Infinite-bus-and-parallel-operations.ppt

Infinite-bus-and-parallel-operations.ppt

• 1.
  Synchronous Machines Mrs.P.Vijayapriya Asst.Prof (SG) SELECT RoomNo. TT 130B UNIT – I Synchronous Generator Constructional details – Types of rotors – mmf of distributed AC winding – emf equation – synchronous reactance – Armature reaction – Voltage regulation – e.m.f , m.m.f, z.p.f and A.S.A methods – synchronizing and parallel operation – Synchronizing torque – Change of excitation and mechanical input – Two reaction theory – Determination of direct and quadrature axis synchronous reactance using slip test – Operating characteristics – capability curves
• 2.
  Parallel operation ofsynchronous generators There are several major advantages to operate generators in parallel: • Several generators can supply a bigger load than one machine by itself. • Having many generators increases the reliability of the power system. • It allows one or more generators to be removed for shutdown or preventive maintenance. • A single generator not operating at near full load might be quite inefficient. While having several generators in parallel, it is possible to turn off some of them when operating the rest at near full-load condition
• 3.
  Before connecting agenerator in parallel with another generator, it must be synchronized. A generator is said to be synchronized when it meets all the following conditions: • The rms line voltages of the two generators must be equal. • The two generators must have the same phase sequence. • The phase angles of the two a phases must be equal. • The oncoming generator frequency is equal to the running system frequency. Synchronization A diagram shows that Generator 2 (oncoming generator) will be connected in parallel when the switch S1 is closed. However, closing the switch at an arbitrary moment can severely damage both generators!
• 4.
  If the phasesequences are different, then even if one pair of voltages (phases a) are in phase, the other two pairs will be 1200 out of phase creating huge currents in these phases. If voltages are not exactly the same in both lines (i.e. in a and a’, b and b’ etc.), a very large current will flow when the switch is closed. Therefore, to avoid this, voltages coming from both generators must be exactly the same. If the frequencies of the generators are different, a large power transient may occur until the generators stabilize at a common frequency. The frequencies of two machines must be very close to each other but not exactly equal. If frequencies differ by a small amount, the phase angles of the oncoming generator will change slowly with respect to the phase angles of the running system. If the angles between the voltages can be observed, it is possible to close the switch S1 when the machines are in phase.
• 5.
  General procedure forparalleling generators • When connecting the generator G2 to the running system, the following steps should be taken: 1. Adjust the field current of the oncoming generator to make its terminal voltage equal to the line voltage of the system (use a voltmeter). 2. Compare the phase sequences of the oncoming generator and the running system. This can be done by different ways: 1) Connect a small induction motor to the terminals of the oncoming generator and then to the terminals of the running system. If the motor rotates in the same direction, the phase sequence is the same; 2) Connect three light bulbs across the open terminals of the switch. As the phase changes between the two generators, light bulbs get brighter (large phase difference) or dimmer (small phase difference). If all three bulbs get bright and dark together, both generators have the same phase sequences.
• 6.
  If phase sequencesare different, two of the conductors on the oncoming generator must be reversed. 3. The frequency of the oncoming generator is adjusted to be slightly higher than the system’s frequency. 4. Turn on the switch connecting G2 to the system when phase angles are equal. The simplest way to determine the moment when two generators are in phase is by observing the same three light bulbs. When all three lights go out, the voltage across them is zero and, therefore, machines are in phase. A more accurate way is to use a synchroscope – a meter measuring the difference in phase angles between two a phases. However, a synchroscope does not check the phase sequence since it only measures the phase difference in one phase. The whole process is usually automated…
• 7.
  Concept of theinfinite bus When a synchronous generator is connected to a power system, the power system is often so large that the operation of the generator does not have much of an effect on the power system. An example of this situation is the connection of a single generator to the Indian power grid. Our Indian power grid is so large that no reasonable action on the part of one generator can cause an observable change in overall grid frequency. This idea is idealized in the concept of an infinite bus. An infinite bus is a power system so large that its voltage and frequency do not vary regardless of how much real or reactive power is drawn from or supplied to it.
• 8.
  Synchronization Load Generator Rest of the powersystem Generator Xs1 Ef1 Xs2 Ef2 Xsn Efn Infinite bus V, f are constant Xs eq =0 G