Voltage changes produced by single-phase loads

Abstract

MAR V I NFl SHE R, J R. ASSOCIATE AlEE~1 . S. HE L M ASSOCIATE AlEET H E PROBLEM of determining the voltage change produced by a single-phase load has become one of considerable importance to the utility engineer. One of the principal reasons for this situation is the application of single-phase resistance welding to 11lany new processes. The usual type of single-phase resistance welder requires for its operation large intermittent values of kilovoltamperes at a low power factor. This type of load "Till, of course, produce voltage fluctuations on the power system adjacent to the welder. If these variations are severe enough, they may cause objectionable lighting flicker to other customers served from the same portion of the power system. The usual range of permissible voltage variation is from 0.5 to 1.5 volts on a 120-volt base. The actual value is dependent upon the type of welder load-whether cyclic or noncyclic-and if cyclic, upon the frequency of the load application.Since such variations are objectionable, it is necessary that the power company engineer be able to determine accurately what the variation in voltage will be in order to provide adequate service at the lowest cost. In general, these problems have been handled satisfactorily with considerable ingenuity in devising schemes to keep the cost of service reasonable. However, in some cases the voltage drop measured after a welder was installed was appreciably lower than had been calculated. It was reasoned by many pO'\Ter cOlllpany engineers that this discrepancy was caused by motor loads which were adjacent to the welder and which had been neglected in calculating the voltage drop. A study of the effect of such 3-phase motor loads has been made in co-operation with the Utilities Research Commission of Chicago, Ill.The solution of this problem was accomplished by the use of symmetrical components in a manner similar to that used for unbalanced fault calculations. This was done by considering the single-phase load as a line-to-line fault having an impedance equal to that of the single-phase load. A typical system for supplying a large single-phase load such as a welder is shown in Figure 1. Equations have been developed which give the voltage changes produced at both P and F by the single-phase load. The change in voltage that is calculated by this method includes the effect of the motor loads at both P and F. The method of analysis used for the particular circuit shown in the figure can be extended to other types of circuit configurations that Blight be encountered.This method of solution has been checked by both laboratory and field tests. Field tests were conducted at Digest of paper 50-236, "Effect of 3-Phase Motor Loads on Voltage Changes Produced by Single-Phase Loads," recommended by the AlEE Committees on Electric Welding and Transmission and Distribution and approved by the AlEE Technical Program Committee for presentation at the AlEE Fall General Meeting, Oklahoma City, Okla., October 23-7, t 950. Scheduled for publication in AlEE Transaction...