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Automatic Voltage Control of OLTC Power Transformers between Substations

Requirements of the voltage regulator operation for power transformer OLTCs are completely different when we compare them during power system disturbances and during the normal operation. Inadequate functioning during disturbances can result in transmission system voltage breakdowns. That is evidenced by disorders caused by loosing of power generating units, transmission lines or by an increasing demand for the reactive power resulting in long voltage recovery times. Such disturbances have been happening all over the world during recent decades and they clearly indicate the need for a different operating mode of voltage regulators. Furthermore, the goal is to achieve a voltage regulator functioning that will be valid during abnormal operation as well as during disturbances. This paper addresses the problem of common voltage regulators under abnormal conditions and proposes certain remedy countermeasures. Remedy countermeasures are described and designed in the form of an algorithm based on the communication between substations according to IEC 61850. Voltages at both sides of power transformers shall be also measured. The proposed algorithm is explained in great detail together with the communication between substations for automatic voltage regulation of power transformers. It is independent from numerical device vendors. The proposed algorithm is presented in the form of an easily applicable flow chart and it can be implemented in a distributed or centralized version of the power transformer automatic voltage control system. The developed algorithm performs corrective measures based on a predefined percentage of voltage drops at HV side of a power transformer. Compared results of the common and the proposed advanced voltage regulation of power transformers achieved by computer simulations considering different parameters of the voltage drop at the primary side of the power transformer are presented at the end. The main improvement compared to the existing common voltage regulation of OLTC power transformers is application of a secure and reliable communication between substations using GOOSE messaging in accordance with IEC 61850. The analysis is focused on the automatic voltage regulation and its effects on the voltage, the tap changer position and the amounts of active and reactive power during a given time period. Proposed countermeasures such as changing of the set up value of voltage regulation and total blocking have confirmed its flexibility, applicability and proved the expected positive effect on the conservation of voltage stability under disturbed power network conditions.

Voltage Stability and Control; OLTC; Smart Grid; Intelligent Electronic Device; IEC 61850; GOOSE messaging; SIPS

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