A PQ Case Study · 2017-01-05 · harmonic filters are combination of series and shunt elements of...

Case study 47

De-tuned Capacitor Bank – Economical Way to Mitigate Harmonics

A PQ Case Study

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A PQ Case Study CS 47 ENG 17


Case study 47


A PQ Case Study


47 Abstract:

TANGEDCO – Distribution Utility for Tamil

Nadu State has given a regulation

regarding Harmonic Mitigation, under

direction of tariff order by Tamil Nadu

Electricity Regulatory Commission

(TNERC), wherein any HT

consumer/generator need to meet the

specified Harmonic limits as per norms

stipulated in CEA regulation on grid code.

TANGEDCO measures the level of

Harmonics and issues a notice if the value

exceeds the CEA specified limit and if the

consumer is unable to mitigate Harmonics

within the specified limit after notified

period, penalty is imposed every month.

Due to above tariff penalty mechanism

the HT consumers started looking at

mitigation techniques wherever the

harmonic distortion is reportedly high.

Most of these facilities already had

reactive power compensation in terms of

capacitor banks installed. These capacitor

banks too suffered from failure etclike

other electrical equipment failure due to

high harmonic distortion levels.

In some cases based on harmonic

spectrum, installed capacitor banks can be

used to mitigate harmonics. After detailed

and proper analysis of harmonics in the

system mitigation techniques like tuned

harmonic filters or capacitor with reactor

can also used as harmonic filteration

technique.

This case study explores such possibility

that can be beneficial for industry to look

at available resources for addressing

harmonic mitigation and optimize their

expenses in deploying fresh investment to

avoid harmonic penalty.

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A PQ Case Study


Introduction

Distribution Company :

With drastic change in type of load used by electricity consumers in last one decade i.e.

adoption of more non linear loads, generation of current harmonics has increased

significantly. These current harmonics generated by consumers, not only have a negative

impact on the consumers own electrical system but also impacts electricity distribution

companies adversely. Higher levels of current harmonics in the system results in distorted

voltage waveform that results in overall poor distribution grid power quality.

Presence of harmonics in the electricity distribution companies not only increases

distribution losses in the system but also decreases life of electrical equipments.

Malfunctioning of circuit breakers and relayes, overheating of conductors and cables,

premature failure of transformers, motors do have a root cause as harmonics in major

cases. If harmonics in the system are beyond a limit, realibility of the electrical system also

decreases. All above effects of harmonics have an impact on overall operation of consumer

and distribution companies.

Tamil Nadu Generation and Distribution Corporation Limited (TANGEDCO) is first state in

India which has enforced a harmonic control regulation for its HT power consumers

empowered byTa il Nadu Ele tri ity Regulatory Co isio ’s TNERC Tarrif a d Har o i

Level order dates 20th

June 2013. Through this order, limits were placed for current and

voltage harmonics and penalty was enforced for violation of prescribed limits. The limits

spe ified for har o i s ere as per Ce tral Ele tri ity Authority’s CEA Notification

TheCe tral Ele tri ity Authority Te h i al “ta dards for Connectivity to the

Grid Regulatio s, 2007 No: 12/X/STD(CONN)/GM/CEA (21-Feb-07).

As per notification:

Voltage Harmonics: The total harmonic distortion for voltage at the connection point shall

not exceed 5% with no individual harmonic higher than 3%.

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Current Harmonics: The total harmonic distortion for current drawn from the transmission

system at theconnection point shall not exceed 8%.

Above limits were set in accordance with definition and guidelines provided in IEEE- 519 –

1992.

TANGEDCO also set the same limits as mentioned in the tariff order. Based on these orders,

TANGEDCO started measurement of harmonics on designated consumers and issued notices

where harmonics were found to be beyond these limits. Text from a sample notice to the

consumer is as below.

As per the lause 4 1 i of the Ta il Nadu Ele tri ity “uppply ode, you are lia le to pay

additio al harges for du pi g of har o i s i to TANGEDCO’s distri utio syste ore

than the permissible limit specified the the CEA regulations and if you fail to provide

adequate harmonic suppression units.

Hence it is hereby informed that,

i) As per the provisions contained in the tariff order in T.P.No 1 of 2013 dt.

20.6.2013, the consumers under category of HT-1 & HT-III are liable to pay

compensation charges after three months period from the date of

measurement, if the measured harmonics is beyond the permissible limit as

specified by CEA regulations.

ii) On 03.12.2013, the measurement of current and voltage harmonics was done

by MRT and field engineer using measuring meters/equipment in the

presence of your representatives and it was found that the Total Harmonics

Distortio THD for urre t is …… %, hi h is a o e the per issi le li it of

8% specified by the CEA regulations.

iii) You are requested to provide adequate harmonic filtering equioment to avoid

du pi g of har o i s i to Li e see’s et ork eyo d per issi le li it as

specified by CEA regulations. The three month period from the date of

measurement expires on 02.03.2014.

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A PQ Case Study


iv) As per the regulation 4, (1) (iv) of Tamil Nadu Electricity Supply Code, if you

fail to provide adequate harmonic filerting equipment to avoid dumping of

har o i i to Li e see’s et ork eyo d the per issi le li its as spe ified y

CEA regulations, you are liable to pay compensation at 15% of the respective

tariff on expiry of the three months perios and as and when you bring down

the harmonics within the limit, the levy of compensation charges will be

withdrawn.

Consumer:

There is a general perception among the industries that mitigation of harmonics is a capital

intensive project. However depending on severity of harmonics, type of harmonics and

objective of consumer, there are number of solutions available in the market which can

meet the requirement. Each solution has its own benefits and limitations. Various harmonic

mitigation techniques that are available can be listed as below. Few technologies need to be

adopted at design stage where as few technologies can be implemented as a retrofit or

additional component/equipment.

Line reactors.

Isolation transformers.

K-Factor transformers.

Phase shifting transformers.

Tuned harmonic filters.

Low pass harmonic filters.

Active harmonic filters

As mentioned each solution has its own advantages and limitations. There is no single

solution that is better than other solution. A careful and detailed study of the harmonics in

the system along with consumer requirement can result in selection of best possible

solution. The selected solution could be either one or combination of few technologies

mentioned above.

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A PQ Case Study


Section below shows brief about each technology and its application.

Line reactors:

Line reactors are one of the simplest and lowest cost technology for mitigation of

harmonics. These are used at the load end and in line with the load that is generating

harmonics. Line reactors are very common with variable frequency drives. They operate on

the principle of increasing impedance of the circuit. If a plant has a 1000 kVA transformer

with an impedance of 7%, and a drive of 100 kVA. This drive will have approximate

impedance of only 0.7%. As transformer impedance will be very small relative to each load,

a line reactor in series will improve impedance of the circuit.

One of major advantage of line filter is their low(est) cost and reduction in harmonic levels

to a great extent. Another advantage is that since they are installed right at the load end,

harmonic distortion is reduced in the distribution system. One of their major limitation is

with normal impedance (5-6%), they can reduce total current harmonics distortion up to

35%. Also their harmonic mitigation capacity reduces drastically with reduction in load

current.

Isolation Transformer:

Isolation transformer are one of very effective means of not only mitigating harmonics but

also other power quality issues like transients, common mode noise and zero sequence

currents.

Operating principle of isolation transformer is also based on line reactor, which increases

system impedance. The leakage inductance of isolation transformers increases circuit

impedance so that harmonics are reduced to a great extent. Typically, a delta – wye

configuration is used in isolation transformers. Since, primary is delta connected and

secondary is wye configuration, zero sequence currents (Triplen harmonics) are not

transferrent to the primary side of the network.

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A PQ Case Study


Due to capacitive coupling between winding and shield with proper grounding, transients,

common mode noise and zero sequence currents are not reflected on the other side of the

transformer.

Isolation transformer has added advantage of eliminating other power quality issues in

addition to harmonics over line reactors. Their harmonic mitigation capability is same as of

line reactor but can be very useful where zero sequence currents and other power quality

issues as mentioned above are present. Their cost of be around 5 -6 times higher as

compared to line reactor.

Tuned Harmonic Filter:

Tuned harmonic filters are basically combination of capacitor and reactor (inductor) which

are connected in series to form a tuned circuit. This tuned filter is connected in parallel to

the main system. As the LC circuit is tuned to particular frequency to form resonance, it

offers lowest impedance to that particular frequency (harmonic). Harmonic current instead

of flowing to the utility, flows to the filter.

Low Pass (De-tuned) Harmonic Filter:

All above mentioned methods filters out particular order of frequency. Low pass filter or de-

tuned filters have capability to attenuate all order of harmonic frequencies. De-tuned

harmonic filters are combination of series and shunt elements of reactors and capacitors.

The shunt circuit prevents attraction of harmonics from other sources where are series

circuits filters of primarily 5th

and 7th

order harmonic. They are one of most economical

means of mitigating harmonics.

Active Harmonic Filter:

Active harmonic filters are latest among all technologies for mitigation of harmonics. They

continuously monitor load current along with fundamental and harmonic current. Based on

the order of harmonic and their magnitude, they will inject equivalent magnitude of

individual harmonics with phase shif of 180 degrees. These injected currents will nullify the

Case study 47


A PQ Case Study


harmonics in the system. Active harmonic filters can filter out harmonics up to 50th

order of

harmonics. So AHFs need some energy to drive itself.

As acti e filters uses fast s it hi g IGBT’s, they produ es oise hile s it hi g hi h a

cause disturbances in the system. These noise signal can also be harmful to the sensitive

equipments. Presence of high voltage distortion can also affect performance of active

harmonic filter. In terms of cost, they are costliest among all available options.

Apart from above technologies, some other techniques apply principle of de-rating where

equipments are capable of handling harmonics. One of the example is K-rated transformers

which basically de-rates a transformer to withstand higher temperatures generated due to

harmonics.

Background

The case study involves one of the leading leather shoe manufacturing plant in Tamil Nadu.

The plant receives power at 11kV which is stepped down to 440 volts. Base demand of the

plant was 320 kVA where as sanctioned demand is 400 kVA. Average registered demand

during the last 12 months was around 328 kVA.

Rated capacity of the transformer is 500 kVA.

Figure – 1 shows basic SLD of the plant.

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A PQ Case Study


Load at the pla t o stitutes ai ly otors ith dri es. VFD’s or dri es are o li ear loads

which generates current harmonics. As Tamil Nadu distribution company has norms for

harmonic that are being injected in the grid by the consumers. They carry out routine

measurements at the consumer incomer (PCC) and if harmonics are found to be high, they

ask the consumer to reduce the harmonics.

The shoe manufacturing plant also got a notice from TANGEDCO asking them to reduce the

harmonics below specified limit. Specified limit of current harmonics by TANGEDCO is 8%. It

is mandatory for all HT consumers to keep current and voltage harmonics as specified by the

TANGEDCO.

Based on the notice, the shoe manufacturing plant carried out detailed harmonic study of

the plant.

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A PQ Case Study


Case study 47


A PQ Case Study


Harmonic Study Summary:

The facility receives power from electricity board at 11kV and stepped down to 440 Volts

using a transformer.

A detailed harmonic study was carried out by the plant. During the study, harmonics were

measured at different locations in the plant. The measurements were carried out at

following locations.

1. Main incomer

2. SSB – 1

3. SSB – 2

4. SSB – 3

5. Lighting load

6. Air Compressors

Section below shows measurement details at individual sections.

Main Incomer:

At the ai i o er, easure e ts ere arried out ith apa itors i ON o ditio a d

OFF o ditio . Graph – 1 & 2 shows voltage and current waveforms at the main incomer

he apa itors are ON a d OFF .

Figure – 2 Wa e for he apa itor is ON Figure – 3 Wa e for he apa itor is OFF

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A PQ Case Study


As see fro the a e for , the urre t a efor is distorted he apa itors are i ON

condition.

Figure 4 & 5 shows i di idual order of har o i s ith apa itor ON a d OFF .

Figure – 4 I di idual order of har o i s he apa itors are ON

Figure – 5 I di idual order of har o i s he apa itors are ON

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A PQ Case Study


As see fro the a o e graphs he apa itors are i ON o ditio , the % THD voltage

and current are 3.28% and 13.53%. Whe apa itors are OFF , % THD oltage a d urre t

values are 2.44% and 3.48%. Predominant harmonics were 5th

and 7th

order.

Above measurements clearly indicates that significant current harmonics are not generated

by the load but are amplified due to capacitor banks. The plant could not isolate capacitor

a k as hile apa itor a ks ere i OFF o ditio , po er fa tor dropped to 0.79 from

0.98 which would result in higher maximum demand by the plant.

Table – 1 shows summary of measurement at other locations in the plant.

Location Capacitor

Voltage

(V)

Current

–

(Amp)

%

THDv % THDi PF kVA

Harmoni

c current

–

Amperes

Main incomer ON 404.3 404 3.28 13.53 0.98 255.22 54.66

OFF 405.2 496.1 2.44 3.48 0.79 334.99 17.26

SSB 1 410.6 257.3 3.61 6.41 0.75 175.19 16.49

SSB 2 391.6 34.75 4.32 13.48 0.81 19.18 4.68

SSB 3 421.2 46.63 4.69 9.08 0.74 26.17 4.23

Compressor 1 431.5 60.07 5.86 11.92 0.82 41.99 7.16

Compressor 2 430.7 59.91 4.75 10.24 0.82 43.29 6.13

Lighting Load 402.7 79.53 4.23 29.49 0.73 40.96 23.45

UPS 417.5 14.79 4.58 135.41 0.56 9.53 20.03

Table – 1 Summary of harmonic measurement

As seen from the table – 1, total percentage harmonic distortion at current main incomer is

13.53% which is more than limit specified by TANGEDCO. Measurements at different load

centers and shows % THDi more than 8%. In terms of harmonic current, major contributors

are lighting load, UPS and SSB-1.

Based on harmonic measurement study, plant team decided to mitigate harmonics.

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A PQ Case Study


Selection of Solution:

After carrying out harmonic study and based on the outcome, plant team decided to

mitigate harmonics. Next challenge for the plant team was to select a technology that would

be technically and financially feasible.

Theplant team had three options available for mitigation of harmonics. The available

options were:

1. Active harmonic filter

2. Passive harmonic filter

3. Detuning filters

The plant team compared above three technologies based on different parameters like

technical feasibility, harmonic mitigation capability, ease of operation, maintenance cost,

operating cost and initial cost.

Table – 2 shows comparison summary of above technologies.

Technology Active harmonic filter

Passive harmonic

filter De-tuned filter

Technically feasibile YES YES YES

Harmonic mitigation

capability Up to 50th order of harmonics

5th, 7th 11th and

13th

5th, 7th 11th and

13th

Ease of operation Very Easy Very Easy Very Easy

Maintenance cost High in case of any failure Low - LC Circuit Low - LC Circuit

Operating cost

High - Power consumption of

IGBT's Negligible Negligible

Initial cost. 800,000 400,000 200,000

Table – 2 Comparison of various technologies

Harmonic spectrum of the plant consisted of mainly 3rd

, 5th

and 7th

order harmonics as

predominant harmonics. Other harmonics present in the system were 11th

and 13th

.

Based on above comparison, the plant team decided to install detuned filters of 100 ampere

capacity.

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A PQ Case Study


Post Implementation Results:

After installation of detuned filter, the plant team carried out harmonic measurement at the

incomer. Figure – 6 shows voltage and current wave form after installation of detuned filter.

Figure – 6 Voltage and current waveform after installation of detuned filter

Figure 7 shows individual harmonic order.

Figure – 7 Individual order of harmonics

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Table 3 shows summary of post mitigation result.

Location Voltage

Current -

Amperes % THDv % THDi PF kVA

Harmonic current

- Amperes

Main

incomer 419 531.5 2.4 4.69 1.00 347.89 24.93

As seen from the table, after installation of detuned filter, total percentage current

harmonic distortion has come down to 4.69% which is well below the limit specified by

TANGEDCO. Photographs below shows detuned harmonic filter installed at the facility.

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A PQ Case Study


Conclusion:

It can be concluded from the case study that harmonic mitigation is not a costly and capital

extensive project. With detailed study, proper understanding of the subject and

implementation technologies, a cost effective and technically strong solution can be

adopted.

About the Author:

Shaikh Shamsher Ali

Shamser is an Ex-Indian Naval Artificer having 33 years hands-on

experience in energy conservation, project management,

operations & maintenance, sales & marketing, manufacturing and

profit center operations. He has worked in India, UK and Middle

East in Marine, Oil & Gas, Utilities and Heavy Fabrications sectors.

He is BEE Certified Energy Auditor & Energy Manager, PMI USA

Certified Project Manager and Professional Engineer from UK

chapter. Presently he is pursuing his PhD in energy conservation from University of

Petroleum & Energy Studies at Dehradun. He has worked with 3 ESCOs for over 10 years in

India & abroad and presently working as freelance energy conservation consultant with 3

companies engaged in energy conservation business in India. Shamser makes regular

presentation on power quality, harmonic mitigation, energy conservation, energy efficient

lighting and maintenance management systems at various seminars / platforms. He also

trains students and professionals in soft skills.

Case study 47


A PQ Case Study


Disclaimer: The sole responsibility for the content of this document lies with the authors. It does not represent the opinion of

the Asia Power Quality Initiative and /or ICA-I/ICA network. APQI and ICA network are not responsible for any use that may

be made of the information contained therein.

A PQ Case Study · 2017-01-05 · harmonic filters are combination of series and shunt elements of...

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