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A survey of voltage and current harmonics in various industries connected to a state electrical grid.

Introduction

Harmonics levels are increasing in the power systems due to proliferation of power converter circuits which are based on switched mode topologies [1]. These circuits offer definite advantages of high efficiency, controllability, compact size and less maintenance. But they also generate harmonics which pollute the quality of the power supply. These power converter circuits are inherently nonlinear. When the nonlinear loads are connected to the sinusoidal supply voltage, they draw current in nonsinusoidal manner. There are many non linear loads in industries and the ill effects of harmonics are many.

Considering these ill effects, Electricity regulatory Commissions and Utilities, all over the world have started imposing penalty for harmonics dumping by the user into the supply lines. In India, Central Electricity Authority through its statutory body Central Electricity Regulatory Commission has notified through legislature, about the allowable limits for harmonics in the electrical system [2]. It is essential for both the Utility and User Industry to understand the related standard and to know the limits specified therein.

Section 2 lists the sources and effects of harmonics. Section 3 explains the indices for harmonic distortion and the IEEE standard for harmonic limits. Section 4 lists the regulatory requirements in India. Section 5 presents the measurements taken at different types of loads at bus voltages of 110kV, 22kV and 415V connected to the Tamilnadu state electrical network. Section 6 discusses about the measurements and draws the conclusion.

Sources and Effects of Harmonics [2]

Sources of harmonics

The following types of electrical equipment generate considerable amount of harmonics:

* Variable speed drives (VSD)

* Uninterruptible power supplies (UPS) and SMPS

* Arc / Induction furnaces

* Fluorescent lamps with electronic ballasts

Effects of harmonics

* Very high neutral currents in 3P-4W system

* Over heating of neutral conductor

* Reduced power factor

* Overloading of distribution transformers

* Failure of power factor correction capacitors.

* High neutral to ground voltages

* Distorted Voltage waveforms to other loads

* Nuisance breaker-tripping (with no overload)

* Incorrect meter functioning.

* Failure of Protection relays.

* Increased system losses as heat, etc.

Indices and Limits for Harmonics

Two important indices, THD and TDD, are used to measure the amount of harmonics in power systems [3]. These indices are used to measure the deviation of a periodic waveform containing harmonics from a perfect sine wave. For a perfect sine wave, the deviation (or the distortion) is zero.

Total Harmonic Distortion in voltage (THD) in percentage is given by,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Where [V.sub.1] is the RMS value of the fundamental component and [V.sub.h] are RMS values of the harmonic components at the Point of Common Coupling (PCC)

Total Demand Distortion in current (TDD) in percentage is given by,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [I.sub.L] is maximum RMS demand load current at the fundamental frequency and [I.sub.h] are harmonic components at the Point of Common Coupling.

In many references and standards THDv (THD in voltage) is interchangeably used for THD and THDi (THD in current) is used for TDD.

Limits for Harmonics--IEEE Standard

IEEE 519, "Recommended Practices and Requirements for Harmonic Control in Electrical Systems" was published in 1981. With the increased usage of non- linear loads, considering the effects of harmonic currents producing distortion in voltages due to finite system impedance, IEEE 519 was revised and published in 1992 [3]. This has set the limits for both harmonic voltages on the utility transmission and distribution system and harmonic currents within the industrial distribution systems. By controlling the harmonic currents or impedances within the system, harmonics in the voltages can be minimized.

The allowable limits for voltage and current distortion as specified by IEEE 519 - 1992 are given in Tables 1 and 2. Harmonics are measured at the Point of Common Coupling (PCC). PCC can be either the primary or secondary of a utility transformer or at the service entrance of the facility or between the non-linear loads and other loads of an industrial plant.

Regulatory Requirements in India

In India, Central Electricity Authority (CEA) is the apex body that stipulates the norms for the Public Electric System both at national and state levels. The Central Electricity Regulatory Commission (CERC), a statutory body under CEA has notified the limits for harmonics in the grid. The limits have been drawn by adopting the recommendations as in IEEE 519. This has been followed by the Regulatory Commissions in all the 28 states. The notification has also set a time frame, in a phased manner, for implementing the control, starting from the highest transmission bus voltages in the grid to the lowest distribution level voltages. [4-11]

Survey of Harmonics

Measurements were taken at the PCC of loads connected to the Utility grid at 110 kV, 22kV, 415V and 230V using a Power Quality Analyser [12]

Harmonics Measurements in loads connected to 110kV bus

Harmonics were measured at the PCC of (1) a foundry, (2) a railway traction feeder and (3) an integrated steel plant. The secondaries of the metering Potential Transformer (PT) and current transformer (CT) at these users were selected as PCC.

The figures given below are the snapshots of the voltage and current waveforms recorded in the PQA. The "yellow" highlighted selection menu at the right side indicates the 3 phase line-line voltages (3U), 3 phase load currents for delta connection (3A) or 3phase load currents for star connection (4A). For 1 phase connection the right side selection menu does not appear. The "yellow" highlighted selection menu at the bottom indicates either the RMS value of voltage or current which appears phase wise (RMS) or the Total Harmonic Distortion (THD) of the voltage or current.

Harmonics measured at the PCC of a Foundry

The bus voltage at the PCC of the foundry is 110kV. There is a large number of induction furnaces and VSDs. The sanctioned demand for the consumer is 7.5 MVA. The measurements were taken at the secondary of the 110kV / 110V PT and 50A/5A CT. The metering system is a two element method. Hence three are only two CTs. The THD in the voltage, as in fig. 5.1, is within limits. THD in all the three voltages are 0.0%. Fig. 5.2 shows the distortions in currents are 6.7% and 7.5%, which are higher than the limits.

[FIGURE 5.1 OMITTED]

[FIGURE 5.2 OMITTED]

Harmonics measured at the PCC of a Traction Feeder

The utility is feeding the Railways Traction supply transformer at a bus voltage of 110kV. The loads to this transformer are electromotive units (EMU). The sanctioned demand to this consumer is 6.7MVA The supply is given as a single phase. Since the EMUs are non linear loads, the current distortion is high. Due to the high impedance of the feeder transformer (12[OMEGA]), the voltage is also distorted. The distortion in voltage is 0.7% and current is 18.1%.

Harmonics measured at the PCC of a Steel Plant

The bus voltage of the PCC of the steel plant is 110kV. The sanctioned demand was 15MVA.. The distortion in all the three phase voltages is 0.0% as in Fig. 5.4. The plant has lot of VSDs and hence the distortion in the current is 6.9% in 1 phase as seen in Fig.5.5.

[FIGURE 5.3 OMITTED]

[FIGURE 5.4 OMITTED]

[FIGURE 5.5 OMITTED]

Harmonics Measurements in loads connected to 22kV bus

Measurements in Utility Feeder Transformer

The following measurements were taken in the substation transformer of the utility. The substation is feeding to an industrial area. The bus voltage is 22kV. The distortion in currents is high in R phase, equal to 1.8% as in Fig. 5.6. The distortion in current is high in Y phase, equal to 1.0% as shown in Fig. 5.7.

[FIGURE 5.6 OMITTED]

[FIGURE 5.7 OMITTED]

Measurements taken at the PCC of a Steel Alloy Plant

The bus voltage at the PCC of a steel alloy mill is 22kV and the sanctioned demand is 4.75 MVA. There are two induction furnaces. As can be seen in fig. 5.8, the distortion in voltage is high in B phase,. The values of voltage distortion are within the limits. The current distortion are within limits as seen from fig. 5.9. The metering system uses two element method and hence current from two phases only available at the metering CT.

[FIGURE 5.8 OMITTED]

[FIGURE 5.9 OMITTED]

Harmonics in loads connected to 415V bus

At the distribution level, 22kV / 415V transformers are used to feed 3 phase loads at 415V or 1 phase loads at 230V. The transformers are owned and maintained by the consumers if the sanctioned demand is more than 250kVA. If the demand is less, the transformers are owned and maintained by the Utility. In either case, the transformers are connected in delta at the primary and in star at the secondary. The higher amount of harmonics at the secondary causes circulating currents in the primary. This increases the losses and reduces the life of the transformers. Harmonics were directly measured at bus voltages of 415V and 230V and the load currents at the PCCs of several types of consumers.

Harmonics in a Recycling Paper Mill

The paper mill has a large number of VFDs. The supply transformer belongs to the consumer. The distortion in voltage is within limits as in fig.5.10 but the distortion in current is higher than the limits as seen from fig.5.11.

[FIGURE 5.10 OMITTED]

[FIGURE 5.11 OMITTED]

Harmonics in a shopping complex

The supply bus voltage is 415V with 3phase 4 wire system. The shopping complex has 800 numbers of fluorescent lamps, all of them fitted with electronic ballasts. There are other linear loads like air-conditioners and fans in a small ratio. The supply transformer is owned by the consumer. The distortion in voltage is within the limits as given in fig.5.12. The harmonics in the loads have created two issues. Fig. 5.13 shows the distortion in current is higher than the limits and fig. 5.14 shows that there is a significant current in the neutral.

[FIGURE 5.12 OMITTED]

[FIGURE 5.13 OMITTED]

[FIGURE 5.14 OMITTED]

Harmonics measured in a bank

The supply the bank is at a bus voltage of 415V with 3 phase 4 wire system. This bank had 34 computers for its operation. The sanctioned demand was 75kVA. The supply transformer belongs to the Utility. The distortion in voltage is shown in fig. 5.15 and it is within limits. The distortion in current is above the limits in all the three phases. In B phase it is highest which is equal to 15.1%. Due to harmonics generated by the non linear loads, there is a significant amount of harmonics in the neutral conductor as seen in fig.5.17.

[FIGURE 5.15 OMITTED]

[FIGURE 5.16 OMITTED]

[FIGURE 5.17 OMITTED]

Harmonics in a lathe work shop

The work shop had a separate supply to electric arc welding sets (non linear loads). The sanctioned demand was 32 kVA. The utility transformer is supplying to this consumer with a three phase three wire system. The transformer is owned and maintained by the Utility. The distortion bus voltage at the PCC is within the limits as shown in fig. 5.18. The distortion in current is above the limits as shown in fig.5.19.

[FIGURE 5.18 OMITTED]

[FIGURE 5.19 OMITTED]

Discussion

The summary of measurements taken at the PCCs of the representative loads are given in table 3 and table 4.

THD in Supply Bus voltage

The THD measured in the supply voltages of various buses are within the IEEE:519 limits as seen from table 3.

TDD in Load Current

The measured TDD values in all the industries were above the allowable limits as seen from table 4. Though, these industries have not registered any tangible problems, the hidden power losses and slow degradation of equipment and system are bound to occur.

Conclusion

From the actual measurements it is evident that the many of the installations do not conform to the limits specified in the standards. There is a likelihood that these installations also suffer from other problems such as large neutral currents, appreciable heat generation, significant losses, etc. An abnormal heat generation in an electrical system is usually attributed to an over load condition. But in a system with large harmonics, even with sub-normal loading, excessive heat will be generated leading to material failure and possible fire hazard. It has been recommended to the individual industries to carry out en extensive long time measurement and monitor the harmonics levels. It may be also useful to think in a new perspective of higher harmonics leading to many problems in the electrical system that have not been explained by conventional over voltage, over current, etc..

The utilities have also proposed to impose penalty for harmonics dumping into the utility lines. Hence on both the accounts, the above type of installations is to be retrofitted with suitable filters. It is also imperative that the new designs of nonlinear loads are made with built-in filters.

Acknowledgement

The "Measurement of Harmonics in the State Grid" project was funded by the Tamilnadu Electricity Board. Their financial grant and technical assistance for accessing the PCCs at the industries metering points are gratefully acknowledged.

References

[1] Ned Mohan, Undeland and Robbin, 1995,Power Electronics: Converters, Applications and Design, John Wiley, New York.

[2] J.Arrillaga, D.A.Bradley, P.S.Bodger, 1985, "Power System Harmonics", John Wiley & Sons, pp.110-133

[3] IEEE Std 519-1992--(revision of IEEE Std 519-1981) "IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems".

[4] Central Board of Irrigation and Power, India, CBIP publication No.276,1999, "Guide for Limiting Current Harmonics", pp. 19.

[5] Andhra Pradesh Electricity Regulatory Commission, India, Regulation 7, 2004

[6] Assam Electricity Regulatory Commission, India, The Assam Gazette, February 4, 2005

[7] Gujarat Electricity Regulatory Commission, India, Distribution Code, Notification 6 of 2004

[8] Maharashtra Electricity Regulatory Commission, India, Regulation 2005

[9] Jharkahnd Electricity Regulatory Commission, India, Regulation 2005

[10] Orissa Electricity Regulatory Commission, India, The Orissa Gazette, May 28,2004

[11] Tamilnadu Electricity Regulatory Commission, India, Tamilnadu Govt. Gazette No.34A, Sep. 1, 2004, Part IV, Section -2(Supplement), 2004

[12] User's Manual for Power Quality Analyser CA 8332, ed. 1, Chauvin-Arnoux, France, 2003

Biographies

K.R. Valluvan obtained BSc from Madras University in 1982, BTech from Anna University in 1985, PG Diploma from Indian Institute of Science in 1986 and ME from Bharathiar University in 2001. Between 1986 and 1997 he has worked in ABB, SPA Computers and Wipro GE Medical Systems in the areas of embedded system design. He is currently Professor in Kongu Engineering College and pursuing research in soft computing methods for power quality issues.

Dr. A.S. Kandasamy obtained BE (Electrical Engg.) and ME (Power Systems) from Madras University and PhD (EEE) from The International University, Washington in 1966, 1980 and 2004 respectively. He has 35 1/2 years of experience in Planning, Procurement, Installation, Commissioning, Operation and Maintenance of power systems in Tamilnadu Electricity Board. He represented India to the Energy Summit at Washington DC, USA in 2000. He was Chief Engineer (Transmission & Commercial) in 2001 and was a member of The Steering Committee, Accelerated Power Development Program, Ministry of Power, Govt. of India. Since 2002 he is a Professor in Kongu Engineering College. His areas of interest are Power Systems, Power Quality Issues and Protection

Dr. A.M. Natarajan is Chief Executive and Professor in Bannari Amman Institute of Technology, Coimbatore, India. He obtained PhD in Systems Engineering from P.S.G College of Technology, Coimbatore in 1984. He has published more than 100 papers in national and international journals and conferences. He was awarded "The Best Engineering College Principal Award" for the year 2000 by ISTE, New Delhi. He is member of various scientific and professional societies. He has guided more than 75 M.E and M.C.A students. Presently he is guiding many PhD and MPhil students. His research areas include Software Engineering, Soft Computing, Operating Systems, Software Project Management and Networking.

K.R. Valluvan (1) *, A.S. Kandasamy (2) and A.M. Natarajan (3)

(1) Professor, Dept. of IT, Kongu Engineering College, Perundurai- 638052, India * Corresponding AuthorE- mail : krvalluvan@yahoo.co.in (2) Professor, Dept. of ECE (PG), Kongu Engineering College, Perundurai- 638052, India (3)Chief Executive and Professor, Bannari Amman Institute of Technology, Coimbatore, India
Table 1: Voltage Distortion Limits

Bus Voltage at PCC     Individual voltage     Total Harmonic voltage
                       Distortion (%)         Distortion THD (%)

[less than or equal    3.0                    5.0
to] 69 kV

69.0001 kV - 161 kV    1.5                    2.5

> 161.001 kV           1.0                    1.5

Table 2: Current Distortion Limits For General Distribution Systems
(120V - 69 kV)

[I.sub.sc] /  Maximum Harmonic Current Distortion in % of [I.sub.L]
[I.sub.L]

              Individual Harmonic order                  TDD
              (Odd   Harmonics)

              <11    11 - 17   17 - 23   23 - 35   >35

<20           4.0    2.0       1.5       0.6       0.3    5.0

20-50         7.0    3.5       2.5       1.0       0.5    8.0

50-100        10.0   4.5       4.0       1.5       7      12.0

100-1000      12.0   5.5       5.0       2.0       1.0    15.0

>1000         15.0   7.0       6.0       2.5       1.4    20.0

Table 3: Summary of THD measured in various loads

Type of            Type of load         Bus       Measured   Maximum
consumer                                voltage   THD        limit

Foundry            Induction            110kV     0.0%       2.5%
                   Furnaces

Railway            EMUs                 110kV     0.7%       2.5%
Traction Feeder

Integrated Steel   Drives, PLCs         110kV     0.0%       2.5%
Plant

Utility            SS feeder to         22kV      1.8 %      5.0 %
                   industrial area

Steel Alloy Mill   Induction furnace    22kV      1.8 %      5.0 %

Recycling Paper    VFDs                 415V      3.2%       5.0 %
Mill

Shopping           Fluorescent          415V      2.0 %      5.0 %
complex            lamps with
                   electronic ballast

Lathe              Welding sets         415 V     1.7 %      5.0 %
Workshop

Bank               Computers            230V      2.0%       5.0 %

Table 4: Summary of TDD measured at PCCs of various industries.

Type of consumer         Type of load              [I.sub.SC]/
                                                   [I.sub.L]

Foundry                  Induction Furnaces        12

Railway Traction         Electromotive Units       7.9
Feeder

Integrated Steel Plant   Drives, PLCs              < 20

Utility                  SS feeder to              < 20
                         industrial area

Steel Alloy Mill         Induction furnace

Recycling Paper Mill     VFDs                      16.3

Shopping complex         Fluorescent lamps         16.6
                         with electronic ballast

Lathe Workshop           Welding sets              16.9

Bank                     Computers                 16.8

Type of consumer         Measured                  Maximum
                         TDD                       limits

Foundry                  7.5%                      2.5%

Railway Traction         18.1%                     2.5%
Feeder

Integrated Steel Plant   6.9%                      2.5%

Utility                  2.2 %                     5%

Steel Alloy Mill         21.0 %                    5%

Recycling Paper Mill     13.1%                     5%

Shopping complex         7.9 %                     5%

Lathe Workshop           46.7 %                    5%

Bank                     15.1%                     5%
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Author:Valluvan, K.R.; Kandasamy, A.S; Natarajan, A.M.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2008
Words:3150
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