Parallel Operation of Synchronous Generators

Parallel Operation of Synchronous Generators

Objective: To study the th e conditions necessary to parallel two three-phase generators and the contribution of each machine to the load power. Requirements for parallel operation of synchronous generators: Before any two generators can be connected together, the oncoming unit terminal voltages must: running generator, 1. have the same magnitude as the running u nit, 2.  be in phase with the corresponding phases of the running unit, 3. have the same phase sequence as the running generator, the running generator. 4. have equal frequency that of the If the requirements 1 through 3 are not met, there will be circulating current between the generators sufficiently large that might damage the machines. The following discussion explains a method to check the above requirements: One ³dark´ two ³bright´ lamps method In this method, one lamp is connected across the a-phase of the two generators, the second lamp between between the b-phase b -phase of the running generator to the c-phase of the oncoming generator, and the third lamp  between the c-phase of the running machine and the b-phase of the oncoming machine as shown in the following figure. With this arrangement, the generators are paralleled when one lamp is dark and the other two are bright.

Dark

 A

Running Generator 

 A

Bright

B

B

Oncoming Generator 

Bright C

C Running F2

ener  t r 

¡   

¦  

¥  

 A2

RL

V Sync . n.

¤   

  

£  

M t r  ¥  

¡   

¥  

Shunt Fi ld

RL

¢  

 A1

¢  

 A Fi ld Winding

RL

¢  

F1

If 

 A2

F2

Synch. n.

¤   

£  

M t r  ¥  

Shunt Fi ld

¡   

¥  

¢  

¢  

 A1 Synchr  nizing Switch ¥  

Fi ld Winding If  ¢  

F1 ¨   

nc ¥

§   

ing

¡   

ener  t r  ¦  

¥  

Thr  -p hse W tt eter  ¢

Procedure:

¢  

¦  

¦  

§   

1. Couple one of the generators to the dc motor. Adjust its speed to 1800 rpm and its terminal voltage to rated value. Connect a 0.2 A resistive load. This generator will be the running generator. 2. Start the second generator and adjust its voltage and speed to rated values. This generator will be the oncoming generator. 3. Using the ³combined Dark and Bright Lamp´ method, parallel the oncoming generator with the running generator. Measure the line voltage, line current, and three-phase power. Also measure the field current of the generator. 4. If the two generators are properly synchronized, the oncoming generator will be ³floating´ neither supplying nor receiving power. 5. Increase the speed of the oncoming generator until it supplies half of  the load power. Adjust the field current of the oncoming unit for unity  power factor. For both generators measure the speed, armature current, terminal voltage, field current, and dc motor field current. 6. Keep the field excitation of the oncoming generator constant and vary its input power (speed of the dc motor) such that its armature current varies from 10% to 150% of the generator rated current. Plot generator output power vs. dc motor field current. What is the significance of this curve? 7. Readjust the input of the generator for 25% of its rated current. Holding this input constant, vary the excitation of the generator and obtain data to draw curves of generator armature current as a function of its field current. What is the significance of this curve? 8. Reduce the speed of the running generator until it is ³floating´ on the other machine and open its line circuit breakers. Reduce the load to zero and shutoff the two machines.

Discussion: 1. Discuss the conditions necessary to parallel and to divide the load  between two three-phase synchronous generators. 2. What will be the effect of synchronizing two generators without having all the necessary conditions fulfilled? Explain each condition separately. 3. Explain how the ³Combined Dark and Bright´ method indicates the correct synchronizing instant. 4. When a synchronous generator is operated with constant field current, why does the power factor vary with the load? Use a phasor diagram to explain. 5. What is the effect of varying the generator field current at constant load? 6. Assume two generators sharing a load. If the field current of primemover No. 1 is adjusted so that generator No. 1 takes on more load, show graphically how the field current of prime-mover No.2 should  be readjusted to maintain the same frequency?