Sub Station n detailed specification on its vital components.

1. SUB STATION
• General Specification
• Power Factor Correction
ER. DURGAPRASAD, SE(E), CPWD

2. What it is ?
• The power system is a constituent of power generation, transmission and
distribution systems. For all the power system operations, substations are
required for their course of action. Substations are congregation of electrical
equipment through which consumers get supply of electrical power from
generating stations. By varying the voltage levels or frequency or any other
aspects, the required electrical quantity can be altered in substations to
provide quality power to consumers.

3. Types of Sub Station
• Based on the application of substations, they are classified into different types:
• Generation substation
• Indoor substation
• Outdoor substation
• Pole mounted substation
• Switching substation
• Transmission substation
• Converter substation and Distribution substation.
• In rare cases like wind farm power generation system, multiple hydroelectric and thermal power plants one
can observe the collector substation which is used for transferring power from multiple turbines into one
transmission unit.

4. Major Equipment
• Electrical Power transformers
• Instrument transformers
• Conductors & Insulators
• Isolators
• Bus bars & LT, HT panels
• Lightning arresters
• Circuit breakers
• Relays
• Capacitor banks and miscellaneous equipment

5. A static electrical machine used for transforming power from one circuit to
another circuit without changing frequency is termed as Power transformer.
Transformers are generally used to step down or step up the voltage levels
of a system for transmission and generation purpose.
Current Transformer
Current transformer is used for the measurement of the alternating current by
taking samples of the higher currents of the system. These reduced samples
are in accurate proportions with the actual high currents of the system. These
are used for installation and maintenance of the current relays in substations
for protection purpose.
Potential Transformer
Potential transformer is quite similar to the current transformer, but it is used
for taking samples of high voltages of a system for providing low-voltage to
the relays of protection system and also to the low-rating meters for voltage
measurement.

6. Isolator is a manually operated
mechanical switch that isolates
the faulty section or the section
of a conductor or a part of a
circuit of substation meant for
repair from a healthy section. It
is also called as a disconnector
or disconnecting switch.
The conductor carrying
current and having multiple
numbers of incoming and
outgoing line connections can
be called as bus bar, which is
commonly used in
substations.
To protect substation
equipments such as
conductors, transformers, etc.,
from external light surges.

7. For the protection of substation and its components from the over currents
or over load due to short circuit or any other fault the faulty section is
disconnected from the healthy section either manually or automatically.
Different types of circuit breakers are designed based on different criteria
and usage. But in general mostly used circuit breakers are Oil circuit
breaker, Air circuit breaker, SF6 circuit breaker, Vacuum Circuit Breaker,
and so on.
Relays are used for disconnecting the circuits by manual or automatic
operation. Relay consists of the coil which is excited or energized and
such that making the contacts of relay closed activates the relay to break
or make the circuit connection. There are different types of relays such
as over current relays, definite time over current relays, voltage relays,
auxiliary relays, reclosing relays, solid state relays, directional
relays,inverse time over current relays, microcontroller relays, etc. The
above figure shows some basic relays and their operation.

8. Capacitor banks
A Capacitor bank is a set of many identical capacitors connected
in series or parallel within a enclosure and is used for the power
factor correction and basic protection of substation. These
capacitor banks are acts as a source of reactive power, and thus,
the phase difference between voltage and current can be
reduced by the capacitor banks. They will increase the ripple
current capacity of the supply. It avoids undesirable
characteristics in the power system. It is the most economical
method for maintaining power factor and of correction of the
power lag problems.

9. Relevant Acts
Indian Electricity Act 2003
Indian Electricity Rules 1956
Wherever IE rule is mentioned, it is based on IE rule 1956, amended up to
date.

10. Current Transformers
• Burden should not be less than 15 VA.
• The secondary load of a current transformer is termed the
"burden" to distinguish it from the primary load.
• The burden in a CT metering circuit is the
largely resistive impedance presented to its secondary winding.
Typical burden ratings for IEC (International Electrotechnical
commission ) CTs are 1.5 VA to 60 VA. ANSI/IEEE (Institute of
Electrical and Electronics Engineers Standards Association) burden ratings are
B-0.1 to B-4.0. This means a CT with a burden rating of B-0.2 can
tolerate an impedance of up to 0.2 Ω on the secondary circuit
before its accuracy falls outside of its specification.. Items that
contribute to the burden of a current measurement circuit are
switch-blocks, meters and intermediate conductors. The most common
cause of excess burden impedance is the conductor between the meter and the
CT. When substation meters are located far from the meter cabinets, the
excessive length of cable creates a large resistance
• Accuracy class 5P 10.
• for the protection CT given in example, the ratio error is less
than 5% at 10 times of rated current (In) if the real load
consumes 15 VA at In .

11. Accuracy class in measuring
instruments
A statement of a power meter for 0.5% FS accuracy means that its inherited accuracy is
half percent of the full scale. For example, if the full scale of the meter is 50A, its error
is 0.25A. If the network load is 25A, the error is 1% (0.25/25). Since accuracy depends
on loading, IEC placed several standards to check the accuracy under various load
conditions. This is known as "Accuracy Class". Class 0.5 means that the accuracy is
0.5% from reading under full load and unity power factor, similar to 0.5% FS above, but
also adds levels of accuracy under lower (typical) load and different power factor. Table
1 shows the levels of Class 0.5, according to IEC 62053-11

12. As can be seen from the table, when the power factor is
unity and the load is above 10% the accuracy is 0.5%.
However, when the power factor is less than unity, which is
the case in every single site due to harmonics (harmonics
reduce the power factor), the accuracy become worse - 0.8%.
This means that Class 0.5 meter will be 0.8% accurate under
normal conditions. In order to provide more accurate
information, IEC published standard 62053-22 which
defines the Class 0.5S accuracy as shown in table-2
This means that Class 0.5S meter will be 0.6%
accurate compared to 0.8% of the Class 0.5 under
normal load conditions (the difference is even bigger
in lower load).

13. Transformers
• Dry type transformers
• Vacuum pressure impregnated
(VPI)
• Cast Resin dry type for capacity >
400 KVA
• Oil cooled transformers

14. Losses in Transformers
• Iron loss
• Hysteresis loss
• Eddy current loss
• Copper loss
• Stray loss
• Dielectric loss

15. Tap changing in Transformers
Transformer Rating : 2000kVA, Voltage Ratio :- 33 /0.433 kV with ON-Load Tap Changing of +10 % to - 10% @ 2.5%
Steps ? How Calculate Tap Changer Levels ?
According to CERC & CEA Norms, Highest Voltage level for the 33kV system is 36kV, So Maximum Tap level should
be equal to or greater than 36kV, Also Lowest Voltage level is 30 kV. Now we will see Calculations.
One Percentage Voltage level of 33kV is = 33000 / 100
= 330 Volt.
@ Every 2.5 % of Tap will be = 2.5 x 330 Volt = 825 Volt.
+10 % will be = 10 x 330 Volt = 3300 Volt.
Now Add 3300 Volt with 33000 Volt [i.e. 33kV], then Final O/P will be
i.e. Maximum Tap Level is = 33000 + 3300 = 36300 Volt [i.e. 36.3 kV]
-i.e. Minimum Tap Level is = 33000 - 3300 = 29700 Volt [i.e. 29.70 kV]
Now Tap Level will be:

16. • Iron loss :-
Iron losses are caused by the alternating flux in the core of the transformer as this loss occurs in the core it is also
known as Core loss. Iron loss is further divided into hysteresis and eddy current loss.
• Hysteresis Loss:-
Hysteresis loss is due to reversal of magnetization in the transformer core. This loss depends upon the volume and
grade of the iron, frequency of magnetic reversals and value of flux density. It can be given by, Steinmetz formula:
Wh= ηBmax
1.6fV (watts)
where, η = Steinmetz hysteresis constant
V = volume of the core in m3
• Eddy Current Loss:-
AC current is supplied to the primary winding which sets up alternating magnetizing flux. When this flux links with
secondary winding, it produces induced emf in it. But some part of this flux also gets linked with other conducting
parts like steel core or iron body or the transformer, which will result in induced emf in those parts, causing small
circulating current in them. This current is called as eddy current. Due to these eddy currents, some energy will be
dissipated in the form of heat.
Ke – co-efficient of eddy current. Its value depends upon the nature of magnetic material like volume and resistivity
of core material, thickness of laminations
Bm – maximum value of flux density in wb/m2T – thickness of lamination in meters
F – frequency of reversal of magnetic field in Hz, V – volume of magnetic material in m3

17. •Copper Loss Or Ohmic Loss:-
These losses occur due to ohmic resistance of the transformer windings. If I1 and I2 are the primary and the secondary current. R1 and
R2 are the resistance of primary and secondary winding then the copper losses occurring in the primary and secondary winding will be
I1
2R1 and I2
2R2 respectively.
Therefore, the total copper losses will be
These losses varied according to the load and known hence it is also known as variable losses. Copper losses vary as the square of the
load current.
•Stray Loss:-
The occurrence of these stray losses is due to the presence of leakage field. The percentage of these losses are very small as compared
to the iron and copper losses so they can be neglected.
•Dielectric Loss:-
Dielectric loss occurs in the insulating material of the transformer that is in the oil of the transformer, or in the solid insulations.
When the oil gets deteriorated or the solid insulation get damaged, or its quality decreases, and because of this, the efficiency of
transformer is effected.

18. Transformer Efficiency
The Efficiency of the transformer is defined as the ratio of useful power output to the input power, the two being
measured in the same unit. Its unit is either in Watts (W) or KW. Transformer efficiency is denoted by Ƞ.
Where,
V2 – Secondary terminal voltage
I2 – Full load secondary current
Cosϕ2 – power factor of the load
Pi – Iron losses = hysteresis losses + eddy current losses
Pc – Full load copper losses = I2
2Res
If x is the fraction of the full load, the transformer efficiency
at this fraction is given by the relation shown below

19. Maximum Efficiency Condition of a Transformer
The transformer efficiency at a given load and power factor is given by the relation shown below
The value of the terminal voltage V2 is approximately constant. Thus, for a given power factor the Transformer
efficiency depends upon the load current I2. In the equation (1) shown above the numerator is constant and the
transformer efficiency will be maximum if the denominator with respect to the variable I2 is equated to zero.
i.e. Copper losses = Iron losses

20. From equation (2) the value of output current I2 at which the transformer efficiency will be maximum is given as
If x is the fraction of full load KVA at which the efficiency of the transformer is maximum
Then, copper losses = x2Pc (where Pc is the full load copper losses)
Iron losses = Pi
For maximum efficiency x2 Pc = Pi
Therefore, Output KVA corresponding to maximum efficiency
Putting the value of x from the above equation (3) in equation (4) we will get

21. Power Factor

22. Power Triangle of an AC Circuit
•P is the I2R or Real power that performs work measured in watts, W
•Q is the I2X or Reactive power measured in volt-amperes reactive, VAr
•S is the I2Z or Apparent power measured in volt-amperes, VA
•θ is the phase angle in degrees. The larger the phase angle, the greater the reactive power
•Cosθ = P/S = W/VA = power factor, p.f.
•Sinθ = Q/S = VAr/VA
•Tanθ = Q/P = VAr/W
The power factor is calculated as the ratio of the real power to the apparent power because this ratio equals
cosθ.

23. •It should be noted that power factor, like all ratio measurements, is a unitless quantity.
•For the purely resistive circuit, the power factor is 1 (perfect), because the reactive power equals zero. Here,
the power triangle would look like a horizontal line, because the opposite (reactive power) side would have zero
length.
•For the purely inductive circuit, the power factor is zero, because true power equals zero. Here, the power
triangle would look like a vertical line, because the adjacent (true power) side would have zero length.
•The same could be said for a purely capacitive circuit. If there are no dissipative (resistive) components in the
circuit, then the true power must be equal to zero, making any power in the circuit purely reactive. The power
triangle for a purely capacitive circuit would again be a vertical line (pointing down instead of up as it was for the
purely inductive circuit).
Reactive power Q, (sometimes called wattles power) is the power consumed in an AC circuit that does not
perform any useful work but has a big effect on the phase shift between the voltage and current waveforms.
Reactive power is linked to the reactance produced by inductors and capacitors and counteracts the effects of
real power. Reactive power does not exist in DC circuits.

24. In general power is the capacity to do work. In electrical domain, electrical power is the amount of electrical energy
that can be transferred to some other form (heat, light etc) per unit time. Mathematically it is the product
of voltage drop across the element and current flowing through it.
Considering first the DC circuits, having only DC voltage sources, the inductors and capacitors behave as short
circuit and open circuit respectively in steady state. Hence the entire circuit behaves as resistive circuit and the entire
electrical power is dissipated in the form of heat.
Here the voltage and current are in same phase and the total electrical power is given by

25. The inductor stores electrical energy in the form of magnetic energy and capacitor stores electrical energy in the
form of electrostatic energy. Neither of them dissipates it. Further there is a phase shift between voltage and
current. Hence when we consider the entire circuit consisting of resistor, inductor and capacitor, there exists some
phase difference between the source voltage and current. The cosine of this phase difference is called electrical
power factor.
This factor (-1 < cosφ < 1 ) represents the fraction of total power that is used to do the useful work.
The other fraction of electrical power is stored in the form of magnetic energy or electrostatic energy in inductor
and capacitor respectively.
Now coming to AC circuit, here both inductor and capacitor offer certain amount of impedance given by,
The total power in this case is,

26. This is called apparent power and its unit is VA (Volt Amp) and denoted by ‘S’.
A fraction of this total electrical power which actually does our useful work is called as active power. It is denoted
as ‘P’.
P = Active power = Total electrical power.cosφ and its unit is watt.
The other fraction of power is called reactive power. This does no useful work, but it is required for the active work
to be done. It is denoted by ‘Q’ and mathematically is given by,
Q = Reactive power = Total electrical power.sinφ and its unit is VAR (Volt Amp Reactive).
This reactive power oscillates between source and load.
To help understand this better all these power are represented in the form of triangle.

27. Power Factor Improvement
The term power factor comes into picture in AC circuits only. Mathematically it is cosine of the phase difference
between source voltage and current. It refers to the fraction of total power (apparent power) which is utilized to do
the useful work called active power.
Need for Power Factor Improvement Real power is given by P = VIcosφ. To transfer a given amount of power at
certain voltage, the electrical current is inversely proportional to cosφ. Hence higher the pf lower will be the current
flowing. A small current flow requires less cross sectional area of conductor and thus it saves conductor and money.
From above relation we saw having poor power factor increases the current flowing in conductor and thus copper
loss increases. Further large voltage drop occurs in alternator, electrical transformer and transmission and
distribution lines which gives very poor voltage regulation.
Further the KVA rating of machines is also reduced by having higher power factor as,
Hence, the size and cost of machine also reduced. So, electrical power factor should be maintained close to unity.

28. Methods of Power Factor Improvement
1. Capacitors:
Improving power factor means reducing the phase difference between voltage and current. Since majority of loads
are of inductive nature, they require some amount of reactive power for them to function. This reactive power is
provided by the capacitor or bank of capacitors installed parallel to the load. They act as a source of local reactive
power and thus less reactive power flows through the line. Basically they reduces the phase difference between the
voltage and current.
2. Synchronous Condenser:
They are 3 phase synchronous motor with no load attached to its shaft. The synchronous motor has the
characteristics of operating under any power factor leading, lagging or unity depending upon the excitation. For
inductive loads, synchronous condenser is connected towards load side and is overexcited. This makes it behave like
a capacitor. It draws the lagging current from the supply or supplies the reactive power.
3. Phase Advancer:
This is an ac exciter mainly used to improve pf of induction motor. They are mounted on shaft of the motor and is
connected in the rotor circuit of the motor. It improves the power factor by providing the exciting ampere turns to
produce required flux at slip frequency. Further if ampere turns are increased, it can be made to operate at leading
power factor.

29. Power Factor Calculation
In power factor calculation, we measure the source voltage and current drawn using
a voltmeter and ammeter respectively. A wattmeter is used to get the active power.
Now, we know P = VIcosφ watt
Hence, we can get the electrical power factor. Now we can calculate the reactive power Q = VIsinφ VAR
This reactive power can now be supplied from the capacitor installed in parallel with load in local. Value of
capacitor is calculated as per following formula:

32. End of Part-I