Class 4 (4th Engineer) MET 📅 Apr 2023

Exam Question

(a) Shunt generators having drooping characteristics are best suited for parallel operation. Discuss. (6)

(b) Two 220 V d.c. generators each having linear external characteristics, operated in parallel. One machine has a terminal voltage of 270 V on no-load and 220 V at a load current of 35 A, while the other has a voltage of 280 V at no-load and 220 V at 50 A.

Calculate the output current of each machine and the bus bar voltage when the total load is 60 A. What is the kW output of each machine under this condition. (10)

Reference Answer

### Part (a): Suitability of Shunt Generators for Parallel Operation
Shunt-wound DC generators are inherently suited for parallel operation primarily due to their **drooping voltage-load characteristic**. This characteristic is fundamental to ensuring stable load sharing, preventing large circulating currents, and maintaining overall system stability.
#### 1. The Drooping Characteristic
The terminal voltage ($$V_t$$) of a DC shunt generator is given by the equation: $$V_t = E_g - I_a R_a$$, where $$E_g$$ is the generated EMF, $$I_a$$ is the armature current, and $$R_a$$ is the armature circuit resistance. As the load on the generator increases, the load current ($$I_L$$) and consequently the armature current ($$I_a$$) increase. This leads to two effects:

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