IDENTIFICATION OF HARMONIC SOURCES IN DEREGULATED POWER SECTOR

Abstract

In this paper identification of origin of harmonic source in deregulated power sector has been carried out. Critical impedance method (CIM) for detection of harmonic disturbances caused between utility and source side has already been reported in literature. The method is based on direction of reactive power flow in the power system when there is the presence of harmonic load. Taking the CIM as the base method harmonic sources are identified in deregulated power sector. Due to unavailability of practical data simulation approach has been adopted. A five bus system model with two Generation ompanies (GENCOS) has been simulated for power transfer from GENCO to the considered five bus system. Harmonic current is injected into the system by using a three phase diode rectifier. The results obtainedusing the CIM clearly identifies whether the harmonic source is present in GENCO side or the load side.

Full text

Page 1

IDENTIFICATION OF HARMONIC
SOURCES IN DEREGULATED POWER
SECTOR

KRITI VAID, Y. R. SOOD and R. K. JARIAL
Department of Electrical Engineering, National Institute of Technology,
Hamirpur, Himachal Pradesh, India, 177005


Abstract:
In this paper identification of origin of harmonic source in deregulated power sector has been carried out.
Critical impedance method (CIM) for detection of harmonic disturbances caused between utility and
source side has already been reported in literature. The method is based on direction of reactive power
flow in the power system when there is the presence of harmonic load. Taking the CIM as the base
method harmonic sources are identified in deregulated power sector. Due to unavailability of practical
data simulation approach has been adopted. A five bus system model with two Generation companies
(GENCOS) has been simulated for power transfer from GENCO to the considered five bus system.
Harmonic current is injected into the system by using a three phase diode rectifier. The results obtained
using the CIM clearly identifies whether the harmonic source is present in GENCO side or the load side.
Keywords: Critical impedance, deregulation, harmonic identification, power quality, simulation.
1. Introduction
The supply of power with high reliability and quality is one of the main aspects of a power system. It is being
ages that the electric power system existed. There are changes in the methodology of power generation,
transmission, and distribution has changed. Not only that the power sector is transferred to business sector also it
is privatized in many countries in the present era. The pace at which each and every nation is moving towards
deregulation is very high. The term deregulation indirectly infers that the complete power system has to be
redefined in terms of business and unbundling. It also compels the power system to get completely
interconnected as deregulated power sector deals with business of power. Another term which is closely
associated with deregulation is restructuring. Both the terms deregulation and restructuring are almost seen
together.
In order to deregulate there must be more means of generations, distributions and transmissions. This cannot
be achieved if all the three functionaries have to be controlled under a single wing. The only possibility is to
reshape the structure of the power system. The restructuring of power system as generation, distribution and
transmission will act as a helping hand to the deregulated power sector. This restructuring is also called as
unbundling [Padhy and Sood, Wills et al (1999), Arrilaga (2000), Torre et al. (1999)]. The power system is
made to change its shape in this way so that it can meet many demands at a time and also to provide some
flexible options. The main objective of the deregulated power sector market is to decrease the cost of electricity
through competition. The market environment typically consists of a pool and privately negotiated contracts.
Using the advantage of such subdivisions and functions it is able to completely operate and control the power
system with economy. However, due to complete interconnection of power system it is obvious that the
disturbances are also carried from one side to the other. The disturbances can be of any form like faults, power
quality problems etc.
Out of the above factors faults in a power system cannot be predetermined but can be identified. In the case
of power quality problems there are only few events which can be predetermined. Harmonics are the special
case whose identification is a cumbersome process. The harmonic sources may be present in any of the part of a
power system. When it is the case of deregulated power sector there are many generations and hence the
identification of harmonic sources will be more difficult. The origin of harmonic source may be from any of the
generation side or directly from the load side. Hence, finding the origin of harmonic loads is of great importance
Kriti Vaid et al. / International Journal of Engineering Science and Technology (IJEST)
ISSN : 0975-5462Vol. 3 No. 3 Mar 2011 2435

Page 2

and the present work stresses on this aspect. Identification of harmonic sources using critical impedance method
has been proposed for a classical power system [Li et al. (2004), Hamzah et al. (2004), Xu (2000), Xu and Liu
(2000), Stagg and El-Abiad (1968)]. However, its usage is not seen in deregulated power system. In this paper,
section II discusses about main points in deregulated power sector, section III describes the methodology used,
section IV is dedicated for results and discussions and finally conclusions are drawn in section V.
2. Deregulated power sector
Deregulation of the electric power sector offers the possibility of improving the system operation efficiency.
This efficiency requires the development of an infrastructure that can address the information needs of the sector
in a less centralized and more organized sense. Deregulation involves removing government legislation and laws
in a particular market. Deregulation often refers to removing barriers to competition. In deregulation
environment, generation, transmission and distribution are independent activities and to implement deregulation
the system has to be restructured. It is well known that the primary components of any electricity supply ---
generation, transmission, distribution, and retail supply. However, similar structure components representing
various segments of the electricity market are also defined as given below: -
2.1. Generation Companies (GENCOS)
GENCOS are responsible for operating and maintaining generating plants in the generation sector and in most
of the cases are the owners of the plants. In some case individual generators do not market their output, but only
GENCO market the output of all its generators.
2.2. Build operate and transfer (BOT) plant or independent power producers (IPPs)
BOT or IPPs can act as its own generator-serving entity and independently market its output to a trading entity
or to a load–serving entity.
2.3. Transmission companies (TRANSCOS) and transmission owners (TOs)
TRANSCOS moves power in bulk quantities from where it is produced to where it is delivered. In most
deregulated industry structures, the Transmission companies owns and maintain transmission lines under
monopoly franchise and are called Transmission Owners (TOs), but they do not operate them. The independent
system operators do that Distribution companies (Discos) and retailers Discos assume the same responsibility on
the distribution side as in a traditional regulatory supply utility. However, a trend in deregulation is that Discos
may now be restricted to maintain the distribution network and provide facilities for electricity delivery while
retailers are separated from Discos and sell electric energy to end consumers.
2.4. Independent system operator (ISO)
The ISO is the supreme entity in the control of transmission system. The basic requirement of an ISO is
disassociation from all market participants and absence from any financial interest in the generation and
distribution business.
2.5. Power exchanger (PX)
The PX handles the electric power pool, which provides a forum to match electrical energy supply and demand
based on bid prices. The time horizon of the pool market may range from half an hour to a week or longer. The
most usual is the day-ahead market to facilitate energy trading one day before each operating day.
2.6. Scheduling coordinators (SCs)
SCs aggregate participants in the energy trade and are free to use protocols that may differ from pool rules. In
other words market participants may enter an SC’s market under SC’s rules through bilateral and multilateral
transactions. Fig. 1 shows a rough block diagram of a restructured power system. There is a competition among
the above said GENCOS, DISCOS and TRANSCOS for providing power for customers. Main benefits from the
deregulation are many and a few of them are: -

•
Cheaper electricity, efficient capacity expansion planning, cost minimization, more choice and better
service.
•
Highest possibility of improving the system operation efficiency and power quality.
•
Low-cost techniques for electricity generation.
Kriti Vaid et al. / International Journal of Engineering Science and Technology (IJEST)
ISSN : 0975-5462Vol. 3 No. 3 Mar 20112436

Page 3

•
More system reliability

Because of security, market operations and privacy, the data like system network parameters, actual generation
and connected load data cannot be shared by other electric companies. Thus it is very clear that due to
deregulation irrespective of the geographical conditions the power system will become completely
interconnected. Consequently, due to efficient interconnections the customers will not suffer with any power
interruptions. However, any origin of disturbance will transmit to maximum possible distance thus affecting the
healthy power system side.
Such problem arises due to usage of non-linear loads in the power system and as the spread of power system is
very huge it is very difficult to identify the origin of the non-linear loads. A private GENCO may transfer power
with good quality and it is possible that the time it reaches the customer it may get polluted. The harmonic
source may be from the customer side or utility side and identification of such source is of vast importance.
Thus a methodology is required to find whether the GENCO is producing polluted power or the customer itself.
Identification of harmonic sources using critical impedance method is found out to be more effective in this
aspect. However, the method is not completely used in deregulated power sector for identification of harmonic
sources. The present paper throws light on this aspect. The subsequent section describes about the methodology.
3. Critical Impedance Method (CIM)
The critical impedance method (CIM) is based on the concept of reactive power flow. In this method it is
investigated that how far the reactive power generated by source can travel along the impedance, if it is
imagined that the impedance is uniformly distributed between and as an “impedance line”. It deals with the
values of reactive power (Q ) < 0 which occurs only in certain cases. The method uses the terms Z , E and I
which are the impedance, voltage and current with subscripts of c andu . These subscripts corresponds to
customer side and utility side thus giving
c Z ,
u
Z ,
c
E ,
E ,
determine the relative magnitudes of the above said parameters. Using the above calculated parameters critical
impedance is formulated which decides the origin of harmonic source. The method is able to provide correct
uc I and
u I . The objective of this method is here to
answers even if the range of the combined utility and customer impedance is very large. The calculation of
critical impedance will be clearer by observing Fig. 2 where,
reactances at the utility and customer side respectively.
m
V is the voltage at PCC,
1
X and
2
X are the

Fig. 2 An equivalent representation of a power system

Fig. 1 A pictorial representation of a deregulated or restructured power
system. The system is also called as unbundled system.
Kriti Vaid et al. / International Journal of Engineering Science and Technology (IJEST)
ISSN : 0975-5462 Vol. 3 No. 3 Mar 20112437

Page 4

It is the supposition of the method that if X , where X =
( x > X /2), the utility source is expected to have a larger magnitude since the source can “push” its reactive
power output beyond halfway ( X /2) of the “impedance line.” Here x is different from X and is the reactance at
the lowest voltage point. Similarly, if or is located closer to the utility side, the customer source is expected to
have a large magnitude. The CIM method is used to determine the relative magnitude of the two sources can. It
is established on the basis of comparing the magnitudes and mathematical analysis. The criterion discussed has
been proven to be the necessary and sufficient condition on which one can conclude. It is also proved that
2 x > X if
c
E >
u
E . Thus finally from all the above equations an index named CI was derived which is given
by: -



customer side is the dominant harmonic source. If CI < 0, the utility side generates reactive power. In summary,
for the case of Z = jX , the CI method can be implemented as follows: -

Calculate the utility side voltage source by using
E
Calculate the reactive power absorbed by
u
E where θ is the phase by which
Calculate CI.
•
If CI > 0, the utility source absorbs reactive power, the customer side is the main harmonic contributor.
•
If CI < 0, the utility generates reactive power

The methodology discussed still now is used to determine the origin of harmonic source in restructured
environment. As the GENCOS also form the utility side for a power system hence we state that the same
concept is applicable for restructure power system. The results and discussions are presented in the next section.
1
X +
2
X , is located closer to the customer side
(1)


•
uPCC PCC U
IUZ
=−
where
U
Z is known.
E leads I.
•
u
•
4. Results and Discussions
A five bus system as shown in Fig. 3 is considered to verify the above discussed method. The power system is
inspired from the standard five bus system designed given in [Stagg and El-Abiad (1968)]. The system operating
frequency is 60Hz. In order to convert the classical power system into restructured system two GENCOS are
modeled with a power rating same as that of generators G1 and G2. The loads L4 and L5 are connected with
dotted lines to resemble switched load for a small interval of time. The same convention is also used with
GENCOS as they are not completely being in usage. The GENCOS are used only when there is a power demand
caused by the loads L4 and L5. The time interval for switching of both the loads and GENCOS is from 0.18 to
0.3s. The buses B1 and B2 are considered as the point of common coupling (PCC) as the upper half of the
system forms the utility side. All the measurements are taken only at bus B1 and B2. The complete simulation is
done in MATLAB/SIMULINKN® 7.6 ver.
In the above shown system the harmonic sources are also included to verify the CIM. A three phase diode
rectifier is taken as the harmonic load to inject harmonic current into the system. The simulation is carried out
for various cases which are discussed further. Throughout the paper deregulated or restructured terms are used
on the same meaning.
2
2Q
I
CI =


Fig. 3 Five bus system with two GENCOS and PCC measurements at
B1 and B2.
Kriti Vaid et al. / International Journal of Engineering Science and Technology (IJEST)
ISSN : 0975-5462Vol. 3 No. 3 Mar 2011 2438

Page 5

4.1. System under normal condition with additional load, without inclusion of GENCOS and
harmonic source
The case considered is a normal real-time behavior of the power system. The load changes in a power system
are dynamic and cause power quality problems like sag and swell which depends on the type of load. Here, a
heavy load is switched on suddenly to create a sag event in the system. Such type of events occurs practically in
a system and is to be rectified or compensated. In order to balance the sudden demand the GENCOS are used for
supplying additional power. Fig. 4 shows the voltage and critical impedance versus time plot of bus B1. The
critical impedance is calculated as given in eqn. (1). Both current and reactive power is measured at the bus B1
to find out the CI. The same procedure is adopted for measurements and finding the CI at bus B2 and is shown
in Fig. 5.
The values of voltages and CI at both bus B1 and B2 have been scaled to 1p.u. to make it more practical and
throughout this paper the same convention is maintained for all cases. From Fig. 4 and Fig. 5 it is seen that the
voltage rose to 0.9 p.u at t=0.02s and increased upto t=0.18s. Later, it will reach to a steady state value of 1 p.u.
This behavior can be accounted for voltage build up of the alternators. After, there is voltage dip which is
caused by sudden load switching. The behavior of CI at B1 and B2 is also similar. The slight oscillation in CI is
due to sudden variation of load. The plot characteristics of both the PCC measurements are similar which is due
to usage of alternators with same ratings and characteristics. The effect of inclusion of GENCOS in the system
is clearly discussed in the next section.
4.2. System under normal condition with additional load, with inclusion of GENCOS and in the
absence harmonic source
Previously, it is seen that due to inclusion of sudden additional load there is a voltage dip in the system. It is also
stated that a power transfer would uplift the voltage profile. In this case two GENCOS at bus B1 and B2 are
switched ON to supply the additional power required. The results are shown in Fig. 6 and Fig. 7. Fig. 6 shows
the plot of voltage profile and CI of PCC1 versus time. Here it is seen that the voltage is lifted to a value slightly
greater than 1p.u. Hence, it can be stated that the designed system is healthy and is very near to practical power
system in terms of power balance.
In the case of critical impedance there is no change in the overall average value. The convention of the
impedance is still positive in this case and there are no heavy changes in the values. Any other type of switching
other than the normal operating loads may lead to impedance variations. The reason lies in the way the critical
impedance is formulated. It depends directly on the reactive power and proportional indirectly to the current.
And it is well known that presence of any type of harmonic load will affect the reactive power which is
described when discussing about the CIM. Hence, it can be affirmed that there are no harmonic sources present
in the system as CI is same as that of the normal operation. The behavior of voltage and CI at PCC2 is as same
as that of PCC1 and reason is same as discussed in the earlier case. Simulation of the system with the presence
of harmonic sources is discussed from the next section.


(a)

(b)
Fig. 4 Plot of (a) Voltage and (b) critical impedance measured at
PCC1 (bus B1) versus time due sudden load switching in the
absence of GENCOS

(a)

(b)
Fig. 5Plot of (a) Voltage and (b) critical impedance measured at
PCC2 (bus B2) versus time due sudden load switching in the
absence of GENCOS
Kriti Vaid et al. / International Journal of Engineering Science and Technology (IJEST)
ISSN : 0975-5462Vol. 3 No. 3 Mar 20112439