Parallel Operations

Parallel Operation
Operating characteristics
triangle of a DC generator
 Normally, as the DC generator
load increases, the terminal
voltage of the said generator
decreases (except on a series
dc generator)
Parallel Operation
Parallel operation of DC generators
 In order to increase the capacity of a system serving different loads, a number
of generators are connected in parallel.

3 Basic requirements:
1. The same external characteristics (i.e. Same type, etc.)
2. The terminal polarity must be the same.
3. The terminal voltage must be equal in magnitude
Parallel Operation
Case1.
 With external characteristics given
(i.e. Kw rating, voltage rating,
regulation etc.)
 Parallel Operation
A. Before Bus Load Changes

𝑃𝑎 = 𝑉𝑏𝑢𝑠 𝐼𝑎 𝑃𝑡 = 𝑃𝑎 + 𝑃𝑏 𝐼𝑡 = 𝐼𝑎 + 𝐼𝑏
𝑃𝑏 = 𝑉𝑏𝑢𝑠 𝐼𝑏 𝑃𝑡 = 𝑉𝑏𝑢𝑠 𝐼𝑡
where:
𝐼𝑎 and 𝐼𝑏 - current delivered by generators A and B
𝑉𝑏𝑢𝑠 - operating voltage prior to change in bus load
𝑃𝑎 & 𝑃𝑏 - power shares of generator A and B
 Parallel Operation
B. After Bus Load Changes

Δ𝐼 = Δ𝐼𝑎 ± Δ𝐼𝑏 𝐼𝑡 𝑛𝑒𝑤 = 𝐼𝑡 ± Δ𝐼 𝐼𝑎 𝑛𝑒𝑤 = 𝐼𝑎 ± Δ𝐼𝑎

𝐼𝑏 𝑛𝑒𝑤 = 𝐼𝑏 ± Δ𝐼𝑏 𝐼𝑡 𝑛𝑒𝑤 = 𝐼𝑎 𝑛𝑒𝑤 + 𝐼𝑏 𝑛𝑒𝑤

where:
Δ𝐼 – change in bus load (current)
Δ𝐼𝑎 & Δ𝐼𝑏 - change in generator currents due to changes in bus load
+Δ𝐼 – if there is an increase in bus load.
− Δ𝐼 – if there is a decrease in bus load.
Parallel Operation
Parallel Operation
Case 2.
 (With internal characteristics given
(generated emf, resistance of
windings, etc.)
Parallel Operation
IL= IL1 +IL2

Ia1= IL1+ Ish1 Ia2= IL2+ Ish2

IL1= Ia1- Ish1 IL2= Ia2- Ish2

Eg1= VL+ Ia1(Ra1) Eg2= VL+ Ia2(Ra2)

Ia1= (Eg1- VL)/ Ra1 Ia2= (Eg2- VL)/ Ra2

Ish1= Vsh1/ Rsh1 Ish2= Vsh2/ Rsh2

Ish1= VL/ Rsh1 Ish2= VL/ Rsh2

IL= (Ia1- Ish1) + (Ia2- Ish2)

IL= [{(Eg1- VL)/ Ra1}- VL/ Rsh1] + [{(Eg2- VL)/ Ra2}- VL/ Rsh2]
Parallel Operation
Sample Problem No. 1
Two 250-V dc generators are operating in parallel and taking equal shares of a 3000 A, 250 V bus load. The ratings of the generator are as
follows:
generator 1 generator 2
% Vr 2% 4%
Rated power 500 kw 400 kw
The bus load increases to 3600 A. Assuming no control adjustments had been made, determine a). New bus voltage. b).Current carried by each
generators.
Parallel Operation
Sample Problem No. 1
Two 250-V dc generators are operating in parallel and taking equal shares of a 3000 A, 250 V bus load. The ratings of the generator are as
follows:
generator 1 generator 2
% Vr 2% 4%
Rated power 500 kw 400 kw
The bus load increases to 3600 A. Assuming no control adjustments had been made, determine a). New bus voltage. b).Current carried by each
generators.
Parallel Operation
Sample Problem No. 2
Two identical shunt generator running in parallel supply a total load current of 2500A. Each machine has a
field resistance of 50 ohms and an armature resistance of 0.01 ohm. The induced emfs generated are 230
and 235V, respectively. Calculate the common operating voltage of each machine.