30917947 Reactive Power Compensation Using Capacitor Banks
Transcript of 30917947 Reactive Power Compensation Using Capacitor Banks
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INTRODUCTION
In this chapter we are going to discuss about power system in short and about A.P
TRANSCO and its role in maintaining power in state from buying and selling the power.
1.1 INTRODUCTION TO POWER SYSTEM
Electrical power is a little bit like the air one breathes. One doesn't really think
about it until it is missing. Power is just "there," meeting ones daily needs, constantly. It
is only during a power failure, when one walks into a dark room and instinctively hits the
useless light switch, that one realizes how important power is in our daily life. Without it,
life can get somewhat cumbersome.
Electric Energy is the most popular form of energy, because it can be transported
easily at high efficiency and reasonable cost. The power system of today is a complex
interconnected network as shown in fig. 1.
1Figure 1 Power System interconnected
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A Power System can be subdivided into four major parts:
i. Generation.
ii. Transmission and Sub transmission.
iii. Distribution.
iv. Loads.
Power is generated at generating stations, usually located away from the actual
users. The generated voltage is then stepped up to a higher voltage for transmission,
as transmission losses are lower at higher voltages. The transmitted electric power is then
stepped down at grid stations.
The modern distribution system begins as the primary circuit, leaves the sub-
station and ends as the secondary service enters the customer's meter socket. First, the
energy leaves the sub-station in a primary circuit, usually with all three phases.
The most common type of primary is known as a wye configuration.The wye
configuration includes 3 phases and a neutral (represented by the center of the "Y".) The
neutral is grounded both at the substation and at every power pole. The primary and
secondary (low voltage) neutrals are bonded (connected) together to provide a path to
blow the primary fuse if any fault occurs that allows primary voltage to enter the
secondary lines. An example of this type of fault would be a primary phase falling across
the secondary lines. Another example would be some type of fault in the transformer
itself.
The other type of primary configuration is known as delta. This method is older
and less common. In delta there is only a single voltage, between two phases (phase to
phase), while in wye there are two voltages, between two phases and between a phase
and neutral (phase to neutral). Wye primary is safer because if one phase becomes2
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grounded, that is, makes connection to the ground through a person, tree, or other object,
it should trip out the fused cutout similar to a household circuit breaker tripping. In delta,
if a phase makes connection to ground it will continue to function normally. It takes two
or three phases to make connection to ground before the fused cutouts will open the
circuit. The voltage for this configuration is usually 4800 volts.
Transformers are sometimes used to step down from 7200 or 7600 volts to 4800
volts or to step up from 4800 volts to 7200 or 7600 volts. When the voltage is stepped up,
a neutral is created by bonding one leg of the 7200/7600 side to ground. This is
commonly used to power single phase underground services or whole housing
developments that are built in 4800 volt delta distribution areas. Step downs are used in
areas that have been upgraded to a 7200/12500Y or 7600/13200Y and the power
company chooses to leave a section as a 4800 volt setup. Sometimes power companies
choose to leave sections of a distribution grid as 4800 volts because this setup is less
likely to trip fuses or reclosers in heavily wooded areas where trees come into contact
with lines.
For power to be useful in a home or business, it comes off the transmission grid
and is stepped-down to the distribution grid. This may happen in several phases. The
place where the conversion from "transmission" to "distribution" occurs is in a power
substation. A power substation typically does two or three things:
i. It has transformers that step transmission voltages down to distribution voltages
ii. It has a "bus" that can split the distribution power off in multiple directions.
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iii. It often has circuit breakers and switches so that the substation can be
disconnected from the transmission grid or separate distribution lines can be
disconnected from the substation when necessary.
It often has circuit breakers and switches so that the substation can be
disconnected from the transmission grid or separate distribution lines can be disconnected
from the substation when necessary. The primary distribution lines are usually in the
range of 4 to 34.5 KV and supply load in well defined geographical area. Some small
industrial customers are served directly by the primary feeders.
1.3 APTRANSCO
Government of Andhra Pradesh enacted the AP Electricity REFORMS ACT in
1998.As a sequel the APSEB was unbundled into Andhra Pradesh Power Generation
Corporation Limited (APGENCO) & Transmission Corporation of Andhra Pradesh
Limited (APTRANSCO) on 01.02.99. APTRANSCO was further unbundled w.e.f.
01.04.2000 into "Transmission Corporation" and four "Distribution Companies"
(DISCOMS).
a.)CURRENT ROLE
From Feb 1999 to June 2005 APTRANSCO remained as Single buyer in the state
-purchasing power from various Generators and selling it to DISCOMs in accordance
with the terms and conditions of the individual PPAs at Bulk Supply Tariff (BST) rates.
Subsequently, in accordance with the Third Transfer Scheme notified by Go AP,
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APTRANSCO has ceased to do power trading and has retained with powers of
controlling system operations of Power Transmission.
1.4 CONCLUSION
In this chapter we discussed about the power system and role of A.P TRANSCO
in the state of A.P.
In next chapter we are going to discuss about the salient features of
A.PTRANSCO.
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INTRODUCTION
In this chapter, we are going to discuss about the salient feature of A.P
TRANSCO/A.PGENCO/DISCOMS.
The object of reform and restructure of power sector in the state is to create
conditions for sustainable development of the sector through promoting competition,
efficiency, transparency and attracting the much needed private finances into power
sector. The ultimate goal of the reform program is to ensure that power will be supplied
under the most efficient conditions in terms of cost and quantity to support the economic
development of the state and power sector ceases to be a burden on the States budget and
eventually becomes a net generator of resources.
A key element of the reform process is that the government will withdraw from
its earlier role as a regulator of the industry and will be limiting its role to one of policy
formulation and providing directions.
In accordance with Reform Policy, the Government of A.P entacted the A.P
Electricity Reforms Act 1998 and made effective from 1.2.1999. Transmission
Corporation of A.P Ltd (APTRANSCO and APGENCO) were incorporated under
Companies Act, 1956. The assets, liabilities and personnel were allocated to these
companies. Distribution companies have been incorporated under Companies Act as
subsidiaries to distribution to APTRANSCO and the assets, liabilities and personnel have
been allocated to distribution companies through notification of a second transfer scheme
by the Govt. on 31.3.2000.
The Government of A.P established the A.P Electricity Regulatory Commission
(APERC) as per the provision of the act and the Commission started functioning from
3.4.1999. Regular licenses have been issued to APTRANSCO by APERC for
Transmission and Bulk supply and Distribution and Retail supply from 31.1.2000. The
commission has been issuing yearly Tariff orders since then based on Annual Revenue
Requirement (ARR) and tariff proposals of these companies.
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2.2 SALIENT FEATURES OF A.P TRANSCO/A.PGENCO/DISCOMS
Table 2.2 (a) features of A.P power system
PARAMETER UNITS 2008-09
(UPTO
MARCH
09)
31.03.09
(PROVL)
2009-10
(UPTO
MARCH
10)
31.03.10
(PROVL)
Energy generated (cumulative) MU - - -
1. Thermal MU - 23325.67 - 24180.38
2. Hydel MU - 7785 - 5510.46
3. Wind MU - - - -
Total MU - 31110.67 - 29690.84
Energy purchased and imported
(includingothers energy handled)
MU - 36511.56 - 45075.68
Energy available for use (2+3) MU - 67622.23 - 74766.52
Maximum demand during the year
(at generation terminal) MW
ME - 9997
(27-03-
2009)
- 10880
(21-03-2010)
PercpaitaConsumption (includes
captive generation)
KWH - 746 - -
APTRANSCO LINE (EHT) - - - - -
400kv CKM 21.44 3008.20 24 3032.79
220kv CKM 265.88 1250.25 19068 12693.18
132kv CKM 233.02 14938.57 164.88 15103.45
DISCOMS Lines # - - - - -
33kv Km 1421.78 38628 1230 39858
11kv Km 19521.82 248670 10596 259266
LT km 10166.53 527852 4212 532064
TOTAL - 26630.14 845599.15 6418.17 862017.32
Table 2.2 (b) load generation and sharing of A.P with other state
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2.3 CONCLUSION
In this chapter, we discussed about the salient features of
A.PTRANSCO / A.PGENCO / DISCOMS.
In next chapter we are going to discuss about the need for compensation and types
of compensations used.
3.1 INTRODUCTION
In this chapter, reactive power compensation, mainly in transmission systems
installed at substations is discussed. Reactive power compensation in power systems can
be either shunt or series.
Except in a very few special situations, electrical energy is generated, transmitted,
distributed, and utilized as alternating current (AC). However, alternating current has
several distinct disadvantages. One of these is the necessity of reactive power that needs
to be supplied along with active power. Reactive power can be leading or lagging. While
it is the active power that contributes to the energy consumed, or transmitted, reactive
power does not contribute to the energy. Reactive power is an inherent part of the total
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power. Reactive power is either generated or consumed in almost every component of
the system, generation, transmission, and distribution and eventually by the loads. The
impedance of a branch of a circuit in an AC system consists of two components,
resistance and reactance. Reactance can be either inductive or capacitive, which
contribute to reactive power in the circuit. Most of the loads are inductive, and must be
supplied with lagging reactive power. It is economical to supply this reactive power
closer to the load in the distribution system.
3.2 TYPES OF COMPENSATION
Shunt and series reactive compensation using capacitors has been 3 widely
recognized and powerful methods to combat the problems of voltage drops, power losses,
and voltage flicker in power distribution networks. The importance of compensation
schemes has gone up in recent years due to the increased awareness on energy
conservation and quality of supply on the part of the Power Utility as well as power
consumers. This amplifies on the advantages that accrue from using shunt and series
capacitor compensation. It also tries to answer the twin questions of how much to
compensate and where to locate the compensation capacitors.
i.) SHUNT CAPACITOR COMPENSATION
Since most loads are inductive and consume lagging reactive power, the
compensation required is usually supplied by leading reactive power. Shunt
compensation of reactive power can be employed either at load level, substation level, or
at transmission level. It can be capacitive (leading) or inductive (lagging) reactive power,
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although in most cases as explained before, compensation is capacitive. The most
common form of leading reactive power compensation is by connecting shunt capacitors
to the line.
Fig. 3.2(i) represents an A.C generator supplying a load through a line of series
impedance (R+jX) ohms, fig.3.2(ii) shows the phasor diagram when the line is delivering
a complex power of (P+jQ) VA and Fig. 3.2(iii) shows the phasor diagram when the line
is delivering a complex power of (P+jO) VA i.e. with the load fully compensated. A
thorough examination of these phasor diagrams will reveal the following facts which are
higher by a factor of2
Cos
1
compared to the minimum power loss attainable in the
system.
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Figure 3.2 (i) represents an A.C generator supplying a load through a line of
series.
Figure.3.2 (ii) shows the phasor diagram when the line is delivering a complex
power of (P+jQ)
Figure. 3.2 (iii) shows the phasor diagram when the line is delivering a complex
power of (P+jQ)
The loading on generator, transformers, line etc is decided by the current flow.
i. The higher current flow in the case of uncompensated load necessitated by the
reactive demand results in a tie up of capacity in this equipment by a factor of
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Cos
1i.e. compensating the load to UPF will release a capacity of (load VA
rating X Cos ) in all these equipment.
ii. The sending-end voltage to be maintained for a specified receiving-end voltage is
higher in the case of uncompensated load. The line has bad regulation with
uncompensated load.
iii. The sending-end power factor is less in the case of an uncompensated one. This is
due to the higher reactive absorption taking place in the line reactance.
iv. The excitation requirements on the generator are severe in the case of
uncompensated load. Under this condition, the generator is required to maintain a
higher terminal voltage with a greater current flowing in the armature at a lower
lagging power factor compared to the situation with the same load fully
compensated. It is entirely possible that the required excitation is much beyond
the maximum excitation current capacity of the machine and in that case further
voltage drop at receiving-end will take place due to the inability of the generator
to maintain the required sending-end voltage. It is also clear that the increased
excitation requirement results in considerable increase in losses in the excitation
system.
It is abundantly clear from the above that compensating a lagging load by using
shunt capacitors will result in
i. Lesser power loss everywhere upto the location of capacitor and hence a more
efficient system.
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ii. Releasing of tied-up capacity in all the system equipments thereby enabling a
postponement of the capital intensive capacity enhancement programs to a later
date.
iii. Increased life of equipments due to optimum loading on them.
iv. Lesser voltage drops in the system and better regulation.
v. Less strain on the excitation system of generators and lesser excitation losses.
vi. Increase in the ability of the generators to meet the system peak demand thanks to
the released capacity and lesser power losses.
Shunt capacitive compensation delivers maximum benefit when employed right
across the load. And employing compensation in HT & LT distribution network is the
closest one can get to the load in a power network. However, various considerations like
ease of operation end control, economy achievable by lumping shunt compensation at
EHV stations etc will tend to shift a portion of shunt compensation to EHV & HV
substations. Power utilities in most countries employ about 60% capacitors on feeders,
30% capacitors on the substation buses and the remaining 10% on the transmission
system. Application of capacitors on the LT side is not usually resorted to by the utilities.
Just as a lagging system power factor is detrimental to the system on various
counts, a leading system pf is also undesirable. It tends to result in over-voltages, higher
losses, lesser capacity utilization, and reduced stability margin in the generators. The
reduced stability margin makes a leading power factor operation of the system much
more undesirable than the lagging p.f operation. This fact has to be given due to
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consideration in designing shunt compensation in view of changing reactive load levels in
a power network.
Shunt compensation is successful in reducing voltage drop and power loss
problems in the network under steady load conditions. But the voltage dips produced by
DOL starting of large motors, motors driving sharply fluctuating or periodically varying
loads, arc furnaces, welding units etc can not be improved by shunt capacitors since it
would require a rapidly varying compensation level. The voltage dips, especially in the
case of a low short circuit capacity system can result in annoying lamp-flicker, dropping
out of motor contactors due to U/V pick up, stalling of loaded motors etc. and fixed or
switched shunt capacitors are powerless against these voltage dips. But thyristor
controlled Static VAR compensators with a fast response will be able to alleviate the
voltage dip problem effectively.
a.) SHUNT CAPACITORS
Shunt capacitors are employed at substation level for the following reasons:
i. Voltage regulation: The main reason that shunt capacitors are installed at
substations is to control the voltage within required levels. Load varies over the
day, with very low load from midnight to early morning and peak values
occurring in the evening between 4 PM and 7 PM. Shape of the load curve also
varies from weekday to weekend, with weekend load typically low. As the load
varies, voltage at the substation bus and at the load bus varies. Since the load
power factor is always lagging, a shunt connected capacitor bank at the substation
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can raise voltage when the load is high. The shunt capacitor banks can be
permanently connected to the bus (fixed capacitor bank) or can be switched as
needed. Switching can be based on time, if load variation is predictable, or can be
based on voltage, power factor, or line current.
ii. Reducing power losses: Compensating the load lagging power factor with the
bus connected shunt capacitor bank improves the power factor and reduces
current flow through the transmission lines, transformers, generators, etc. This
will reduce power losses (I2R losses) in this equipment.
iii. Increased utilization of equipment: Shunt compensation with capacitor banks
reduces KVA loading of lines, transformers, and generators, which means with
compensation they can be used for delivering more power without overloading
the equipment.
Reactive power compensation in a power system is of two typesshunt and series.
Shunt compensation can be installed near the load, in a distribution substation, along the
distribution feeder, or in a transmission substation. Each application has different
purposes. Shunt reactive compensation can be inductive or capacitive. At load level, at
the distribution substation, and along the distribution feeder, compensation is usually
capacitive. In a transmission substation, both inductive and capacitve reactive
compensation are installed.
b.) SHUNT CAPACITOR INSTALLATION TYPES:
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The capacitor installation types and types of control for switched capacitor are
best understood by considering a long feeder supplying a concentrated load at feeder end.
This is usually a valid approximation for some of the city feeders, which emanate from
substations, located 4 to 8 Kms away from the heart of the city.
Absolute minimum power loss in this case will result when the concentrated load
is compensated to up by locating capacitors across the load or nearby on the feeder. But
the optimum value of compensation can be arrived at only by considering a cost benefit
analysis.
Figure 3.2 (iv) long distribution feeder supplying a concentrated load
It is evident from fig. 3.2 (v) that it will require a continuously variable capacitor
to keep the compensation at economically optimum level throughout the day. However,
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this can only be approximated by switched capacitor banks. Usually one fixed capacitor
and two or three switched units will be employed to match the compensation to the
reactive demand of the load over a day. The value of fixed capacitor is decided by
minimum reactive demand as shown in Fig 3.2 (v)
Figure. 3.2 (v) reactive demand
Automatic control of switching is required for capacitors located at the load end
or on the feeder. Automatic switching is done usually by a time switch or voltage
controlled switch as shown in Fig 3.2(v). The time switch is used to switch on the
capacitor bank required to meet the day time reactive load and another capacitor bank
switched on by a low voltage signal during evening peak along with the other two banks
will maintain the required compensation during night peak hours.
ii) SERIES CAPACITOR COMPENSATION
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Shunt compensation essentially reduces the current flow everywhere upto the
point where capacitors are located and all other advantages follow from this fact. But
series compensation acts directly on the series reactance of the line. It reduces the transfer
reactance between supply point and the load and thereby reduces the voltage drop. Series
capacitor can be thought of as a voltage regulator, which adds a voltage proportional to
the load current and there by improves the load voltage.
Figure 3.2 (vi) Aerial view of 500-kV series capacitor installation
Series compensation is employed in EHV lines to
i. Improve the power transfer capability
ii. Improve voltage regulation
iii. Improve the load sharing between parallel lines.
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Economic factors along with the possible occurrence of sub-synchronous
resonance in the system will decide the extent of compensation employed.
Series capacitors, with their inherent ability to add a voltage proportional to load
current, will be the ideal solution for handling the voltage dip problem brought about by
motor starting, arc furnaces, welders etc. And, usually the application of series
compensation in distribution system is limited to this due to the complex protection
required for the capacitors and the consequent high cost. Also, some problems like self-
excitation of motors during starting, ferro resonance, steady hunting of synchronous
motors etc discourages wide spread use of series compensation in distribution systems.
3.3 ECONOMIC JUSTIFICATION FOR USE OF CAPACITORS:
Increase in benefits for 1KVAR of additional compensation decrease rapidly as
the system power factor reaches close to unity. This fact prompts an economic analysis to
arrive at the optimum compensation level. Different economic criteria can be used for
this purpose. The annual financial benefit obtained by using capacitors can be compared
against the annual equivalent of the total cost involved in the capacitor installation. The
decision also can be based on the number of years it will take to recover the cost involved
in the Capacitor installation. A more sophisticated method would be able to calculate the
present value of future benefits and compare it against the present cost of capacitor
installation.
When reactive power is provided only by generators, each system component
(generators, transformers, transmission and distribution lines, switch gear and protective
equipment etc) has to be increased in size accordingly. Capacitors reduce losses and
loading in all these equipments, thereby effecting savings through powerless reduction
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and increase in generator, line and substation capacity for additional load. Depending on
the initial power factor, capacitor installations can release at least 30% additional
capacity in generators, lines and transformers. Also they can increase the distribution
feeder load capability by about 30% in the case of feeders which were limited by voltage
drop considerations earlier. Improvement in system voltage profile will usually result in
increased power consumption thereby enhancing the revenue from energy sales.
Thus, the following benefits are to be considered in an economic analysis of
compensation requirements.
a) Benefits due to released generation capacity.
b) Benefits due to released transmission capacity.
c) Benefits due to released distribution substation capacity.
d) Benefits due to reduced energy loss.
e) Benefits due to reduced voltage drop.
f) Benefits due to released feeder capacity.
g) Financial Benefits due to voltage improvement.
Capacitors in distribution system will indeed release generation and transmission
capacities. But when individual distribution feeder compensation is in question, the value
of released capacities in generation and transmission system is likely to be too small to
warrant inclusion in economic analysis. Moreover, due to the tightly inter-connected
nature of the system, the exact benefit due to capacity release in these areas is quite
difficult to compute. Capacity release in generation and transmission system is probably
more relevant in compensation studies at transmission and sub-transmission levels and
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hence are left out from the economic analysis of capacitor application in distribution
systems.
a.) BENEFITS DUE TO RELEASED DISTRIBUTION SUBSTATIONCAPACITY:
The released distribution substation capacity due to installation of capacitors
which deliver Qc MVARs of compensation at peak load conditions may be shown to be
equal to
c2c
c
2/1
2c
22cc S1
S
SinQ
S
CosQ1S
+
=
In general and SinQS cc when10
SQ CC