10/9/2017

Major Electric Plant in

Power Stations
BY: EARL JOSEPH PIÑAS

Introduction
In this chapter, the main electrical plant required in power
stations is discussed and overall specifications are given for
guidance in the choice of the required plant.

The major electrical plant in power station consists of

generators, exciters for the generators, power transformers,
reactors, circuit breakers, switch board and control-board
equipment. In addition station transformers, auxiliary equipment,
emergency lighting equipment, etc. are required.

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Generators and Exciters

Generator constants – to determine the performance of a

generator in the power system under different loading
conditions at different power factors, under transient
conditions and under faulty conditions. They are also used in
studies of system stability.

Generators and Exciters

Short circuit constant – this is the ratio of the field current at rated open-circuit
voltage and rated frequency to the field current at rated armature current on
sustained symmetrical short-circuit at rated frequency. For large generators
short circuit ratio varies between 0.8 and 1.2.

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Generators and Exciters

When studying the performance of a synchronous generator under
asymmetrical faulty conditions such as a line-to-line or line-to-earth fault, the
negative and zero sequence reactance of the machine should be known to
enable one to work out the fault current and voltage conditions of the machine.
The negative sequence reactance X2, and the zero sequence reactance X0, are,
therefore other important constants of the synchronous machine which should
be given in the detailed specifications. The negative sequence reactance X 2, is the
ratio of the fundamental component of the reactive armature voltage, due to the
fundamental negative-sequence component of armature current, to this
component of armature current at rated frequency.

Generators and Exciters

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Specifications of Generators
Hydro-Electric Generator
Besides the normal rating and other items in the specifications mentioned in previous chapters
the specifications of a hydro-electric water-wheel generator should include the following
particulars:
 Type, such as vertical shaft, thrust bearing below rotor.
 Line-charging capacity
 synchronous compensator capacity
 Guaranteed overall efficiencies at 50%, 75%, 100% of load and at 15% overload at unity
and 0.9 power factors
 Reactances - Xd, X’d, X’’d, X2 , X0 , Xq, X’q, X’’q
 Reactances – stator and rotor.

Specifications of Generators
 Short-circuit ratio
 Losses
 Type of voltage regulator
 Full load regulation at rated voltage and speed. Time response of voltage regulator.
 Field currents at full load at unity and 0.9 power factor.
 Mechanical characteristics
 Main exciter
 Pilot exciter

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Earthingof a Power System

A power system is earthed at a suitable point by a suitable method. Some of the
advantages of earthing are:
1. Reduce operation and maintenance expenditure.
2. Improved service reliability.
3. Greater safety.
4. Better system and equipment overcurrent protection.
5. Improved lightning protection

The methods commonly used for earthing the system neutral are:
1. Solid earthing
2. Resistance earthing
3. Reactance earthing
4. Earth fault neutralizer earthing

Earthingof a Power System

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Earthingof a Power System

Power Transformers: Types and

Characteristics
When single phase transformers are operating in parallel:
◦ Voltage rating is identical
◦ Percentage impedance voltages is equal
◦ The ratio of reactance to resistance must be equal.

This can also be used in three single-phase transformer banks.

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Transformer Connections

Vector Groups in Transformer

Connections
Three phase transformer consists of three sets of primary windings, one
for each phase, and three sets of secondary windings wound on the same iron
core. Separate single-phase transformers can be used and externally
interconnected to yield the same results as a 3-phase unit.
The primary windings are connected in one of several ways. The two most
common configurations are the delta, in which the polarity end of one winding is
connected to the non-polarity end of the next, and the star, in which all three
non-polarities (or polarity) ends are connected together. The secondary windings
are connected similarly. This means that a 3-phase transformer can have its
primary and secondary windings connected the same (delta-delta or star-star), or
differently (delta-star or star-delta).

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Vector Groups in Transformer Connections

It’s important to remember that the secondary voltage waveforms are in
phase with the primary waveforms when the primary and secondary windings
are connected the same way. This condition is called “no phase shift.” But when
the primary and secondary windings are connected differently, the secondary
voltage waveforms will differ from the corresponding primary voltage waveforms
by 30 electrical degrees. This is called a 30 degree phase shift. When two
transformers are connected in parallel, their phase shifts must be identical; if not,
a short circuit will occur when the transformers are energized.”

Vector Groups in Transformer Connections

Phase rotation is always counterclockwise (internationally adopted
convention) and indicates multiples of 30 degree lag for low voltage winding
using the high voltage winding as the reference.
Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.
As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For
example, a step-up transformer with a delta-connected secondary, and wye-
connected primary, is written as 'Dy1'. The 1 indicates the LV winding lags the HV
by 30 degrees.

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Harmonics in Transformers
The non-sinusoidal nature of the magnetizing current
necessary to produce a sine wave of flux produces harmonics in
current and voltage waveforms of the three-phase transformers. The
effect is more pronounced in 3-phase transformers whose phases are
magnetically separate as in the case of a bank of single-phase
transformers. Third harmonics present in waveforms, their ill effects
and suppression of the third harmonics should be considered. The
nature of the harmonics present in the transformers depends on
their connections.

Harmonics in Transformers
Current harmonics

oIncreased heating of windings

oInductive interference with communication circuits
oIncreased iron loss

Voltage harmonics

oVoltage stress on the insulation is increased

oCapacitive interference with communication circuits
oProduction of large resonant voltages

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Suppression of Harmonics
o Use very low flux densities in transformers
o Connect one of the winding in delta
o The inductive way to do this will be to place a third winding on the same
magnetic core on which the primary and secondary are wound and not in
delta.
o The conductive way to do this is to use the arrangement of star/delta
earthing transformer.
o Star/zigzag connection of transformer may be used to reduce harmonics.

Parallel Operation of Transformers

Same voltage ratio of transformer.
Same percentage impedance.
Same polarity.
Same phase sequence.

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Specification of Transformers
A bank of three single-phase transformers or a 3-phase transformer unit.
A type of transformer: Outdoor or indoor.
Type of cooling: self-oil-cooled, forced-air-cooled, forced-oil-cooled or water-cooled.
Rating
Connections
Voltage
Frequency
Temperature rise: 40 OC, 50 OC or 75 OC, etc.
Percentage impedance voltage at 75 OC

Specification of Transformers
The transformer will operate in parallel with others (give particulars) under varying power
factors and voltages, and the transformers will divide the load approximately in proportion to
their respective capacities.
Coordination of insulation
Mention whether the system is solidly earthed or not, whether the star point is to be earthed
on the H.V side of the transformer.
Magnetizing current for the H.V winding.
Efficiency at normal ratio under 25%, 50%, 75% and 100% loads at 75 OC.
Losses
Weight and dimensions

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Specification of Transformers
Oil circulation and cooling system used, and the requirements for this purpose.
When the capacity of a power transformer is large, instead of specifying the efficiency
limitation, the total loss, iron loss and copper limitations are given.

Principles of Circuit Breaker

The circuit breaker mainly consists of fixed contacts and moving contacts. In normal "ON" condition of circuit
breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the
moving contacts. There is an arrangement stored potential energy in the operating mechanism of circuit
breaker which is released if switching signal is given to the breaker. The potential energy can be stored in the
circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydraulic pressure.
But whatever the source of potential energy, it must be released during operation. Release of potential energy
makes sliding of the moving contact at extremely fast manner. All circuit breaker have operating coils (tripping
coils and close coil), whenever these coils are energized by switching pulse, and the plunger inside them
displaced. This operating coil plunger is typically attached to the operating mechanism of circuit breaker, as a
result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic
energy, which makes the moving contact to move as these moving contacts mechanically attached through a
gear lever arrangement with the operating mechanism. After a cycle of operation of circuit breaker the total
stored energy is released and hence the potential energy again stored in the operating mechanism of circuit
breaker by means of spring charging motor or air compressor or by any other means. Till now we have
discussed about mechanical working principle of circuit breaker. But there are electrical characteristics of a
circuit breaker which also should be considered in this discussion of operation of circuit breaker.

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Circuit Breaker Types and Characteristics

Types of Circuit Breaker
According to their arc quenching media the circuit breaker can be divided as:
Oil circuit breakers.
Air circuit breakers
SF6 circuit breakers
Vacuum circuit breakers.

Circuit Breaker Types and Characteristics

According to their services the circuit breaker can be divided as:
Outdoor circuit breaker
Indoor breaker.

According to the operating mechanism of circuit breaker they can be divided

as:
Spring operated circuit breaker.
Pneumatic circuit breaker.
Hydrolic circuit breaker.

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Circuit Breaker Types and Characteristics

According to the voltage level of installation types of circuit breaker are
referred as:
High voltage circuit breaker.
Medium voltage circuit breaker.
Low voltage circuit breaker.

Circuit Breakers Selection and Specification

The choice of a CB is made in terms of:
Electrical characteristics of the installation for which the CB is intended
Its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic
conditions, etc.
Short-circuit current breaking and making requirements
Operational specifications: discriminative tripping, requirements (or not) for remote control
and indication and related auxiliary contacts, auxiliary tripping coils, connection
Installation regulations; in particular: protection of persons
Load characteristics, such as motors, fluorescent lighting, LED ligthing, LV/LV transformers
The following notes relate to the choice LV circuit-breaker for use in distribution systems.

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Circuit Breakers Selection and Specification

Circuit breakers come with a list of several specifications, and understanding them is vital in
order to select the proper breaker for each application. The technical specifications most
commonly found in miniature (MCB) and molded-case (MCCB) circuit breakers are the following:
1) Rated Operational Voltage (Ue)
Self-Explanatory: The operating voltage which the circuit breaker is designed. A single breaker
may be rated for several voltages, or may be compatible with both AC and DC voltages.
2) Rated Insulation Voltage (Ui)
This is the voltage at which the circuit breaker is tested under laboratory conditions. For safety
reasons, this value is always higher than the rated voltage.
3) Rated Impulse-Withstand Voltage (Uimp)
Maximum peak voltage the circuit breaker can withstand without being damaged. The Uimp
often has a value of several thousand volts.

Circuit Breakers Selection and Specification

4) Rated Current (In)
Maximum current the circuit breaker allows without tripping. Anything above this value will
eventually cause it to trip. Low overcurrent levels trip the thermal protection in a few minutes, while
drastic peaks in current (line fault, short circuit) cause an instant trip.
5) Service breaking capacity (Ics)
This is the largest fault current the circuit breaker can interruptwithout suffering damage.
6) Ultimate breaking capacity (Icu)
Maximum fault current that the circuit breaker can interrupt. However, the unit is damaged
permanently for all fault currents above the service breaking capacity.
7) Mechanical Life
Average number of times the circuit breaker handle can be operated manually before failure.
8) Electrical Life
Average number of times the circuit breaker can trip before failure.

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Short Circuit Currents and Size of Reactor

Problem: Find the reactance per phase, external to a generator with
6% internal reactance voltage. Such that the steady state current on
the short circuit shall not exceed 8 times the full load current. The
generator rating is 3-phase, 10MVA, 11KV.

Short Circuit Currents and Size of Reactor

Problem: Find the reactance per phase, external to a generator with
6% internal reactance voltage. Such that the steady state current on
the short circuit shall not exceed 8 times the full load current. The
generator rating is 3-phase, 10MVA, 11KV.
Solution:
𝑆 107
Full load current, I= = = 525 A.
𝑉 11𝑥103 3
11𝑥103
Voltage per phase, V= = 6350V
3

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Short Circuit Currents and Size of Reactor

8000𝑥100
Short-circuit kVA on low voltage side of breakers = = 154 MVA
5.2

Reactance voltage of 3000 kVA transformer = 4% on 3000 kVA base

8000
= 4x
3000

= 10.66% on 8000 kVA base

If fault occurs on the high voltage side of the transformer, the total reactance voltage upto the
point of fault will be 5.2% + 10.66% or 15.86%
8000 𝑥 100
Short circuit kVA on the high voltage side of transformer = = 50.441 MVA
15 .86

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