KIOT
School of Electrical & Computer Engineering
Power Engineering stream
ECEG 4232 Power System Protection & Control
Shegaw M.
Email: smelak8@gmail.com
shegawm@kiot.edu.et
1
Lecture 4: Power System Protections
November-2020
Contents
Protection Zone
Protection of generators
Protection of transmission lines
protection of transformers
Protection of motors
Protective Zones
What Do you think About Zones
of Protection?
Protection zones (primary protection zones) are regions
of primary sensitivity.
3
Protection Zone in Power System
 Protection zone is defined as the part of the power system which is
protected by a certain protective scheme, and it established around each
power system equipment.
 When the fault occurs on any of the protection zones then only
the circuit breakers within that zone will be opened. Thus, only the
faulty element will be isolated without disturbing the rest of the system.
 The protection zone cover the entire power system, and no part of the
equipment is left unprotected. It usually consists one or more element of the
power system. The protection zone of the power system mainly depends
upon the rating of the machine, its location, the probability of
faults and abnormal condition of the equipment.
4
Cont’d…
5
Figure 1, One line diagram of a system showing primary protection zones.
Cont’d…
Primary relays are relays within a given protection zone that should
operate for prescribed abnormalities within that zone.
 In Figure 1, for example, consider a fault on line JK. For this condition,
relays supervising breakers J and K should trip before any others and these
relays are called primary relays.
6
Cont’d…
Backup relays are relays outside a given primary protection zone, located
in an adjacent zone, which are set to operate for prescribed abnormalities
within the given primary protection zone and independently of the
primary relays.
 For example, suppose a fault on line JK of Figure 1 cannot be cleared by
breaker J due to relay or breaker J fault. Assume that breaker K operates
normally leaving the fault connected to the bus terminated by breakers I
and M.
 Backup relays at locations I and M should be set to operate for the fault on
line JK.
7
Cont’d…
 Local backup relays are an alternate set of relays in a primary protection zone that
operate under prearranged conditions in that protection zone.
 Often such local backup relays are a duplicate set of primary relays set to operate
independently for the same conditions as the primary set.
 This constitutes an OR logic trip scheme and is an effective safeguard against
relay failures.
Failure to trip is a protective system fails to take appropriate action,
 This type of failure may result in extensive damage to the faulted
component if it is not rectified by backup protection.
8
Cont’d…
Undesired tripping (false tripping) results when a relay trips
unnecessarily for a fault outside its protection zone or when there is no
fault at all.
 This can occur when the protective system is set with too high a
sensitivity. Such operation may cause an unnecessary load outage, for
example, on a radial circuit, or may cause overloading of adjacent lines of
a network.
 Thus, in some cases, unnecessary tripping may not cause serious damage
or overloading. In other cases, where an important line is falsely tripped,
it can lead to cascading outages and very serious consequences.
9
Zones of protection…
10
Cont’d…
 To limit the scope of the power
system that is disconnected
when a fault occurs, protection
is arranged in zones. The
principle is shown in Figure 2.
11
Figure 2 Division of power systems into protection zones
Cont’d…
Protection zones follow common logical boundaries to cover
such equipment as Lines, Transformers, Buses, Generators,
Motors and any combinations of the above equipment.
For such boundaries to be genuine there must be:
 Measuring devices such as Current Transformer or Voltage
Transformer.
 Isolating devices such as breakers.
12
Exercise
 Consider the system portion shown in Figure below.
a) Sketch the zones of protection.
b) Describe a possible backup scheme for failure of breaker 7 for a fault
on line BC. Is load 1 interrupted? load 2?
G
13
14
Generator Protection
 The AC windings are subject to the same types of failures as transformers, and
protection must be provided to remove faulted machines to preserve system
integrity as well as to limit the damage to the equipment.
 Synchronous generators may subjected for different types of faults, so, different
types of protection are required.
15
Cont’d…
 All generators will not have the same level of protection.
 As a general rule, the larger, more expensive machines will have the
greatest number of different protective systems. Because, serious damage
of these units is very costly, both in terms of the repair and also the cost
due to the unavailability of the unit.
 Mainly problems occurred in a synchronous generator parts are
I.
Stator winding problems
II. Rotor problems
16
Cont’d…
 Stator winding problems
17
 Rotor problems
A.
Phase fault
I.
Shorted field winding
B.
Ground fault
II.
Grounded field winding
C.
Tum-to-turn fault
III. Open field winding
D.
Open circuit
IV. Overheating
E.
Overheating
V.
F.
Overvoltage
G.
Unbalanced current
Overspeed
 STATOR PROTECTION
 All synchronous generators must have stator protection, and most units have
several types of protection, each of which guards against different types of
problems.
 Stator faults result from insulation breakdown that causes an arc to develop,
either from phase to phase or from the phase conductor to the grounded
magnetic steel laminations of the stator.
 The cause of the insulation breakdown may be due to overvoltage, overheating,
or mechanical damage of the winding insulation due to faults.
18
Cont’d…
What can
go wrong?
• Stator
winding
problems
19
A) Phase Fault Protection
 Phase faults in generators are rare, but they can occur and must be protected against.
 Phase faults can develop in the winding end turns, where all three phase windings
are in slots, if there are two coils in the same slot.
Phase faults often change to ground faults.
 The standard method of protection against phase faults is the differential relay, and
usually the percentage differential type of relay. Most of the manufacturers
recommend this type of protection for all units larger than about 1 MVA event.
20
B) Ground Fault Protection
 Most of the generator stator winding faults are phase-to-ground faults. This is
true because the windings are always in close contact with steel slots that are
at ground potential.
 Phase to-phase faults are less difficulty, in a sense, since the damage due to
these faults can sometimes be repaired by carefully taping the damaged
insulation.
 This is not true for ground faults, which often melt the steel laminations and
may cause so much steel damage that the core laminations will have to be rebuilt,
21
resulting in a very long outage.
Cont’d…
 Note that the generator is grounded through
an impedance, which is installed to limit the
ground fault current.
 If the impedance is too high, the fault current
is low and the phase relaying lacks sensitivity.
 Generator ground fault protection using
overcurrent relays with carefully selected
pickup and time setting.
22
Fig. 1 Grounding a generator neutral through a distribution transformer.
C) Turn-to-Turn Fault Protection
 Differential stator winding protection will not detect turn-
to-turn faults until they become shorted to ground.
 Turn-to-turn faults are rare, but they can have high currents and
produce significant core damage.
 Since turn-to-turn faults will eventually develop into ground
faults, so it can be protected using overcurrent relays
23
D) Stator Open Circuit Protection
 Stator open circuit protection is usually not provided, since no
permanent damage.
 it is possible that damage will accumulate due to a high resistance
connection.
 An open circuit in one of the stator windings will cause single phasing of
the generator and high negative-sequence currents. This will cause an
alarm to alert the operator of the condition.
24
E) Overheating Protection
 Overheating of a synchronous generator may occur due to
one of the following causes:
1. Overload
2. Failure of the hydrogen cooling system
3. Shorted laminations in the stator iron
4. Core bolt insulation failures in the stator iron
25
Cont’d…
 Failure of the cooling system is also likely to be detected by operator
alarms, The other failure, involving core failures and heating will
develop slowly and must be detected by temperature measurements of
some kind.
 Temperature detection is often accomplished using embedded
thermocouples in the stator winding slots, placing the thermocouples
throughout the windings in several locations
26
F) Unbalanced Current Protection
 Unbalanced loading of a synchronous generator causes negative-sequence currents to
flow in the stator windings.
 These harmful conditions may be caused by
1. One open phase of a line
2. One open pole of a circuit breaker
3. A close-in unbalanced fault that is not on time cleared
4. A stator winding fault
 The first three of these causes are for failures external to the machine. Unbalanced
faults near a generator cause more heat in synchronous machines than balanced faults
27
G) Overvoltage Protection
One type of overvoltage in a generator is that due to transient
surges caused by lightning or switching surges.
 These transients are protected by surge protective devices that are
designed for this purpose.
 Generator over voltage fault protection using overvoltage relays
28
 ROTOR PROTECTION
 There are several different types of rotor protection, each type guarding the rotor
from a particular type of fault.
I.
Shorted Field Winding Protection
 Shorted turns in the generator field winding have the potential for distorting the
field across the air gap.
 This is due to the unsymmetrical ampere turns of mmf in different parts of the
field winding.
 The mechanism that causes the shorted winding is often due to grounding of
29
the winding at two different places.
Generator protection…
 Rotor
30
II. Grounded Field Winding
 The field winding of a synchronous machine is usually fluctuating with respect to
ground .
 There are several methods of detecting a rotor circuit ground
 Potentiometer method
 AC injection method
 DC injection method
DC voltage relay (64F) the field ground relay is connected from the negative side of the
field to DC ground. Detects voltage from the field to ground.
31
III. Overheating of the Field Winding
 The temperature of the field winding can be monitored by an
ohmmeter type of detector that measures the winding resistance of
the field .
 Such an instrument is often calibrated in temperature, rather than
ohms, for a direct estimation of the winding temperature.
32
LOSS OF EXCITATION PROTECTION
 The problems associated with the loss of excitation in a synchronous
generator. Loss of field protection is often provided using a mho type of
distance relay.
 OTHER GENERATOR PROTECTION SYSTEMS
 Overspeed Protection
 Vibration Protection
 Bearing Failure Protection
 Coolant Failure Protection
33
 Fire Protection
Bus Bar Protection Zone
The most common fault within this zone is the phase
to ground fault generally caused by flash over
on insulators as a result of lightening.
Other causes of this flash over are:
• Cracked insulators, birds and reptiles, dirty or broken
insulators or animals that may walk close to the bus.
• The bus has several lines/feeders tied to it.
• The current transformers on the bus get easily saturated due
to these lines.
34
Cont’d…
The usual type of protection for the bus zone is the
differential type.
This method compares the current entering the bus
zone with that leaving it.
Current transformers installed on each bus feed are used to
make this comparison.
35