Electricity Today Magazine Article -- Desgining an Excitation System

When generator applications are in the design and proposal stages, what needs to be done to put excitation system theory into practice? How do the application and specification dictate the design of a generator system? The following will work through some examples to illustrate the way these questions are answered.

Example #1 - A Simple Standby Generator

A common generator set application provides back-up power to a building or load when a commercial power outage occurs. A simple standby system (for this article) is defined as a standby generator that is specified to require only a minimum amount of equipment to perform its backup role. A sample specification for such a generator might read as follows: "A diesel engine driven generator shall be provided to supply building loads during a commercial power failure. The generator shall be rated for: 277/480 Vac, four wire, grounded neutral; 60 Hertz; 60kW @ 0.8 PF; 1800 RPM; Brushless Excited; and Full Load Excitation: 63 Vdc @ Four Adc. The generator set and all auxiliary equipment shall be provided to make up an unattended standby power system."

Before we can begin to specify the excitation system, we must first review the types of loads the generator will be supplying, any economic factors that may exist, and what "auxiliary equipment" is required to make up the most functional/affordable design.

The building under consideration probably has a variety of loads, typical in most residential or industrial facilities. Several areas of concern could appear. If the generator is expected to pick up large block loads of kW and the engine is near its limit on available torque from no-load, a frequency compensated regulator should be selected. An adjustable underfrequency circuit can tune the reguator/generator/engine to obtain optimum performance. This allows for faster load pick-up with reduced speed variations.

The main feature of any underfrequency circuit is to protect the field windings from excessive current and to protect the regulator from overload. When a generator is operated at reduced frequency for extended periods of time, there is a very real danger that an excessive current will be supplied by the voltage regulator to the exciter field winding. This excessive current can be above the continuous rating of the voltage regulator and, if operated for some time, could cause the regulator to fail. The second danger is with the regulator delivering excessive current to the exciter field winding. In an attempt to have the generator deliver 100 per cent voltage during this underspeed condition, it causes excessive current to flow in the main rotor (field) of the generator. This excessive current is flowing at a time when the cooling air moving past the rotor by the generator fan is decreased. Therefore, at a time when the rotor needs cooling air most, the cooling air is reduced and, in turn, could cause the rotor or field windings of the main generator to fail.

During a power outage, a standby generator is usually started and brought up to rated speed immediately, with no warm-up period. If the engine is going to be warmed up when testing the generator system or cooled down before shutdown, the generator and excitation system will be operated at idle speed for extended periods of time. During such operation, the excitation to the generator should be shut down, with or without a frequency compensated regulator. A speed switch operating above the idle speed and below normal speed can be used to turn the excitation on and off (See Figure 2). The adjustment of the speed switch pull-in and dropout points should be selected for positive shutoff of excitation above any cool-down or warm-up speed and below 80 per cent of nominal speed.

If the building loads have a large content of induction motors or if the generator will be required to provide fault current to operate load breakers, the regulation system must be designed to provide for these needs. When a motor of significant size is started across the generator terminals, the voltage across the generator terminals can drop severely. If this happens and the regulator is powered from the generator terminals, the regulator may not have sufficient input power to maintain generator output and the system voltage could collapse completely. It may be necessary to add a current boost system to the voltage regulator to ensure system performance during motor starting and line fault circumstances.

This boost system can be in one of two forms. It can be either a Current Boost System (CBS) or a Series Boost System (SBS). The CBS system applies DC current directly to the field and does not rely on the regulator during those times when extra boosting of the excitation is required. The series boost system uses similar design theories, except it powers the regulator with a constant power supply and allows the regulator to stay in control of the generator at all times. To examine the effects of a boost system, let's look at the excitation system shown in Figure 3.

As load increases, the regulator is called upon to increase power to the exciter field. If a large block load were to be applied to the generator at one time, a voltage drop will occur. As this happens, the available power to the exciter is decreased and the ability of the system to recover is reduced. If this load is large enough, the system voltage will collapse and the generator will be unable to pick up the load, as shown in Figure 4.

If this were to occur on a system with multiple loads, for example, a hospital, high-rise housing complex or a shipboard application, the entire system could be jeopardized by a short circuit or heavy load on only one line. Looking at a simple example in Figure 5, it is seen that if the system voltage should collapse due to a fault on the line feeding Load #1, we would most likely be forced to open all three feeder breakers to the main generator breaker before restoring generator voltage. This is because the other loads on the system would make it impossible to restart from residual. If, however, an external power source were provided for the regulator, as shown in Figure 6, the regulator would continue to provide excitation to the exciter's field. This would allow the generator to continue to provide fault current although the generator terminal voltage is low or near zero. This means that, as soon as the fault is cleared, the voltage can return to normal as shown in Figure 7, thus restoring system integrity.

Another type of load condition that may cause a temporary overloading of the generator is the locked rotor condition encountered when starting a large motor. During the initial surge caused by starting a large motor on your system (large is relative to generator size), the motor winding may look to the generator much like a short circuit. This apparent short circuit can cause the same effect as a fault on your own system. However, if an external power source were connected to the field to apply additional excitation to the field during this time, you would have the same effect as providing a constant power source to the regulator. This is shown in Figure 8.

The above example is the basic principle used for the SBO and CBS systems. The only major difference is that we let the generator itself provide the power for the support system. This is done by use of current transformers in the generator's output lines. During a fault or motor starting condition, a high current is drawn from the generator. Power current transformers can be used to convert this high current to a lower level and then power the input of a regulator, or it can be rectified and injected directly into the exciter field. The first method to be discussed is the CBS feeding directly into the exciter field.

In this example, the power provided to the exciter field will always be channeled through the boost system. If the regulator senses an incorrect low voltage condition, the boost option is turned on and the power provided by the current transformer is rectified and directly injected into the field. This is an additive type of action and may provide a higher excitation level than either the boost option or regulator could provide alone. This may be very beneficial when motor starting is a major consideration. With this type of system, the regulator is in control of the boost option at all times. A typical interconnect for this system can be seen in Figure 9. If the regulator senses an underfrequency condition and reduces its excitation, thereby reducing the generator output voltage, you will not erroneously energize the boost option defeating the purpose of the underfrequency circuit in the regulator.

There is another method of providing forcing level excitation to the field of the exciter. This is to provide the regulator with a constant power source. This can be accomplished by using an SBO. This requires both current transformers and a reservoir assembly based on a ferroresonant circuit. As shown in Figure 10, there are two inputs to the SBO. One is from the generator terminal voltage and one from the power current transformer(s).

The ferroresonant circuit, consisting of Inductive, Resistive and Capacitive components, takes the voltage output from the generator and adds the power from the CTs to provide a relatively constant output voltage to the regulator input. Figure 11 shows the two CT connections for the SBO and an SRA type regulator. The output of the SBO is a constant voltage square wave. This square wave is an acceptable input to the SCR firing circuit of a voltage regulator. It also has the advantage, due to the L.C. filtering circuit, of not being noticeably affected by the notching usually introduced from UPS systems and the SCR loads placed on your generator. This has made the SBO a very useful tool when applied with this type of load.

One CT is required, as shown in Figure 12, if two phases of the generator may be safely passed through the available window. Two CTs may be used if safety dictates separating the two phases.

All of the systems discussed have a current transformer in two of the three power leads from the generator. If proper connections of the equipment are made, excitation may be maintained for any single phase fault by providing regulator power (or SBO input voltage) from the same phase used by the CTs for current. This will allow the regulator to maintain field power for any fault that may be applied to the generator output.

A short involving any one or two of the lines with a CT will provide power through that source. In the event of a short of the other lead to neutral, the two remaining power leads should provide adequate excitation power for the system.

Whether you use a CBS or SBO system, there are two major design characteristics that must be observed. The first is that all of the boost systems are phase sensitive. This is because they use the principle of phasor addition in order to ensure that the system actually provides a boosting action. Therefore, the correct phase relationships as shown in the interconnections must be followed. If this is not done, damage may be caused to the boost system, or a failure to hold excitation during a fault condition may be experienced.

As discussed in the previous pages, there are two basic methods of providing excitation support. One is the current boost type, providing power directly to the field. The other is the SBO type, which provides a relatively constant power source to the regulator. Either one of these systems can provide adequate power to the exciter field of your machine to ride through the disturbance caused by either a fault condition or application of large inductive loads as with motor starting. However, the SBO, with its large capacitors and inductor, will eliminate the adverse effects that a distorted power input waveform may have on the regulator's SCR controlled output stage.

The specification suggested that some "auxiliary equipment" is also required. This "auxiliary equipment" may refer to protective relaying that protects the generator and loads from being damaged from abnormal operating conditions or faults on the power system. At least two protective functions should be considered on every generator. These are as essential to generator and loads as the overspeed, low lube oil, and high water temperature are for the engine. The first protective device is overvoltage. If the excitation system fails to control the generator voltage, the voltage may go to the maximum of the generator's capability (saturation voltage) ranging from 40 per cent to 80 per cent above rated. The generator and the owner's load will suffer damage if this condition persists. An overvoltage sensor or relay is recommended to trip the generator circuit breaker and remove excitation.

Finally, after knowing all the system loads and generator requirements, we can select a voltage regulator. An APR 63-5 and CBS 305 Current Boost System were selected because of the concern about motor starting, block load pick-up and, of course, price. ET

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