Power System Fault and Protection
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Power System Fault Protection
Per Unit Fault Protection
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Per Unit
Problem
Computation for a power system having 2 or more voltage levels become cumbersome Need to convert currents to a different voltage level wherever they flow through a transformer
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Per Unit
Solution
A set of base values is assumed for each voltage class All values express in per unit (i.e. no dimension)
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Per Unit
To completely define a per unit system, minimum 4 base quantities are required
Voltage Current Impedance Power
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Per Unit
General
Per Unit= Actual / Base
Vpu=Vactual / Vbase Ipu=Iactual / Ibase Zpu=Zactual / Zbase Spu=Sactual / Sbase Sbase= Vbase Ibase Vbase= Ibase Zbase
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Specific
Example
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Example
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Example
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Example
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Example
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Example
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Example
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Example
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Example
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Fault
Cause of Fault Effect of Fault Fault Level
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Causes of Faults
Transient over voltages
the failure of insulation, resulting in fault current or short-circuit current.
Insulation aging External object
tree branches bird
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Effect of Fault
Equipment be damaged due to overheating or insulation breakdown
Generator Transformer Busbar Cable Circuit Breaker etc.
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Effect of Fault
Power System becomes unstable
Voltage Reduction for the whole power system. Frequency change
results in cascade tripping of generators.
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Type of Fault
Shunt Fault Series Fault
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Fault Study
Determine the maximum fault current for short circuit fault Determine open circuit fault Determine circuit breaker rating Determine scheme of protection
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Fault Level
The fault level is
the three-phase fault value expressed in MVA at the system voltage at the fault point in the system
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Fault Level Limiting
Problem
Large interconnected system
Large number of generators and thus large power rating Large fault current fed into the fault point
Solution
Reduce fault current
Increase reactance of a system
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Fault Level Limiting
Reactor arrangement
Normal condition
Small voltage drop across Reactor Smaller short circuit current fed into the faulty point Smaller breaking currents for circuit breaker
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Short circuit condition
Reactance in Generator Circuit
Reactors
Feeders
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Reactance in series with Feeders
Reactors
Feeders
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Busbar reactors in Ring System
Reactors
Feeders
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Busbar reactors in a Tie Bar System
Reactors
Feeders
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Example
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Example
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Example
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Protection
Problem
Fault cause abnormal current flows in the system
many undesirable effects are likely to occur
Solution
remove the fault from the system quickly The equipment that performs the functions of fault clearing from the system
Protection System
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Component for Protection
Circuit Breaker Transducer
Voltage Transformer Current Transformer
Relay
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Circuit Breaker
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Voltage Transformer (VT)
Voltage transformers are connected across the points at which the voltage is to be measured
Ns Vs Vp Np
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Current Transformer (CT)
Primary winding
Consists of a single turn Consist of multiple-turn
Secondary winding
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Relay
A protective relay is an electrical device
Abnormal condition
to initiate isolation of a part of the power system to operate an alarm signal
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Sequence of Protection Operation
Fault occurs Voltage or current signal transmits to relay Relay operates Circuit Breaker Fault clear
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Type of Protection
Over current protection Differential protection Primary and backup protection
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Over Current Protection
Function
monitor the current continuously issue a tripping command to the circuitbreaker when triggered
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Time Over Current Protection
Time over current relay
definite time
ta
ta=constant
In the definite time scheme, the operating ta" time is constant regardless of the current flowing through the relay providing it is higher than the pick-up setting
Iset
(a)
I
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Time Over Current Protection
Time over current relay
inverse time
The operating time of an inverse time-over current relay is inversely proportional to the level of the fault current ta t a"
Iset
I
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Inverse Definite Multiple Time Relay (IDMT)
Two possible adjustments :
Current setting Time setting
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Discrimination by Time
Objective
To delay between successive relay
Circuit breaker trip time
OCB: 150 ms SF6 CB: 50 ms
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Discrimination by Time
Relay trip time
for EM Relays with OCB: 0.4 – 0.5s for Solid State Relays with vacuum or SF6 switchgear: 0.25s
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Differential Protection
Compare the current between upstream and downstream of the protected zone
Protection Zone
CT 1
F
F
CT 2
F
IA Trip Coil
IB
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Differential Protection
Normal condition
Current different = 0
Fault condition
Current different 0 Differential current triggers the trip coil
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Differential Protection
Features
Sensitive to faults only within a particular section of a power system Greatly increase the reliability of the protection of that particular section Insensitive to faults outside that particular section Do not contribute to back-up protection of the remainder of the power system
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Example
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Example
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Solution
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Solution
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Solution
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Primary and Back-up Protection
Main protection
Cover smaller section of the power system Shorter operating time Covering larger section of the power system Longer operating time
Back-up protection
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