Paralleling of Generators and Synchronization

 

Paralleling of Generators and Synchronization

Connecting generators in parallel increases the power capacity, control in load management, ease of maintenance, and redundancy. The process involves the physical connection of two or more electric generators, and the synchronization of their outputs. The synchronization matches the waveform of the output voltage of one generator with the voltage waveform of the other generator (s).

Factors considered when paralleling generators

Capacity: easy of synchronizing the system based on load demand; powering on a set when demand increases and switching off a set when demand decreases. Redundancy: ensuring a soft power transfer as the outgoing generator is switched off and the incoming one is powered on. The design should avoid a situation where the load is not powered or allowed to run on the UPS. Compliance with the electrical standards in terms of safely, protection and operation.

Figure 1: Basic connection of parallel connected generators | image: people.ucalgary.ca

Requirements for connecting generators in parallel

Ideally, any type of a generator can be paralleled together with another type as long as their frequencies and voltage are the same at the point of interconnection. However, there are some practical limitations such as incompatibility between older and newer models or when the cost of making them compatible is not justifiable. Among the things to consider are;

     

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Speed control: The generators may have equal or different engine speeds, but must be locked into the final speed of the system.

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Load balance: The entire load should be shared by all the generators according to the capacity of the individual units.

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Synchronization: Synchronization is required to ensure that the resulting output is in phase, has the same frequency and voltage hence compatible with load.

 

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Voltage regulation

Compatible generators

The easiest approach is to use similar generators or at least sets with the same alternator pitch, output voltage and frequency. Generators are said to be compatible when they share some properties such as compatible alternators, engines, speeds, load sharing controls, interfaces to other control and monitoring systems, etc Incompatible Incompatibl e generators

Paralleling generators with different characteristics is a bit complex. Apart from the compatibility issues, there are other system level problems the sizes. If the sizes are the same, the system is capable of supporting a higher priority load in case of an emergency. However, if the sizes (in KW) are different, the maximum priority load is limited by the smallest generator; otherwise a higher load will overload the generator unless it is prevented from coming up. In such situations, the control is quite complex and might involve changing the sequence of operation. In addition, manual operation as well as dealing with failure modes becomes difficult. The design should ensure that larger loads are dropped out first in the event of a failure of the bigger generator; this ensures that the smaller generator will be left with a load it can support. Synchronizing parallel connected generators

This is the process in which the voltage and frequency of the generators are matched to provide a standard AC output waveform. For the generators to be paralleled and synchronized correctly, each of the sets should have the below characteristics

       

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Frequency: the frequencies must be the same

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Voltage: The generators should produce or be set to produce the same voltage

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Phase number: The two systems should have the same number of phases, either three or single phase. Phase rotation: For three phase systems, each of the three phases must be matched. This prevents excessive mechanical and electrical stresses as well as avoiding power surges.

 

 

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Voltage Phase angle: The waveforms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero.

Figure 2: Matching phase angle | image: cpower.com Advantages of Parallel generator systems

Increased reliability and redundancy: Improving reliability and redundancy for both critical and non-critical loads. The system ensures that there is always constant power to critical loads, and if one of the generators fails, the load is supplied by the other one or more parallel unit.

Low cost of power generation: Cost of generation increases with generator size and is more for ratings above 600KW.This is due to the fact that the market for smaller engines is higher, hence more are manufactures, resulting in lower cost. Using several smaller generators becomes more economical than a single large generator.

Decreased light loading of the generator prime mover: In most installations, the load varies from time to time and it is common to have a generator running at 30% of its capacity when the load is low. This may cause wet stacking. The efficiency of the prime mover is normally higher when the load is between 75% and 100%, and a light load means that the generator operation is inefficient. Using a smaller generator increases efficiency, hence a cost reduction.

More control and savings on generating costs: The total amount of power supplied from several small generators is equal to the power supplied by one large unit. However, there is more control and balancing in the smaller generators. One can balance the load over the different circuits, and decide on the power to generate at any particular time based on the load.

 

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The parallel system can achieve huge savings when all generators are operated above 75% of their rated load. This is the point at which generators use the minimum fuel. Expandability and flexibility: The use of several generators makes it possible to supply a varying load without piling up costly units or spending too much on a big generator whose full capacity is rarely used. Generators can be added gradually as demand increases.

Conclusion

Setting up parallel generators is a complex procedure that requires a qualified electrical professional. A well designed system provides backup power and a variable output. Using the correct paralleling switchgear helps the consumer to achieve the maximum output when the power demand is at its peak while providing the minimum output when the load requirements are low.

 

 

Parallel Operation of DC Generators

system,, power is usually supplied by many  many  synchronous generators  generators connected in parallel which helps to maintain continuity of of In modern  modern power system plant operations. Now a day’s, isolated one large gener ator is outdated. When we connect two generators in parallel, they have a tendency to remain in step. By doing some changes in their armature current and by connecting generators properly to the existing bus bar, this problem could be solved. Connection of Parallel DC Generators 1. 

The generators in a  a power plant, plant, connected by heavy thick copper bars, called bus-bars which act as positive and negative terminals. t erminals. To connect the generators in parallel, Positive terminal of the th e generators are connected to the positive terminal of the bus-bars and negative terminals of generators are connected to negative terminal of the t he bus-bars, as shown in the figure.

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To connect the 2 generators with the 1 existing e xisting working generators, first we have to bring the speed of the prime mover of the 2nd generator to the rated speed. At this point switch S4 is closed. The  circuit breaker The breaker  V2 (voltmeter voltmeter)) connected across the open switch S 2 is closed to complete the circuit. The excitation of the generator 2 is increased with the help of field  field rheostat rheostat  till it generates  generates voltage  voltage equal to the voltage of bus-bars. The main switch S2 is then closed and the generator 2 is ready to be paralleled with existing generator. But at this point of time generator 2 is not taking any load as its it s induced e.m.f. is equal to bus-bar voltage. The present condition is called floating, that means ready for supply but not supplying  supplying current  current to the load. In order to deliver current from generator 2, it i t is necessary that its induced e.m.f. E should be greater than the bus-bars voltage V. By strengthening the field current, the induced e.m.f. of generator 2 could be improved and the current supply will get started. To maintain bus-bar voltage, the field of generator 1 is weakened so that value remains constant.

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Field current I given by

resistance  of  armature winding winding..  Where, Ra is   is resistance

Load Sharing of Parallel Connected DC Generators The load gets shifted to another generator by adjusting induced e.m.f., but in modern  modern   power plant, everything has been done by “sychroscope” which gives instruction to governor of the prime mover. Let us suppose that two generator having different load load   voltage voltage.. Then the load sharing

between these generators will be The value of current output depends upon the values of E1 and E3 which could be managed rheostats  to keep the bus-bars voltage constant. by field  field rheostats Advantages of Parallel Operation of DC Generators Cost Effectiveness:-  Effectiveness:- Electric power power  cost reduces when generator produce electricity according to their capacity and easily manage demand and supply requirement. If there is less requirement of power, one or o r more units can be taken on hold or startup visa versa. Smooth Supply of Power:- In case of breakdown of generator, the supply of power will not get disrupted. If something goes wrong in one generator, the continuity of power can be continued by other healthy units. Easy to Maintain:- The routine maintenance of generator is required time to time. But for that supply of power cannot be obstructed. In parallel generators the routine checkup could be done one by one.

 

Easy to Increase Plant Capacity:- The demand of electricity is increasing day by day. To meet the requirement of  power generation, generation, additional new unit can run paralleled with the running units. Precautions During Parallel Connection 1.  2.  3.  4.  5. 

The specification of each generator is different from one another. When they are synchronized together, their speed are locked into the overall speed of the system. The entire load of the system should be distributed di stributed in all the generators. There should be a controller for keeping k eeping check on parameters of the engine. This can be done with modern digital controllers which are available in market. Voltage regulation in the whole system plays an important role. In case of   of  voltage drop drop  in one unit compare with other units, end up bearing the whole  whole voltage  voltage load of the system of parallel generators. While connecting terminals to the bus-bars, extra precaution should be made. If generator is connected with wrong polarity of the bar, it may result to a short circuit.