Circulating current paralleling failures— conditions & consequences

When an applied transformer paralleling method is inappropriate, or incorrectly set, the operation of paralleled power transformer tapchangers can result in detrimental effects on system voltages and power transfer capability. In some cases, these operations can also contribute to the premature failure of the power transformer. One of the most popular paralleling methods is the circulating current method of paralleling. There are several conditions that make the circulating current method application inappropriate or to cause it to mis-operate. These conditions include system configurations, transformer construction, or equipment ratings or settings incompatibility. The problems with system configurations can occur during normal or contingency/emergency operation of the power system. The transformer construction issues can be addressed by recognizing the variations that can cause mis-operation and either specifying a suitable transformer design or changing the paralleling method used. Many other problems can be overcome by additional auxiliary equipment or commissioning procedures. This paper discusses the effects of several system configurations and equipment variations on the operation of the circulating current method of paralleling. The effects of automatic operation of system equipment, such as breaker operations or distributed generation, will be considered. Solutions for each condition are suggested.