The switchyard functions to transmit power from generating stations to the electrical grid at incoming voltages and allow for switching via switchgears. It interfaces between the power plant and grid. There are two main types of switchyards: air insulated and gas insulated. The switchyard contains various equipment like busbars to distribute power, insulators to support conductors, surge arresters to protect from overvoltage, isolators to isolate lines, wave traps to prevent communication interference, and circuit breakers and relays to isolate faulty lines. Differential protection relays compare currents at the boundaries of protected equipment like power lines or transformers to accurately and quickly detect internal faults.

2. The main function of switchyard is to transmit &
distribute the power at incoming voltage from the
generating station and provide facilities of switching by
the help of switchgears.
Switchyard acts as interface between the power plant
electrical system and electrical grid.
Types of Switchyard:
AIS (Air Insulated Switchyard)
GIS (Gas Insulated Switchyard).

4. 1.Primary power lines
2.Ground wire
3.Overhead lines
4.Transformer for measurement of
electric voltage
5.Disconnect switch
6.Circuit breaker
7.Current transformer
8.Lightning arrester
9.Main transformer
10.Control building
11.Security fence
12.Secondary power lines

5. Bus-bar
Bus-bars receive the power from incoming circuits or we can say a grid and
deliver the power to an outgoing Circuit. It can be single phase or three phases,
but most of the cases it is 3-phase and one bus consists of 3 conductors(R-Y-
B).There are also various types of bus bar arrangements depending upon their
requirements. The various arrangements are mentioned below.
Insulator
The insulators mainly serve two purposes. First of all they support the
conductor and confined the high current of the line to the conductor. The
most common material for the manufacturing of insulators is Porcelain.
Below mentioned are the types of Insulators used in switchyard:
•Pin Insulator
•Post Type
•Suspension Insulator
•Strain Insulator

6. Surge Arrester
This will protect the equipment from transient, surge and high voltages. They are generally connected
in parallel to the equipment to be protected and function to divert the surge current safely to ground.
Isolator
Isolators are generally no- load switches which are used to isolate the electrical lines during
maintenance. They are only operated for isolation after the circuit breaker is operated. They are
operated by means of a motor present below the isolator assembly.
Wave trap
Wave trap is a resonant circuit connected in series with the HV transmission line to prevent the
transmission of high frequency signals. The communication wave is having a high frequency of 150
KHz to 200 KHz and the electrical power has a frequency of 50 Hz. So to avoid the communication
waves to travel to the electrical equipment we are using this Wave-trap. It creates a high reactance
path for the High –frequency signal and blocks it. We are employing these Wave-traps is due to the
use of PLCC (Power Line Carrier Communication).
PLCC
PLCC stands for Power Line Carrier Communication. This technology is used in all sub-
stations for communication with other sub-stations. The information regarding the generation
and other parameters are transmitted to other sub-stations or grid by the help of this
PLCC. By the help of PLCC, we are avoiding the use of an extra wire for communication
and the information flows through the transmission lines and it is much faster than any other
medium

7. Circuit breaker
A circuit breaker is equipment which can make and break a circuit under normal as well as during
fault conditions. It is operated during on load condition and the arc generated while opening the
circuit is quenched by a strong di-electric medium. The most widely used arc quenching mediums for
HV Lines are Vacuum and SF6 Gas.
Current transformer
A current transformer is a type of transformer that is used to reduce or multiply an alternating
current. It produces a current in its secondary which is proportional to the current in its primary.
Potential transformer
The potential transformer may be defined as an instrument transformer used for the transformation
of voltage from a higher value to a lower value.
Protective relay
A protective relay is a device that detects the fault and initiates the operation of the Circuit breaker to
isolate the faulty element from the rest of the system. Whenever there is a fault in the bus,the relay
senses it and gives the command to the circuit breaker and the circuit breaker is operated. The relay
receives the command from the instruments transformers (i.e CTs & PTs).

10. Protection does not mean prevention, but minimizing the duration of the
trouble, the five basic objectives are:
i) Reliability: assurance that the protection will perform correctly.
ii) Selectivity: maximum continuity of service with minimum system
disconnection.
iii) Speed of operation: minimum fault duration and consequent equipment
damage.
iv) Simplicity: minimum protective equipment and associated circuitry to achieve
the protection objectives.
v) Economics: maximum protection at minimum total cost.
PROTECTIONSCHEME
Basic objectives of system protection :

11. Classification can be done by different ways, such as by function, input,
performance characteristics an operating be divided into five types:
i) Protective Relays:
Protective relays and fuses operate on the intolerable power system conditions.
They are applied to all parts of the power system; generates, buses, TFs, TLs,
distribution lines and feeds, motors, loads, capacitors banks and reactors. Fuses
are usually used for low voltage level (480 V).
ii) Regulating Relays:
Regulating relays are associated with tap changer of TFs, on governor of generating
equipment to control the voltages level with varying load (used during normal
conditions).
Classification of Relays:

12. iii)Reclosing, synchronism check,
synchronizing relays:
Relays of this type are used in energizing
or restoring lines to service after an
outage and in interconnecting pre-
energizing parts of the systems.
iv) Monitoring Relays:
Relays of this type are used in energizing
or restoring lines to service after an
outage and in interconnecting pre-
energizing parts of the systems.
v) Auxiliary Relay:
There are two categories: contact
multiplication (repeat contactors) and
circuit isolation.

13. Protection Relays and Circuit Breakers
Upon detection of a fault, protection relays provide the trip signals for the
circuit breakers. These are switching devices designed to be able to carry
and interrupt safely the often very high currents, which flow during faults.
Modern high capacity circuit breakers are capable of interrupting currents
of up to 60 000 amps at a nominal voltage of 750kV+ within 40
milliseconds.
The purpose of the circuit breakers is:
To disconnect the faulty element or circuit, leaving the rest of the power
system intact .
To prevent other healthy equipment from being damaged by the fault
currents they must carry.
To reduce damage at the point of fault, important because the less
damage caused by the fault the greater the probability of successful
repair.

14. There are numerous types of relay protection
schemes, the most popular being:
1.Overcurrent Protection
2.Distance Protection
3.Differential Protection:
– Current differential protection
– Voltage differential protection
4.Undervoltage Protection
5.Overvoltage Protection
6.Underfrequency Protection

15. Differential Protection
For further improvement of relay protection schemes, let’s consider a differential
protection which has the following principle: differentiation between external and
internal faults in reference to protected equipment by comparing the direction of
currents on boundaries of protected equipment:
For power lines – on both ends of the lines
For transformers – on high and low voltage sides
Advantages of differential protection:
 High accuracy
 High speed of clearing the fault
Disadvantages:
 Relative complexity
 Need in communication between ends of the line

16. The dotted line shows the section which is used to be protected.
The current transformer is placed at both the ends of the protection
zone. The secondary of the transformers is connected in series with
the help of the pilot wire. Thereby, the current induces in the CTs
flows in the same direction. The operating coil of the relay is
connected on the secondary of the CTs.
In the normal operating condition, the magnitude of current in the
secondary of the CTs remains same. The zero current flows through
the operating coil. On the occurrence of the fault, the magnitude of
the current on the secondary of CTs becomes unequal because of
which the relay starts operating.