creating the perfect wave PDI W a v eStar ™ Static T r ansfer Switch LOW INRUSH TRANSFER UTILIZING THE PDI VOLT SECOND SYNCHRONIZATION (VSS) METHOD Introduction One of the limiting factors in applying static transfer switch- es (STS) in mission critical facilities has been the inability of the electrical infrastructures to withstand the transformer inrush when switching occurs on the primary (or 480 volt) side of the transformer. Inrush currents can reach as high as 10-12 times the transformer rating, causing breakers and molded case switches in the STS (or devices upstream) to trip, creating an outage in the facility. PDI’s patented Volt Second Synchronization (VSS) transfer algorithm controls magnetic inrush current in transformer loads and limits it to 1.5 times the rated current. The transfer outage time and the waveform distortions during transfers have an effect on non-linear loads that are connected to the secondary side of the transformer. Short transfer outage times and waveform peak conservation algorithms are nec- essary for any STSs that feed non-linear loads. Typically, non-linear loads, servers, and other computers use Switch Mode Power Supplies (SMPS) which generate non-linear load currents. The SMPS receives AC power from the transformer and generates DC power for the internal logic. SMPS only draw power from the peaks of the input power waveforms; RMS values are associated with linear loads and evaluations based on linear RMS loads will not provide accurate data for evaluation of SMPS applications. To conserve all voltage waveform peaks, the transformer outage time of each phase must be short enough to assure that no peaks are lost. Since SMPS generally use single phase power, each individual phase must maintain power at the waveform peaks. There are four methods of achieving low transformer saturation (inrush) currents when the transformer is connected to the load bus of an STS. These methods are as follows: • Phase Delay Method (referred to as phase displacement method) • Volt Second Balance Method • Volt Second Wave Shaping Method (with RMS conservation) • PDI Volt Second Synchronization (VSS) Method (with Peak conservation) Phase Delay Method This method measures the half cycle phase delay from the time of the disconnect from one source to the reconnection of the other source with approximately the same half cycle delay; this will maintain volt second continuity. When transferring from one source to the other when the connecting source lags the disconnect source, there is a direct relationship between the size of the phase shift and the length of the outage. • If the source phase shifts are small, the transfer outage is of short duration. • If the source phase shifts are large, the transfer outage is of long duration. The transfer outage time of at least one phase can exceed 15 Ms, depending on the source phase shift and detect time. When the connecting source leads the disconnecting source, the transfer outage is large at small phase angles. This method is simple to implement but does not support SMPS very well. Volt Second Balancing Method This method determines the volt seconds applied to the magnetic load by the disconnecting source, and determines the volt seconds that will be supplied by the connecting source. The volt seconds applied to the magnetic load can not exceed the maximum rated volt seconds. This basic algorithm must be optimized to obtain the desired waveform during the transfer outage. The Volt Second Wave Shaping Method (with RMS conser- vation) and PDI Volt Second Synchronization (VSS) Method (with Peak conservation), described below, are two separate and distinct solutions to optimize the limitations of the Volt Second Balancing Method. Page 1 vss 9/06