This document outlines procedures for conducting an open circuit saturation test on a synchronous generator. The test establishes the relationship between armature winding voltage and excitation current under no load conditions. Key measurements taken include the open circuit saturation characteristic, direct axis synchronous reactance, short circuit ratio, and voltage regulation. The test involves gradually increasing and decreasing the field current while measuring armature voltage and other quantities. Deviations from nominal values are addressed through a nonconformity report.

1. Open Circuit saturation test
Table of Content:
1 Objective...................................................................................................
2 Scope........................................................................................................
3 Referenced Documents ............................................................................
4 Definitions and Abbreviations....................................................................
5 Preparation................................................................................................
6 Conduction of test ....................................................................................
7 Proceeding in case of deviations...............................................................
8 Copyright...................................................................................................
1 Objective
This procedure describes the open-circuit (no-load) test, which is performed to establish
the relationship between the voltage at the terminals of the open-circuited armature
winding and the excitation current.
From the measurement data of the open-circuit test the following machine
characteristics and
quantities are determined:
- open circuit saturation characteristic (with separate and/or brushless excitation)
- direct-axis synchronous reactance Xd
- short-circuit ratio Kc
- Potier reactance Xp rated voltage regulation ∆UN
Note:
During the open-circuit test also the follow ing quantities are measured (see
corresponding procedures):
- Shaft voltage (with saturation characteristic)
- Balance of phases (at rated voltage)
- Voltage wave form, harmonic analysis and THF-factor (at rated voltage)
- Residual voltage
- Sequence of phases (preferably at residual voltage)
- Constant losses for turbogenerators w ith calibrated machine method (loss curve,
included in Short Commercial Test for GE orders)
- Vibration (at rated voltage)
- Noise (at rated voltage)
2 Scope
All types of synchronous machines during final tests in workshop or at site.
3 Referenced Documents
IEC 34-4, 25*), 27
(EN 60034-4, 25, 27)
IEEE Std. 115, 4.2.5
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2. 4 Definitions and Abbreviations
None.
5 Preparation
- Set the generator in the three-phase armature winding open-circuit condition
- Install the precision instruments for the armature voltage and field current
measurement respectively.
- The voltage should be measured with potential transformers
- Check electrical clearances around generator terminals.
- For work shop tests either install temporary slip rings for separate excitation, or install a
rotating shunt with a signal transmission system for the shunt voltage drop (telemetric
system or measuring slip rings).
6 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 be conducted acc. IEC 34-4 *),
running the machine as a motor (unity power factor test) or during OC-excited
retardation.
- Supply small field current and check that instruments, excitation system and generator
are in the proper condition
- After that, gradually change field current and measure armature voltages and field
currents.
If not otherwise specified in the contract, measurement voltage steps shall be app. 10 %
UN.
- Maximum armature voltage is limited to 130 % of the rated armature voltage, however
with turbogenerators do not exceed 120 % UN.
- Readings shall be taken at increasing excitation currents, from residual to maximum
voltage, continuing with decreasing excitation currents as well
- During the OC-tests, the field current should not exceed the rated value either.
- Check the armature voltage balance in the three phases at approximately rated
voltage.
The voltages on each phase should be the same ±1 %.
- When the saturation characteristic has been measured with separate excitation, repeat
with excitation from own exciter (brushless), after the original state of the excitation
system is reestablished.
- Measure the shaft voltage
- At residual voltage level check the sequence of the voltages on the generator terminals
with phase sequence meter. Time sequence of phases must be in correspondence to
the terminal numbering and the sense of rotation of the rotor as specified in the contract.
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3. Appendix for site testing:
- When a work shop test was performed but the OC-test is repeated on site (this holds
generally for commissioning of turbogenerators), it is sufficient to measure only one (or a
few points), including approximately rated voltage, in order to check correspondence
with the originally measured characteristic and verify balance of generated voltages. For
comparison data see the original test report of the tested machine, or the type test report
of another machine and exciter of the same design. Sometimes temporary slip rings are
not installed on site, so field current cannot be measured directly. Only a rough check of
the excitation current will be possible by measuring the field current of the exciter
machine.
- Excitation of the generator is from its own exciter (brushless) or from an external
(static) excitation system as appropriate.
- Secondary voltages are taken from the potential transformers installed in the plant.
7 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.
8 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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