1. Sustained three phase short circuit test
Table of Content:
1 Objective...........................................................................................................
2 Scope................................................................................................................
3 Referenced Documents....................................................................................
4 Definitions and Abbreviations...........................................................................
5 Sustained three-phase short-circuit test and overcurrent test.........................
5.1 Preparation.......................................................................................
5.2 Conduction of test.............................................................................
5.3 Appendix for site testing...................................................................
6 Proceeding in case of deviations.....................................................................
7 Copyright.........................................................................................................
1 Objective
The sustained three-phase short-circuit test is performed to establish the relationship
between the armature short-circuited winding current and the excitation current (short-
circuit saturation characteristic). It is also used to verify the capability of the machine to
withstand occasional excess current without damages. From the data of the short-circuit
test (as well as the open-circuit saturation test) the following machine characteristics and
quantities are determined: − Three-phase short-circuit saturation characteristic − Direct-
axis synchronous reactance Xd − Short-circuit ratio Kc − Potier reactance Xp Note:
Continuous operation of the generator in 3-phase short-circuit condition is performed for:
− SC-heat run of turbo generators in the works − Determination of load loss of (vertical)
HYDRO generators by calorimetric method (see P GEN QC3 6102DEFS) − Drying of
wet windings (see P GEN QC3 6111DEFS)
2 Scope
All types of synchronous machines during final tests in workshop or at site.
3 Referenced Documents
IEC 34-4, 26*), 27 (resp. EN 60034-4, 26, 27)
IEC 34-1, 8.2 (resp. EN 60034-1, 8.2).
IEC 34-3, 14 (resp. EN 60034-3, 14)
IEEE 115, 4.2.8, 4.3.15
Efficiency Determination on Hydro generators by Calorimetric Method
Insulation Resistance Testing
4 Definitions and Abbreviations
None
5 Sustained three-phase short-circuit test and overcurrent test
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2. 5.1 Preparation
- Set the generator in three-phase armature winding short-circuit condition - Install the
precision instruments for the armature and field current measurement - Unlock trip order
from protection relays
5.2 Conduction of test
− Operate the unit at rated speed with the manual control of the excitation system
Note: When a prime mover is not available, this test can (acc. IEC, *) also be conducted
during retardation
− Supply small field current, then check, whether instruments, excitation system and
generator are in the proper condition
− After that, gradually change field current and measure the armature and field currents
− Check the armature current balance in the three phases at approximately rated
current.]
− Maximum armature current is limited to 120 % of the rated armature current, with
exception of the short-time overcurrent test, which is only performed on special request
by the customer with current magnitude and duration as defined in the contract.
Note: The overcurrent test can only be conducted when a prime mover is available.
5.3 Appendix for site testing
− When a work shop test was performed and this test is repeated at site, or generally for
all turbogenerators during commissioning tests, it is sufficient to check correspondence
with the original measured characteristic by measuring one (or a few) point(s), including
approximately rated current. For comparison data see the original test report of the
tested machine, or the type test report for this type of machine and exciter respectively.
− Excitation of the generator is from its own exciter (brushless) or excitation system
(static) as appropriate.
− When the short circuit, contrary to international standards (IEC, IEEE), can only be
made on the HV side of the step-up transformer, measure also the voltage between the
generator terminals.
The generator is loaded with the short –circuit stray impedance of the transformer. To
induce the voltage that appears at the generator terminals, additional excitation current
is required. The relationship between the generator voltage and the excitation current is
(with sufficient accuracy) represented by the zero p.f. curve U=f(If), which is obtained by
sliding (the vector from the top of) the Potier (short-circuit) triangle along the open-circuit
saturation characteristic. The excitation current for shorted generator terminals is thus
simply obtained by drawing a parallel to the air gap line through the measured point of U
and If and extrapolating back to zero voltage.
This procedure is to be repeated for different armature currents. The linear dimensions
of the Potier triangle vary proportionate to the armature current. The relationship
between the armature current and the excitation current for short-circuit at the HV side of
the transformer I = f(If) is a straight line, which is flatter than the generator synchronous
impedance line. However, when also separate load losses are to be determined either
by retardation or calorimetric method, the short circuit must be realized as near as
possible to the generator main terminals, but in any case before the machine
transformer.
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3. 6 Proceeding in case of deviations
Any nominal value, minimum value, maximum value, tolerance etc given in an instruction
is intended to serve for quality control purposes only. In the case of deviations, a
nonconformity report has to be written according to PNG Power Standard
17 Copyright
2This Document is properly of PNG Power internal use only and must neither be copied.
Neither is it to be handed over nor in other way communicated to a third party.
Infringement will lead to prosecution.
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