power quality-report

Study and interaction with industry for application of power electronics in electricity distribution system for improvement in power quality. -A REPORT NESCL, Noida

Engg. Department (2012-13)

NESCL/ NTPC MoU/R&D/Power Quality NESCL:Engg. of 43

A study report on application of power electronics in electricity distribution system for improvement in power quality

A REPORT

On

“Study and interaction with industry for application of power electronics

in electricity distribution system for improvement in power quality.”

By

M. Nageswar Rao (Manager) S.Lokanatham (Manager)

Under the guidance of

Shri C.D. Murthy (HOD-Engg) Shri A.K. Parhi (AGM-Engg)

Shri B.M. Singh (AGM-Engg.)

ENGINEERING DEPARTMENT NTPC ELECTRIC SUPPLY COMPANY LIMITED



ACKNOWLEDGEMENTS We are thankful to Shri C.D. Murthy, HOD (Engg.), NESCL for his

valuable suggestions in accomplishing this project. We express our

sincere thanks and acknowledge with deep sense of gratitude for the

guidance and encouragement rendered by Shri A.K. Parhi (AGM-

Engg.), NESCL and Shri B.M. Singh (AGM-Engg.), NESCL for sparing

their valuable time at every stage besides their friendly co operation

and guidance to complete our work successfully.

NESCL expresses its sincere gratitude to M/s L&T, M/s ABB, M/s

P2Power Solutions for their valuable support extended which helped

in preparation of the report.

Finally, we would like to thank all the people who directly and

indirectly co operated us in completing our study successfully.

Power Quality Study Team NESCL-Engg.



PROFILE OF NTPC ELECTRIC SUPPLY COMPANY LIMITED

NTPC Electric Supply Company Ltd. (NESCL) a wholly owned subsidiary of

NTPC was formed in the year 2002, as the electricity distribution arm of power

generation major, NTPC Ltd. NESCL, is engaged in the Rajiv Gandhi Grameen

Vidyutikaran Yojana (RGGVY) of Govt of India and other Consultancy works in

the electricity distribution sector.

NESCL has also been associated in the field of design, engineering and turn-key

execution of various works in the area of construction of EHV substations and

transmission lines from concept to commissioning. Some of the works executed

by NESCL as well as those under execution are as indicated below:

Project management and engineering consultancy work including site

supervision for BPCL Kochi refinery for setting up 220/33 kV

Substation with 2X50 MVA 220/33 kV power Transformer along with

33 kV Gas insulated substation for supply arrangement of BPCL Kochi

refinery.

Power supply arrangement for International Container

Transshipment Terminal, Vallarpadam on behalf of Cochin Port Trust.

Execution of Power supply arrangement of all the five coal mining

projects of NTPC. NESCL has already taken up works related to Pakri-

Barwadih Coal mines in Jharkhand state involving construction of 220

kV Transmission Line and 220/33kV substation with 2X50 MVA

220/33 kV Power Transformer and associated 220/33 kV switch

yard.

Construction of 66 kV transmission lines (both underground and

overhead) along with 66/11 kV substation in UT Chandigarh,

commissioned in year 2007-08.

Turnkey execution of 3x10 MVA power transformers along with 33 kV

line at Mega Sports Complex, Hotwar Ranchi, commissioned in Dec

2008.



Turnkey execution of following substation works on behalf of UT-

Chandigarh is under implementation:

1. 2X20 MVA, 66/11 kV Grid Sub Station at Raipur Kalan.

2. Augmentation of 66/11 KV Sub Station at Sec-52 with 1x30

MVA Power Transformer along with the associated bay.

The company is also involved in providing supply of electricity in 5

km. area around NTPC power plants under a Government of India

scheme.

NESCL has also positioned itself in providing consultancy and

advisory services for assisting Electricity Department of Union

Territory of Pondicherry in preparation and filing of Annual Revenue

Requirement petition and its submission/presentation and defense

before the concerned Regulatory Commission.

NESCL has offered its services to Orissa Power Transmission

Corporation Limited (OPTCL) for rendering Pre-Award Contract

Management Services in respect of major capacity addition

undertaken by OPTCL by putting up new EHT substation, associated

lines and associated works for eleven such packages. NESCL has also

extended its services in the area of Quality Assurance and 3rd party

inspection of stock material to UPCL, UHBVN, MPMKVVCL,

MPPoKVVCL, MPPKVVCL and all the Discoms of Karnataka.

The entire gamut of services offered by NESCL is tailored to meet the

individual needs of the customer and we have created a benchmark being

receptive to our customers in dealing with various cultural issues related to

Quality Assurance, Project Management, System orientation and

Management Information System.



Table of Contents Page No.

INTRODUCTION ....................................................................................................... 8 1. Background ................................................................................................... 8 2. Validation of the Report ................................................................................ 8 3. Introduction ................................................................................................... 8 4. Roadmap ....................................................................................................... 9 5. Power Quality Study Team: ........................................................................... 9

1. POWER QUALITY ............................................................................................... 10

1.1 Introduction ......................................................................................... 10 1.2 Effect of poor Power Quality ................................................................. 10

a. Utility concerns .............................................................................. 11 b. Industrial consumer concerns ....................................................... 11 c. Commercial/ Residential consumer concerns ................................ 11

1.3 What is Power Quality .......................................................................... 11 a. Voltage sag (dip) ........................................................................... 13 b. Voltage swell.................................................................................. 14 c. Interruption .................................................................................. 14 d. Undervoltage ................................................................................ 14 e. Overvoltage ................................................................................... 14 f. Waveform distortion (Harmonics) ................................................ 15

i. Sources of Harmonics ............................................................... 15 ii. %THD ....................................................................................... 16 iii. Recommended Limits ............................................................... 17

g. Voltage Imbalance ......................................................................... 18 h. Voltage Fluctuations...................................................................... 18 i. Power Frequency Variations .......................................................... 19

2. POWER ELECTRONIC SOLUTIONS .................................................................... 20

3. DISTRIBUTION STATIC VAR COMPENSATORS (D-SVC) ................................... 21

3.1 SVC using TCR/ FC ................................................................................ 21 3.2 SVC using TCR/ TSC .............................................................................. 24

4. DISTRIBUTION STATIC SYNCHRONOUS COMPENSATORS (D-STATCOM) or

ACTIVE -FILTERS ........................................................................................... 25 4.1 Principle of operation ........................................................................... 25 4.2 Topology ............................................................................................... 26 4.3 Operating characteristics ..................................................................... 27 4.4 Modes of D-STATCOM operation .......................................................... 28

a. Load compensation mode .............................................................. 28 b. Voltage regulation mode ............................................................... 30

4.5 Product Survey ..................................................................................... 31



5. Dynamic Voltage Restorer (DVR) ...................................................................... 32

6. Power Quality Measurements............................................................................ 33

6.1 CASE STUDY-1: NTPC EOC Substation .................................................. 33 6.2 CASE STUDY-2: M/s Goldwyn Ltd., NSEZ, Noida ................................... 35 6.3 CASE STUDY-3: M/s Karna Apparel, NSEZ, Noida ................................ 40 6.4 Observations: ........................................................................................ 43 6.5 Recommendations: ................................................................................ 43 6.6 Conclusions: .......................................................................................... 43



INTRODUCTION

1. Background

As per MoU for the year 2012-13 finalized by MoP, GoI, NESCL is required to

take up this study and interact with industries and to prepare a report on

application of power electronics in improving power quality in electricity

distribution systems.

2. Validation of the Report

As a part of validation, a technical paper has been prepared based on this report

and has been submitted to CPRI, Bangalore for presentation in a conference

“National Conference on Power Distribution” organized by CPRI during 8-9th

Nov’2012.

Upon scrutiny by Technical Review Committee of the CPRI, the paper was

selected for presentation in the conference vide their mail dated 16.10.2012. A

copy of the mail is attached herewith at Annex-I.

Accordingly, the paper was presented in the Conference held at CPRI,

Bangalore on 8th Nov’2012. The technical paper presented in the CPRI

conference is attached herewith at Annex-II.

3. Introduction

Power Quality mainly deals with supply voltage magnitude disturbances (short

term) and waveform distortion of supply voltage and currents. Power quality

can only be maintained with combined effort of utilities and the consumers.

Utilities have to maintain quality supply even under increased renewable

generation and grid disturbances. Similarly, consumers have to prevent the

electrical disturbances and distortions from spreading into the distribution

system.



EN 50160 defines the quality of power and the max. acceptable levels at the

consumer’s supply terminals. Also, IEEE 519-1992 stipulates max. acceptable

levels of harmonic distortions.

This paper discusses about the spectrum of power quality, causes of power

quality problems, solutions using various power electronic technologies

suitable for distribution systems. Also, this paper presents case studies

conducted with measurements taken at three industries.

4. Roadmap

As a part of this study, following industries have been interacted to understand

various technologies and power electronic products available those help in

improving power quality in electrical distribution sector.

a) M/s P2Power Solutions, Noida

b) M/s L&T, New Delhi and

c) M/s ABB, New Delhi.

Also, Literature survey was conducted and had a detailed study of various IEEE

papers, text books, journals and industry brochures.

A detailed report has been prepared with documenting theory, various

technologies and products helpful in improving power quality in electrical

distribution system.

5. Power Quality Study Team:

Members of team are Sh. M.Nageswar Rao Manager (Engg.), NESCL, Noida Sh. S.Lokanatham Manager (Engg.), NESCL, Noida

The team worked under the guidance of Sh. C.D.Murthy HOD (Engg.), NESCL, Noida Sh. A.K.Parhi AGM (Engg.), NESCL, Noida Sh. B.M.Singh AGM (Engg.), NESCL, Noida



1. POWER QUALITY This chapter discusses importance of power quality in electrical distribution

system. Also, this chapter discusses about harmonics, their sources and

methods of estimation of them. Also, but not the least, the effects of poor power

quality are also discussed in this chapter.

1.1 Introduction

Power Quality has gained tremendous concern in distribution utilities as well as

consumers, and is also mandated by international standards like EN 50160 and

IEEE 519.

Generally, the power quality disturbances are caused by industries like

Automobile, Cement Steel/ foundries, Pulp processing, Printing press etc. Also,

wave form distortions are generally caused, as identified by IEEE 519:1992

standard are power converters, arc furnaces, static VAR compensator, inverters

of dispersed generation, electronic phase control of power, switched mode

power supplies and Pulse wide modulated drives.

1.2 Effect of poor Power Quality

The poor power quality in turn increases the losses in the system as well as

technical losses in the electrical product/ equipment itself. The electrical

disturbances & distortions caused by one consumer/ industry are not only

pollutes the power supply of other equipment of his own, but also pollutes the

power supply of neighboring consumers. And all the distortions are transmitted

back to the source through distribution transformers, distribution &

transmission network, thereby polluting the entire system.

Poor power quality affects badly to Utilities as well all consumers in the system,

as follows:



a. Utility concerns

i. Frequent failures of equipment

ii. Reduced life time of equipment

iii. Reduced safety levels of installations

iv. Increased carbon footprint

v. Increased kWh losses in network components like DTs and cables

etc.

vi. Reduced system capacity

vii. Nuisance tripping of safety devices

viii. Vibration and audible noise in electrical machines like motors,

transformers etc.

ix. Large neutral currents

b. Industrial consumer concerns

i. Production loss

ii. Non-compliance with utility regulations

iii. DG hunting

iv. Frequent failures of equipment

v. Reduced life time of equipment

vi. Vibration and audible noise in electrical machines like motors,

transformers etc.

vii. Low p.f. and hence penalty

c. Commercial/ Residential consumer concerns

i. Increased kWh consumption and billing charges

ii. Low p.f. and hence penalty

iii. Reduced life time of equipment

1.3 What is Power Quality

Power quality is defined by

a) Magnitude variations in fundamental voltage of power supply, and

b) Waveform distortion of fundamental voltage and current of power supply.



The term Power Quality is rather nebulous and may be associated with

reliability by electric utilities. Power Quality refers to those characteristics of

power supply that enable the equipment to work properly. Reliability refers to

the non-availability of electricity supply to consumers because of sustained

interruptions.

The common power quality issues are

a) Transients

b) Short-duration variations

a. Voltage sag

b. Voltage swell

c. Momentary interruptions

c) Long-duration variations

a. Interruption, sustained

b. Under voltages

c. Over voltages

d) Voltage unbalance

e) Waveform distortions

a. Harmonics, Inter-harmonics

b. Notching

c. Noise

f) Voltage fluctuations

g) Power frequency variations



IEEE 1159-1995 stipulates typical characteristics like time duration and voltage

magnitude variations of the above power quality distortions, as tabulated

below.

Instantaneous (0.5–30 cycles)

Momentary (30 cycles–

3 s)

Temporary (3 s–1 min)

Sustained (>1 min)

Others

Voltage sag

0.1–0.9 pu

0.1–0.9 pu

0.1–0.9 pu

-- --

Voltage swell

1.1–1.8 pu

1.1–1.4 pu

1.1–1.2 pu

-- --

Interruptions -- <0.1 pu

<0.1 pu

0.0 pu

--

Under voltages

-- -- -- 0.8 pu

--

Overvoltage

-- -- -- 1.1–1.2 pu

--

Waveform distortion

-- -- -- Steady state

Voltage fluctuations

-- -- -- Intermittent 0.1–7%

Power frequency variations

-- -- -- <10 s

All the above power quality issues are described below in detail.

a. Voltage sag (dip)

A Voltage sag (dip) is defined as a decrease in the root-mean-square (rms) voltage

at the power frequency for periods ranging from a half cycle to a minute.

It is caused by voltage drops due to fault currents or starting of large motors.

Sags may trigger shutdown of process controllers or computer system crashes.



b. Voltage swell

A voltage swell is defined as an increase up to a level between 1.1 and 1.8 pu in

rms voltage at the power frequency for periods ranging from a half cycle to a

minute.

c. Interruption

An interruption occurs when the supply voltage decreases to less than 0.1 pu for a

period of time not exceeding 1 min.

Interruptions can be caused by faults, control malfunctions, or equipment

failures.

d. Undervoltage

An Undervoltage is a decrease in the rms ac voltage to less than 90% at the power

frequency for duration longer than 1 min.

These can be caused by switching on a large load or switching off a large

capacitor bank. Undervoltages are sometimes due to a deliberate reduction of

voltage by the utility to lessen the load during periods of peak demand. These

are often referred to by the nontechnical term brownout.

An undervoltage will lower the output from capacitor banks that a utility or

customer will often install to help maintain voltage and reduce losses in the

system by compensating for the inductive nature of many conductors and loads.

e. Overvoltage

An overvoltage is an increase in the rms ac voltage to a level greater than 110%

at the power frequency for a duration longer than 1 min.

These are caused by switching off a large load or energizing a capacitor bank.

Incorrect tap settings on transformers can also cause undervoltages and

overvoltages.



As these can last several minutes, they stress computers, electronic controllers,

and motors. An overvoltage may shorten the life of power system equipment

and motors.

f. Waveform distortion (Harmonics)

With the increased use of non-linear loads like electronic goods, Switch mode

Power supplies, power electronic switching of power supplies, in these modern

years, the quality of power supply has been disrupted and distorted heavily.

Any distortion caused in the wave shape of voltage or current is analyzed and

synthesized (by Fourier series expansion) using harmonics.

A harmonic component in an AC power system is defined as a sinusoidal

component of a periodic waveform that has a frequency equal to an integer

multiple of the fundamental frequency of the system.

Sub-integral multiple of fundamental frequency of voltage & currents are called

Inter-harmonics or Sub-Harmonics.

i. Sources of Harmonics

Sources of harmonics are broadly categorized as follows

b) Supply side harmonics : mainly causes voltage quality distortion,

c) Load side harmonics : mainly causes current quality distortion.

Supply side harmonics are mainly caused because of

i. Generator torque pulsations, eccentricity, non-sinusoidal winding

patterns etc. causes distortions in voltage generated. No generator

can generate perfect sinusoidal voltage.



ii. Power transformer & Distribution transformers, because of non-

linear (B-H curve) magnetic properties of core, causes severe

distortions in secondary voltage and currents as shown below.

Figure 1: Harmonics Generated by Transformers

Load side harmonics are mainly caused by electrical consumers deploying

equipment/ machines having

i. Electronic switching of power supplies (SMPS) or

ii. Power electronic switching of power supply (power conditioning

devices etc.).

iii. Adjustable speed drives

ii. %THD

The most common harmonic index, which relates to the voltage waveform, is

the THD, which is defined as the root mean square (r.m.s.) of the harmonics

expressed as a percentage of the fundamental component, i.e.

1

2

2

V

VTHD

N

nn

….Eq.(1)

where,

Vn is the single frequency r.m.s. voltage at harmonic n,

N is the maximum harmonic order to be considered,

V1 is the fundamental line to neutral r.m.s. voltage.



Current distortion levels can also be characterized by a THD value but it can be

misleading when the fundamental load current is low. A high THD value for

input current may not be of significant concern if the load is light, since the

magnitude of the harmonic current is low, even though its relative distortion to

the fundamental frequency is high. To avoid such ambiguity a total demand

distortion (TDD) factor is used instead, defined as:

R

N

nn

I

ITDD

2

2

…Eq.(2)

This factor is similar to THD except that the distortion is expressed as a

percentage of some rated or maximum load current magnitude, rather than as a

percentage of the fundamental current. Since electrical power supply systems

are designed to withstand the rated or maximum load current, the impact of

current distortion on the system will be more realistic if the assessment is

based on the designed values, rather than on a reference that fluctuates with

the load levels.

iii. Recommended Limits

As per IEEE 519-1992, the recommended limits for Voltage THD for various

classes of loads are depicted below.

Application Class THDV % (max.)

Special System 3%

General System 5%

Dedicated System 10%



Also, the same standard recommended limits for current harmonic distortions

and %TDD are as given below.

. where,

Isc: Maximum short-circuit current at the Point of Common Coupling (PCC).

IL: Maximum demand load current (fundamental) at the PCC.

g. Voltage Imbalance

Voltage imbalance (unbalance) is defined as the ratio of a negative- or zero-

sequence component to a positive-sequence component.

The voltage imbalance is due to single-phase loads. Uneven distribution of

single phase loads on 3-ph system leads to heavy voltage unbalance and large

neutral current.

h. Voltage Fluctuations

Voltage fluctuations are defined by their rms magnitude expressed as a

percentage of the fundamental magnitude.

Loads that exhibit continuous, rapid variations in load current can cause voltage

variations erroneously referred to as flicker. ANSI C84.1-1992 recommends that

the system voltages should lie in the range 0.9–1.1 pu.

Arc furnaces are the most common cause of voltage fluctuations in the

transmission and distribution system.



i. Power Frequency Variations

At any instant, the frequency depends on the balance between the load and the

capacity of the available generation. When dynamic balance changes, small

changes in frequency occur. In modern interconnected power systems,

frequency is controlled within a tight range as a result of good governor action.

Frequency variations beyond ±0.1 Hz are likely to occur under fault conditions

or from the loss of a major load or generating unit.



2. POWER ELECTRONIC SOLUTIONS

Conventional solutions like APFC panels, Voltage boosters, Static Balancer

Transformer etc. have poor dynamic response and are limited in improving

power quality. On the other hand, with improvement in Power Electronic

systems and microcontroller development, various solutions have been evolved

for power quality in electrical distribution systems.

The Power electronic solutions for power quality improvement can generally be

categorized in two Shunt controllers and series controllers.

a) Shunt controllers:

a. Distribution Static VAR Compensators (D-SVC)

b. Distribution Static Synchronous Compensators (D-STATCOM)

(or) Active -Filters

b) Series Controllers

a. Dynamic Voltage Restorer (DVR)

Shunt controllers protect the utility electrical system from the unfavorable

impact of customer loads. They are recommended mainly for mitigation of the

causes of disturbances, and not their effects in distanced nodes of a power-

electronics system. Series controllers are preferred in case when reduction of

disturbances effects is required, that leads to protection of sensitive loads from

the deterioration in the supply-side voltage.



3. DISTRIBUTION STATIC VAR COMPENSATORS (D-SVC)

The Static VAR Compensators have been widely used by utilities since the mid

1970s in the world. SVC provides reactive power, load balancing, power factor

improvement, and also helps in reducing voltage variations and associated light

flicker due to arc furnace loads.

SVC is based on conventional capacitors and inductors combined with thyristor

switching facilities. Basically, there are two variations of SVC as follows:

i) SVC using a TCR/ FC

ii) SVC using a TCR/ TSC

3.1 SVC using TCR/ FC

In this arrangement, two or more FC (fixed capacitor) banks are connected to a

TCR (thyristor controlled reactor) through a step-down transformer, as shown

in the figure below.

The rating of the reactor is chosen larger than the rating of the capacitor by an

amount to provide the maximum lagging VARS that have to be absorbed from

the system. By changing the firing angle of the thyristor controlling the reactor

from 90° to 180°, the reactive power can be varied over the entire range from

maximum lagging VARS to leading VARS that can be absorbed from the system

by this compensator.



It is common to use wye–delta transformers with SVCs because the delta

windings provide a path to circulate zero-sequence components of the

fundamental and other harmonic currents.

If all the three-phase currents are balanced even though they are distorted with

harmonics, then all the triplen harmonics are of zero-sequence nature.

However, if all the three-phase currents do not contain balanced harmonic

currents, then the triplen harmonics not only contain zero-sequence

components but will also contain both positive- and negative-sequence triplen

harmonics. Positive- and negative-sequence components of the triplen

harmonics can still flow into the system from nonlinear harmonic-producing

loads even with wye–delta transformers.

The sinusoidal steady-state characteristics such as voltage–current (or voltage

and MVArs supplied by the SVC) relationship of an SVC is shown in Figure

below. It consists of three parts. In the regulated region, the voltage and current

are linearly related. Outside the regulated interval, output current (VAR) versus

voltage characteristic of the compensator is the same as that of the capacitor

(low voltage) or an inductor (high voltage).



Disadvantages:

a) The main disadvantage of this configuration is the significant harmonics

that will be generated because of the partial conduction of the large

reactor under normal sinusoidal steady-state operating condition when

the SVC is absorbing zero MVAr.

These harmonics are filtered in the following manner. Triplex harmonics

are canceled by arranging the TCR and the secondary windings of the

step-down transformer in delta connection. The capacitor banks with

the help of series reactors are tuned to filter fifth, seventh, and other

higher-order harmonics as a high-pass filter.

b) Further losses are high due to the circulating current between the

reactor and capacitor banks. The losses in these types of SVCs are shown

in Figure below.

c) These SVCs do not have a short-time overload capability because the

reactors are usually of the air-core type. In applications requiring

overload capability, TCR must be designed for short-time overloading, or

separate thyristor-switched overload reactors must be employed.



3.2 SVC using TCR/ TSC

This compensator comprises of Thyristor controlled reactor and Thyristor

switched Capacitor.

This type of SVC overcomes two major shortcomings of the earlier

compensators by

a) reducing losses under operating conditions and

b) better performance under large system disturbances.

Figure below shows the arrangement of this SVC with a TCR in parallel with

several TSC banks (say, n).

In view of the smaller rating of each capacitor bank, the rating of the reactor

bank will be 1/n times the maximum output of the SVC, thus reducing the

harmonics generated by the reactor. In those situations where harmonics have

to be reduced further, a small amount of FCs tuned as filters may be connected

in parallel with the TCR.



4. DISTRIBUTION STATIC SYNCHRONOUS

COMPENSATORS (D-STATCOM) or ACTIVE -FILTERS

D-STATCOM and Active filters are synonymously called. Development & design

wise, it is called as D-STATCOM and the same is called as Active filter in the

industrial market. By topology of the circuit, D-STATCOMs are called as shunt

Active filters.

D-STATCOM is the most important controller for distribution networks. It has

been widely used since the 1990s.

D-STATCOM helps in precisely

i) regulate the system voltage

ii) Improve voltage profile

iii) Reduce voltage harmonics

iv) Reduce transient voltage disturbances, and

v) Load compensation.

In most applications, a DSTATCOM can use its significant short-term transient

overload capabilities to reduce the size of the compensation system needed to

handle transient events. The short-term overload capability is up to 325% for

periods of 1 to 3 seconds, which allows applications such as wind farms and

utility voltage stabilization to optimize the system’s cost and performance.

Due to its lesser power handling requirement, PWM converters (at higher

switching frequencies) are used (with IGBTs in D-STATCOM as compared to

Thyristors used in STATCOM (FACTS controllers).

4.1 Principle of operation

The equivalent circuit of a power system with a DSTATCOM is shown below.



DSTATCOM generates a variable voltage, Vd, that is very nearly in phase with

the source voltage, Vs. The inductance in this simplified circuit, L, consists of the

inductance of the coupling transformer and filter. The voltage across the

inductance, VL, equals Vs-Vd and is small in per-unit terms. . . of the order of 5-

20%.

If Vs > Vd, VL is in phase with Vs and current IL lags Vs by 90°; DSTATCOM, acting

as a generator, produces leading (inductive) reactive current.

If Vs < Vd, VL is antiphase with Vs and current IL leads Vs by 90°; DSTATCOM

produces lagging (capacitive) reactive current.

4.2 Topology

DSTATCOM controllers can be constructed based on both VSI and CSI

topologies, as shown below.



The VSI converter is connected to the feeder via a reactor Lf and has a voltage

source (Capacitor CD) on the dc side. On the other hand, the CSI converter is

connected on the AC side via capacitor Cf and has a current source (inductor LD)

on the dc side.

In practice, CSI topology is not used for D-STATCOM due to

a) Higher loses on the dc reactor compare to the dc capacitor of VSI

b) Requirement of reverse-blocking semiconductor switches, which have

higher losses than reverse-conducting switches of VSI.

c) VSI has advantage that Inductance of coupling transformer (if present)

adds to Lf (of ac filter), thus reducing the size of ac filter inductance.

4.3 Operating characteristics

The static V-I characteristic of D-STATCOM reactive power is symmetrical as

shown in Fig. below.



Assuming lossless operation, the averaged (but not instantaneous) active power

has to be zero. There are no similar limitations for reactive power, because it is

only exchanged between phases, and is not converted between the AC and DC

sides of D-STATCOM VSI.

4.4 Modes of D-STATCOM operation

There are two modes of D-STATCOM operation: load compensation in current

control mode and voltage regulation in voltage control mode.

a. Load compensation mode

In the load-compensation mode, D-STATCOM is controlled in current mode. In

this current control mode, the feeder currents are made proportion to the

fundamental, positive component of terminal voltage. The control system of D-

STATCOM has to generate

i) reference currents,

ii) compensating harmonic, unbalance and fundamental reactive

components of non-linear load supply currents.

The required rated power of load-compensating D-STATCOM depends only on

i) reactive power,

ii) harmonic distortion and

iii) power of the compensated load.



In general, D-STATCOM is capable of compensating current disturbances from

harmonics to long duration effects, including active power transients.

The possibility and effectiveness of compensation of a particular voltage-quality

problem depends on

i) the topology and

ii) rated power of the controller, as well as on

iii) the capacity of the energy-storage (ES) system connected on the D-

STATCOM DC side.

Load compensation provides also a reduction of voltage distortion related with

the feeder voltage drop. The level of distortion reduction depends on

i) the configuration of the distribution network, as well as

ii) The ratio between the power of the compensated nonlinear load and the

feeder short-circuit power.

The block diagram of a control system for load compensating D-STATCOM is

given in the Figure below.

Despite current compensation, a D-STATCOM controller can be used at the

same time for AC/DC power conversion, for example providing a supply for a



DC feeder or micro-DC distribution system, especially in distributed generation

systems.

b. Voltage regulation mode

The idea of voltage regulation using D-STATCOM is consistent with D-SVC,

discussed in previous chapter. It is realized by compensating reactive power

(i.e. by injecting or absorbing reactive power). The advantage of D-STATCOM

over D-SVC is also V-I characteristics and dynamics, but this controller is more

expensive.

D-STATCOM in voltage regulation mode, requires higher compensating power

than for load compensation.

The block diagram of a voltage-regulating D-STATCOM is presented in Figure

below.



4.5 Product Survey

PureWave DSTATCOM of M/s S&C Electric Company utilizes ±1.25 MVA / 3.3

MVAR PWM Inverters in a modular arrangement. The product specifications are

as follows:

Specifications

System Voltage Continuous Output 480 V to 35 kV, 50 or 60 Hz

Short-Term Current Rating 264% for 2 seconds, ramping to 100% at 4 seconds; or 264% for 3 seconds, stepping directly to 100%

Reactive Current Response Time 2 to 4 milliseconds

Inverter IGBT, pulse-width modulated at 4860 Hz

Temperature Range -40° to +50° C

Efficiency >98% typical

Cooling Ambient air cooling



5. Dynamic Voltage Restorer (DVR)

DVR is a series power electronic controller, protects sensitive loads from all

supply-side disturbances other than outages. They are connected in series to

the feeder between supply and load. They operate as synchronous voltage

source and inject voltage into the feeder in phase with supply voltage and with

required waveform to mitigate supply side disturbances, and thereby

maintaining quality power at the load side. A typical location and operation

principle of DVR is shown in figure below.

DVRs can be divided into two groups with and without energy storage (ES). ES

devices like batteries, capacitors or flywheels are used to store and deliver

energy during disturbances. In cases of DVR without internal ES, the energy is

taken from the supply grid during disturbances.



6. Power Quality Measurements

Power Quality Measurements were carried out at following locations on Low

voltage side of the incomers.

1. NTPC EOC Substation (33/0.4 kV)

2. M/s Goldwyn Ltd., NSEZ, Noida- An LED manufacturing unit

3. M/s Karna Apparels (P) Ltd. , NSEZ, Noida- A garment factory

6.1 CASE STUDY-1: NTPC EOC Substation

Measurements were carried out at 33/0.4kV substation of NTPC EOC building

on the low voltage side of incomer 9R using Power analyser KRYKARD.

The results of the measurement are tabulated as follows:

R Y B N R Y B R Y B R Y B PF1 PF2 PF3INCOMER OF 9R(27july) #1 307 360 348 53 5 6 6 240 240 240 1.3 1.4 1.4 0.86 0.84 0.81

#2 270 308 327 15 15 14 255 256 254 1.9 2.1 1.8 0.82 0.82 0.74#3 91 105 86 71 59 68 257 256 257 3.2 3.3 3.1 0.52 0.75 0.59

INCOMER OF 9R(28july) #4 202 229 254 44 13 13 13 247 248 246 2.1 2.1 2.2 0.88 0.86 0.78

CASEContentsArms iTHD% Vrms vTHD% PF

The phase currents variations are recorded and are as follows:

R Phase Y Phase B Phase N Phase Critical PointsLegend



100.0

150.0

200.0

250.0

300.0

350.0

400.0

A

3:29:30.000 PM7/27/2012

12:40:58.000 PM7/28/2012

4 h/Div21:11:28 (h:min:s)

The current THD% of the three phases is also recorded and is as follows:

5.000

10.00

15.00

20.00

25.00

30.0035.00

40.00

45.0050.00

55.00

60.0065.00

70.00

75.00

80.00

85.00

%

3:29:30.000 PM7/27/2012

12:40:58.000 PM7/28/2012

4 h/Div21:11:28 (h:min:s)

Observations:

It is inferred from above measurements and recordings that

i) % Unbalance of phase currents is very high, of the order of 15%.

ii) Loads are highly non-linear and pulsating.

iii) % Current THD increases to abnormal values upto 75%.



6.2 CASE STUDY-2: M/s Goldwyn Ltd., NSEZ, Noida

Measurement was carried out on incomer LT cables housed inside the LT

distribution panel. These LT cables are run from the 11/0.433kV Distribution

Transformer and are 2 runs of single core type for R-ph, Y-ph, B-ph and Neutral.

These 2 runs of cables are terminated on common LT busbars inside the panel.

Due to limitation in clamp-on CT diameter, measurement was conducted in 2

stages. One set of data recorded on one set of cables and the other set of data on

the 2nd set of cables inside the panels. Neutral current was calculated by the

Instrument, based on the three phase current measurements.

Measurements on 1st set of cable (R1,Y1,B1) and 2nd set of cable (R2,Y2,B2)

are as follows:

Measurement on 1st set of cable (R1,Y1,B1):

Figure 2: Measurement results on 1st set of cables

Note: -ve sign in currents & power is due to clamp of meter CTs in reverse

direction, hence they should be considered +ve for power drawal from UPPCL.

The voltage waveform measured is as follows:



Basic scope / File: 43.DAT

-400

-360

-320

-280

-240

-200

-160

-120

-80

-40

0

40

80

120

160

200

240

280

320

360

400

U1 I 1 U2 I 2 U3 I 3 Figure 3: Voltage waveforms on 1st set of cable

CT current setting in the meter was 10A whereas the clamp on CT used with the

instrument was of 1000A rating, hence multiplying factor was derived as 100

(1000A/10A).


-2

-2

-2

-1

-1

-1

-1

-1

0

0

0

0

0

1

1

1

1

1

2

2

2

U1 I 1 U2 I 2 U3 I 3 Figure 4: Current waveforms on 1st set of cable

M.F. for current readings = 100.

Measurement on 2nd set of cable (R2,Y2,B2):



Figure 5: Measurement results on 2nd set of cables

Note: -ve sign in currents & power is due to clamp of meter CTs in reverse

direction, hence they should be considered +ve for power drawal from UPPCL.

The voltage waveform measured is as follows: Basic scope / File: 45.DAT

-400

-360

-320

-280

-240

-200

-160

-120

-80

-40

0

40

80

120

160

200

240

280

320

360

400

U1 I 1 U2 I 2 U3 I 3 Figure 6: Voltage waveforms on 2nd set of cable




-2

-2

-2

-1

-1

-1

-1

-1

0

0

0

0

0

1

1

1

1

1

2

2

2

U1 I 1 U2 I 2 U3 I 3 Figure 7: Current waveforms on 2nd set of cable

M.F. for current readings = 100.



Harmonic measurement was also carried out, and the current-harmonic

spectrum was recorded. The bar-graphs for 1st set of cable (R1,Y1,B1) and 2nd

set of cable (R2,Y2,B2) are as follows:

Phase

Current Harmonic spectrum (measured) 1st set of cable (R1,Y1,B1) 2nd set of cable (R2,Y2,B2)

R-ph

Harmonics / Curr Ph1 / File: 43.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 520

3.3

6.7

10.0

13.3

16.7

20.0

23.3

26.7

30.0

33.3

36.7

40.0

43.3

46.7

50.0

H armonics / C urr Ph1 / File: 45.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 560

3.3

6.7

10.0

13.3

16.7

20.0

23.3

26.7

30.0

33.3

36.7

40.0

43.3

46.7

50.0

Y-ph

H armon ics / Curr Ph2 / File: 43 .DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 560

0.7

1.3

2.0

2.7

3.3

4.0

4.7

5.3

6.0

6.7

7.3

8.0

8.7

9.3

10.0

Har monics / C urr Ph2 / File: 45.D AT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 560

0.7

1.3

2.0

2.7

3.3

4.0

4.7

5.3

6.0

6.7

7.3

8.0

8.7

9.3

10.0

B-ph

Har mon ics / Cur r Ph3 / File : 43.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 600

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

14.7

16.0

17.3

18.7

20.0

Har monics / C urr Ph3 / File: 45.D AT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 560

3.3

6.7

10.0

13.3

16.7

20.0

23.3

26.7

30.0

33.3

36.7

40.0

43.3

46.7

50.0

NOTE: %THD(I) is displayed in the bar-graphs at 64th harmonic no.



6.3 CASE STUDY-3: M/s Karna Apparel, NSEZ, Noida

Measurement was carried out on incomer LT cables of both LT feeders

separately.

a) 1st LT feeder is that connected on 250kVA distribution transformer with

sanctioned load of 175kVA.

b) 2nd LT feeder is that connected on common 1000kVA distribution

transformer (common to 4 neighboring industries). Sanction load for M/s

Karna from this transformer is 89kVA.

The measurements on 1st & 2nd LT feeder are carried out separately, and the

meter recordings are as follows:

Measurement on 1st LT feeder:

The phase-wise electrical parameters are recorded in the meter, and are as

follows:

The scope of waveforms of voltage and currents of all phases is as follows:



Basic scope / File: 01_01_01.DAT

-400

-360

-320

-280

-240

-200

-160

-120

-80

-40

0

40

80

120

160

200

240

280

320

360

400

U1 I 1 U2 I 2 U3 I 3

M.F. for voltage & current readings = 1.

Measurements carried out on 2nd LT feeder:



Scope:

Basic scope / File: 01_01_05.DAT

-400

-360

-320

-280

-240

-200

-160

-120

-80

-40

0

40

80

120

160

200

240

280

320

360

400

U1 I 1 U2 I 2 U3 I 3

Also, the current harmonics are recorded for both 1st & 2nd LT feeders and are as follows: Phase Current Harmonic spectrum (measured)

1st LT feeder source 2nd LT feeder source R-ph H armonics / C urr Ph1 / F ile : 01_0 1_01 .D AT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 640

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

14.7

16.0

17.3

18.7

20.0

H ar monic s / C ur r Ph1 / File: 01_01_05.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 640

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

14.7

16.0

17.3

18.7

20.0

Y-ph Harmonics / Curr Ph2 / File : 01_01_01.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 640

3.3

6.7

10.0

13.3

16.7

20.0

23.3

26.7

30.0

33.3

36.7

40.0

43.3

46.7

50.0

H armonics / Cur r Ph2 / File: 01_01_05.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 640

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

14.7

16.0

17.3

18.7

20.0



B-ph

H armonics / C urr Ph3 / F ile : 01_0 1_01 .D AT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 640

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

14.7

16.0

17.3

18.7

20.0

Har monics / Curr Ph3 / File: 01_01_05.DAT

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 640

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

14.7

16.0

17.3

18.7

20.0

6.4 Observations:

In all the above case studies, it found that the

a) load currents are generally polluted with heavy harmonic currents.

Current THD levels are predominantly higher than 15%, much higher

than recommended limit of 5%.

b) Loads are unbalanced.

6.5 Recommendations:

Because of above observations, it is recommended in all the locations with D-

STATCOM (active filter), as it can mitigate load unbalance, harmonic currents.

Also, the solution provides dynamic reactive support.

6.6 Conclusions:

It is generally observed from the case studies conducted that the current

distortions are very high at consumers’ PCC, and the loads are high unbalanced.

Also, due to increased use of computers, switch mode power supplies and

controlled supplies, lot of harmonics are being injected into the system.

It is high time for distribution utilities as well as consumers to install power

electronic controllers to mitigate power quality problems and to restrict the

disturbances from spreading into the system.

---*---*---

Dear Sir/Madam, Your paper has been accepted. Kindly convert this paper into 6 to 8 pages and revert the paper to us. Kindly register by paying registration fees. Please send your power point presentation which is to be presented during 8‐9th November 2012 conference. You also send your detailed CV about qualification ,experience, Area of interest, No.of Publications etc With regards, P.Chandhra Sekhar 9480619140 From: nageswar [mailto:[email protected]] Sent: Friday, October 12, 2012 4:16 PM To: [email protected]; [email protected] Cc: [email protected]; [email protected]; lokanatham; [email protected] Subject: Paper for NCPD CPRI

From: P Chandra sekher Date: 10/16/2012 9:52:40 AM To: 'nageswar' Subject: RE: Paper for NCPD CPRI

Dear Sir, Please find the Paper prepared by us for submission to National Conference on Power Distribution (NCPD), to be organized by CPRI. The scope of the paper covers power quality issues & concerns in electrical distribution and various power mitigation technologies implemented in distribution area. Please consider the paper for presentation in the conference. With regards, M.Nageswar Rao, Manager (Engg.) NESCL, Noida N.T.P.C. Mobile: 9650992103

Page 1 of 2

10/17/2012file://C:\Documents and Settings\NESCL\Local Settings\Application Data\IM\Runtime\...

NTPC ELECTRIC SUPPLY COMPANY Ltd.

1

Abstract— Power Quality mainly deals with supply voltage magnitude disturbances (short term) and waveform distortion of supply voltage and currents. Power quality can only be maintained with combined effort of utilities and the consumers.

Utilities have to maintain quality supply even under increased renewable generation and grid disturbances. Similarly, consumers have to prevent the electrical disturbances and distortions from spreading into the distribution system.

EN 50160 defines the quality of power and the max. acceptable levels at the consumer’s supply

terminals. Also, IEEE 519-1992 stipulates max. acceptable levels of harmonic distortions.

This paper discusses about the spectrum of power quality, causes of power quality problems, solutions using various power electronic technologies suitable for distribution systems. Also, this paper presents case studies conducted with measurements taken at three industries.

Index Terms— Power Quality (PQ), THD, D-SVC, D-STATCOM, Active filters, DVR.

I. INTRODUCTION

ower Quality has gained tremendous concern in distribution utilities as well as consumers, and is also mandated by international standards like EN 50160 and IEEE 519.

Generally, the power quality disturbances are caused by industries like Automobile, Cement Steel/

foundries, Pulp processing, Printing press etc. Also, wave form distortions are generally caused, as identified by IEEE 519:1992 standard are power converters, arc furnaces, static VAR compensator, inverters of dispersed generation, electronic phase control of power, switched mode power supplies and Pulse wide modulated drives.

The poor power quality in turn increases the losses in the system as well as technical losses in the electrical

product/ equipment itself. The electrical disturbances & distortions caused by one consumer/ industry are not only pollutes the power supply of other equipment of his own, but also pollutes the power supply of neighboring consumers. And all the distortions are transmitted back to the source through distribution transformers, distribution & transmission network, thereby polluting the entire system.

M. Nageswara Rao has been working with NTPC Electric Supply Company Ltd (wholly owned subsidiary of NTPC), Noida, India as Manager (Engg.). (Mobile:

+91-9650992103; e-mail: [email protected] ).

Application of Power Electronics in electricity distribution system for improvement in power quality

M. Nageswara Rao

P


2

Poor power quality affects Utilities with Frequent failures of equipment, Reduced life time of equipment,

Reduced safety levels of installations, Increased carbon footprint, Increased kWh losses in network components like DTs and cables etc., Reduced system capacity, Nuisance tripping of safety devices, Vibration and audible noise in electrical machines like motors, transformers etc., Large neutral currents. Similarly, poor power quality affects industrial consumers with Production loss, Non-compliance with utility regulations, DG hunting , Frequent failures of equipment, Reduced life time of equipment, Vibration and audible noise in electrical machines like motors, transformers etc., Low p.f. and hence penalty. Commercial/ Residential consumers also get affected with Increased kWh consumption and billing charges, Low p.f. and hence penalty, Reduced life time of equipment etc.

II. POWER QUALITY

Power quality is defined by a) Magnitude variations in fundamental voltage of power supply, and b) Waveform distortion of fundamental voltage and current of power supply.

The term Power Quality is rather nebulous and may be associated with reliability by electric utilities.

Power Quality refers to those characteristics of power supply that enable the equipment to work properly. Reliability refers to the non-availability of electricity supply to consumers because of sustained interruptions.

The common power quality issues are

a) Transients b) Short-duration variations

a. Voltage sag b. Voltage swell c. Momentary interruptions

c) Long-duration variations a. Interruption, sustained b. Under voltages c. Over voltages

d) Voltage unbalance e) Waveform distortions

a. Harmonics, Inter-harmonics b. Notching c. Noise

f) Voltage fluctuations g) Power frequency variations

IEEE 1159-1995 stipulates typical characteristics like time duration and voltage magnitude variations of

the above power quality distortions, as tabulated below.

Instantaneous (0.5–30 cycles)

Momentary (30 cycles–3 s)

Temporary (3 s–1 min)

Sustained (>1 min)

Others

Voltage sag 0.1–0.9 pu 0.1–0.9 pu 0.1–0.9 pu -- --


3

Voltage swell 1.1–1.8 pu 1.1–1.4 pu 1.1–1.2 pu -- --

Interruptions -- <0.1 pu <0.1 pu 0.0 pu --

Under voltages -- -- -- 0.8 pu --

Overvoltage -- -- -- 1.1–1.2 pu -- Waveform distortion (Harmonics)

-- -- -- Steady state

Voltage fluctuations -- -- -- Intermittent 0.1–7%

Power frequency variations

-- -- -- <10 s

Various international standards have been evolved to maintain power quality in electrical systems. They are listed as follows:

i) IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems established limits on harmonic currents and voltages at the point of common coupling (PCC), or point of metering. This standard stipulates max. acceptable levels of Total Harmonic distortions of voltage and Total demand distortion of currents of supplies as follows:

Application Class THDV % (max.)

Special System 3%

General System 5%

Dedicated System 10%

. where, Isc: Maximum short-circuit current at the Point of Common Coupling (PCC). IL: Maximum demand load current (fundamental) at the PCC.

ii) IEC 61000-3-2 and IEC 61000-3-4: These standards specify limits for harmonic current

emissions applicable to electrical and electronic equipment, and intended to be connected to public low-voltage distribution systems.

iii) IEEE Standard 1159-1995, Recommended Practice for Monitoring Electric Power Quality iv) IEEE Standard 1250-1995, Guide for Service to Equipment Sensitive to Momentary Voltage

Disturbances


4

III. POWER ELECTRONIC SOLUTIONS

Conventional solutions like APFC panels, Voltage boosters, Static Balancer Transformer etc. have poor dynamic response and are limited in improving power quality. On the other hand, with improvement in Power Electronic systems and microcontroller development, various solutions have been evolved for power quality in electrical distribution systems.

The Power electronic solutions for power quality improvement can generally be categorized in two Shunt

controllers and series controllers.

i) Shunt controllers: a. Distribution Static VAR Compensators (D-SVC) b. Distribution Static Synchronous Compensators (D-STATCOM) (or) Active -Filters

ii) Series Controllers

a. Dynamic Voltage Restorer (DVR)

Shunt controllers protect the utility electrical system from the unfavorable impact of customer loads. They are recommended mainly for mitigation of the causes of disturbances, and not their effects in distanced nodes of a power-electronics system. Series controllers are preferred in case when reduction of disturbances effects is required, that leads to protection of sensitive loads from the deterioration in the supply-side voltage.

IV. DISTRIBUTION STATIC VAR COMPENSATORS (D-SVC)

The Static VAR Compensators have been widely used by utilities since the mid 1970s in the world. SVC provides reactive power, load balancing, power factor improvement, and also helps in reducing voltage variations and associated light flicker due to arc furnace loads.

SVC is based on conventional capacitors and inductors combined with thyristor switching facilities. TCR

(thyristor controlled reactor) is connected to either Fixed Capacitor banks (FC) or Thyristor Switched Capacitor banks (TSC) through a step-down transformer to the system, as shown in the figure below.


5

The rating of the reactor is chosen larger than the rating of the capacitor by an amount to provide the maximum lagging VARS that have to be absorbed from the system. By changing the firing angle of the thyristor controlling the reactor from 90° to 180°, the reactive power can be varied over the entire range from maximum lagging VARS to leading VARS that can be absorbed from the system by this compensator. It is common to use wye–delta transformers with SVCs because the delta windings provide a path to circulate zero-sequence components of the fundamental and other harmonic currents.

The major disadvantages of SVC are the significant harmonics that will be generated because of the partial

conduction of the large reactor under normal sinusoidal steady-state operating condition when the SVC is absorbing zero MVAR. These harmonics can either be reduced by using delta winding in the transformer or by using TSC instead of FC banks. Further losses are high due to the circulating current between the reactor and capacitor banks. These SVCs do not have a short-time overload capability because the reactors are usually of the air-core type.

V. DISTRIBUTION STATIC SYNCHRONOUS COMPENSATORS (D-STATCOM) (OR) ACTIVE -FILTERS

D-STATCOM and Active filters are synonymously called. Development & design wise, it is called as D-

STATCOM and the same is called as Active filter in the industrial market. D-STATCOM is the most important controller for distribution networks. It has been widely used since the 1990s.

D-STATCOM helps in precisely regulate the system voltage, Improve voltage profile, Reduce voltage

harmonics, Reduce transient voltage disturbances, and Load compensation.

The main advantage of D-STATCOM is its significant short-term transient overload capabilities, that helps in reducing the size of the compensation system needed to handle transient events. The short-term overload capability is up to 325% for periods of 1 to 3 seconds, which allows applications such as wind farms and utility voltage stabilization to optimize the system’s cost and performance. The other major advantage is its lesser power handling requirement.

Due to lesser power handling requirement, D-STATCOM is built with PWM converters (at higher

switching frequencies) with IGBTs as against Thyristors used in STATCOM (FACTS controllers) for transmission systems.

The principle of operation of D-STATCOM is explained with following equivalent circuit of a power

system with a DSTATCOM. DSTATCOM generates a variable voltage, Vd, that is very nearly in phase with the source voltage, Vs. The inductance in this simplified circuit, L, consists of the inductance of the coupling transformer and filter. The voltage across the inductance, VL, equals Vs-Vd and is small in per-unit terms of the order of 5-20%.

i) If Vs > Vd, VL is in phase with Vs and current IL lags Vs by 90°; DSTATCOM, acting as a

generator, produces leading (inductive) reactive current. ii) If Vs < Vd, VL is antiphase with Vs and current IL leads Vs by 90°; DSTATCOM produces lagging

(capacitive) reactive current.


6

The general arrangement of DSTATCOM (in VSI topology) is shown in figure below. The VSI converter is connected to the feeder via a reactor Lf and has a voltage source (Capacitor CD) on the dc side.

There are two modes of D-STATCOM operation: load compensation in current control mode and voltage

regulation in voltage control mode. In the load-compensation mode, D-STATCOM is controlled in current mode. In this current control

mode, the feeder currents are made proportion to the fundamental, positive component of terminal voltage. The control system of D-STATCOM has to generate reference currents, and compensating harmonic, unbalance and fundamental reactive components of non-linear load supply currents. The block diagram of a control system for load compensating D-STATCOM is given in the Figure below.


7

In Voltage Regulation mode, the operation of D-STATCOM is consistent with D-SVC, discussed in

previous section. It is realized by compensating reactive power (i.e. by injecting or absorbing reactive power). The advantage of D-STATCOM over D-SVC is also V-I characteristics and dynamics, but this controller is more expensive. D-STATCOM in voltage regulation mode, requires higher compensating power than for load compensation. The block diagram of a voltage-regulating D-STATCOM is presented in Figure below.

VI. DYNAMIC VOLTAGE RESTORER (DVR) DVR is a series power electronic controller, protects sensitive loads from all supply-side disturbances

other than outages. They are connected in series to the feeder between supply and load. They operate as synchronous voltage source and inject voltage into the feeder in phase with supply voltage and with required waveform to mitigate supply side disturbances, and thereby maintaining quality power at the load side. A typical location and operation principle of DVR is shown in figure below.


8

DVRs can be divided into two groups with and without energy storage (ES). ES devices like batteries, capacitors or flywheels are used to store and deliver energy during disturbances. In cases of DVR without internal ES, the energy is taken from the supply grid during disturbances.

VII. POWER QUALITY MEASUREMENTS

Power Quality Measurements were carried out at following locations.. i) NTPC, Noida EOC Substation ii) M/s Goldwyn Ltd., NSEZ, Noida- An LED manufacturing unit iii) M/s Karna Apparels (P) Ltd. , NSEZ, Noida- A garment factory

KRYKARD/ AMPROBE Power Analyzers have been used for conducting measurements on Low voltage

side of the incomers at above locations.

CASE STUDY-1: NTPC EOC Substation Measurements were carried out on the low voltage side of incomer 9R and the results of the measurement

are tabulated as follows:

R Y B N R Y B R Y B R Y B PF1 PF2 PF3INCOMER OF 9R(27july) #1 307 360 348 53 5 6 6 240 240 240 1.3 1.4 1.4 0.86 0.84 0.81

#2 270 308 327 15 15 14 255 256 254 1.9 2.1 1.8 0.82 0.82 0.74#3 91 105 86 71 59 68 257 256 257 3.2 3.3 3.1 0.52 0.75 0.59

INCOMER OF 9R(28july) #4 202 229 254 44 13 13 13 247 248 246 2.1 2.1 2.2 0.88 0.86 0.78

CASEContentsArms iTHD% Vrms vTHD% PF

The phase currents variations are recorded and are as follows:

100.0

150.0

200.0

250.0

300.0

350.0

400.0

A

3:29:30.000 PM7/27/2012

12:40:58.000 PM7/28/2012

4 h/Div21:11:28 (h:min:s)

The current THD% of the three phases is also recorded and is as follows:

R Phase Y Phase B Phase N Phase Critical PointsLegend


9

5.00010.0015.00

20.0025.0030.0035.0040.0045.0050.0055.00

60.0065.0070.0075.0080.0085.00

%

3:29:30.000 PM7/27/2012

12:40:58.000 PM7/28/2012

4 h/Div21:11:28 (h:min:s)

Following observations are inferred from above measurements and recordings that

i) % Unbalance of phase currents is very high, of the order of 15%. ii) Loads are highly non-linear and pulsating. iii) % Current THD increases to abnormal values upto 75%.

CASE STUDY-2: M/s Goldwyn Ltd., NSEZ, Noida Measurement was carried out on incomer LT cables housed inside the LT distribution panel. These LT

cables are run from the 11/0.433kV Distribution Transformer and are 2 runs of single core type for R-ph, Y-ph, B-ph and Neutral. These 2 runs of cables are terminated on common LT busbars inside the panel.

Due to limitation in clamp-on CT diameter, measurement was conducted in 2 stages. One set of data

recorded on one set of cables and the other set of data on the 2nd set of cables inside the panels. Neutral current was calculated by the Instrument, based on the three phase current measurements.

Measurements on 1st set of cable (R1,Y1,B1) and 2nd set of cable (R2,Y2,B2) are as follows:


10

Note: -ve sign in currents & power is due to clamp of meter CTs in reverse direction, hence they should be considered +ve for power drawal from UPPCL.

CT current setting in the meter was 10A whereas the clamp on CT used with the instrument was of 1000A

rating, hence multiplying factor was derived as 100 (1000A/10A). (M.F. for current readings = 100.). Basic scope / File: 43.DAT

-2

-2

-2

-1

-1

-1

-1

-1

0

0

0

0

0

1

1

1

1

1

2

2

2

U1 I 1 U2 I 2 U3 I 3 Following observations are inferred from above measurements and recordings that

i) % Unbalance of phase currents is very high. ii) Loads are highly non-linear and pulsating. iii) % Current THD is very high.

CASE STUDY-3: M/s Karna Apparel, NSEZ, Noida

Measurement was carried out on incomer LT cables of both LT feeders separately. i) 1st LT feeder is that connected on 250kVA distribution transformer with sanctioned load of

175kVA. ii) 2nd LT feeder is that connected on common 1000kVA distribution transformer (common to 4

neighboring industries). Sanction load for M/s Karna from this transformer is 89kVA.

The measurements on 1st & 2nd LT feeder are carried out separately, and the meter recordings are as follows:


11

The scope of waveforms of voltage and currents of all phases is as follows: Basic scope / File: 01_01_01.DAT

-400

-360

-320

-280

-240

-200

-160

-120

-80

-40

0

40

80

120

160

200

240

280

320

360

400

U1 I 1 U2 I 2 U3 I 3 Following observations are inferred from above measurements and recordings that

i) % Unbalance of phase currents is very high. ii) Loads are highly non-linear and pulsating. iii) % Current THD is very high.

VIII. CONCLUSION It is generally observed from the case studies conducted that the current distortions are very high at consumers’ PCC, and the

loads are high unbalanced. Also, due to increased use of computers, switch mode power supplies and controlled supplies, lot of harmonics are being injected into the system.

It is high time for distribution utilities as well as consumers to install power electronic controllers to mitigate power quality

problems and to restrict the disturbances from spreading into the system.

ACKNOWLEDGMENT Author expresses deep gratitude to NESCL (NTPC) for the extended support and motivating to present this paper in the

forum. Also, author is indebted to M/s L&T, M/s ABB, M/s P2Power Solutions for their valuable support extended which helped in preparation of the report.

REFERENCES [1] IEEE 519:1992 “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”. [2] Power Quality: Mitigation Technologies in a Distributed Environment – By Antonio Morento-Munoz (Ed.) M. Nageswara Rao (S’11) received B.E.(EEE) from Andhra University, Visakhapatnam in and then joined NTPC in 2001. After training, he is posted to NTPC Electric Supply Company Ltd (wholly owned subsidiary of NTPC) in Engineering dept, Noida. Also, the author received M.Tech (Power electronics & Electrical Machine Drives) from IIT-Delhi in 2011. The author is currently working as Manager (Engg.) and deals with load flow studies of power system networks, designing of Transmission & Distribution networks and Substations upto 220kV. The author also deals with BOQ finalization, Cost estimate preparations, Tender document preparations etc. The main interests of author are renewable power generation like solar & wind power generation technologies, Active filters, Smart Grid technologies etc.