Haruhiko Kohyama

Influence of system transients on the residual flux of an unloaded transformer

Developments of smart grids and integration of renewable energy sources will increase the amount of switching operations of unloaded transformer systems. A random energization of an unloaded power transformer can cause inrush currents and related voltage disturbances. State of the art controller systems control these phenomena taking into account the residual flux after a transformer de-energization, which is generally assumed as a stable and “locked in” value. However, the residual flux can be influenced by system transients passing through a circuit breakers grading capacitors. Furthermore, the frequency content of system transients can also change the transformer impedance at the instant while the system transients appear. These changes determine how extent system transients influence the residual flux and therefore change the optimal re-energization instant. The paper evaluates these phenomena based on frequency response analysis and proposes a filter for an improved approximation of the residual flux level.

Field verification of controlled switching GCB

A controlled switching system (CSS) with an independent-pole-operated gas circuit breaker (GCB) was applied to a 121 kV-capacitor bank and tested in the field. Complete system performance checks have been conducted according to the CIGRE recommendations for evaluation tests. The compensation algorithm of minimizing the scatter of the switching instant as well as the practical approach of determining the optimum switching targets are proposed based on the GCB characteristics identified in the type tests. Any drifts in operating times that associated with the past operations and the idle time of the drives should be included in the compensation algorithm along with the variations of the operating times caused by the operating conditions. The drifts of the operating times caused by a number of consecutive operations can be compensated using an adaptive control, which can predict the operating time for the next operation based on several previous operations. Innovative operating mechanisms such as stable high-pressure hydraulic drives and low-friction spring drives have been developed and demonstrated to minimize the delay of the operating time after a long idle time. The proposed method of determining the optimum switching targets can be applied for the CSS and its effectiveness was demonstrated by the controlled switching tests. The paper also describes the design requirements and the results of the field performance tests in detail.

Field application of a synchronous controller based on the measurement of residual fluxes for the energization of a step-up transformer

A random energization of unloaded transformers may cause large inrush currents and overvoltages; they lead to malfunction of protection relays and possibly to mechanical damages of the transformers windings. The paper describes the reduction of those stresses by the implementation of a controller system installed in a 400kV pumped storage power plant — Transformer, circuit breaker and controller system are from independent manufacturers. Firstly, the assessment of the circuit-breaker parameters (on site) is presented. Secondly, specific operating conditions of the 400kV circuit breaker have been performed, leading to non zero residual flux values. Thirdly, several energizations of the step-up transformer have been performed, leading to a substantial reduction of inrush currents, when the synchronous controller is active. The principles of such a synchronous controller are also addressed, and a comparison with EMTP simulations is presented.

Development of 550 kV 63 kA one-break gas circuit breaker

The 550 kV one-break gas circuit breaker (GCB) has been developed to meet advanced mechanical and electrical reliability requirements for GCBs. An improvement of the interruption capability reduces the number of break points in the GCB, and also realizes an elimination of the grading capacitor. An application of a synchronous switching scheme eliminates the closing resistor. Introduction of these innovative technologies into the 550 kV GCB realizes a reduction in the number of parts and minimizes the maintenance effort. The reduction of the break points and the reduction of the size of the SF/sub 6/ gas enclosure contribute directly to decreased gas usage.

Compact switching technologies, state-of-art and future trends

The development of EHV and UHV substation equipment have been proceeding to cope with the electricity demand expansion due to economic growth in the world. Simultaneously, the requirements for substation equipment compactness, cost reduction and efficiency also have been emphasized in the competitive market. This paper introduces several examples of recent developments on the compact and reliable switching technologies. They include advanced diagnostic techniques based on field data for operation and maintenance, and monitoring and diagnostic systems, controlled switching supported by information technology with the latest sensor technology and information network.