OUTPUT SHORT-CIRCUIT CURRENT BEARABLE BY THE UPS

Transcription

1 OUTPUT SHORT-CIRCUIT CURRENT BEARABLE BY THE UPS

2 Abstract According to the standards, an overcurrent is a current which exceeds its rated value. Overcurrents can arise in different modalities, depending on whether they are the result of an overload or a short-circuit. While the former is a normal situation for an electrical circuit, the latter occurs after a negligible impedance fault between two points of the circuit where there is a voltage in ordinary operating conditions. One of the features that must be carefully evaluated when choosing a UPS is its capability to properly withstand a short circuit current on its output for a certain amount of time. This capability depends on whether the output short circuit current is withstood solely by the inverter or by the source through the static bypass. In the first case the capability strictly depends on the UPS design, while in the second it also depends on the source prospective short circuit current (named Icp). The Icp is independent of the UPS and, because of its electrodynamic and thermal effects on the components, in some cases it is appropriate to define its limits and/or to implement particular solutions which allow limiting its extent within the UPS or the entire site. However, in other cases limiting the prospective short circuit current is necessary in order to stay within the limits of the protective devices downstream the UPS. All the choices mentioned above risk negatively affecting the short circuit current that the electrical distribution downstream the UPS requires to guarantee the correct coordination between the various site protective devices. Therefore you need to take into consideration how the maximum source-icp can be mitigated by the UPS while operating in double conversion mode. The aims of this white paper are to clarify the concept of prospective short circuit current bearable by the UPS, to explain the role of fuses and to indicate the requirements of the international standard IEC

3 Devices run across by short circuit current and their sizing Throughout the current path, between the bypass input and the UPS output, there are various components and devices which are run across by short circuit current. Among these we can mention cables, switching devices, static devices (named SCR after the acronym of Silicon Controlled Rectifiers ) and possible fuses (Figure1). Their sizing indirectly fixes a limit to the maximum perspective short circuit current of the source. There are mainly two guidelines for this sizing: 1. Protect as many devices as possible while accepting to penalize the maximum output current; 2. Achieve the maximum output current possible while accepting that the fault may spread over more than one device, but still guaranteeing their insulation. Maintenance Bypass QS3 Static Switch U1, V1, W1 QS2 Rectifier Inverter U, V, W QS1 QS4 U2, V2, W2 Battery converter N Battery N Figure 1: UPS block diagram. Short-circuit current effects Short-circuit current effects are both electro-dynamic and thermal. The thermal effects consist of heat dissipation over the conductors by Joule effect and, as short-circuit currents are much higher than rated current, the component overheats and degrades. On the contrary, the electro-dynamic effects are attractant or repulsive mechanical actions between the components and are function of the square of the current peak. Depending on the prospective short-circuit current of the source (Icp), the current peak value must be evaluated according to Table 7 of the international standard CEI EN here below: Up to a Icp 10kArms, Ipk max = 1.7 x Icp Up to a Icp 20kArms, Ipk max = 2.0 x Icp Up to a Icp 50kArms, Ipk max = 2.1 x Icp Up to a Icp 100kArms, Ipk max = 2.2 x Icp 3

4 High speed fuses When there is the need for maximum device protection, it is necessary to use high speed fuses, which are capable of limiting the prospective short circuit current peak whenever it rises above a certain level. Figure 2 portrays the curve of some high speed fuses which show the feature mentioned above. For example, the 1000 A fuse does not limit the maximum current peak up to the prospective short-circuit current of 5 ka, while above 5 ka the value gradually reduces so that in case the prospective short circuit current value is 100 ka it becomes equal to 31 ka only. If a UPS with a rated current of 1000 A is paired with a 1000 A high speed fuse, the UPS itself could be compatible with a source prospective short-circuit current of 45 ka if the peak current limit of the static bypass devices is higher than 24 ka, while it could be compatible with 100 ka if the peak current limit of the static bypass devices is higher than 31 ka. If the same UPS was paired with a 1250 A high speed fuse and the maximum peak current limit was still 31 ka, it could be compatible with a source prospective shortcircuit current of about 50 ka maximum, and not 100 ka as in the case of the 1000 A fuse. The use of high speed fuses inevitably reduces the current peak and its duration (which is usually some ms long), therefore the selectivity downstream can be accomplished for the short-circuit only by using some more high speed fuses or electromechanical devices, which are also capable to open the circuit with characteristics dependent on the I 2 t value. In these two cases the current peak will be mainly limited by the high speed fuses downstream of the device. How to increase the short-circuit current As fuses are not resettable after they intervene and Service intervention to replace them is always required, a valid solution to achieve a higher short-circuit current is to increase the fuse size at the expense of SCR protection Im = 2.3 Ip 1400 A 1250 A 1100 A Max peak let trhu current (Im) (A) A 900 A 800 A 700 A 630 A 550 A 500 A e + 06 Available current in RMS symmetrical amperes (Ip)(A) Figure 2: Short-circuit current peak limitation of a typical commercial high speed fuse 4

5 In this scenario, in the rare case of a short circuit taking place right at the UPS output, both fuses and SCR will need to be replaced. For example, if a 1400 A fuse is used then the maximum current peak becomes about 45 ka while the prospective short-circuit current is approximately equal to 100 ka; it is then clear that both the peak value and the I 2 t value are increased, but both values remain much lower than the maximum current peak value with no fuses, which is approximately equal to 220 ka in the worst case. Manual bypass short-circuit current The maximum current peak value without fuses, equal to 220 ka in the example above, is the value to be considered for any possible manual bypass, be it inside or outside the equipment, as a manual bypass is not typically protected by fuses. In this scenario it is the very switch, with its typical short-circuit withstand current (called Icw), to fix the limit of the maximum prospective short circuit current: when it is lower than the static bypass short circuit current It becomes the limit for the entire installation / site. In case of systems with a prospective current higher than the switch Icw, then one of the two following solutions is needed: yadding fuses on the manual bypass; yadding an inductance in series with the source whose value may be enough to lower the maximum prospective short-circuit current to the desired value. The inductances mentioned above are typically realized in the air, in order to avoid their saturation in short-circuit conditions. Their presence upstream of the UPS can give the opportunity to reconsider the static bypass fuse sizing as well. Short-circuit current in a parallel UPS system In those situations in which there are more UPS connected in parallel, once again considering just the short-circuit-tothe-output of the parallel system, the maximum prospective short-circuit current of the source may be considered equal to about 80% of that of the single UPS multiplied by the number of UPS in the parallel system (without considering any redundancy). 80% is a safety margin which takes into consideration that there can be a short-circuit current imperfect distribution due to asymmetries in the equipment connections both at the bypass input and at the system output. These asymmetries have always to be taken into consideration when balancing the UPS static bypasses in nominal operating conditions, in order to keep them within a 10% range. If the site layout does not allow symmetry between the bypass input and UPS output connections, solutions have to be implemented in order to compensate for these deviations. In case inductances are chosen as a solution, they must not saturate in a short circuit scenario. International standard IEC The international standard IEC :2008 (EN : ) was published in 2008 under the name Uninterruptible Power Systems (UPS) - Part 1: General and Safety Requirements for UPS. In September 2011 it replaced the first editions of IEC and IEC , published in This standard applies to UPS units which are movable, stationary, fixed or for building-in, for use in low-voltage distribution systems and intended to be installed in any operator accessible area or in restricted access locations as applicable. Amendment IEC :2008/A1:2013 (Date of publication, DOP) must be taken into consideration mandatorily by law, in addition to the original standard as of 14/02/2016 (Date of withdrawal, DOW) in order to assess UPS safety conformity through harmonized standards. This amendment introduces requirements to the UPS output short-circuit withstand current. In particular, it forces the manufacturers to guarantee a minimum value depending on the UPS rating. Important final note When talking about prospective short-circuit current, considering only the source can be penalizing because the softening action carried out by the impedance of the connection between source and bypass input would be left out. The above mentioned impedance depends both on the distance between source and UPS, and, above all, on the connection type, (i.e. if this is made with a low impedance busbar or with cables). In the latter scenario, the way in which cables are arranged into the support conduit is another factor to be taken into consideration because it could cause significant impedance depending on the specific solution implemented. Therefore, the declared UPS performance has to be intended as a prospective short-circuit current available to the bypass input. 5

6 Conclusion With this whitepaper the concept of prospective short-circuit current withstand capability has been clarified for both a single UPS and a parallel system, by illustrating the role of fuses and by giving an indication about what the IEC international standard requirements are. All the topics above are important because one of the features to be carefully evaluated when choosing a UPS is its capability to bear a short circuit current to its output properly for a certain amount of time. 6

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