Alternator synchronization
The process of connecting an AC generator (alternator) to other AC generators is known as synchronization and is crucial for the generation of AC electrical power.
Conditions
There are five conditions that must be met before the synchronization process takes place. The alternator must have equal line voltage, frequency, phase sequence, phase angle and waveform to that of the system to which it is being synchronized.
Process
In the past, synchronization was performed manually using three-lamp method. Nowadays, the process is automatically operated and controlled with the aid of synchronization relays.
During installation of a generator, careful checks are made to ensure the generator terminals and all control wiring are correct so that the order of phases (phase sequence) matches the system. Connecting a generator with the wrong phase sequence will result in a short circuit as the system voltages are opposite to those of the generator terminal voltages. [1]
The sequence of events is similar for manual or automatic synchronization. The generator is brought up to approximate synchronous speed by supplying more energy to its shaft - for example, opening the valves on a steam turbine, opening the gates on a hydraulic turbine, or increasing the fuel rack setting on a diesel engine. The field of the generator is energized and the voltage at the terminals of the generator is observed and compared with the system. The voltage magnitude must be the same as the system voltage.
Synchronizing lamps
Formerly, three light bulbs were connected between the generator terminals and the system terminals (or more generally, to the terminals of instrument transformers connected to generator and system). As the generator speed changes, the lights will rise and fall in intensity at a rate proportional to the difference between generator frequency and system frequency. When the voltage at the generator is opposite to the system voltage (either ahead or behind in phase), the lamps will be bright. When the voltage at the generator matches the system voltage, the lights will be dark. At that instant, the circuit breaker connecting the generator to the system may be closed and the generator will then stay in synchronism with the system. [2]
Synchroscope
Another manual method of synchronization relies on observing an instrument called a "synchroscope", which displays the relative frequencies of system and generator. The pointer of the synchroscope will indicate "fast" or "slow" speed of the generator with respect to the system. To minimize the transient current when the generator circuit breaker is closed, usual practice is to initiate the close as the needle slowly approaches the in-phase point. An error of a few electrical degrees between system and generator will result in a momentary inrush and abrupt speed change of the generator.
Synchronizing relays
Synchronizing relays allow unattended synchronization of a machine with a system. Today these are digital microprocessor instruments, but in the past electromechanical relay systems were applied. A synchronizing relay is useful to remove human reaction time from the process, or when a human is not available such as at a remote controlled generating plant. Sometimes as a precaution against out-of-step connection of a machine to a system, a "synchro check" relay is installed that prevents closing the generator circuit breaker unless the machine is within a few electrical degrees of being in-phase with the system. (Synchro check relays are also applied in places where several sources of supply may be connected and where it is important that out-of-step sources are not accidentally paralleled).
Synchronous operation
When the generator is synchronized, the frequency of the system will change depending on load and the average characteristics of all the generating units connected to the grid. If a large load is suddenly added, the energy required to supply that load for a time is drawn out of the mechanical stored energy in all the system's connected machines, and the machines will all slow down. Speed-regulating equipment (the governor) at each machine will detect this slow down, and increase the mechanical power supplied to the machines to maintain frequency. The reverse happens on a load decrease; the machines speed up, the governors close valves or throttles to maintain frequency.
Each speed governor has a characteristic sensitivity to speed change called "droop". It can be mathematically shown that if all machines synchronized to a system have the same droop, they will share load proportionate to the machine ratings. [3]
For stable operation of the electrical grid of North America, power plants operate with a five percent speed droop. In a small island grid one machine can be run as an "isochronous" governor; it will maintain a constant speed and system frequency. The machine size must be comparable to the size of expected load fluctuations to be an effective regulator of system frequency. [4].[5]
In a large system with hundreds of generators connected, the connection of any one machine will have negligible influence on the rest of the machines. Power systems engineers sometimes refer to a large system as an "infinite bus" which notionally maintains a constant voltage and frequency independent of load. At any given connection point to the system, it can be modeled as an infinite bus feeding through an equivalent impedance to the point in question.
References
- ^ Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07013932-6 pages 7-45 through 7-49
- ^ Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07020974-X pp. 3-64,3-65
- ^ William D. Stevenson, Jr. Elements of Power System Analysis Third Edition,McGraw-Hill, New York (1975) ISBN 0070612854 page 378-379
- ^ http://www.control.com/thread/1026221805
- ^ http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Frequency/Freq60.htm