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Chap17-280-315 17/06/02 10:43 Page 280
17
•
Generator and
Generator Transformer Protection
•
Introduction
17.1
Generator earthing
17.2
Stator winding faults
17.3
Stator winding protection
17.4
Differential protection of
direct-connected generators
17.5
Differential protection of generator
–transformer units
17.6
Overcurrent protection
17.7
Stator earth fault protection
17.8
Overvoltage protection
17.9
Undervoltage protection
17.10
Low forward power/reverse
power protection
17.11
Unbalanced loading
17.12
Protection against inadvertent energisation
17.13
Under/Overfrequency/Overfluxing protection
17.14
Rotor faults
17.15
Loss of excitation protection
17.16
Pole slipping protection
17.17
Overheating
17.18
Mechanical faults
17.19
Complete generator protection schemes
17.20
Embedded generation
17.21
Examples of generator protection settings
17.22
Chap17-280-315 17/06/02 10:44 Page 281
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17
•
Generator and
Generator-Transformer Protection
17.1 INTRODUCTION
The core of an electric power system is the generation.
With the exception of emerging fuel cell and solar-cell
technology for power systems, the conversion of the
fundamental energy into its electrical equivalent
normally requires a 'prime mover' to develop mechanical
power as an intermediate stage.
The nature of this machine depends upon the source of
energy and in turn this has some bearing on the design
of the generator. Generators based on steam, gas, water
or wind turbines, and reciprocating combustion engines
are all in use. Electrical ratings extend from a few
hundred kVA (or even less) for reciprocating engine and
renewable energy sets, up to steam turbine sets
exceeding 1200MVA.
Small and medium sized sets may be directly connected
to a power distribution system. A larger set may be
associated with an individual transformer, through
which it is coupled to the EHV primary transmission
system.
Switchgear may or may not be provided between the
generator and transformer. In some cases, operational
and economic advantages can be attained by providing
a generator circuit breaker in addition to a high voltage
circuit breaker, but special demands will be placed on
the generator circuit breaker for interruption of
generator fault current waveforms that do not have an
early zero crossing.
A unit transformer may be tapped off the
interconnection between generator and transformer for
the supply of power to auxiliary plant, as shown in
Figure 17.1. The unit transformer could be of the order
of 10% of the unit rating for a large fossil-fuelled steam
set with additional flue-gas desulphurisation plant, but
it may only be of the order of 1% of unit rating for a
hydro set.
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required. The amount of protection applied will be
governed by economic considerations, taking into
account the value of the machine, and the value of its
output to the plant owner.
The following problems require consideration from the
point of view of applying protection:
a.
stator electrical faults
b.
overload
c.
overvoltage
d.
unbalanced loading
e.
overfluxing
f.
inadvertent energisation
e.
rotor electrical faults
f.
loss of excitation
g.
loss of synchronism
h.
failure of prime mover
j.
lubrication oil failure
l.
overspeeding
m.
rotor distortion
n.
difference in expansion between rotating and
stationary parts
o.
excessive vibration
p.
core lamination faults
Generator
Main transformer
HV busbars
Unit transformer
Auxiliary
supplies switchboard
Figure 17.1: Generator-transformer unit
Industrial or commercial plants with a requirement for
steam/hot water now often include generating plant
utilising or producing steam to improve overall
economics, as a Combined Heat and Power (CHP)
scheme. The plant will typically have a connection to the
public Utility distribution system, and such generation is
referred to as ‘embedded’ generation. The generating
plant may be capable of export of surplus power, or
simply reduce the import of power from the Utility. This
is shown in Figure 17.2.
Utility
PCC
Generator
Rating: yMW
17.2 GENERATOR EARTHING
The neutral point of a generator is usually earthed to
facilitate protection of the stator winding and associated
system. Earthing also prevents damaging transient
overvoltages in the event of an arcing earth fault or
ferroresonance.
For HV generators, impedance is usually inserted in the
stator earthing connection to limit the magnitude of
earth fault current. There is a wide variation in the earth
fault current chosen, common values being:
1.
rated current
2.
200A-400A (low impedance earthing)
3.
10A-20A (high impedance earthing)
The main methods of impedance-earthing a generator
are shown in Figure 17.3. Low values of earth fault
current may limit the damage caused from a fault, but
they simultaneously make detection of a fault towards
the stator winding star point more difficult. Except for
special applications, such as marine, LV generators are
normally solidly earthed to comply with safety
requirements. Where a step-up transformer is applied,
Industrial plant
main busbar
•
17
•
Plant feeders - total
demand: xMW
PCC: Point of Common Coupling
When plant generator is running:
If y>x, Plant may export to Utility across PCC
If x>y, Plant max demand from Utility is reduced
Figure 17.2: Embedded generation
A modern generating unit is a complex system
comprising the generator stator winding, associated
transformer and unit transformer (if present), the rotor
with its field winding and excitation system, and the
prime mover with its associated auxiliaries. Faults of
many kinds can occur within this system for which
diverse forms of electrical and mechanical protection are
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Chap17-280-315 17/06/02 10:44 Page 283
the generator and the lower voltage winding of the
transformer can be treated as an isolated system that is
not influenced by the earthing requirements of the
power system.
sufficient that the transformer be designed to have a
primary winding knee-point e.m.f. equal to 1.3 times the
generator rated line voltage.
17.3 STATOR WINDING FAULTS
Failure of the stator windings or connection insulation
can result in severe damage to the windings and stator
core. The extent of the damage will depend on the
magnitude and duration of the fault current.
(a) Direct earthing
Typical setting
(% of earthing
resistor rating)
10
17.3.1 Earth Faults
The most probable mode of insulation failure is phase to
earth. Use of an earthing impedance limits the earth
fault current and hence stator damage.
An earth fault involving the stator core results in burning
of the iron at the point of fault and welds laminations
together. Replacement of the faulty conductor may not
be a very serious matter (dependent on set
rating/voltage/construction) but the damage to the core
cannot be ignored, since the welding of laminations may
result in local overheating. The damaged area can
sometimes be repaired, but if severe damage has
occurred, a partial core rebuild will be necessary. A
flashover is more likely to occur in the end-winding
region, where electrical stresses are highest. The
resultant forces on the conductors would be very large
and they may result in extensive damage, requiring the
partial or total rewinding of the generator. Apart from
burning the core, the greatest danger arising from failure
to quickly deal with a fault is fire. A large portion of the
insulating material is inflammable, and in the case of an
air-cooled machine, the forced ventilation can quickly
cause an arc flame to spread around the winding. Fire
will not occur in a hydrogen-cooled machine, provided
the stator system remains sealed. In any case, the length
of an outage may be considerable, resulting in major
financial impact from loss of generation revenue and/or
import of additional energy.
I>>
I>
5
(b) Resistance earthing
Loading
resistor
U>
(c) Distribution transformer earthing
with overvoltage relay.
Loading
resistor
I>
(d) Distribution transformer earthing
with overcurrent relay
Figure 17.3: Methods of generator earthing
An earthing transformer or a series impedance can be
used as the impedance. If an earthing transformer is
used, the continuous rating is usually in the range 5-
250kVA. The secondary winding is loaded with a resistor
of a value which, when referred through the transformer
turns ratio, will pass the chosen short-time earth-fault
current. This is typically in the range of 5-20A. The
resistor prevents the production of high transient
overvoltages in the event of an arcing earth fault, which
it does by discharging the bound charge in the circuit
capacitance. For this reason, the resistive component of
fault current should not be less than the residual
capacitance current. This is the basis of the design, and
in practice values of between 3-5
I
co
are used.
It is important that the earthing transformer never
becomes saturated; otherwise a very undesirable
condition of ferroresonance may occur. The normal rise
of the generated voltage above the rated value caused by
a sudden loss of load or by field forcing must be
considered, as well as flux doubling in the transformer
due to the point-on-wave of voltage application. It is
•
17
•
17.3.2 Phase-Phase Faults
Phase-phase faults clear of earth are less common; they
may occur on the end portion of stator coils or in the
slots if the winding involves two coil sides in the same
slot. In the latter case, the fault will involve earth in a
very short time. Phase fault current is not limited by the
method of earthing the neutral point.
17.3.3 Interturn Faults
Interturn faults are rare, but a significant fault-loop
current can arise where such a fault does occur.
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Conventional generator protection systems would be
blind to an interturn fault, but the extra cost and
complication of providing detection of a purely interturn
fault is not usually justified. In this case, an interturn
fault must develop into an earth fault before it can be
cleared. An exception may be where a machine has an
abnormally complicated or multiple winding
arrangement, where the probability of an interturn fault
might be increased.
calculation, after measurement of the individual CT
secondary currents. In such relay designs, there is full
galvanic separation of the neutral-tail and terminal CT
secondary circuits, as indicated in Figure 17.5(a). This is
not the case for the application of high-impedance
differential protection. This difference can impose some
special relay design requirements to achieve stability for
biased differential protection in some applications.
17.5.1 Biased Differential Protection
The relay connections for this form of protection are
shown in Figure 17.5(a) and a typical bias characteristic
is shown in Figure 17.5(b). The differential current
threshold setting
I
s1
can be set as low as 5% of rated
generator current, to provide protection for as much of
the winding as possible. The bias slope break-point
threshold setting
I
s2
would typically be set to a value
above generator rated current, say 120%, to achieve
external fault stability in the event of transient
asymmetric CT saturation. Bias slope
K
2
setting would
typically be set at 150%.
17.4 STATOR WINDING PROTECTION
To respond quickly to a phase fault with damaging heavy
current, sensitive, high-speed differential protection is
normally applied to generators rated in excess of 1MVA.
For large generating units, fast fault clearance will also
maintain stability of the main power system. The zone
of differential protection can be extended to include an
associated step-up transformer. For smaller generators,
IDMT/instantaneous overcurrent protection is usually the
only phase fault protection applied. Sections 17.5-17.8
detail the various methods that are available for stator
winding protection.
I
1
II
17.5 DIFFERENTIAL PROTECTION OF DIRECT
CONNECTED GENERATORS
The theory of circulating current differential protection is
discussed fully in Section 10.4.
Stator
A
(a): Relay connections for biased differential protection
B
C
I
difff
I
=
I
1
+
II
Operate
KK
Restrain
K
1
I
S1
I
•
17
•
III >
III >
III >
I
S2
I
I
1
+
2
I
BIAS
I
=
(b) Biased differential operating characteristic
Figure 17.4: Stator differential protection
Figure 17.5: Typical generator biased
differential protection
High-speed phase fault protection is provided, by use of
the connections shown in Figure 17.4. This depicts the
derivation of differential current through CT secondary
circuit connections. This protection may also offer earth
fault protection for some moderate impedance-earthed
applications. Either biased differential or high
impedance differential techniques can be applied. A
subtle difference with modern, biased, numerical
generator protection relays is that they usually derive the
differential currents and biasing currents by algorithmic
17.5.2 High Impedance Differential Protection
This differs from biased differential protection by the
manner in which relay stability is achieved for external
faults and by the fact that the differential current must
be attained through the electrical connections of CT
secondary circuits. If the impedance of each relay in
Figure 17.4 is high, the event of one CT becoming
saturated by the through fault current (leading to a
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