When paralleling A.C. Generators, the engine governors must have the same speed droop characteristics if the sets are to divide the load in proportion to their ratings throughout the entire operating range.
It is very important that two basic facts be understood concerning load division between generators operating in parallel. First, the power supplied to the generator and thus to the load is a function of the engine. The engine governor settings and the positions of the governor controls determine the amount of power delivered by the engine and the KW load carried by the generator. If the governor control setting is advanced, the engine and generator will assume more KW load. Likewise, decreasing the governor control setting will result in a reduction of load on the unit. Any other units on the line will, conversely, either reduce or gain load at the same time, assuming no change in total load or no change in the governor settings of the other units has taken place. Second, the division of power is not determined by generator excitation or terminal voltage. The Power Factor at which an generator will operate when paralleled with other generators is determined by its excitation. For more discussion on this subject, refer to the section on Parallel Operation of generators.
As mentioned previously, governors furnished with Caterpillar Powered Electric Sets can be either of two types, governors with fixed speed droop (mechanical) or governors with adjustable speed droop (hydraulic). The values of speed droop used are commonly 3% and 0%. Governors with adjustable speed droop can be adjusted so their characteristics match quite closely the characteristics of governors with fixed speed droop. The operating characteristics of the following combinations of governors on paralleled electric sets will be described.
- Two 3% mechanical governors
- One 3% mechanical or hydraulic governor and one 0% (isochronous) hydraulic governor
In the diagrams that follow, the per cent of rated load of one unit will be shown across the bottom. The system capacity will, however, be the sum of the capacities of the units operating in parallel. Percentage is used in the general cases discussed. For any particular installation this can be converted to kilowatts as shown in the following:
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| Illustration 1 | g01059009 |
Load Division - 3% Mechanical Governors
The governor speed droop characteristics of these two units are similar. This is shown by lines "A" and "B" coinciding in 2. If both units are stated, set for high idle speed and paralleled at no load, the system frequency will be 61.8 cycles or 103% of rated frequency. As load is applied to the system, the frequency will decrease along the speed droop characteristic until the frequency at full load is 60 cycles.
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| Illustration 2 | g01059016 |
Paralleling two units with similar speed droop characteristics | |
If Unit A had been operating alone carrying full load, the system frequency would be 60 cycles as shown by Line "A" at 60 cycles and 100% load. Now, if the frequency of Unit B is adjusted by the engine speed control to be equal to that of Unit A and circuit breaker of Unit B is closed, the system would be operating under the following conditions:
- Unit A is at 60 cycles and 100% load
- Unit B is at 60 cycles and 0% load. The characteristic of the governor of Unit B at this time is shown by the dotted line B
In order for Unit B to carry load, it is necessary to advance the speed setting of the governor. If it is advanced to the full load position, the governor characteristic of B will coincide with the characteristic of A. Since the load on the system was 100% of one unit, no change was made in total load, and the available capacity is now 200%, the system will operate at 50% load on each unit, and 60.9 cycles for two units of equal capacity (Point E). For units of unequal capacity, the load will be divided in proportion to the ratio of the capacity of each unit to the total capacity, and the system frequency will be determined by the points on the governor characteristic corresponding to these loads. The frequency will be the same for both units since paralleled governors must operate at the same speed.
If unit A had been a 60 KW unit fully loaded and a 100 KW Unit B was paralleled with it and the governor adjusted to the full load position, the final load division and frequency would be determined as follows:
- System load - 60 KW.
- System capacity - 160 KW.
- Refer to Illustration 3 in order to determine what the 60 KW unit will carry
- 100 KW unit will carry (60 ÷ 160) × 100 KW = 37.5 KW.
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| Illustration 3 | g01059044 |
- The system frequency can be determined readily from step c or step d. The load carried by each unit is 22.5 ÷ 60 or 37.5 ÷ 100 which figures out to be 37.5% of the capacity of either unit. Again, using Illustration 5 for the governor characteristics of the 100 KW unit and reading up from the value of load (37.5% to point F), we find the system frequency to be 61.126 cycles.
Load Division - 3% Mechanical Governor and Isochronous Governor
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| Illustration 4 | g01059059 |
Paralleling two units, one with a 3% mechanical governor and the other with a hydraulic governor set for isochronous operation | |
The characteristics of the 3% governor (Unit A) is shown by line "A" of Illustration 4 and the characteristics of the isochronous governor (Unit B) is shown by line "B". Only at full load, 60 cycles, do the frequencies of the units have the same value. It is customary to operate a system of this type with a system load greater than the capacity of Unit A. In this way Unit A carries its full load at 60 cycles and the additional load and load swings are handled by Unit B, also at 60 cycles.
The system can maintain constant frequency with load changes because of the speed droop characteristics of the governors. In the system described in this example, if the load is less than the capacity of Unit A (which has the 3% governor) and can be carried by Unit B, (isochronous governor) Unit A can be disconnected from the system. If the system load is reduced to the point where Unit A is not operating at the 60 cycle point of its governor in a power system is to maintain constant frequency.
The preceding discussion and examples of governor operation can be summarized as follows:
- The simplest governor combination for paralleled electric sets is to have a 3% speed droop characteristic for each of each governor. If a constant frequency from no-load to full-load is required, one governor can be adjusted for isochronous operation. This is referred to as lead time.
- In order for all parallel units to accept their full share of the load, the following governor adjustments are required:
- The same full load speed.
- The same high idle (no-load) speed in the case of mechanical governors or hydraulic governors adjusted for speed droop operation.
- Governor controls set to the high idle position so the full governor rang is available.
- Operation of an isochronous governor in parallel with speed droop governors requires the special techniques described in step 2.
- Any number of electrical sets can be operated in parallel. However, only one governor of the group can be adjusted for isochronous operation except in the special case of electric-hydraulic and electronic governors.