HIGH-POTENTIAL (HIPOT) TEST REQUIREMENTS FOR EXCITATION SYSTEMS FOR SYNCHRONOUS MACHINES VOLTAGE STANDARDS

Exciter power circuit

The exciter power circuit includes all components not electrically isolated from the exciter output. For static exciters, this includes the rectifier and thyristor circuits, transformer windings, line filters, shaft current suppressors, and any auxiliary components connected to either the input or output of the rectifier/thyristor bridge.

For rotating exciters, it includes armature windings, commutators, and brushes. For rotating exciter rated outputs 350 V dc or less, the ac rms test voltage shall be 10 times the rated output voltage of the exciter, but with a minimum of 1500 V.

For static exciter rated outputs 350 V dc or less, the ac rms test voltage shall be the greater of 10 times the rated output voltage of the exciter, but with a minimum of 1500 V, or twice the rated ac rms input voltage of the exciter plus 1000 V.

For rotating exciter rated outputs greater than 350 V dc, the ac rms test voltage shall be 2800 V plus twice the rated output voltage of the exciter. For static exciter rated outputs greater than 350 V dc, the ac rms test voltage shall be the greater of 2800 V plus twice the rated output voltage of the exciter, or twice the rated ac rms input voltage of the exciter plus 1000 V.

The synchronous-machine field winding is not included as it is covered by ANSI C50.10-1990. The exciter rated output voltage (for determination of the test voltage) shall not be less than the voltage

required at the associated generator field terminals when the generator is operated at rated kilovolt amperes, rated power factor, and rated voltage with the generator field winding at

- 75°C for field windings designed to operate at rating with a temperature rise of 60°C or less, or

- 100°C for field windings designed to operate at rating with a temperature rise greater than 60° C.

The exciter rated input voltage shall not be less than the voltage at the exciter input terminals when the generator is operated at rated kilovolt-amperes, rated power factor, and rated voltage with the generator field winding at

- 75°C for field windings designed to operate at rating with a temperature rise of 60°C or less, or

- 100° C for field windings designed to operate at rating with a temperature rise greater than 60° C.

All other circuits (electrically isolated from the exciter power circuit)

For circuits rated above 60 V or above 60 VA and not greater than 600 V, the ac rms test voltage shall be 1000 V plus twice the rated voltage. For circuits rated above 600 V, the ac rms test voltage shall be 2000 V plus 2.25 times the rated voltage.

Circuits rated at 60 V or less and 60 VA or less need not be given a high-potential test.

TESTS FOR SHORT-CIRCUITED FIELD TURNS AND POLARITY TEST FOR FIELD INSULATION OF SYNCHRONOUS GENERATORS

The purpose of these tests is to check for field-coil short-circuited turns, for number of turns/coil, or for short-circuit conductor size. Besides tests at standstill, a test at rated speed is required, as short circuited turns may occur at various speeds.

There are DC and alternating current (AC) voltage tests for the scope. The DC or AC voltage drop across each field coil is measured.

A more than +2% difference between the coil voltage drop indicates possible short-circuits in the respective coils. The method is adequate for salient-pole rotors.

For cylindrical rotors, the DC field-winding resistance is measured and compared with values from previous tests. A smaller resistance indicates that short-circuited turns may be present.

Also, a short-circuited coil with a U-shaped core may be placed to bridge one coil slot. The U-shaped core coil is placed successively on all rotor slots.

The field-winding voltage or the impedance of the winding voltage or the impedance of the exciting coil decreases in case there are some short-circuited turns in the respective field coil.

Alternatively, a Hall flux probe may be moved in the airgap from pole to pole and measures the flux density value and polarity at standstill, with the field coil DC fed at 5 to 10% of rated current value.

If the flux density amplitude is higher or smaller than that for the neighboring poles, some field coil turns are short-circuited (or the airgap is larger) for the corresponding rotor pole.

If the flux density does not switch polarity regularly (after each pole), the field coil connections are not correct.

COUPLING TO TEST GENERATORS BASIC TUTORIALS

Application of the test wave to the relay under test shall be done using coupling/isolating networks or capacitive coupling clamps as described following.

Coupling/isolating networks

This method shall be used when testing current, voltage, power supply, input circuit, or output external connection groups.

Capacitive coupling clamp

The capacitive coupling clamp is intended as a means for coupling test waveforms to shielded and other circuits where direct galvanic connection to the equipment under test (EUT) terminals may impair operation.

Typical use of the clamp for common mode tests is shown in Figure 7. The use of the capacitive clamp shall be limited.

The capacitive clamp consists of two conductive surfaces (made of galvanized steel, brass, copper, or aluminum) insulated from each other by means of high quality insulating supports.

The coupling effect is established by placing an insulated cable (feeding the signals to the device under test) through the V shaped duct on top of the clamp, and subsequently closing the clamp as much as possible in order to maximize the coupling capacitance between the clamp and the cable.

The lower conductive surface of the clamp shall be placed on (and solidly connected to) a ground plane with a minimum area of 1 m2. The reference ground plane shall extend beyond the clamp by at least 0.1 m on all sides.

The high voltage coaxial connector intended for connection to the test generator may, for convenience, be provided on both ends of the clamp. Regardless of the design however, during the test, the generator shall be connected to that end of the clamp that is nearest to the device under test.

Due to high voltages involved, the external insulation on the cable to which the transients are being coupled must be capable of withstanding a 5 kV 1.2/50 μs pulse.

VERIFICATION OF TEST GENERATOR CHARACTERISTICS BASIC INFORMATION

It is necessary to establish a simple test procedure intended to verify and document the SWC generator performance prior to its use for these tests. Two verification tests shall be defined:

a) SWC waveform validity tests

b) SWC test generator performance verification

Waveform validity tests

The waveform validity tests shall be performed before and after each SWC test session, with the results recorded and included with the SWC test report. The tests described are intended to verify that there is no major insulation breakdown, damage, or test system component failure that may have occurred during the physical test setup, test application, or prolonged instrument storage.

The waveform validity tests shall include

a) Measuring system feedthrough test

b) Oscillatory SWC test generator open circuit voltage waveform test

c) Fast transient SWC test generator open circuit voltage waveform test

Measurement methods different from those described in the Annex A are acceptable, as long as the same level of precision and the same parameters are used to document the results.

SWC test generator performance verification

A full set of SWC test generator characteristics shall be verified as per manufacturer’s recommendations or at least once every year, with the test results dated, recorded, and included with each SWC test report. These tests are intended to precisely document the test generator characteristics and are used to track the SWC generator’s compliance with this standard.

The characteristics to be verified are given in Table 1.

Table 1—Test generator characteristics to be verified

Fast transient Oscillatory waveform

Rise time Rise time of the first peak

Peak voltage level (no load) Peak voltage level (no load)

Output impedance Output impedance

Impulse duration Waveform envelope decay

n/a Oscillation frequency

Repetition rate during the burst Repetition rate

Burst duration n/a

Burst period n/a

Test duration Test duration

TESTING OF AC GENERATORS BASIC AND TUTORIALS

Tests are performed on generators to establish conformance with projected performance and dynamic performance parameters. Details of such tests are contained in IEEE Standard 115, IEC 60034-2 and IEC 60034-4 standards.

Resistance
Field and armature resistances are typically small, so measurements should be made using a 4-wire technique. It is important that resistance be measured at a known temperature so that correction can be made to actual operating temperature.

Open-Circuit Saturation Curve
The generator is driven by a motor to rated speed and excitation varied to produce terminal voltage over a range, typically from perhaps 30% to 120% that of rated. Some caution is required here, particularly for large machines in which excessive flux can damage the core. Open-circuit losses may be established by this test if the drive motor is well characterized and input power is measured.

Short-Circuit Saturation Curve
This test is similar to the open-circuit test, except the armature terminals are short-circuited and excitation varied to produce armature current over some convenient range. Windage and friction losses may be inferred from power input at zero excitation.

Stray load loss may be estimated as the difference between input power at rated armature current and the sum of friction and windage and armature I2R.

Zero Power Factor Saturation Curve
For a relatively small generator, the zero power factor saturation curve can be determined by running the machine with its shaft unloaded, driven by a second generator. By adjusting the excitation on the ac generator under test and excitation on the second generator, it is possible to measure the zero power factor saturation curve.

Rather extensive discussion of this method is described in IEEE 115. For large generators for which this “back-to-back” method is not practical, the zero power factor curve is usually determined by numerical methods. Often those methods employ finite elements.

Deceleration
Deceleration may be used for determining losses if the inertia of the machine is known. Since, if the shaft of a machine is unloaded, power dissipated is

where wm is mechanical speed, deceleration through synchronous speed can give a good measure of dissipation. The test may be run with the machine operating either at open-circuit or short-circuit conditions, or at zero excitation. It is usually run from a slight overspeed.

This test can be used to determine an unknown inertia from known losses and observed deceleration.

Heat Runs
These are tests performed by operating the generator at some condition until the temperature stabilizes. Heat runs at open-circuit, short-circuit, and zero power factor may be combined to estimate temperature rise in actual operation.

In large machines, good estimates of dissipation may be made by measuring the temperature rise of coolant (e.g., water). This is an alternative or supplement to measuring input power to the drive motor or machine deceleration. wm