|
ANSI & IEC Co-operate on Protection Relay System Tests
By Nicholas Wright, Product Manager, Haefely Test, Switzerland
IEC 61000-4-12, product standards IEC 60255-1, IEC 60255-4 and ANSI C37.90 have been available for many years as parallel publications. They relate to the testing of electronic equipment to be used in a power substation environment. A revision of the ANSI standard has eliminated the differences between the IEC and ANSI Oscillating Wave Generator definitions. There are, however, still some differences in the test methodology.
Waveforms Switching in power circuits generates Damped Oscillating and Fast Transient waves. These are characterized by very fast rise time, low energy, bursts of interference, having a high repetition rate. The new ANSI C37.90 standard specifies two waveform types to be used for this application. Examples of these waveforms are shown as Figures 1 & 2. Test levels for the waveforms at all interfaces are 2.5kV Oscillatory wave and 4kV EFT/Burst.
Fig. 1 Oscillatory waveform 1MHz frequency
Fig. 2 Fast Transient 2.5kHz (as in IEC 61000-4-4)
These waveforms are separately specified in the parallel IEC product standards 60255-1(Oscillatory waves) and 60255-4 (EFT/Burst).
Coupling Any coupling network employed must be capable of injecting the test signal into the interface being tested and preventing test signals being propagated into the protection relay supply or auxiliary control interfaces.
Two coupling methods are used for both test types, Common and Transverse(Differential) modes.
Common mode coupling can be performed in BOTH cases using either a classical CDN employing inductors and capacitors as coupling and de-coupling elements, or using a capacitive coupling clamp as normally used for IEC 61000-4-4, EFT testing. This latter is possible because the test signal is always referenced to ground. The only drawback of the capacitive clamp is that it provided coupling, but no de-coupling, meaning the test impulse is transmitted into both the EUT and any auxiliary equipment being used.
Transverse mode requires a classical CDN because the test signal is applied between cables or groups of cables. Transverse mode tests are not to be applied to data communication and signal interfaces, so the standard power line CDN can be used using both the oscillatory and EFT impulses. Because of the application, this means a three phase CDN should be employed.
Test System Verification In order to ensure repeatability, ANSI C37.90 requires a verification of test system performance before AND after each test session. This is not to be confused with a system calibration which entails more detailed measurements undertaken annually or at a specific time period. The verification requires testing of:
· Oscillatory generator open circuit waveform
· EFT generator open circuit waveform
· System feed-through test
In contrast to the IEC standards, verification of waveform parameters is specified as to be performed including any coupling and decoupling networks used during a test session. Indicating clearly that the waveform has to be correct at the point where the EUT is connected (the IEC standards are unclear on this point and can be interpreted as either with or without CDNs).
For each generator a waveform verification should include measurements of:
1. Waveform rise time (EFT)
2. Rise time of first peak (Oscillatory wave)
3. Peak voltage level with no load
4. Impulse duration (EFT)
5. Waveform decay (Oscillatory wave)
The waveforms should be recorded and included with the test report.
According to Annex A of ANSI C37.90, the calibrations are performed as a common mode measurement only as shown in figures 3 & 4.
Fig. 3 Oscillatory Waveform verification
Fig. 4 EFT Waveform verification
To avoid interference signals appearing due to extraneous pick-up, the feedthrough test must be conducted with the shortest cable possible between the generator ground terminal and measurement system input.
Test Procedure Test impulses should be applied to the EUT at all relevant ports using the appropriate coupling device (CDN or Capacitive clamp). The EUT should be, as far as possible, under usual operating conditions, that means connected to any necessary power supplies and auxiliary equipment needed to exercise a particular interface.
Smaller units may be tested as so called “Table-top” equipment in a similar manner to the EFT test as described in IEC 61000-4-4. Larger units are tested on the floor. In both cases, a Ground Reference Plane (GRP) must be used which extends at least 0.1m beyond the EUT. In similar fashion to the EFT test requirements, the EUT is insulated from the GRP. For “Table-top” equipment the insulation must be 0.5mm and for “Floor-standing” equipment 10cm.
A Table-top test set-up is shown in figure 5.
Fig. 5 ANSI C37.90 Test set-up for power lines
Conclusion The changes in ANSI C37.90 have suddenly propelled this type of testing into the foreground. Apart from rectifying errors that have made the standard unworkable for many years, the new changes are also a step towards greater cooperation between ANSI and IEC standard committees.
This article is a bare bones introduction to the ANSI standard for manufacturers. There are of course many different interface types that could be employed to transmit data from the remote relay back to a control center. In the spirit of this standard, any interface employing metallic connections is to be included within the scope of test. For example an Ethernet or telephone interface would require testing as part of the EUT, ideally with data transmission taking place. Fortunately, such tests are required only in Common mode so the capacitive coupling clamp could be used. The alternative to a capacitive coupling clamp is a specialist wide bandwidth CDN capable of operating at the high data rates experienced in a modern communications system.
For further information contact: Alex Straumann, tel: +41 61 373 4538, email: straumann.alex@haefely.com
In the UK: Roland Brunisholz, tel: +44 (0)1462 486866, email: htt-emc@dial.pipex.com
|
