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Wye, Too (not to be confused with Y2K) Richard P. Bingham, Dranetz-BMI In the previous article, we reviewed some of the differences between a wye and delta wiring configuration. Now we'll take a look at the effect of the monitored values, depending on what side of the transformer you are connected to, as well as some other effects from different configurations. The most common type of power quality disturbance is the sag, a temporary reduction in the voltage, with a duration ranging from one cycle to a minute. On the electrical distribution system, a common cause of this is single line to ground (SLTG) faults. This means that there is an unintentional connection between one of the phase conductors and earth. It may be a low or high impedance path, but the result is that excessive current is flowing in the conductor. This current produces a larger voltage drop in the wiring, which leaves less voltage remaining for the load, hence, the sag. Monitoring is often done on the secondary side of the transformers where the voltage levels are within the input range of the monitor without having to add extra potential transformers. Therefore, the data that will be recorded will be affected by the transformer type, as shown in the table below. TRANSFORMER SECONDARY VOLTAGES (pu or per unit) WITH Single-Line-to-Ground FAULTS TRANSFORMER CONNECTION Phase-to-Phase Phase-to-Neutral (primary - secondary) Va Vb Vc Van Vbn Vcn Grounded wye - grounded wye 0.58 1.00 0.58 0.0 1.0 1.0 Grounded wye - ungrounded wye 0.58 1.00 0.58 0.0 1.0 1.0 Ungrounded wye - ungrounded wye 0.58 1.00 0.58 0.33 0.88 0.88 Ungrounded wye - grounded wye 0.58 1.00 0.58 0.33 0.88 0.88 Delta - delta 0.58 1.00 0.58 ---------------Ungrounded wye - delta 0.33 0.88 0.88 ---------------Grounded wye - delta 1.00 0.5 0.5 ---------------Delta - grounded wye 0.88 0.88 0.33 0.58 1.00 0.58 Delta - ungrounded wye 0.88 0.88 0.33 0.58 1.00 0.58 Table 4 from an IEEE paper on "The Impact of Voltage Sags of Industrial Plant Loads" by Charles Smith, Jeff Lamoree, Paul Vinett, Tom Duffy, and Mike Klein. The table shows what the transformer secondary voltages would be with SLTG faults and the tradeoffs on the impact to the phase voltages that occur, based on the wiring configuration of the transformer. For example, where there are wye-wye or delta-delta connections, two phase-tophase voltages will drop to 58% of nominal, while the other phase-to-phase is unaffected. However, for delta-wye and wye-delta connections, one phase-tophase voltage will be as low as 33% of nominal, while the other two voltages will be 88% of nominal. It is the circulating fault current in the delta secondary Richard P. Bingham is the manager of technology and products for Dranetz-BMI. © copyright January 2000 Electrical Contractor Magazine www.ecmag.com windings that results in a voltage on each winding. An example of this can be seen in the below figure, where the faulted phase (C) is approxinately. 33% of its pre-fault value, and the other two phases are approximately 88% . Richard P. Bingham is the manager of technology and products for Dranetz-BMI. © copyright January 2000 Electrical Contractor Magazine www.ecmag.com