Harmonic Filters

Harmonic Measurements and Filter design

for an utility system - a case study

J. Sreedevi, K.S.Meera, B.S.Manjunath

Power Systems Division

Central Power Research Institute

Bangalore, India

[email protected]

P. Kanungao

PGCIL, NER

Shillong, India

Abstract— This paper presents the harmonic filter design study

results carried out for an electric utility for suppression of

harmonics in the system. Harmonic measurements were carried

out at 132 and 33kV substations of utility system. These

measurement results were analyzed to find the maximum voltage

and current distortions among the various measurement points.

Based on the predominant harmonics of significance harmonics

filters are designed. The filter performance results are discussed in

this paper.

Keywords-components: Reactive power, Harmonic filters,

Voltage distortion, current distortion

I. INTRODUCTION

Electric utilities are always concerned about having power

factor which has the advantage of reducing required equipment

ratings, line losses and voltage drops thus lessening the need

for voltage regulation equipment. This is however accompanied

by the industries increasing use of variable speed drives and

electronic equipment which can interact with power factor

correction capacitor banks to result in voltage current

amplification.

The steep development that the industries of solid-state

electronics has undergone lately resulted in the introduction of

delicate appliances which are more sensitive to the quality of

power supplied by electrical utilities in comparison with the

robust devices of the older days. At the same time these electric

appliances result in the distortion of the steady–state ac current

and voltage wave forms which results in harmonics.

The sources of harmonics are Thyristor controlled devices such

as rectifier and inverter station of HVDC schemes, Thyristor

controlled reactor, Thyristor switched capacitors and static var

compensators which introduces the harmonics into the power

circuit which results in the distorted wave forms. The other

sources of harmonics are non-linear loads like Induction

Furnaces & Arc Furnaces, Steels mills & Rolling mills, single

Phase uncompensated railway loads and Switching equipment

& electronics etc.

Linear loads draw currents that are proportional to applied

voltages example incandescent lighting heating and motor

loads and non-linear loads draw currents only a part of voltage

cycle and introduce harmonics. The resulting current from non-

linear loads contains 3rd, 5th, 7th Harmonics. These harmonic

currents permeate into source currents and source currents

having harmonic content impact source voltages.

Excessive harmonic voltage and/or current can cause damage to

equipment and the electrical system. IEEE Std 519-199210

gives application guidelines. One of the common ways of

controlling harmonic distortion is to place a passive shunt

harmonic filter close to the harmonic producing load(s). The

harmonic-producing device can generally be viewed as a source

of harmonic current.

The objective of the harmonic filter is to shunt harmonic

current from the load into the filter, thereby reducing the

amount of harmonic current that flows into the power system.

The simplest type of shunt harmonic filter is a series

inductance, capacitance (LC) circuit which is a single tuned

filter. More complex harmonic filters may involve multiple LC

circuits.

Measurements are carried out for 9 days using Power analyzer

―PowerPro‖ instrument simultaneously at different load points

during different generating conditions. Trends in Voltage,

Current, Power and Frequency, harmonics Voltage sags and

swells can be recorded using the instrument.

The harmonic studies assume a steady state solution

corresponding to a typical operating condition. Harmonic loads

are modeled as harmonic current sources in the study. The

currents injected into the frequency dependant network are

computed for harmonic currents at each frequency and checked

for violation of limits for distortion in voltage and current. If

violation of limits is observed, the study is repeated with

appropriate tuned filters. Filters are chosen to mitigate the

harmonics, especially lower order harmonics.

II. SYSTEM CONFIGURATION

The Single line diagram of the system considered for the study

is given in Fig. 1 below. The system consists of one 132 kV

feeder from BUS1 to BUS2. There is one more feeder

connecting to BUS2, but is used only in case of emergency and

TR2 132/33 kV 10 MVA

Z=8.6%

630 KVA 630 KVA

Feeder

F1

22.29 km

(HARMONIC PRODUCING LOADS)

CONSTANT LOADS

BUS1 1500

BUS4

BUS2

BUS3

TR1,132/33 kV 10 MVA

Z1=8.6%

Feeder

F2

Feeder

F3

Feeder4

Measurement points

hence not considered in the study. The loads at 33 kV buses are

fed from 132 kV substation through two numbers of 10 MVA,

132/33 KV transformers. There are four feeders Feeder1 to

Feeder4 among which for Feeder 3 presently no load is

connected. Feeder1 and Feeder2 are connected to one section of

the 33 kV bus, and Feeder3 and Feeder4 are connected to the

second section of the 33 kV bus (bus coupler between section 1

and section 2 of the 33 kV bus is kept open).

The harmonic loads are represented as current sources with

sum of fundamental, 2nd harmonic, 3rd harmonic up to 16th

harmonic currents as obtained from the field harmonic currents.

The system behind the incoming feeder is represented by the

thevenin equivalent i.e as a voltage source behind short circuit

impedance. SKM Power tools – HIWAVE software is used to

carryout the filter design studies.

Fig 2.1: SLD of the system considered for the study

III. HARMONIC MEASUREMENTS

Harmonic measurements are recorded at 18 locations and

finally found that feeder1, feeder2 and feeder4 have harmonic

content. The parameters that are measured are RMS Voltage,

Current, frequency, Power (MW), Power factor, Apparent

Power (MVA), Reactive Power (MVAR), Total Voltage and

Current Harmonic Distortion (THD), Voltage & Current FFT,

various individual Voltage & Current harmonics, V & I Phasors

and Voltage & Current unbalance over the measurement.

Records of Maximum current THD for rated load currents and

voltage THD of Feeders F1, F2 and F4 are given in Figure 2 to

7.

THD values alone will not be sufficient to conclude the

presence of harmonics in the system. The current distortion

level can be characterized by THD values but can often be

misleading. In certain cases, there may be instances where the

THD may exhibit a high value for input current when they are

operating at a very low/light loads. This is not necessarily a

significant concern because the magnitude of harmonic current

is low, even though its relative distortion is high. To handle this

concern for characterizing harmonic current in a consistent

fashion, IEEE standard 519- 1992 defines another term, the

Total Demand Distortion (TDD) as defined in

L

h

h

h

I

I

TDD

max

2

2

This term is the same as the THD except that the distortion is

expressed as a percent of sum rated load current IL rather than

as a percentage of fundamental current magnitude at the

instance of measurement.

Fig 2: Current THD graph(Ia, Ib, Ic) of feeder F1

Fig 3: Voltage THD graph(Va, Vb, Vc) of feeder F1


Fig 5: Voltage THD graph(Va, Vb, Vc) of feeder F2


Fig 7: Voltage THD graph(Ia, Ib, Ic) of feeder F4

This measured harmonic data is used for modeling the

harmonic loads as current sources, injecting the harmonic

currents into the network. The Maximum current THD’s and

currents RMS for F1, F2 and F4 feeders are shown in Table 1.

Table 1: Maximum THD’s of Harmonic loads

It is observed from the ITHD given in Table above, that current

harmonics are less on F1, F4 feeders compared to that on F2

feeder. As the harmonics are more on F2 feeder, filter design is

carried out for different cases of harmonics of F2 feeder as

given in Table 2. The detailed data extracted for F2 feeder from

the measurements is given below:

Table 2 : Highest 11th & 13th Harmonics recorded in F2 feeder

Sl

No.

Current Magnitude

in kA

11th

Harmonics

(Highest)

13th

Harmonics

(Highest)

THD Record

Number

1 I1 (R) 0.084 6.06% 6.06% 10.9% F2_3202

2 I3 (B) 0.115 7.89% ----- 10.5% F2_3145

3 I3 (B) 0.107 ------ 5.71% 10.3% F2_3142

In Sl no. 1 of the above Table, maximum 11th

and 13th

harmonics are present simultaneously in recorded period. In Sl

no 2 and 3, maximum 11th

harmonics and maximum 13th

harmonics are observed at different instants of time.

The Magnitude and Phase angle of individual harmonic

component currents up to 16th order harmonics of F1, F2 and

F4 loads for all the record numbers mentioned above are given

in Table 3 and 4 respectively. These individual harmonics are

considered as harmonic current sources in the simulation of

harmonic loads in the study.

III. HARMONIC FILTER DESIGN:

Harmonic Filter Design studies for arriving at the specifications

of the filter to suppress harmonics generated by Loads at utility

substation is carried out for following cases depending on THD

levels as mentioned in previous section.

Table 3: Individual Harmonic load data for F1 and F4 Harm

onic

order

Record Number

F1_2024 F4_4641 F4_1498

Magnitude Angle Magnitude Angle Magnitude Angle

1 100.00 0.0 100.00 101.6 100.000 89.7

2 0.375 0.0 0.307 -24.7 2.229 -8.9

3 1.281 42.4 0.556 30.3 1.360 -60.4

4 0.140 0.0 0.381 0.0 1.307 0.0

5 3.312 158.3 0.726 176.6 1.390 -75.0

6 0.202 0.0 0.174 14.4 0.824 14.4

7 0.668 0.0 0.748 -91.1 1.082 -129.6

8 0.213 0.0 0.216 0.0 0.192 0.0

9 0.637 0.0 0.081 0.0 0.170 0.0

10 0.149 0.0 0.074 90.9 0.513 135.5

11 2.183 0.0 0.114 -53.5 2.527 86.1

12 0.382 0.0 0.167 78.7 0.813 -3.3

13 1.422 0.0 0.426 8.8 1.392 86.1

14 0.266 0.0 0.076 -50.2 0.111 5.7

15 0.137 0.0 0.138 96.2 0.329 176.6

16 0.568 0.0 0.086 -82.0 0.267 -37.0

Table 4: Individual Harmonic load data for F2

Harm

onic

order

Record Number

F2_3202 F2_3142 F2_3145

Magnitude Angle Magnitude Angle Magnitude Angle

1 100.000 -28.8 100.000 82.7 100.000 85.8

2 0.448 150.9 0.26246 -64.8 0.224 -64.8

3 1.383 118.8 0.78266 -61.7 1.037 -57.7

4 0.194 17.5 0.51623 1.7 0.678 1.7

5 2.108 21.9 2.15282 35.5 1.962 29.8

6 0.099 -168.5 0.12110 21.4 0.879 21.4

7 1.597 48.4 1.12586 79.3 1.001 71.2

8 0.222 0 0.76640 0 0.787 0

9 0.401 -141.5 0.51795 0 0.476 0

10 0.245 0 0.35060 0 0.587 0

11 7.892 101.2 7.94101 105.7 7.713 105.9

12 0.871 -95.2 0.73808 -160.3 0.251 158.9

13 5.966 102.8 5.47117 100.3 6.022 103.1

14 0.322 67.4 0.10003 17.1 0.218 17.1

15 0.257 143.9 0.26811 -36.8 0.219 -36.8

16 0.067 -3.3 0.23827 -5.1 0.316 -5.1

Magnitude is in % and Angle is in degrees

Two set of cases were considered for filter design studies. In

first set of cases CASE1, CASE2 and CASE3 given below,

harmonic currents at F1and F4 which resulted in highest THD

are considered. And in each case F2 harmonic load is varied as

per the highest 11th & 13th Harmonics recorded in F2 feeder as

given in Table 2.

Feeder Name Current THD Current RMS

Max

ITHD

Record

Number

IRMS

(Amp)

Record Number

F1 4.7 F1_2024 41.15 F1_2024

F4 5.9 F4_4641 65.34 F1_1498

F2 10.9 - 82.73 -

In second set of cases CASE4, CASE5 and CASE6 given

below, harmonic currents at F1and F4 which are present during

average loading condition are considered.

CASE1: F1_2024, F4_4641 & F2_3202 as harmonic loads



CASE4 : F1_2024, F4_1498 & F2_3202 as harmonic loads

CASE5 : F1_2024, F4_1498 & F2_3142 as harmonic loads


In the case of harmonic currents which resulted in highest

THD during the measurement period, highest current THD

recorded is 4.7 on FeederF1, highest current THD recorded is

5.9 and on FeederF4 and highest current THD recoded is 10.9

on Feeder F2(as given in Table 1). When the measured

individual harmonics are observed on the feeders, 5th harmonic

is dominant on F1 and 11th harmonic is dominant on F2and

F4.

Keeping in view of the dominant harmonics present, filters are

designed at the 33 kV load bus. For all cases studied, different

combinations of filters with different MVAR ratings are

considered and V_THD at all buses and I_THD of all branches

are monitored to ensure that they are within the limits as per

IEEE519. The results of the cases are tabulated giving order of

the filter and the associated MVAR rating. For each case,

I_THD on BUS1 to BUS2, V_THD at BUS1, BUS2, BUS3 &

BUS4 are tabulated in Table 5. The 33 kV bus voltages at

substation are also monitored to make sure that there are no

overvoltages with connection of filters. To have an idea of the

possible rise in voltage at 33 kV bus of substation with

connection of filters, 33 kV bus voltages is also tabulated for

different cases studied.

From the simulations carried out it is observed that 11th

harmonic filter of 2 MVAR is sufficient to keep the harmonics

within limits specified in IEEE-519 for CASE1, CASE2 and

CASE3. Whereas for CASE4, CASE5 and CASE6, it is seen

that in addition to 11th

harmonic filter, a 5th

harmonic filter of 5

MVAR is required.

The Distortion Spectrum of harmonics and Impedance

Magnitude Scan and Angle for CASE 1 are provided in Fig 8 to

Fig 13. Distortion Spectrum of harmonics at 132kV bus and

33kbus without filters is given in Fig 8 and with filters is given

in Fig 9. The presence of 11th

and 13th

harmonic magnitudes is

seen in distortion spectrum without filter where as in distortion

spectrum with filters they are absent. Impedance Magnitude

Scan and Angle at 132kV bus and 33kbus without filters is

given in Fig 10 and Fig 11 respectively and with filters is given

in Fig 12 and Fig 13. BUS0006 and BUS0001 in the figures

refer to BUS3 and BUS1 of single line diagram.

Table 5: Results for different cases

Case

Harmonic filters

I_THD

V_THD 33 kV voltage

Orde

r

MVAR BUS1

to

BUS2

BUS1 BUS2 BUS3 BUS4 BUS3

I

- - 5.22 0.56 0.92 0.97 0.92 32.06

11 2 2.39 0.20 0.32 0.34 0.32 32.14

11 1.5 2.51 0.23 0.36 0.38 0.36 32.12

II

- - 5.47 0.61 1.00 1.05 1.00 32.00

11 2 2.30 0.20 0.32 0.33 0.32 32.08

III

- - 5.30 0.57 0.93 0.98 0.93 32.04

11 2 2.37 0.20 0.31 0.33 0.32 32.12

IV

- - 5.5 0.57 0.93 0.97 0.93 32.05

11 2 2.97 0.21 0.33 0.35 0.33 32.14

11 5 3.37 0.2 0.31 0.32 0.31 32.27

11, 5 2, 1 2.52 0.19 0.30 0.32 0.30 32.18

11, 5 2, 1.5 2.36 0.19 0.29 0.31 0.29 32.20

V

- - 6.45 0.70 1.16 1.21 1.16 31.99

11, 5 2, 1.5 2.62 0.21 0.34 0.35 0.34 32.14

11, 5 2,4 2.48 0.2 0.32 0.32 0.32 32.26

VI

- - 6.39 0.67 1.10 1.15 1.11 32.04

11, 5 2,4 2.61 0.2 0.32 0.32 0.32 32.30

11, 5 2,5 2.50 0.2 0.31 0.31 0.31 32.39

Fig 8: Distortion Voltage Spectrum at

BUS3 and BUS1 without filter

Fig 9: Distortion Voltage Spectrum at

BUS3 and BUS1 with filter

Fig 10: Impedance Magnitude Scan at BUS3 and BUS1 without filter

Fig 11: Impedance Magnitude Scan at BUS3 and BUS1 with filter

Fig 12: Impedance Angle Scan at

BUS3 and BUS1 without filter

Fig 13: Impedance Angle Scan at

BUS3 and BUS1 with filter

However, since the filter design study considers the presence of

maximum harmonics simultaneously in the system, which may

not occur in practice the 11th

harmonic filter of 2 MVAR be

implemented in the first stage. Later, in the next stage the 5th

harmonic filter may be considered for implementation after

confirming the modified THD levels (after implementation of

11th

harmonic filter) in the system.

Additionally, as the source of the 11th

and 13th

harmonics is on

feederF2 the suggested 11th

harmonic filter can be installed at

the 33 kV load bus itself so that the filter can be switched in/out

along with the load.

V. RESULTS & DISCUSSION:

5.1 Specifications of 11th harmonic filter:

From the above studies specifications of 11th

harmonic filter are

given in Table 6 below:

Table: 6: Specifications of filter

Sl.No. Parameter

1 Filter Type Single tuned filter

2 3 Phase MVAR rating 2.0 MVAR

3 Rated Voltage 32.417 kV rms

4 Rated frequency 50 Hz

5 Quality factor 20

6 Connection Y

7 Resistor(R) 2.5781

8 Inductor(L) 0.0143 H

9 Capacitor (C) 5.8459 F

The 2MVAR, 11th harmonic single tuned filter designed will

act as a 2MVAR capacitor at fundamental frequency of 50Hz.

The inductance and the quality factor, Q (wL/R at tuning

frequency) for the harmonic filter reactor is specified at the

resonant (tuned) frequency of the harmonic filter network. The

tolerance on the inductance should be selected to ensure the

proper performance of the harmonic filter network across the

range of the tolerance. Inductance tolerance is usually available

as ± 3% or +0% to -6%. The tolerance on the quality factor, Q,

is typically ± 20%

IEEE Std 18-2002 states that, capacitors are intended to be

operated at or below their rated voltage. Capacitors shall be

capable of continuous operation under contingency system and

bank conditions provided that none of the following limitations

are exceeded:

a) 110% of rated rms [root-mean-square] voltage

b) 120% of rated peak voltage including harmonics, but

excluding transients

c) 135% of nominal rms current based on rated KVAR and

rated voltage

d) 135% of rated KVAR.

In Table 7, Filter spectrum Report is provided which gives the

fundamental and harmonic currents through the filter

components. Table 7: Filter spectrum Report

Harmonic filters are generally not tuned to exact harmonic

frequencies, as tuning directly to the harmonic frequency may

have the following undesirable consequences:

a)The low impedance at resonance can result in nearly all

harmonic current at that frequency being absorbed by the

harmonic filter. The harmonic filter is required to be larger and

more expensive than is needed to achieve the required

harmonic performance

b) The harmonic filter interaction with the system impedances

results in a parallel resonance at a frequency just lower than the

tuned frequency. If a harmonic filter is designed exactly at the

harmonic frequency, a variation in the impedance values of the

actual equipment from the design values could retune the

harmonic filter and place the parallel resonant frequency very

close to the harmonic. Instead of low impedance, the combined

harmonic filter-system impedance becomes resonant at the

harmonic frequency, distortion levels become unacceptable,

and damaging voltage amplification may result in severe cases.

Changes in the system supply can shift the parallel resonance

also.

In this regard, literature survey indicated either 0.96 or 0.98

detuning factor being adopted. In this study detuning factors of

0.99, 0.98 and 0.97 are considered and CASE1 presented in

Table 5 is repeated with filter harmonic orders of 10.89, 10.78

and 10.76. The results of this study, considering detuning

factors is presented in Table 8.

From these results it is seen that detuning factor 0.98 results in

I_THD of less than 2.5, and detuning factor of 0.97 results in

I_THD of 2.55 which is violating the limits.

Table 8: Results for detuning frequencies

Cas

e

Harmonic

filters

I_TH

D

V_THD 33 kV

voltage

Order M

VAR

BUS1

to BUS2

BUS

1

BUS

2

BUS

3

BUS

4

BUS3

I 10.89 2 2.43 0.21 0.33 0.35 0.33 32.143

10.78 2 2.48 0.22 0.34 0.36 0.34 32.143

10.67 2 2.55 0.23 0.36 0.37 0.35 32.143

Table: 9: Specifications for detuned filter Sl.No. Parameter

1 Filter Type Single tuned filter

2 3 Phase MVAR rating 2.0 MVAR

3 Rated Voltage 32.417 kV rms

4 Rated frequency 50 Hz

5 Quality factor 20

6 Connection Y

7 Resistor(R) 2.5255

8 Inductor(L) 0.0149 H

9 Capacitor (C) 5.8459 F

From the studies carried out 11th

Harmonic filter is

recommended to mitigate the harmonics as per IEEE 519 guide

lines. A detuning factor of 0.98 is to be adopted for the 11th

harmonic filter. The Filter may comprise of two similar star

connected capacitor banks in parallel with the a CT on the tie

between the two star points.

ACKNOWLEDGEMENTS

Authors wish to thank, CPRI for permitting to publish this

paper. We also wish to thank Dr. R.S. Shivakumara Aradhya

and Sri.R. Deshpande for their support.

VI. REFERENCES

1. ―Report on Harmonic measurement in Powergrid, NE

region, April – 2010‖ CPRI report: DCCD-11341

dated 22-4-1010.

2. IEEE-519-1992: Recommended Practices and

Requirements for Harmonic Control in Electrical

Power Systems.

3. IEEE Std 18.-2002, IEEE Standard for Shunt Power

Capacitors.

Harmonic Filters

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