Protective Relay Defects to Power System Buses

2. Abstract
Power system bus is defined as a physical line or media at which several
components like generators, feeders, power station etc. are attached.
Protective Relays were developed over hundred years ago to provide “last line”
of defense for protection of expensive electrical parts. These relays have the
ability to identify a fault and isolate it so that the processing of the entire
system may not be effected. In other words, these devices called Protective
Relays have artificial intelligence to judge such situations in order to avoid
hazards.
As everything has its pros and cons, similarly there are also some shortcomings
to these Protective Relays attached to Power System buses.

3. Objectives
 Power System Buses Structure and Working.
 Protective Relays Functions and Importance.
 To Understand the proper usage of Protective Relays on Power
System Buses.
 To Avoid excessive use of Protective Relays in order to minimize cost
and hazards.

4. Power System Buses
Power System Buses are classified on the basis of four quantities:
 Magnitude of Voltage
 Phase Angle of Voltage
 Active or True Power
 Reactive Power

5. Magnitude of Voltage
Voltage is the pressure from an electrical
Circuit’s Power Source that pushes charged
electrons (electric current) through a
conducting loop, enabling it to do any work
for example illuminating a light bulb. In other
words, voltage is pressure measured in volts.
The quantity of voltage flowing through the
physical media is termed as its magnitude.

6. Phase Angle of Voltage
The phase angle is the shift between AC
current and Voltage on the measured
impedance(50kHz).

7. Active/True and Reactive Power
Part of Complex power that corresponds to storage and retrieval of
energy rather than consumption. For example on an AC power system,
there are two kinds of power – real power that actually consumes to do
the work, and reactive power which enables transformer to transform,
generators to generate and motors to rotate and vice versa.

8. Power System Bus Classification
The buses are classified into three categories
 Generation Bus
 Load Bus
 Slack Bus.
Type of Buses
Know or Specified
Quantities
Unknown Quantities or
Quantities to be
determined.
Generation or P-V
Bus
P, | V | Q, δ
Load or P-Q Bus P, Q | V |, δ
Slack or Reference
Bus
| V |, δ P, Q

9. Generation Bus
This bus is also called the P-V bus, and on this bus, the voltage
magnitude corresponding to generate voltage and true or active power
P corresponding to its rating are specified.
Voltage magnitude is maintained constant at a specified value by
injection of reactive power. The reactive power generation Q and phase
angle δ of the voltage are to be computed.

10. Load Bus
This is also called the P-Q bus and at this bus, the active and reactive
power is injected into the network. Magnitude and phase angle of the
voltage are to be computed. Here the active power P and reactive
power Q are specified, and the load bus voltage can be permitted within
a tolerable value, i.e., 5 %. The phase angle of the voltage , i.e. δ is not
very important for the load.

11. Slack, Swing or Reference Bus
Slack bus in a power system absorb or emit the active or reactive power from
the power system. The slack bus does not carry any load. At this bus, the
magnitude and phase angle of the voltage are specified. The phase angle of
the voltage is usually set equal to zero. The active and reactive power of this
bus is usually determined through the solution of equations.
The slack bus is a fictional concept in load flow studies and arises because the
I2R losses of the system are not known accurately in advance for the load flow
calculation. Therefore, the total injected power cannot be specified at every
bus. The phase angle of the voltage at the slack bus is usually taken as
reference or zero.

12. Protective Relays
These smart devices are used to protect
electrical equipment like Motors,
Transformers, Transmission Lines, Generators,
Bus-bars, etc. They receive input from sensor
and compare with set value (trip value) then
provide output and isolate electrical
equipment from the system.

13. Classification of Relays
Protective Relays
Regulating Relays
Reclosing, Synchronism Check and
Synchronizing Rleays
Monitoring Relays
Auxiliary Relays

14. Functions of Protective Relays
 To sound an alarm or to close the trip circuit breaker to disconnect
the faulty section.
 To localize the fault by isolating the defective part from the healthy
part of the system to avoid disturbance resulting in entire system
failure.
 To minimize hazards to human resources and downsizing their
overhead by looking after the entire system.
 Provide vital diagnostic information regarding the defects occurred to
be avoided in future.

15. Disadvantages
 Higher skilled manpower required.
 Auxiliary Power Supply is Needed
 Sensitive to Temperature
 False Alarms or System Halts
 Re-Programming as Per System Change.

16. Disadvantages
 Higher skilled manpower required.
 Auxiliary Power Supply is Needed
 Sensitive to Temperature
 False Alarms or System Halts
 Re-Programming as Per System Change.

17. Thank You