# Lec # 07 parallel operation of synchronous generators

Eletrical Engineer at MUET SZAB Campus Khairpur Mir's Sindh Pakistan
Apr. 12, 2017
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### Lec # 07 parallel operation of synchronous generators

• 1. Parallel Operation of Synchronous Generators NOORNABI SHAIKH DEPARTMENT OF ELECTRICAL ENGINEERING MUET
• 2. N.N.SHAIKH 2 An isolated synchronous generator supplying its own load independently from other generators is very rare. This happens only in emergency generators. [standby generators] For all usual generator applications, there is more than one generator operating in parallel to supply the power demand by the loads. Parallel operation of ac generators There are several major advantages of generators operating in parallel  Several generators can supply a bigger load than one machine by itself.  Increased reliability of power system, since failure of any one of them does not cause a total power loss to the load.  For preventive maintenance, one or more of them can be removed form the system.  More efficient then single generator operating alone.
• 3. N.N.SHAIKH 3 Conditions Required For Paralleling Above figure shows a generator G1 supplying power to a load, with another generator G2 about to paralleled with G1 by closing the switch S1.  If switch S1 is closed randomly at some moment, the generators are liable to be severely damaged, and the load may lose power.  If voltages are not exactly the same in each conductor being tied together, there will be a very large current flow when the switch is closed.
• 4. N.N.SHAIKH 4 To avoid problems, following paralleling conditions must be met: 1. The rms line voltages of the two generators must be equal. 2. The two generators must have the same phase sequence. 3. The phase angles of the two a phases must be equal. 4. The frequency of the new generator, called oncoming generator, must be slightly higher than the frequency of the running system.
• 5. N.N.SHAIKH 5 Condition 1 obvious – in order for two sets of voltages to be identical, the voltage in phases a & a’ will be completely identical. If not then very large current will flow when the switch is closed??? Condition 2 This ensures that the sequence in which the phase voltages peak in the 2 – generators is same. If phase sequence is different as shown then even one pair voltages are in phase, the other two pairs of voltages are 120o out of phase. If generators are connected in this manner, there would be no problem with phase a but huge current would flow in phase b & c, damaging both machines.
• 6. N.N.SHAIKH 6 Condition 4 If frequencies are not very nearly equal then, large power transients will occur until generators stabilize at a common frequency. The frequencies of the 2 – machines must be very nearly equal, but not exactly equal. [???]
• 7. N.N.SHAIKH 7 General Procedure for Paralleling Generators Suppose G2 is to be connected to the running system shown in figure. The following steps should be taken to accomplish the paralleling. STEP – 1 Using voltmeters & ammeters, the IF of the oncoming generator should be adjusted until its terminal voltage is equal to the line voltage of the running system. STEP – 2 Phase sequence of the oncoming generator must be compared to the phase sequence of the running system. It can be checked by: a. Phase sequence meter. It is a small 3 – phase induction motor. First connect it with the running system and check its direction of rotation. Then connect it with the oncoming generator and check the direction. If direction of rotation of two separate systems is equal, then the phase sequence is same. If not then interchange the 2 – phases of oncoming generator to make same the phase sequence same. b. Three-lamp method [HW].
• 8. N.N.SHAIKH 8 STEP – 3 Frequency of the oncoming generator is adjusted to be slightly higher than the frequency of the running system. This is done by watching a frequency meter until the frequencies are close and then by observing changes in phase between the systems. [why oncoming generator’s frequency should be slightly higher than running system???] STEP – 4 Phase difference must be same. A Synchroscope is a meter that measures the difference in phase angle between the a phases of the two systems.