1. Sudden three phase short circuit test
Table of Content:
1 Objective..........................................................................................................
2 Scope...............................................................................................................
3 Referenced documents....................................................................................
4 Terms and abbreviations..................................................................................
5 Sudden three-phase short-circuit test...............................................................
5.1 Preparations...................................................................................................
5.2 Conduction of test..........................................................................................
5.3 Evaluation of the test......................................................................................
6 Proceeding in case of deviations ......................................................................
7 Copyright............................................................................................................
1 Objective
The sudden three-phase short-circuit test is performed to determine the following
machine quantities:
- Direct-axis transient reactance Xd'
- Direct-axis subtransient reactance Xd"
- Direct-axis transient short-circuit time constant Td'
- Direct-axis subtransient short-circuit time constant Td"
- Armature short-circuit time constant Ta.
- Evidence of mechanical strength
2 Scope
This procedure applies to all types of synchronous machines during final inspections at
site or in the workshop.
3 Referenced documents
IEC 34-4, 40-41 Rotating electrical machines – Methods for determining
Synchronous machine quantities from tests
IEEE 115, 11.7 Test Procedures for synchronous machines
EN 60034-4 Rotating electrical machines - Methods for determining
Synchronous machine quantities from tests
4 Terms and abbreviations
none
5 Sudden three-phase short-circuit test
5.1 Preparations
Visual inspections:
- End winding supports
- Condition of foundation
- Are the foundation bolts tightened?
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2. - Correct assembly and fixation of all rotor components
- Disconnect the line terminals from the terminal buses (may not be necessary in all
cases).
- Connect the power cable between the line terminals and the circuit breaker, which is
used to make the short-circuit and install measuring devices for recording the short-
circuit currents in wye-connection.
Note: Normally the plant circuit breaker will be used for this purpose. To install a
separate
short-circuit breaker in the plant requires high efforts and good coordination between the
user of the plant and the generator manufacturer, as well as considerable extra costs.
In the workshop a special short-circuit breaker with synchronized phases is used. The
short circuit itself can be made with a special bar across the terminal buses or a
disconnector installed in the current leads (e.g. braking disconnector). This is placed
after the circuit breaker and is inserted before switching in the circuit breaker. Consider
that it is designed to support the current forces and can carry the substained short circuit
current during the recording time.
-
For current transformation the following equipment may be alternatively used:
- Linearized current transformers
- Low inductance precision shunts
- Unlock protection relays as required.
- Remove the grounding cable of the neutral terminals and directly connect the common
terminal of the three-phase short-circuit wye to ground.
- Attach a voltmeter and potential transformer to the generator line terminals (if required)
to measure the armature voltage prior to the sudden short-circuit.
- Connect and set the precision instruments for measuring the terminal voltage and field
current.
Note: The following analog- or digital instruments or equivalent (at least 4 channels) may
be used for recording purposes:
- Digital storage oscilloscope (Hioki 8855)
- Memory Recorder (YEW AR 1100 A)
- Memory Recorder (YEW DL 708)
- Connect the output signals of the current transformer (CT) or equivalent to the
recorder, whereby the current signal of the CT is converted into a voltage signal required
for the digital storage oscilloscope using precision shunts.
- Calibrate and set the trigger of the recording instruments to the expected levels.
5.2 Conduction of test
- Operate the unit at rated speed in no-load condition with the manual control of the
excitation system. In case of static excitation set the ignition anlge of the gate trigger
pulse to achieve the required generator voltage. Neither a current regulator (for the field
current) nor a voltage regulator (for the generator voltage) may be switched on.
Note: If a prime mover is not available, this test can also be performed by retardation
method, initiating the sudden short-circuit, when the decelerating machine passes
through nominal speed.
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3. - Supply small field current and then initiate a trial run of the sudden short-circuit test.
Verify that recorders are triggered and all signals are recorded properly.
- Adjust the excitation to the desired value of the terminal voltage.
Note: The usual test voltage is approx. 0.5 Un. The test may be repeated at various open
circuit voltages as required by the contract. For voltages exceeding this limit value,
explicit agreement of the turbine manufacturer should be reached. Through the machine
set must withstand the short-circuit forces, there is a high risk of damages to the
foundation (especially in case of bulb-type generators). Therefore it is not recommended
to perform a 100% Un sudden short-circuit directly at all stator terminals. Consider that
machine life endurance may be reduced.
- Read off the terminal voltage, frequency and field current immediately before
performing the sudden short circuit test. If still free channels are available on the digital
storage oscilloscope, at least the phase-to-phase voltage of the generator should be
recorded by oscilloscope.
- Close the circuit breaker in order to initiate the sudden short-circuits. Recording
instruments will be triggered and the three armature currents, the field current and the
voltage going to zero should be recorded continuously.
- After approx. 10 sec. record the armature currents manually, then switch off the
excitation and open the circuit breaker.
- Check the recordings in order to ensure that the data was registrated correctly.
Performed by: GP-QS1
5.3 Evaluation of the test
- Mathematical evaluation acc. to IEC-standard.
Note: Since the sudden short-circuit test is mainly performed with reduced voltages, in
general only unsaturated values for the reactance’s can be determined. Saturated
values should be estimated from the calculation values.
- The evaluation of analog short-circuit current characteristics can only be done by hand.
- Digital storage data may be computer evaluated, using a special program for
separating the envelope of the current waveform into 2 exponential functions.
- If a 100% Un sudden short-circuit is performed at the generator terminals (only after
agreement with the customer), despite of the prohibitive aspects mentioned, thorough
visual inspection for cracks, permanent deformations and any other damages shall be
made after the test.
e.g.:
- End winding support
- Condition of winding (ties, spacers …)
- Condition of foundation
- Foundation bolts
- Assembly and fixation of rotor components
- Acc. to international standards, a repetition of the voltage test, preferably 80% of test
voltage, are recommended.
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4. 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 purposes only. In case of deviations, a non-
conformity report has to be written acc. to PNG Power Standard
7 Copyright
This 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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