Conditions for Parallel Operation of Dc Generator :
To connects the generators in parallel to common bus-bars, the generators should satisfy the following conditions.- The incoming generator's voltage should be the same as the bus-bar voltage.
- The +ve and -ve terminals (i.e. polarity) of generators must be connected to +ve and -ve of bus-bars (otherwise a serious short-circuit will occur).
- Equalizer bar should be used for compound and series generators.
- Induced e.m.fs of generators should be preferably the same (otherwise circulating currents result).
The Major Advantages of Many Units Operating in Parallel are :
The advantages of parallel operation of dc generators are,- Service continuity.
- Efficient working.
- Repair facility.
- Extension facility.
- Stand by unit capacity reduces.
Equalizer Bar or Ring :
Two identical series generators G1 and G2 are connected in parallel. Let E1 and E2 are initially equal, generators supply equal currents and have equal shunt resistances. When E1 increases slightly (E1 > E2). In that case, I1 becomes greater than I2.
Consequently, the field through generator G1 is strengthened thus increasing E1 further whist the field of generator G2 is weakened thus decreasing E2 further. At a stage when G1 supplies power not only to the whole load but also supplies power to G2 which starts running as a motor. Here, the two machines will form a short-circuited loop and the current will rise in the loop.
This condition can be prevented by placing an equalizing bar between two generators because of which two machines passes similarly equal currents to the load, the slight difference between the two currents are adjusted in the loop is made by the armatures and the equalizer bar.
Parallel Operation of Two Generators :
Let us see the parallel operation with dc compound generators. Here the generators are taking an equal share of the load.
Let the two compound generators G1 and G2 are connected for parallel operation as shown in the figure. Consider, for instance, the generator G1 takes increased sharing of the load. Due to this, the current through the series winding of that generator G1 increases further. Therefore the emf in the generator also increases, which makes the generator to take more load.
As the load on the two generators is to be constant. It may result in the weakening of the series field of the generator G2. Which in turn further reduces the load on generator G2.
If this condition may continue, at a timing generator G1 takes the entire load, and at that instant, the generator G2 will start working as a motor. This may result in the opening of the circuit breakers of the generator and the entire parallel operation will be stopped.
In order to make the parallel operation, stable and smooth. A conductor of low resistance known as "Equalizer Bar" is connected between the two armature ends of two generators as shown in the figure. The operation of the equalizing bar is to make an equal flow of field currents in two generators series field coils.
Suppose when the current through the winding of generator G1 increases. Due to the sharing of load increases. This current also passes through the equalizing bar. Therefore, this current will be shared by the series field winding of the generator G2.
In this way, the current is balanced in both the windings of the generators with the help of an equalizing bar, and load sharing is made equal on both generators.
