THYRISTOR VALVE

THYRISTOR VALVE :

              INTRODUCTION :

                                                 HVDC Converters are an assembly of valves which have the propertyof conducting in the forward direction and blocking in the reverse directions. The term 'valve'carried over from the mercury are valve days, is applied even now for thyristor valves which are made up of series and parallel connection of many thyristor cells or devices.

                The major problem with the mercury are valves is the occurrence of are backs (or back fire) which results in the destruction of the recifing property of the valves. Are backs are random phenomena which results in failure to block in the reverse direction. Although the incident of are backs can be reduced by carefully controlling the factors that influence them, complete elimination is impossible and the valve cost is also increased. Furthermore, are backs are non self - clearing and result in line faults which stress transformer windings and anodes in the valve. The maintenance requirements for the valves go up and lead to poor reliability.  

                     Thyristor valves which were developed in the late sixties have eliminated all these problems. They have now completely displaced mercury are valves in HVDC transmission.

                     Thyristors that constitute the valves are also not perfect devices. The major problem is that their ratings cannot be exceed even for short durations. However, there is continuing development in the field of power semiconductors which has brought down the cost while improving the reliability. This chapter reviews the principles of operation, characteristic and control of thyristor devices. Some of the design aspects, protection and testing of thyristor valves for DC transmission applications are also presented