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White paper: Isolation in UPS systems

White paper: Isolation in UPS systems. In PW UPS, the addition of an EDCP system provides effective protection from the DC component.

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White paper: Isolation in UPS systems

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  1. White paper: Isolation in UPS systems In PW UPS, the addition of an EDCP system provides effective protection from the DC component There are three distinct types of isolation in a UPS system: the galvanic isolation between input and output, the input isolation between mains and battery, and the isolation between the dc circuit and the UPS output. It is important to understand the distinction between these types of isolation in order to avoid misinterpretation of specifications. Galvanic isolation input/output In transformer-based UPS systems the transformer is used to step up the voltage at the output of the inverter to a voltage compatible with the utility or generator supply voltage. A common misconception is that the transformer is also used to provide galvanic isolation, which is not the case. In transformer-based UPS systems, the neutral line passes through the bypass line and therefore no galvanic isolation between UPS input and output is provided. If a true and total galvanic isolation is required in transformer-based or transformerless UPS, an additional transformer is necessary at the output of the UPS, so that a galvanic isolation from the load is provided for both the inverter and bypass. Input isolation between mains and battery In the early 60s, when only open lead-acid batteries were available, galvanic isolation was required for safety reasons. Since the late 80s, when the maintenance-free lead-acid or nickel-cadmium batteries came into use, input galvanic isolation was abandoned. Today this isolation is very rare. DC-component output isolation Transformer-based technology As mentioned above, in transformer-based UPS systems the transformer is used to step up the voltage at the output of the inverter to a voltage compatible with the utility or generator supply voltage. Furthermore, the transformer isolates DC components, and therefore the inverter transformer isolates the DC circuit from the output load. Figure 1 shows a block diagram of a transformer-based, double-conversion UPS system. It can be seen that the transformer is on the output of the inverter and not on the output of the UPS. Figure 1 Transformer-based UPS There are two possibilities for the DC component to pass from the UPS to the load – when there is an inverter IGBT fault or a bypass thyristor fault. UPS 367-01-00 17/08/10

  2. Figure 2 The effect of an IGBT fault in a transformer-based UPS In the event of an inverter IGBT fault – if, for example, IGBT 2 of the inverter does not conduct – a DC component will be generated, and in the transformer-based UPS the output inverter transformer will isolate the inverter DC component from the load (Figure 2). Figure 3 The effect of a bypass thyristor fault in a transformer-based UPS In the event of a bypass thyristor fault – if, for example, one of the thyristors does not conduct – a considerable DC component will feed the load as the transformer does not isolate the bypass. The transformer-based UPS does not control this DC component (Figure 3). Transformerless technology As transformerless UPS technology (Figure 4) does not provide an inverter output transformer, the DC-component issue must be handled differently. The DC component is blocked at the output by hardware and software regulation and control so that it cannot be fed to the load. The transformerless UPS behaves as follows in the two cases. Figure 4 Transformerless UPS In the case of an inverter IGBT fault – if, for example, IGBT 2 of the inverter does not conduct – a DC component will be generated. Transformerless UPS technology handles the DC component by means of a fully-redundant EDCP (electronic dc protection) system, so that the probability of a DC component appearing at the inverter output is practically zero (Figure 5). How does the EDCP system in PW transformerless UPSs work? UPS 367-01-00 17/08/10

  3. Figure 5 The effect of an inverter IGBT fault in a transformerless UPS The PW UPS is provided with a fully-redundant EDCP system on the inverter side consisting of three parts. Firstly, redundant DC-component regulation continuously detects (double, redundant detection) and regulates (double, redundant regulation) the DC component within a tolerance of ±10mV. Note that a normal mains supply to which all non-protected equipment is exposed has a DC-component tolerance of ±300mV. Secondly, redundant DC-component control continuously detects (double, redundant detection) the DC component, and if it is higher than 4V the DC-component control circuit (double, redundant control) will automatically and instantly transfer the load to bypass. The inverter, rectifier and booster will be switched off, and the battery will be disconnected. The alarm DC-COMPONENT FAULT will appear. To make sure the DC component does not appear on the load side, the EDCP system operates at all times, even if the UPS is in LOAD-OFF mode. Bearing in mind that the DC-component detection, regulation and control circuits are redundant, this makes the EDCP system very safe and secure. Thirdly, a DC component may appear on the output if one IGBT fuse blows and the other IGBT continues to conduct. The PW inverter bridges are designed in such a way that if one of two vertical fuses (F1 or F2) blows the other fuse will also automatically blow, preventing the DC component flowing to the load. Using advanced electronic technology, the EDCP system is extremely reliable. The probability of a DC component passing through a transformerless EDCP system is no higher than the probability of a transformer going short circuit (and allowing the DC component to pass). Furthermore over 6000 PW three-phase transformerless UPS systems are operating and being protected by the EDCP system, without a single case of a DC component appearing on the load. Figure 6 The effect of a bypass thyristor fault in a transformerless UPS In the case of a bypass thyristor fault, PW UPS are provided with an additional EDCP system on the bypass side, which detects if one of the static bypass SCRs is not conducting. In this event, the load will be automatically transferred to inverter within 2 to 5ms in order to avoid a DC component on the load side. UPS 367-01-00 17/08/10

  4. High reliability In the PW transformerless UPS, the addition of the EDCP system on the bypass side provides more effective overall protection from the DC component than simply having a transformer on the inverter side. Contact Kohler Uninterruptible Power 7 Jurong Pier Road Singapore 619159 Phone: +65 6264 6422 Email: salesups.sg@kohler.com Web: www.kohlerups.com UPS 367-01-00 17/08/10

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