Conformity Magazine - March 2009 - (Page 35) Individual impulses are triggered to generate a burst packet then the bursts are triggered with a fixed repetition rate. The fast wave impedance of 50 ohm was selected to be consistent with the EFT/Burst generator as specified in IEC 61000-4-4 and ANSI C37.90. Verification of Damped Oscillatory Waves It is logical that damped oscillatory waves should be calibrated at the point of injection into the test line, ie at the CDN output. All parameters should be verified, but the most important are: rise time, oscillation frequency, decay, burst duration and burst period. Effectively, the Slow and Fast waves can be treated as two independent impulse types. They may share some characteristics, but when it comes to calibration the procedures are very different. The slow waves have to be calibrated as both common and differential mode signals. Figure 6 shows the 1 MHz slow wave calibrated in common mode and Figure 7 the same generator waveform in differential mode. Effectively, the differential generator has two outputs which deliver both positive and negative impulses simultaneously to the EUT. The fast waves are calibrated only as common mode signals. Because of the high frequency nature of these impulses, the voltage can be measured with an effective 1000 ohm load on the generator. For practical purposes and because of similarities, the same 1000 ohm load may be used as specified in IEC 61000-4-4 Edition 2 for EFT verification. The current impulse is measured using a test load of 0.1 ohm. Calibration of the 0.1 ohm load should be performed up to 400 MHz using a network analyzer as indicated in Figure 8. Coupling Decoupling Networks Damped oscillatory waves are superimposed onto power and signal lines, so some form of coupling device is required. Coupling Decoupling Networks (CDNs) provide dual functionality. They enable injection of impulses into the power or data lines without the generator being subjected to AC current levels that would cause damage to the impulse circuits. Secondly, CDNs are a defined impedance ensuring that the impulse is injected into the Equipment Under Test (EUT) and not absorbed into the very low impedance power main. Because the application is generally associated with three phase connections, any CDN must be capable of operating within the voltages available. The CDN must be capable of injecting impulses in common mode (Lx - PE, N – PE ) for both slow and fast waves and in differential mode (Lx – Lx, Lx – N) for slow waves only. As an alternative to the CDN, a capacitive coupling clamp can be used to inject fast waves when it is not essential to provide decoupling for auxiliary equipment. This method could be applicable to signal lines. The capacitive clamp from IEC 61000-4-4 is ideally suited to this application. Test Equipment Damped oscillatory waves are applicable only to products co-located in electricity sub-stations. However, as we have seen from the list of product standards, several additional test impulse are required to fully verify correct operation of protective relays. For example, the EFT is a requirement in ANSI C37.90 and also the products standard IEC 60255-22-4. Other requirements include voltage insulation tests, dip and interrupts, ESD and power line surges. The EFT is specified in all standard with 50 ohm impedance for common mode testing. There are however applications where the EFT is required for differential mode tests. In this case, the 50 ohm grounded output has to be transformed into a 200 ohm floating differential output. This can be achieved using a balun device. The ANSI C37.90 describes the construction of such a balun. A CDN for measuring relay tests should be able to handle the “slow” waves, “fast” waves and EFT as required in the product standards. Nicholas Wright is international sales manager for EMC Partner based in Switzerland, and can be reached at sales@emc-partner.ch. Thomas Revesz is the EMC Sales Manager at HV Technologies, Inc., and can be reached at revesz@hvtechnologies.com. Figure 9: Capacitive coupling clamp FAST Link www.conformity.com/2750 Figure 10: Combined slow and fast waves test system marCh 2009 Conformity 35 http://www.conformity.com/2750
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