ONE facet of a-c generator protection that has not received sufficient study is that of detecting grounds in the field windings. There are many schemes available and in use for detecting field grounds, but all of them have some weakness or shortcoming.
THE REVISION OF THE STANDARD describing “Short-Circuit Requirements” for synchronous rotating machines is of considerable interest to the relay engineer. The quantitative statement of current-time capability of the generator makes available a basis for coordination of system and machine relaying on unbalanced faults. The permissible time duration of unbalanced faults is expressed as a variable based on if I 2 2 t being constant, where I 2 is the negative-sequence component of generator armature current and t is the time in seconds.
WHEN A GENERATOR is subjected to unbalanced short-circuit currents, a negative-sequence current (I2) is caused to flow in the stator windings, establishing a counter rotating flux that induces double-frequency rotor currents. The rotor currents flow in the surface parts of the rotor and cause detrimental heating. The generator is susceptible to varying degrees of damage depending on the magnitude of the negative-sequence current.
THE LOAD capability and performance of high-voltage transmission lines can be improved by the installation of series capacitors. Some reasons for the application of series capacitors to transmission circuits are: 1. To effect the desired load division between parallel circuits. 2. To increase the load capacity of a transmission line by a nominal amount (0 to 50 per cent). 3. To provide increased line capacity so that the system can handle short-time emergency loads. 4. To increase the transient stability limits of a system. 5. To improve voltage regulation and power factor for a given load condition by reducing the operating angle of the line.
In order to pick the proper power transformer for a certain application many factors must be taken into account. Each transformer location on each system must be considered separately in the light of these factors.