Dejan Žukovski

The Kostolac A thermal power plant consists of two units A1 with a capacity of 100 MW and A2 with a capacity of 210 MW. The first synchronization of unit A1 in the TPP Kostolac A thermal power plant to the electrical power system (EES) with a voltage level of 110 kV was carried out in 1967 via a transformer unit marked 5T with an apparent power of 120 MVA. The transformer unit has four coolers with three fans and one oil pump each. Cooling is carried out by forced OFAF (oil forced, air forced). The boiler (housing) protection without a time delay protects the transformer unit from internal ground faults. In order to ensure the proper operation of the boiler protection, it is necessary that the transformer vessel is grounded at only one point via a conductor and a single-phase current transformer. This means that the wheels, coolers with fans, cooling cabinet and busbar trunking on the low-voltage side (10.5 kV) must be insulated from the vessel. Protection of the transformer unit against atmospheric and switching surges was carried out with classic surge arresters (with a spark gaps) on the 110 kV side. Differential and overcurrent protection of the transformer unit was carried out with electromechanical relays. The transformer unit was in operation for over fifty years with minor and major outages. During that period, there were poor voltage conditions in the 110 kV network, which often caused the need for higher reactive power and the generator to leave the PQ diagram and the network. In the period from 2005 to 2007, the revitalization of TPP A1 was carried out. As part of the revitalization of TPP A1, a generator with a higher apparent power of 137 MVA was installed. The increase in the apparent power of the new generator caused a current limitation on the 110 kV side of the transformer unit and increased attention of the electrical control personnel during operation. Taking into account the year of production of the transformer unit, operating conditions, overloads during operation, the appearance of gases and water dissolved in oil, the appearance of hot spots, degradation of solid insulation, reduced dielectric strength, unreliable operation of the boiler (housing) protection, a decision was made to purchase a new transformer unit of appropriate power with a load control switch (OLTC) and a built-in current transformer in the star point for restrictive ground fault protection. Along with the transformer, the latest generation microprocessor protection and zinc-oxide surge arresters with a monitoring device in operation (start-up counter and leakage current measurement) were installed. The installation of new equipment extended the unit’s operating life, increased reliability, efficiency, reduced maintenance costs, increased the number of protective functions, reduced the possibility of inadequate protection operations, and provided information for easier analysis of operating events.

Keywords: efficiency, maintenance, on-load tap changer, protection, reliability, synchronization, transformer unit