Power transformer operation at over and underexcitation, benefits and consequences

One of the most important parameters in the function of a power transformer is it’s voltage transformation, or turn ratio. Optimum use demands that it’s turn ratio be changed to accommodate varying system needs. On a long term basis the power transmission voltage on a system may change due to changes in power supply, transmission system equipment, and load. The DETC (deenergized tap changer) is used for such eventualities. In practice DETC can also be used to compensate for regulation when load is increased to some new constant value in case of a discrete predictable change in load and where de-energization to change the tap setting is possible. LTC (load tap changer), on the other hand, is for maintaining a constant voltage at the power consumers equipment on a daily basis by compensating for regulation as load fluctuates with time of day. LTC can also be used to adjust the transformer turn ratio for system voltage changes as well as for regulation, and indeed, for both simultaneously. The advantages and disadvantages of the resulting variable volts per turn or overexcitation operation on the performance characteristics of the transformer are covered. Methods of estimating the changes in transformer characteristics with varying excitation are outlined. Proposals for showing supplemental nameplate information to help the transformer operator evaluate overexcitation options are offered. The reason some transformers might be operable at higher overexcitation than allowed by ANSI/IEEE C57.12.00 [1] are given. A discussion of the effects of operation at underexcitation is also presented. The possible need for an ANSI/IEEE standard document to supplement the present coverage on the subject of overexcitation of power transformers perhaps in the form of a guide, is addressed.