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A3-302
Session 2004
© CIGRÉ
CONSEQUENCES ON THE VOLTAGE STRESSES IMPOSED ON STEP-UP
TRANSFORMERS DUE TO THE USE OF GENERATOR CIRCUIT-BREAKERS
DIETER BRAUN
ABB SWITZERLAND LTD.
(Switzerland)
1.
GEORG S. KOEPPL
KOEPPL POWER EXPERTS
(Switzerland)
INTRODUCTION
Today the use of generator circuit-breakers mounted between the generator and the low-voltage
terminals of the step-up transformer is widespread because this offers many advantages when
compared to the unit connection such as simplified operational procedures and better protection
against the effects of faults [1]. One of the major reasons for introducing generator switchgear into a
power station is the improved protection it provides both for the generator and the step-up transformer
against damage due to fault currents, whether they arise from short-circuit, short-time unbalanced load
or out-of-phase conditions. Specifically, the rapid and selective clearance of all kind of faults rendered
possible by the use of a generator circuit-breaker helps to avoid expensive secondary damage and the
consequently long down times for repair.
The insertion of a switching device in the connection between the generator and the step-up
transformer however exerts an influence on the type and magnitude of overvoltages that will occur. It
therefore raises the question of the possibility of overvoltages being generated by the generator circuitbreaker itself during switching operations and by its sole presence while being in the open state.
A large number of papers has been published which deal with the voltage stresses imposed on the
high-voltage terminals of step-up transformers. These voltage stresses comprise e.g. lightning surges,
very fast transients caused by the operation of disconnectors, energising transients and overvoltages
due to the switching of transformer magnetising currents. There are also publications, which deal with
overvoltages appearing at the terminals of the generator and other equipment connected to the
generator busbar. This paper specifically examines the voltage stresses occurring on the low-voltage
side of the step-up transformer in power stations equipped with generator circuit-breakers. It
differentiates between
-
Temporary overvoltages
Switching overvoltages
Transient overvoltages transferred through the step-up transformer
In most power stations the step-up transformers are protected by surge arresters fitted at or close to
their high-voltage terminals. These surge arresters may not provide in all cases a sufficient overvoltage
protection for the equipment on the low-voltage side and additional measures must be taken to protect
the generator busbar and the equipment connected to it, such as the generator, the step-up transformer
___________________________________________________________________________
Dieter.Braun@ch.abb.com
(low-voltage winding), the unit transformer(s), the excitation transformer, the voltage transformers,
the current transformers and the generator circuit-breaker. In the present paper recommendations for
an adequate overvoltage protection of this equipment are given.
2.
SURVEY OF THE CONSEQUENCES OF THE USE OF GENERATOR CIRCUITBREAKERS IN POWER STATIONS
The consequences of the use of generator circuit-breakers on the voltage stresses in power stations are
explained based on the circumstances prevailing in thermal power stations. For other types of power
stations similar considerations apply. A simplified single line diagram of a thermal power station is
shown in Figure 1.
High-Voltage
Circuit-Breaker
High-Voltage
Circuit-Breaker
Step-up
Transformer
Step-up
Transformer
∆
Generator
Circuit-Breaker
∆
Unit
Transformer
Generator
∆
∆
Shut-down
Transformer
Unit
Transformer
Generator
Unit
Auxiliaries
a)
Station
Transformer
Unit
Auxiliaries
b)
Rapid Changeover Equipment
Figure 1: Single Line Diagram of a Thermal Power Station
a) Layout with Generator Circuit-Breaker
b) Layout without Generator Circuit-Breaker (Unit Connection)
In a power station equipped with a generator circuit-breaker (Figure 1a)) the generator is switched on
and off by means of this circuit-breaker. The unit auxiliaries are supplied at all times from the unit
transformer, i.e. also during the starting-up and shutting-down of the unit. The station transformer can
therefore either be completely omitted or rated as an emergency shut-down transformer. The circuitbreaker on the high-voltage side of the step-up transformer is normally operated only in case of longer
standstill periods of the unit.
In a power station without a generator circuit-breaker (Figure 1b)) the generator is switched on and off
by means of the circuit-breaker on the high-voltage side of the step-up transformer. It is a
characteristic of this layout that the unit auxiliaries cannot be supplied from the unit transformer unless
the generator is synchronised to the high-voltage system. During the starting-up and shutting-down
periods the unit auxiliaries must therefore be transferred to an alternative source. Usually this is a
station transformer that is connected directly to the high-voltage system.
A comprehensive survey of the consequences of the use of a layout with a generator circuit-breaker in
comparison to a layout without a generator circuit-breaker (unit connection) is given in Tables 1, 2 and
3. Thereby all relevant technical aspects are considered. Specifically, an indication of the voltage
stresses arsing from the various switching operations (transient recovery voltages, switching
overvoltages) and fault conditions (temporary, switching and transient overvoltages) is given. Table 1
covers normal and exceptional operating conditions, Table 2 covers fault conditions and Table 3
covers conditions involving transient overvoltages. From Table 2 particularly the advantages of the
use of a generator circuit-breaker with respect to the protection of power plant equipment from the
effects of fault currents become evident.
Some specific aspects of the use of generator circuit-breakers are dealt with in more detail in Sections
3 (temporary overvoltages), 4 (switching overvoltages) and 5 (transient overvoltages).
Table 1: Consequences of the Use of a Generator Circuit-Breaker – Normal and Exceptional
Operating Conditions
Duty
Connection with Generator Circuit-Breaker
Generator Circuit-Breaker
High-Voltage Circuit-Breaker
Unit Connection
High-Voltage Circuit-Breaker
Normal operating conditions
1.1 Energising unloaded stepup transformer on HV-side
Not applicable
1.2 Synchronising unit with
HV-system
Relatively low voltage
stresses (compared to the
BIL) imposed on circuitbreaker before closing
1.3 Taking unit out of service
Circuit-breaker interrupts
small current (few percent of
the rated current of the
generator), TRV ≤ 1.0 pu 3)
Not applicable
1.4 De-energising unloaded
step-up transformer on HVside
Exceptional operating
conditions
2.1 De-energising unloaded
step-up transformer
2.2 Load rejection
Circuit-breaker interrupts
magnetising current, very
small switching overvoltage
(≤ 2.0 pu) 3)
Temporary overvoltage
(≤ 1.4 pu)
Circuit-breaker interrupts load
current, TRV ≤ 1.9 pu 3)
Flow of inrush current 1)
Possibly high-frequency
oscillations at HV-terminals of
step-up transformer (if circuitbreaker is located in some
distance from power station)
Possibly initiation of
ferroresonance on LV-side of
step-up transformer 2)
Not applicable
Not applicable
Circuit-breaker interrupts
magnetising current, small
switching overvoltage (≤ 2.5 pu)
Not applicable
Relatively high voltage stresses
(compared to the BIL) imposed
on circuit-breaker before
closing (especially critical for
outdoor circuit-breakers in
presence of high pollution)
Circuit-breaker interrupts small
current (few percent of the rated
current of the generator), TRV
≤ 1.0 pu
Not applicable
See 1.4
Not applicable
Not applicable
Temporary overvoltage
(≤ 1.4 pu)
Circuit-breaker interrupts load
current, TRV ≤ 1.7 pu
Notes:
1
) The magnitude of the inrush current can be reduced by the use of synchronised closing [2]
2
) Refer to Section 3.1 regarding the prevention of ferroresonance on the LV-side of the step-up transformer
3
) Applies to SF6 generator circuit-breakers only, air-blast and vacuum generator circuit-breakers may produce higher overvoltages and need
special consideration
Table 2: Consequences of the Use of a Generator Circuit-Breaker – Fault Conditions
Duty
Connection with Generator Circuit-Breaker
Generator Circuit-Breaker
High-Voltage Circuit-Breaker
Unit Connection
High-Voltage Circuit-Breaker
Circuit-breaker interrupts
system-source short-circuit
current, TRV ≤ 2.7 pu
De-excitation of generator
necessary to remove
generator-source short-circuit
current
Circuit-breaker interrupts
generator-source short-circuit
current (short-circuit current
may exhibit delayed current
zeros), TRV ≤ 2.7 pu
No action
Circuit-breaker interrupts
system-source short-circuit
current, TRV ≤ 2.4 pu
De-excitation of generator
necessary to remove generatorsource short-circuit current
Circuit-breaker interrupts
system-source short-circuit
current, TRV ≤ 2.4 pu
Circuit-breaker interrupts
system-source short-circuit
current, TRV ≤ 2.4 pu
De-excitation of generator
necessary to remove generatorsource short-circuit current
Fault conditions
3.1 Short-circuit between
generator and generator
circuit-breaker
3.2 Short-circuit between
generator circuit-breaker
and high-voltage circuitbreaker
Duty
Connection with Generator Circuit-Breaker
Generator Circuit-Breaker
High-Voltage Circuit-Breaker
Unit Connection
High-Voltage Circuit-Breaker
Circuit-breaker interrupts
generator-source short-circuit
current (short-circuit current
may exhibit delayed current
zeros), TRV ≤ 2.4 pu
No back-up available
Opening of line-side highvoltage circuit-breakers
necessary to remove systemsource short-circuit current (loss
of HV-busbar)
Voltage in healthy phases on
LV-side of step-up transformer
rises to 1.8 pu
Circuit-breaker interrupts load
current, TRV ≤ 1.2 pu
De-excitation of generator
necessary to remove fault
Not applicable
Fault conditions (continued)
3.3 Short-circuit between highvoltage circuit-breaker and
line-side high-voltage
circuit-breakers
Back-up for high-voltage
circuit-breaker
Circuit-breaker interrupts
generator-source short-circuit
current (short-circuit current
may exhibit delayed current
zeros), TRV ≤ 2.4 pu
Opening of line-side highvoltage circuit-breakers
necessary to remove systemsource short-circuit current (loss
of HV-busbar)
3.4 Single-phase-to-earth fault
between generator and
generator circuit-breaker,
generator circuit-breaker
closed (or no generator
circuit-breaker present)
Voltage in healthy phases
rises to 1.8 pu
Circuit-breaker interrupts
load current, TRV ≤ 2.4 pu 1)
De-excitation of generator
necessary to remove fault
No action
3.5 Single-phase-to-earth fault
between generator and
generator circuit-breaker,
generator circuit-breaker
open
3.6 Single-phase-to-earth fault
between generator circuitbreaker and LV-winding of
step-up transformer,
generator circuit-breaker
closed (or no generator
circuit-breaker present)
3.7 Single-phase-to-earth fault
between generator circuitbreaker and LV-winding of
step-up transformer,
generator circuit-breaker
open
3.8 Synchronising under out-ofphase conditions (up to an
out-of-phase angle of 180°)
Voltage in healthy phases
rises to 1.8 pu
De-excitation of generator
necessary to remove fault
No action
Voltage in healthy phases
rises to 1.8 pu
Circuit-breaker interrupts
magnetising current *), very
small switching overvoltage
(≤ 2.0 pu) 1)
Circuit-breaker interrupts load
current *), TRV ≤ 1.2 pu
Voltage in healthy phases
rises to 1.8 pu, possibly
intermittent earth fault
conditions
Possibly initiation of
ferroresonance 2)
Circuit-breaker interrupts outof-phase current (fault current
may exhibit delayed current
zeros 3)), TRV ≤ 4.8 pu *)
*) assuming that the HV-circuitbreaker interrupts the current
before the generator circuitbreaker
Circuit-breaker interrupts small
load current, TRV ≤ 1.2 pu
Not applicable
*) across contacts of circuitbreaker only
3.9 Stuck-pole condition
(during load current
switching)
Possibly high temporary
overvoltages (≤ 3.0 pu) 4)
De-excitation of generator
necessary to remove fault
condition
Circuit-breaker interrupts load
current, TRV ≤ 1.2 pu
3.10 Circuit-breaker failure
condition
De-excitation of generator
necessary to remove fault
condition
Circuit-breaker interrupts load
current or short-circuit current,
TRV ≤ 2.4 pu
Voltage in healthy phases on
LV-side of step-up transformer
rises to 1.8 pu
Circuit-breaker interrupts load
current, TRV ≤ 1.2 pu
De-excitation of generator
necessary to remove fault
Not applicable
Possibly extremely high
saturation of step-up
transformer, may lead to
transformer failure
Circuit-breaker interrupts out-of
phase current (critical case for
high-voltage circuit-breaker due
to delayed current zeros 3)),
TRV ≤ 4.8 pu *)
Possibly small temporary
overvoltages (≤ 1.3 pu)
Opening of line-side highvoltage circuit-breakers and deexcitation of generator
necessary to remove fault
condition (loss of HV-busbar)
Opening of line-side highvoltage circuit-breakers and deexcitation of generator
necessary to remove fault
condition (loss of HV-busbar)
Notes:
1
) Applies to SF6 generator circuit-breakers only, air-blast and vacuum generator circuit-breakers may produce higher overvoltages and need
special consideration
2
) Refer to Section 3.1 regarding the prevention of ferroresonance on the LV-side of the step-up transformer
3
) Refer to [3]
4
) Refer to Section 3.2 regarding the recognition of a stuck-pole condition
3.
TEMPORARY OVERVOLTAGES
There are a number of events, which may cause temporary overvoltages of considerable duration at
power frequency or at low-order harmonics or sub-harmonics thereof on the low-voltage side of the
Table 3: Consequences of the Use of a Generator Circuit-Breaker – Conditions Involving Transient
Overvoltages
LV-Side of Step-up
Transformer
Generator Circuit-Breaker
Generator Circuit-Breaker
Closed (Or No Generator
Open
Circuit-Breaker Present)
HV-Side of Step-up
Transformer
Lightning overvoltages
4.1 Transient overvoltages due
to shielding failures
Not applicable
Not applicable
4.2 Transient overvoltages due
to back-flashes
Not applicable
Not applicable
Not applicable
Not applicable
Transient overvoltages up to
BIL, requires adequate surge
protection on HV-side
Transient overvoltages up to
BIL, requires adequate surge
protection on HV-side
Very fast transients
5.1 Very fast transients due to
high-voltage disconnector
operation (only in case of
GIS disconnectors) 1)
Transient overvoltages ≤ 2.5 pu
with extremely high rate of
voltage rise, endanger winding
insulation of step-up
transformer
Transferred surges
6.1 Lightning overvoltages
Small transient overvoltages
Transient overvoltages below
Not applicable
transferred through step-up
(far below BIL)
BIL, may require surge
transformer
protection on LV-side
6.2 Very fast transients due to
Negligible transient
Transient overvoltages below
Not applicable
high-voltage disconnector
overvoltages (far below BIL)
BIL, may require surge
operation transferred
protection on LV-side
through step-up transformer
Notes:
1
) Can possibly be avoided by relocating the switching operations to the low-voltage side of the step-up transformer
step-up transformer. Such events comprise load rejections, single-phase-to-earth faults, non-linear
oscillations (ferroresonance) and circuit-breaker malfunctioning (stuck-pole condition). In the
following the cases of ferroresonance and of a stuck-pole condition are closer looked at.
3.1
Ferroresonance (Relaxation Oscillations)
The phenomenon of ferroresonance is caused by the periodic discharging and recharging of the phaseto-earth capacitances through saturable inductances, resulting in a cyclic displacement of the neutral
point of the system [4]. Ferroresonance can only occur in a three-phase system with an isolated neutral
in which the following conditions prevail:
-
Inductive voltage transformers connected between phase and earth in all three phases
Value of the phase-to-earth capacitances within a certain range
Kick-on event to trigger the relaxation oscillations, for example a switching operation (e.g. the
energisation of a transformer) or the self-extinction of a single-phase-to-earth fault
The overvoltages arising from the most frequent case i.e. that of the second sub-harmonic are
relatively small and not dangerous. However the saturation currents in the voltage transformers can
cause these to overheat and possibly destroy them. Based on the conditions listed above, relaxation
oscillations can only occur on the low-voltage side of the step-up transformer when the generator
circuit-breaker is open, because the part of the system comprising the low-voltage winding of the stepup transformer and the high-voltage winding of the unit transformer normally becomes a system with
an isolated neutral under these circumstances. No relaxation oscillations can occur on the generator
side of the open generator circuit-breaker or when the generator circuit-breaker is closed.
Figure 2 shows the simulation of a case of ferroresonance, which is triggered by the energisation of the
step-up transformer from the high-voltage side. A second sub-harmonic voltage develops across the
open delta winding of the voltage transformer (Figure 2a)). With the fitting of a damping inductance
device or a suitably dimensioned damping resistor across the open delta winding of the voltage
transformer the relaxation oscillations are quickly damped out, as shown in Figure 2b) [5].
a)
Figure 2:
3.2
b)
Voltage Across Open Delta Winding of Transformer Side Voltage Transformer During
Ferroresonance Condition
a) Without Damping Device
b) With Damping Inductance Device Fitted Across Open Delta Winding
Overvoltages Due to Stuck-Pole Conditions
Although being an extremely rare incident, the possibility of a malfunctioning of a circuit-breaker, in
that one phase fails to close or open, does exist. The use of a common operating mechanism for all
three phases reduces but cannot totally eliminate the possibility of such a failure.
Figure 3 shows the results of a simulation of a load rejection at 100% load combined with stuck-pole
condition in one phase of the generator circuit-breaker. After approximately 0.5 seconds a phase
opposition condition is reached and the phase-to-earth voltages at the low-voltage terminals of the
step-up transformer approach 3.0 pu in two phases. At the same time the voltage across the open
a)
Figure 3:
b)
Simulation of a Load Rejection at 100 % Load with a Stuck Pole in Phase R
a) Phase-to-Earth Voltage (Phase T) at Generator Terminals
b) Phase-to-Earth Voltage (Phase T) at Low-Voltage Terminals of Step-up Transformer
contacts of the circuit-breaker rises to approximately 3.8 pu. In practice these values will not be
attained due to the action of the voltage regulator. Further, if a stuck-pole condition occurs when the
loading of the generator is low then the time to reach the maximum voltage is much longer.
An even more critical condition can arise if a single-phase-to-earth fault develops in one of the phases
where the generator circuit-breaker has opened. Surge arresters connected to the generator busbar will
operate under such circumstances and could become thermally overloaded.
Such a fault involves the risk of equipment being damaged and is to be avoided. A simple measure to
recognise a stuck-pole condition that can easily be implemented into the generator protection system is
described in [5].
4.
SWITCHING OVERVOLTAGES
Switching overvoltages can arise during the switching of an unloaded step-up transformer, the
switching of load currents, the switching of fault currents and also during the initiation of singlephase-to earth faults. In the following the cases of the switching transformer magnetising currents and
of the switching of load currents are dealt with.
4.1
Overvoltages Following the Interruption of Transformer Magnetising Currents
Under normal operating conditions the unloaded step-up transformer is de-energised via the highvoltage circuit-breaker and it is only under exceptional circumstances that a generator circuit-breaker
is required to interrupt a transformer magnetizing current. Experience shows that SF6 circuit-breakers
when performing this duty generate negligible overvoltages [6]. Site tests carried out with a SF6
generator circuit-breaker confirm this statement. During 15 interruptions of the magnetising current of
a 240 MVA step-up transformer the overvoltage factor did not exceed 1.0 pu (Figure 4).
Figure 4:
4.2
Oscillogram of the Interruption of the Magnetising Current of a 242/13.8 kV 240 MVA
Step-up Transformer with a SF6 Generator Circuit-Breaker in a Hydro Power Station
Overvoltages Following the Interruption of Load Currents
Independent of the actual power factor, the transient recovery voltage resulting from the interruption
of a load current in a power station is identical to that arising in a highly inductive circuit [7]. It is well
known that vacuum circuit-breakers can initiate multiple reignitions in such circuits. Depending on the
system parameters and the characteristics of the switch, a vacuum circuit-breaker may therefore give
rise to multiple reignitions when interrupting the load current of a generator [8]. Although the
probability of the initiation of this phenomenon is low it cannot be ignored due to high phase-to-earth
voltages associated with it. The use of a vacuum circuit-breaker as a generator circuit-breaker
therefore requires an additional and carefully designed overvoltage protection scheme to protect both
the generator and the step-up transformer. SF6 generator circuit-breakers on the other hand do not give
rise to this phenomenon.
5.
TRANSIENT OVERVOLTAGES TRANSFERRED THROUGH THE STEP-UP
TRANSFORMER
Transient overvoltages hitting the high-voltage terminals of a step-up transformer are transferred both
capacitively and inductively to the low-voltage side [5]. The initial portion of voltage surges with very
steep wave fronts and high amplitudes is thereby transferred via capacitive coupling. During the time
when the generator circuit-breaker is open the large phase-to-earth capacitance of the generator stator
winding is removed from the capacitor divider formed by the capacitance between the high-voltage
and low-voltage windings of the step-up transformer and the phase-to-earth capacitances on the lowvoltage side. The highest transferred surges will therefore appear under these circumstances.
6.
RECOMMENDATIONS FOR OVERVOLTAGE PROTECTION
From Tables 1 and 2 follows that the voltage stresses which are generated by SF6 generator circuitbreakers during switching operations (transient recovery voltages, switching overvoltages) remain far
below the insulation level of the equipment connected to the generator busbar and therefore are of no
concern. Air-blast and vacuum generator circuit-breakers may produce higher overvoltages and need
special considerations.
A switching device in the connection between the generator and the step-up transformer while being in
the open state exerts a negative influence on the amplitude and steepness of transient overvoltages
which are transferred from the high-voltage system through the step-up transformer to the low-voltage
side. To mitigate this effect the connection of surge capacitors with a capacitance of at least 100 nF to
the low-voltage side of the step-up transformer is recommended. The decision whether surge arresters
should also be fitted can in principle be made as the case arises. Because surge arresters are low cost
elements they are normally included in the overvoltage protection scheme without any further
investigations. It is however important that both the surge capacitors and the surge arresters are
connected to the generator busbar between the low-voltage terminals of the step-up transformer and
the generator circuit-breaker so that they remain effective when the circuit-breaker is open.
In power stations with generator circuit-breakers the occurrence of ferroresonance has to be prevented
by selecting properly rated voltage transformers and fitting a damping inductance device or a suitably
dimensioned damping resistor across the open delta winding of the voltage transformers on the
transformer side of the generator circuit-breaker. Finally, it is also recommendable to use an
arrangement of protective equipment that recognises a stuck-pole condition.
7.
CONCLUSIONS
The use of generator circuit-breakers offers many advantages when compared to the unit connection
such as simplified operational procedures and better protection against the effects of faults. But the
insertion of a switching device in the connection between the generator and the step-up transformer
obviously also exerts an influence on the type and magnitude of overvoltages that will occur. By
introducing the described measures the advantages of the use of a generator circuit-breaker can be
fully exploited.
8.
BIBLIOGRAPHY
[l] Braun, D.; Guerig, A: Life Management for Generator Circuit-Breakers. CIGRE 1994 Session,
Report 13-204.
[2] Braun, D.; Koeppl, G.; Azuaje, C.J.; Borges, F.: Inrush Currents of a Large Step-up Transformer
and Means for their Reduction. International Conference on Power System Transients, Rio de
Janeiro, 2001.
[3] Canay, I.M.; Braun, D.; Koeppl, G.S.: Delayed Current Zeros Due to Out-of-Phase
Synchronizing. IEEE Transactions on Energy Conversion, 13(1998)2, pp. 124-132.
[4] Bergmann, C.: Grundlegende Untersuchungen über Kippschwingungen in Drehstromnetzen.
ETZ-A 88(1967)12, pp. 292-298.
[5] Sanders, M.; Koeppl, G.; Kreuzer, J.: Insulation Co-ordination Aspects for Power Stations with
Generator Circuit-Breakers. IEEE Transactions on Power Delivery, 10(1995)3, pp. 1385-1393.
[6] CIGRE WG of Study Committee 13: Interruption of Small Inductive Currents, Chapter 5:
Switching of Unloaded Transformers, Part 1: Basic Theory and Single Phase Transformer
lnterruption without Reignitions. Electra (1990)133, pp. 87-107.
[7] Vadaszi, J.: Load Current Interruption with Generator Circuit-Breakers and Generator Load
Switches. Brown Boveri Review 68(1981)8/9, pp. 316-325.
[8] Glinkowski, M.T.; Gutierrez, M.R.; Braun, D.: Voltage Escalation and Reignition Behavior of
Vacuum Generator Circuit-Breakers During Load Shedding. IEEE Transactions on Power
Delivery, 12(1997)1, pp. 219-226.