Power System Lab II
Synchronizing Two Generators
Theory
In an alternating current electric power system, synchronization is the process of matching the speed and frequency of a generator or other source to a running network. An AC generator cannot deliver power to an electrical grid unless it is running at the same frequency as the network. If two segments of a grid are disconnected, they cannot exchange AC power again until they are brought back into exact synchronization.
It is very difficult even on power plant generators let alone small generators. You have to match frequencies otherwise you'll be tripping breakers left and right from electric waves overlapping each other. Synchronization requires that the generators are producing alternating current at the same frequency, and that the outputs of the generators are in phase with one another. If both conditions are not met, extremely large electrical currents will flow through the generators, potentially tripping circuit breakers within the network, even if it is only two generators, maybe even damaging the equipment.
Fig.1 Two generators are ready to be connected in synchronizing manner .
Basic Principles: 1-
Automatic synchronizing of a generator consists of electrically “coupling” the generator output to another source of electrical energy and operating the generator such that its output adds to the other source.
2 - Automatic synchronizing can encompass a wide variety of conditions such as:
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Power System Lab II a) Two or more equal or similar-sized generators which, when paralleled to each other, will operate as though they were one larger generator. This is the most common application and reason for parallel operation. b) Two or more unequal-sized generators which are operated in parallel as though they were one larger generator. This is also a common Condition. c) Generator systems (which may consist of two or more individually paralleled generators) which are operated in parallel with another electrical system which, by comparison, is infinitely large. This is the case of operation in parallel with the normal electrical utility source. This is commonly done for on-site peak shaving, bottom shaving or cogeneration systems. It may be done momentarily in some special cases.
Benefits of automatically-synchronized (paralleled) systems: 1- Economy
An existing distribution system may not lend itself to being split into several sections and handled by separate non-paralleled units. When the loads are expected to expand substantially, the initial investment is minimized by installing one smaller generator set, and then adding more sets in parallel as the loads increase.
2- Reliability
When a part of the emergency load is deemed very critical, it may be desirable to have more than one generator capable of being connected to that load. When there is a normal source outage, all generators in the system are started. The probability of having a generator start and achieve nominal voltage and frequency is increased according to the number of sets available. The first set ready to handle the essential load does so. As the other generators are running and connected to the bus, the remaining loads are connected in declining order of priority.
Synchronizing Basics: i.
To successfully synchronize a generator to a bus requires some degree of Instrumentation to tell the operator what the phase relationships are between the two sources. The simplest is two voltmeters connected to read voltage between the same phases of the incoming generator and the bus. When the two sources are in phase and at equal voltage, both and will read 0 volts. (The third phase will also be the same since, if any two are correct, the third must be correct.) When the phases are 180° out of sync, the voltmeters will read 2 x normal system voltage. As the phases go in and out of sync the voltmeters will drift from 0 to 2 x to 0 at a rate which depends on the slip frequency (frequency difference). The breaker closure must occur when the voltage difference is at, or very near, 0. Otherwise each source will be subjected to extreme currents and forces which will damage the equipment. Out of sync voltage differences (and resultant forces) increase rapidly with increasing phase to phase mismatch angles. In general the forces are
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Power System Lab II acceptably small if the phase angles are within about ± 15° of true synchronism.
ii.
Three-light-bulb method. Compare the phase sequences of the oncoming generator and the running system by examining the three light bulbs. If all three bulbs get bright and dark together, both generators have the same phase sequences. If not, two of the conductors must be altered.
iii.
A synchroscope is a pointer-type meter that incorporates the two voltmeter movements with a single pointer. The pointer moves to a circular position dependent on the voltage difference. At zero volts it will be located at top dead center. The synchroscope position is representative of voltage difference, not phase displacement angle. Any area within about 30° to 45°of top dead center represents a fairly small voltage difference corresponding to a fairly small phase-to-phase displacement. A synchroscope will rotate at the slip frequency rate.
iv.
All of the foregoing are instrumentation devices which will allow an operator to observe when synchronism occurs and to initiate breaker closure accordingly.
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Power System Lab II The operator must adjust the incoming generator speed (and voltage if necessary) to obtain synchronized conditions). v. For automatic systems, an automatic device must be used to obtain synchronized conditions and initiate breaker closure at the proper time. There are a wide variety of automatic synchronizers available to interface with various types of governors. The synchronizer can also be utilized to match voltages as well as speed.
Synchronizing Procedure K5 contains two voltmeters, one for the A bus and one for the B bus, both with voltmeter selector switches. The instruments are used to check that equal and symmetrical voltages exist on both bus bar systems before paralleling (or synchronizing) by means of CB2 and isolators I3 and I4. The five position selector switch will give a choice of synchronizing around CB1, CB2, CB3 or CB4 by connecting the three synchronizing instruments around these. Position 5 is merely a position for OFF = no instruments or lamps will be connected. The synchronizing lamps are connected for rotating phasing, that is to say, if all three lamps are pumping (= not rotating) at the same time phasing cannot be done because either side has wrong phase order – two line leads has to be changed somewhere to achieve rotating lamps Connecting two lines together may involve some problems to be considered. As can be seen on the figures below, the two voltages may have a certain difference in size, phase and frequency. The voltage difference (presented as a vector below) will force a current through the system limited by the internal impedance in each voltage source if the two voltages are connected together without any precautions. To avoid tripping the frequency must be the same (or at least very close), the voltage must be the same (or at least not too big difference and – finally – the phase sequence must be the same. You also have to have a suitable circuit breaker to perform the operation of “synchronising”. There are four breakers which can be used: CB1 (the generator breaker), CB2 (the synchronising breaker), CB3 (line 1 breaker) and CB4 (line 2 breaker). By means of the selector switch in CB2.K5 you select your breaker. At the same time you connect the synchronising instruments around the breaker: The double voltmeter to see the two voltages The double frequency meter to see the two frequencies The synchronoscope to find the right position for the voltage vectors The “rotating phase lamps” to see “speed” difference together with the right/wrong phase sequence. Exp.1 Synchronizing Two Generators
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Power System Lab II
Vdiff=ZI
The external voltage vector The generator Voltage vector
Voltage vectors and The corresponding
The selector switch For CB1, 2, 3 or 4
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Power System Lab II
Procedure 1. 2. 3. 4. 5. 6. 7.
Find the selector switch, instruments and lamps as described (picture below) Put the system in stand-by operation. Check that all isolators and circuit breakers are open. Start the generator 1 and speed it up to 1500 rpm (or 1800 rpm for 60 Hz). Start the generator 2 and speed it up to 1500 rpm (or 1800 rpm for 60 Hz). Magnetise the generators for 220-230 V The isolators and circuit breakers around the one chosen for synchronizing .You can now note the voltage and the frequency of each generator and compare them together. 8. Choose by the selector switch CB3 as synchronising switch Now the following should be seen: The double voltmeters are indicating the generator voltage and the external voltage The double frequency meters are indicating the corresponding frequencies The synchronoscope LEDs are rotating clockwise or ccw. The phasing lamps are rotating WARNING! If the phase lamps are pumping together instead of rotating the phase sequence is wrong: change two connections of the external network to make the phase lamps rotate!
Then: 1. Adjust the generators voltage to be approximately the same. The same as the external one. 2. Adjust the speed. 3. Find the phasing point by the synchronoscope: the very top LED should be lit up and the two warning LEDs in the middle of the instrument should be dark. 4. Then – if above OK – turn the switch for – in this case – CB3 to ON-position: the phasing procedure is performed!. 5. Try to increase one generator speed: active power will be indicated. 6. Try to increase one voltage: reactive power will be indicated. 7. Disconnect the load. 8. Shutdown the system.
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Power System Lab II
Discussion "to be solved in lab" 1- Mention four advantages of synchronizing operation. 2- What are the required devices to synchronize two generators? 3- In synchronization, why all of the three phases must be the same? 4- What is the angle (between phasor) that makes the magnitude of voltage difference max and min? 5- Determine the magnitude range of voltage difference for any phase between two generators. 6- Draw the electrical diagram that shows the connection on the simulator required to make this experiment. Conclusion Write down conclusion that clearly shows: why synchronizing, when we can make synchronizing and your notes on this operation.
Problem Two generators are connected in synchronizing manner, G1 and G2.The operator decided to increase the field current of G1.About 5% of its value. As an analyzer: 1- How does the power supplied by G1 and G2 will be affected? 2- Draw the house diagram that describes whole of this operation.
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Power System Lab II
Appendix 1 SM225B Sychroscope Description SM225A Synchroscope is applied to indicate the frequency and phase angle relationship of the two sources. This unit provides a solid state display by means of LED lamps and replaces the traditional electro mechanical synchroscope. Ten lamp group, each of three LED’s, are sequentially illuminated to indicate the relative frequency and phase relationship between the two similar AC supplies. Direction of lamp “rotation” pattern indicates if frequency of incoming power is too slow(-) or too fast(+) relative to the common power's. The faster speed of rotating, means the bigger frequency error between them, otherwise means smaller. The lamp at “in phase” position is green, all others are red. The unit is rated for continuous operation and may be left permanently connected to the two supplies. Basic parameter Supply: AC380V Frequency: 45-65HZ Typical connection
Normally use 3 poles circuit breaker
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Power System Lab II
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Power System Lab II
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