AUTOMATIC SYNCHRONIZING RELAYS BASIC TUTORIALS

An automatic synchronizing relay is used for synchronizing an incoming generator to a power system. Automatic synchronizing is applied to generating equipment where the station is unattended; where the element of human error should be ruled out in the start-up procedures of a generating unit; or where consistent, accurate, and rapid synchronization is preferred.

The relays used are multifunctional devices that sense the differences in phase angle, voltage magnitude, and frequency of the sources on both sides of an incoming generator breaker and initiate corrective signals to the prime mover and field in order to adjust the generator frequency and voltage until the systems are in synchronism.

Most automatic synchronizing relays can anticipate an advance angle at which to initiate breaker closing so that, when the circuit breaker is closed, the systems are as close to exact synchronism as possible. A synchroscope is used to monitor the synchronizing process.

From the time the relay initiates a close signal until the breaker’s contacts actually close, the needle travels a certain distance (measured in degrees) around the scope. The distance traveled can be determined based on the speed of rotation and how long it was permitted to rotate.

The scope’s needle rotates at a speed that is directly proportional to the slip frequency between the generator and the system. Therefore, given the circuit breaker’s closing time and the desired slip rate, the rotational distance traveled (or advance closing angle) can be determined.


When the generator is to be connected to the system, the appropriate synchronizing switch is selected and closed. The synchronizing equipment performs the following functions automatically:

a) A speed-matching relay element senses the frequency difference between the sources and adjusts the governor with raise or lower signals to control the speed of the incoming generator and thereby matches the frequency of the generator with the frequency of the running system bus.

b) A voltage-matching relay element compares the running system and incoming generator voltages and provides raise or lower signals to the excitation system of the incoming generator so that its voltage matches the running system voltage.

c) As the phase angle between the two systems approaches zero, the relay energizes the circuit breaker's closing circuit at an advance angle determined by the relay so that when the circuit breaker contacts close, the two systems are in synchronism. The synchronizing relay itself has at least two adjustable settings that should be made for correct performance.

One adjustment permits the relay to accommodate breaker closing time, for example, 0.05 s to 0.4 s, and one adjustment sets the maximum phase-angle advance, from 0° to 30-40°. The advance closing angle is calculated by the following expression:

θ = 360(st)

where
θ is advance angle (˚),
s is slip frequency (cycles/s),
t is breaker closing time (s).

For example, for systems coming into synchronism rapidly, that is, s = 0.5 cycles/s, the closing circuit should be energized well in advance of synchronism. If the circuit breaker has a 0.15 s closing time, the advance angle required would be 27°.

If the slip frequency is much lower, then the advance angle is much smaller. For a 0.1 cycle/s slip frequency, the closing angle is now 5.4°. Thus, precise control of the point of synchronism can be obtained.

Several different schemes for automatic synchronizing can be developed depending on economics, reliability, and operating system requirements. By using electromagnetic relays, several relays are required to perform all functions. Static relays provide all the functions in one unit.