of cases consists

of two cases: i) there is no distribution harmonic loads at other

buses except where the tertiary winding filter is installed; ii) all

the load buses (Bus 4, Bus 5, Bus 9, Bus 10, Bus 11, Bus 12,

Bus 13, Bus 14) are modified as distribution harmonic loads.

The detailed description of these case sets and overloaded

components for different cases are given in Table IV.

Loading assessment for all eight case sets show that the dis-

tribution harmonic loads at other buses do have influence on

the loading level of the tertiary winding filter. But the influ-

ence is different for the tertiary winding filter at different loca-

tions. According to Table IV, indicated by Case Set 4, 5 and 6,

the tertiary winding filter designed based on the distribution

harmonic load information at its own bus will be overloaded

by distribution harmonic loads at other buses. Thus if multiple

distribution harmonic loads exist, the transmission system

harmonic power flow should be incorporated into the tertiary

winding filter loading assessment and components of larger

size should be adopted when overloading issues are identified.

TABLE IV: LOADING ASSESSMENT RESULT

Case

Set Case

Filter

Placement

Distribution Harmonic

Loads Location

Overloaded

Components

1 i Bus 4 Bus 4 None

ii Bus 4 All load buses None






ii Bus 10 All load buses C4









2) Installation Density Study

This subsection presents the sensitivity study to assess the

influence of the installation density of the tertiary winding fil-

ter on the overall transmission system harmonic distortion lev-

el. Simulation results for the following case sets are compared

in Fig. 12.

i. Test system # 2 with the loads at bus 4, bus 5, bus 9, bus

10, bus 11, bus 12, bus 13 and bus 14 all modified as a

distribution harmonic load.

ii~ix, based on i, install one tertiary winding filter at any

one, any two,…, all of the buses with distribution harmon-

ic loads.

In Fig. 12, the average voltage THD is the voltage THD av-

erage over all buses under each cases for each case set and the

average current TDD is the current TDD average over all

transmission lines and transmission transformers under each

cases for each case set, while minimum voltage THD is the

minimum voltage THD average over all buses for each case set

7

and minimum current TDD is the minimum current TDD aver-

age over all transmission lines and transmission transformers

for each case set. As shown in Fig. 12, the more the tertiary

winding filter installed the lower the overall transmission sys-

tem harmonic distortion level is in terms of both the bus volt-

age THD and transmission equipment TDD.

Fig. 12 Average minimum harmonic distortion level for different cases.

Table V further gives the filter placement which results the

minimum harmonic distortion level for Case Set ii ~ viii and

the corresponding overall system voltage distortion level and

current distortion level with such placement.

TABLE V: OPTIMUM FILTER PLACEMENT FOR DIFFERENT CASES

Case Optimum Filter

Placement

Overall System

Harmonic Distortion

Voltage

THD* (%)

Current

TDD* (%)

ii Bus 9 6.70(8.63) 12.14(13.92)

iii Bus 9, Bus 14 5.61(8.07) 10.49(13.50)

iv Bus 9, Bus 10, Bus 14 4.92(7.48) 10.09(12.98)

v Bus 9, Bus 10, Bus 14,

Bus 4 3.86(6.70) 8.83(12.30)

vi Bus 9, Bus 10, Bus 14,

Bus 4, Bus 5 2.93(5.65) 8.37(11.39)

vii Bus 9, Bus 10, Bus 14,

Bus 4, Bus 5, Bus 13 2.25(4.41) 8.31(10.26)

viii Bus 9, Bus 10, Bus 14, Bus

4, Bus 5, Bus 13, Bus 12 2.01(3.15) 7.87(8.88)

*Figure in the bracket is the average value for each case set.

As indicated by Fig.12 and Table V, with optimum filter

placement, better harmonic mitigation effects can be achieved.

Another useful information could be extracted from Table V is

the distribution harmonic load at Bus 9 has the largest impacts

on the overall system distortion level since for all the optimum

filter placement one tertiary winding filter should be placed at

Bus 9.

VI. COMPARATIVE ECONOMIC ANALYSIS

The cost of the proposed tertiary winding filter has been in-

vestigated using test system #1. It is also compared with the

cost of the MV LC filter package.

To obtain a meaningful comparison, the MV LC filter

package is designed to achieve the same harmonic distortion

reduction with the tertiary winding filter specified by Table III.

The designed MV LC filter package shown as Fig. 13 consists

of two inductor grounded single tuned LC filters which are

tuned to filter the 3rd

, 5th

and 9th

and 11th

harmonics respec-

tively and one directly grounded single tuned LC filter which

is tuned to filter the 7th

harmonic. The designed components

size of the MV LC filter package is given in Table VI.

A B C A B CA B C

h=3, 5 h=9, 11h=7

L L L

C C

L L L

C C C C C C C

L L L

0L0L

Fig. 13 MV LC filter package.

TABLE VI: DESIGNED COMPONENT SIZE OF MV LC FILTER PACKAGE

Filter

Design

Tuned Harmonic

Order Component Size

Filter 1 3, 5

L 56.141mH (208A)

C 1875.28kvar (31.5kV)

L0 33.269mH (416A)

Filter 2 7 L 28.071mH (69A)

C 488.77kvar (15.92kV)

Filter 3 9, 11

L 11.228mH (69A)

C 494.84kvar (15.92kV)

L0 1.848mH (69A) a The voltage in the bracket is the capacitor rated voltage. b The current in the bracket is the inductor rated current, the inductor rated

voltage is 14.4kV.

For the cost estimation, the data collected from [27] and the

Internet are used. Table VII shows the approximate expenses

for filter components.

TABLE VII: ESTIMATED COST OF COMPONENTS

Capacitor

Rated

Voltage <1kV 1kV~10kV 10kV~25kV 25kV~50kV

Cost $15/kvar $20/kvar $40/kvar $65/kvar

Inductor Voltage Level

1kV~10kV 10kV~25kV

Rated

Current

<100A $1400 $2800

100A~500A $1700 $3400

500A~1000A $1900 $3800

Other Components 1kV~10kV 10kV~25kV

Switching Device $17,000 $34,000

Protection Device $3,500 $7,000

By the mentioned information, the overall cost of the ter-

tiary winding filter and MV LC filter package is estimated as

presented in Table VIII. As seen from Table VIII, the tertiary

winding filter is found to be much more economical.

TABLE VIII: OVERALL ESTIMATED COST

Tertiary Winding Filter MV LC Filter Package

Total Cost $149,110 $557,910

VII. APPLICATION CONSIDERATIONS

Most substation transformers are custom made and have

tertiary windings. It is relatively easy to make the six terminals

of the tertiary windings available for the proposed application.

Therefore, utility companies could order such a transformer

for new substations. The tertiary winding filter can then be

implemented by connecting corresponding LC components.

The same applies to the existing substations where six termi-

nals of the tertiary windings are accessible.

8

For existing substations where only three terminals of the

tertiary windings are accessible, the delta connection cannot be

achieved. As a result, ZS filtering is not doable. But the NZS

filtering can still be achieved by connecting shunt LC filters to

the tertiary windings. This configuration is similar to what is

proposed in [28]. Since the tertiary winding has lower rated

voltages than the secondary winding, this scheme is still cost

less than the traditional MV passive filter connected to the

secondary bus.

VIII. CONCLUSIONS

This paper presents a novel and effective scheme to prevent

harmonic currents from propagating into the transmission sys-

tems. The basic idea is to utilize the tertiary winding of a sub-

station transformer to create a tuned low impedance path for

the harmonics. The design issues of the proposed filters are

investigated and solved. The main findings and contributions

of this paper could be summarized as follows.

The proposed tertiary winding filter could trap two ZS har-

monics and three NZS harmonics simultaneously. It be-

comes a desirable method to mitigate multiple harmonic

currents.

The filter and its design method have been tested through

two comprehensive test systems. The results have demon-

strated the effectiveness of the filter and also revealed prop-

er placement could achieve the same reduction of the over-

all system harmonic distortion level with fewer filters.

Compared to other applicable schemes, the main benefit of

the proposed tertiary winding filter is its utilization of an

existing substation transformer to construct a low imped-

ance path. As a result, economic analysis showed it as a

cost effective solution to the problem.

IX. REFERENCES

[1] A. Mansoor, W. M. Grady, P. T. Staats, R. S. Thallam, M. T. Doyle, and

M. J. Samotyj, "Predicting the net harmonic currents produced by large

numbers of distributed single-phase computer loads," IEEE Trans.

Power Del., vol. 10, no. 1, pp. 2001-2006, Oct. 1995.

[2] CEATI Technical Report, “Power quality impact from mass penetration

of energy efficient and consumer electronic devices”, Project T084700-

5146 A&B, CEATI International Inc., Sept. 2011.

[3] S. Bhattacharyya, S. Cobben, P. Ribeiro, and W. Kling, “Harmonic

emission limits and responsibilities at a point of connection,” IET Gen.

Transmiss. Distrib., vol. 6, no. 3, pp. 256–264, Mar. 2012.

[4] D. Salles, J. Chen, W. Xu, W. Freitas, and H. E. Mazin, "Assessing the

collective harmonic impact of modern residential loads - Part I&II,"

IEEE Trans. Power Del., vol. 27, no. 4, pp. 1937-1955, Oct. 2012.

[5] H. Sharma, M. Rylander, and D. Dorr, "Grid impacts due to increased

penetration of newer harmonic sources," in Proc. IEEE Rural Electric

Power Conference (REPC), 2013, pp. B5-1-B5-5.

[6] K. D. McBee and M. G. Simoes, "Evaluating the long-term impact of a

continuously increasing harmonic demand on feeder-level voltage dis-

tortion," IEEE Trans. Ind. Appl., vol. 50, no. 3, pp. 2142-2149,

May./Jun. 2014.

[7] M. T. Au, J. S. Navamany, "The impact of small and medium power

loads on distribution network efficiency and harmonic propagation," in

Proc. 19th Int. Conf. Elect. Dis., May. 21-24, 2007, pp. 1-4.

[8] CSA Standard C22.3 No.6-13: Principles and Practices of Electrical

Coordination between Pipelines and Electric Supply Lines, 2013, Note

2 of Page 10.

[9] G. Todeschini, D. R. Mueller, and G. Y. Morris, "Telephone interfer-

ence caused by harmonics in distribution systems: Analysis and simula-

tions," in Proc. IEEE Power Eng. Soc. Summer Meeting, 2013, pp. 1-5.

[10] J. Arrillaga, D. A. Bradley, P. S. Bodger, Power System Harmonics,

John Wiley & Sons Ltd, 1985.

[11] V. E. Wagner, J. C. Balda, D. C. Griffith, A. McEachern, T. M. Barnes,

D. P. Hartmann, et al., "Effects of harmonics on equipment," IEEE

Trans. Power Del., vol. 8, no.2, pp. 672-680, Apr. 1993.

[12] S. B. Gin, J. H. Sawada, and T. R. Treasure, "BC Hydro harmonic reso-

nance experience," in Proc. IEEE Power Eng. Soc. Summer Meeting,

2000, pp. 1088-1093.

[13] Bonneville Power Administration (2013, Nov.). Technical Requirements

for Interconnection to the BPA Transmission Grid. [Online]. Available:

http://www.bpa.gov/transmission/Doing%20Business/Interconnection/D

ocuments/tech_requirements_interconnection.pdf

[14] NGET plc (2014, Aug.). The Grid Code. [Online]. Available:

http://www2.nationalgrid.com/uk/industry-information/electricity-

codes/grid-code/the-grid-code/

[15] E. R. Detjen and K. R. Shah, "Grounding transformer applications and

associated protection schemes," IEEE Trans. Ind. Appl., vol. 28, no. 4,

pp. 788-796, Jul./Aug. 1992.

[16] W. Jewell, W. L. Miller, and T. Casey, "Filtering dispersed harmonic

sources on distribution," IEEE Trans. Power Del., vol. 15, no. 3, pp.

1045-1051, Jul. 2000.

[17] J. Hurng-Liahng, W. Jinn-Chang, W. Kuen-Der, C. Wen-Jung, and C.

Yi-Hsun, "Analysis of zig-zag transformer applying in the three-phase

four-wire distribution power system," IEEE Trans. Power Del., vol. 20,

no. 2, pp. 1168-1173, Apr. 2005.

[18] E. Larsen, "Filter for removing harmonic current from a neutral conduc-

tor," US Patent 5 914 540, Jun. 22, 1999.

[19] P. Bagheri and W. Xu, "A technique to mitigate zero-sequence harmon-

ics in power distribution systems," IEEE Trans. Power Del., vol. 29, no.

1, pp. 215-223, Feb. 2014.

[20] IEEE Guide for Application and Specification of Harmonic Filters,

IEEE Standard 1531-2003, 2003.

[21] J. C. Das, "Passive filters - potentialities and limitations," IEEE Trans.

Ind. Appl., vol. 40, no.1, pp. 232-241, Jan./ Feb. 2004.

[22] A. B. Nassif, W. Xu, and W. Freitas, "An Investigation on the selection

of filter topologies for passive filter applications," IEEE Trans. Power

Del., vol. 24, no.3, pp. 1710-1718, Jul. 2009.

[23] IEEE Standard Terminology for Power and Distribution Transformers,

IEEE Std C57.12.80-2010, 2010.

[24] IEEE Application Guide for AC High-Voltage Circuit Breakers Rated

on a Symmetrical Current Basis, ANSI/IEEE C37.010-1979, pp. 1-54,

1979.

[25] G. C. Montanari and D. Fabiani, "The effect of nonsinusoidal voltage on

intrinsic aging of cable and capacitor insulating materials," IEEE Trans.

Dielectr. Electr. Insul., vol. 6, no. 6, pp. 798-802, Dec. 1999.

[26] R. Abu-Hashim, R. Burch, G. Chang, M. Grady, E. Gunther, M. Halpin,

et al., "Test systems for harmonics modeling and simulation," IEEE

Trans. Power Del., vol. 14, no.2, pp. 579-587, Arp. 1999.

[27] P. Bagheri, "Methods to mitigate harmonics in residential power distri-

bution systems," M. Sc. dissertation, Dept. Elect. Comput. Eng., Univ.

Alberta, Edmonton, AB, Canada, 2013.

[28] L. Helle, J. Nieuwenuizen, and M. Lidholm, "Low-voltage harmonic fil-

ter for full-scale converter systems," US Patent 2010/0118568 A1, May

13, 2010.

Tianyu Ding (S'12) obtained the B.Sc. degree in electrical engineering from

Shandong University, Jinan, China, in 2010. Currently, he is pursuing his

Ph.D. degree in Electrical and Computer Engineering at the University of

Alberta, Edmonton, AB, Canada. His main research interest is power quality.

Wilsun Xu (M’90-SM’95-F’05) obtained the Ph.D. degree from the Universi-

ty of British Columbia, Vancouver, in 1989. Currently, he is a

NSERC/iCORE Industrial Research Chair Professor at the University of

Alberta. His current main research interests are power quality and power

disturbance analytics.

http://www.bpa.gov/transmission/Doing%20Business/Interconnection/Documents/tech_requirements_interconnection.pdf

http://www.bpa.gov/transmission/Doing%20Business/Interconnection/Documents/tech_requirements_interconnection.pdf

http://www2.nationalgrid.com/uk/industry-information/electricity-codes/grid-code/the-grid-code/

http://www2.nationalgrid.com/uk/industry-information/electricity-codes/grid-code/the-grid-code/

A Filtering Scheme for Reducing Harmonics Penetration …apic/uploads/Research/sample14.pdf · low...

Documents

Transcript of A Filtering Scheme for Reducing Harmonics Penetration …apic/uploads/Research/sample14.pdf · low...