Consulting-Specifying Engineer - March 2008 - (Page 36)

Finally, it should be mentioned that most UPS include a static bypass switch. This is a device that automatically bypasses the UPS in the event of a UPS failure and assures that power from the source will be able to get around the disabled unit. Static switches operate at approximately 4 milliseconds or one-quarter of an electrical cycle. This is sufficiently fast for an electronic load to literally not notice the interruption. When a UPS needs to use its static switch, the UPS itself is considered to have failed. The load it is protecting is one outage away from being lost. Key electrical concepts At the top of the list of rudimentary electrical theory is Ohm’s law: voltage is equal to resistance times current (V=RI). Related to this is the relationship during a generator’s transient response to a UPS load and pertaining to harmonics, namely that: Vn = n x Z1 x In where n = order of the harmonic (i.e., n = 2, 3, 4, etc. for the 2nd, 3rd, 4th, etc. harmonic order) Z 1 = impedance of the generator to fundamental frequency current Vn = nth harmonic voltage In = nth harmonic current during the first cycle of this response which is, by definition, its subtransient reactance or X”d. Given the above, the engineer may modify the relationship between individual order, voltage, and current harmonics to the following: Vn = n x X”d x In Inasmuch as a higher X”d results in a higher Vn, the aggregate effect, taking all orders of harmonics into consideration, is that as X”d increases, so too does VTHD (Statement 1). Understand that, as is the case with all harmonic-generating electronics, UPS generate current harmonics (I THD)—not VTHD harmonics. Voltage harmonics result from the interaction of current harmonics with the circuits’ impedance, which in this case is predominantly reactance. As can be understood by considering Statement 1, reactance acts as a sort of volume control for VTHD. For any given ITHD, the higher the reactance, the higher the VTHD. A UPS continually monitors the quality of the power source feeding into it. If one or more of the parameters being monitored does not fall within acceptable limits, the UPS rejects the source and goes to or remains in battery mode. Among these parameters is VTHD. The automatic transfer switch (ATS) between the generator and UPS acts more or less as a traffic cop between the two primary sources. In the scenario being explored, it sets things in motion, but once having done so, plays no further role. The harmonics issue How do these principles and equipment attributes combine to create the cyclical phenomenon under investigation? Many of the events occur instantaneously or nearly so, yet we can break them down by steps as follows: 1. Utility power is lost. Instantaneously, the UPS goes to battery power. 2. The generator receives a start signal from the ATS. Once up to full frequency and voltage, the ATS switches to generator power. 3. The UPS senses acceptable voltage and frequency from the generator and switches internally from battery power back to line (now generator) power. This is the state of things in Figure 2, which shows the schematic of a typical UPS/generator system. 4. Upon making the transition, high harmonic current from the UPS interacts with the high subtransient reactance of the generator. This results in high voltage harmonic Generator VTHD The relationship between the individual orders of voltage harmonics and the total voltage harmonic distortion (VTHD) is as follows: VTHD = SQRT[V22 + V32 + V42 + … VN2 + …] x 100/V1 Note that V1 is the fundamental (60 Hz) voltage. Because generators have negligible resistance and relatively high reactance, one can assume that for all intents and purposes, Z = X. Because we are concerned with what happens the instant a generator responds to the added load of a UPS, we are interested in the reactance Normal Current Flow ITHD UPS Sensitive Load Automatic Transfer Switch Utility Batteries Figure 2: The schematic of a typical UPS/generator system shows how the UPS senses acceptable frequency and voltage and switches to generator power. Source: Michael Shields 36 Consulting-Specifying Engineer • MARCH 2008

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Consulting-Specifying Engineer - March 2008 Contents Viewpoint Letters News M/E Roundtable Grade A HVAC When UPS and Gensets Have Issues Ignite Opportunities To Mentor Fire Protection Engineers Cooling People, Not Buildings Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space Pure Power Supplement Edison Institute Honors Recovery Efforts Canada Launches Clean Energy Software Climate Change Drives Increased Investment Critical Power for R&D Heating Calculations for Electrical Duct Bank Submetering for Energy Profiling New Products

Consulting-Specifying Engineer - March 2008

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