Load-Frequency Control in Power System | Electrical Engineering

In this article we will discuss about the load frequency control in power system.

In a power system, both active and reactive power demands continually vary with the rising or falling trend. Power input (steam input to turbo-generators or water input to hydro- generators) must, therefore, be continuously regulated to match the active power demand; otherwise the machine speed will change with consequent change in frequency, which may be highly undesirable. Also the excitation of generators must be continuously regulated to match the reactive power demand with reactive generation, failing which the voltages at various system buses may go beyond the prescribed limits.

It is necessary to maintain the frequency of the power system constant (maximum permissible variation in supply frequency is ± 0.5 Hz).

The reasons for it are as follows:

1. The frequency control keeps the balance between generation and absorption of real power and thus makes the operation of power station in parallel satisfactory.

2. The speed of synchronous and induction motors, extensively used in industry as prime movers, depends upon supply frequency (the synchronous speed being equal to 120 f/P where f is supply frequency and P is the number of poles) and so change in supply frequency causes variations in speed of motors of consumers- not desirable for consumers, particularly the process industries depending on constant speed drives.

3. The variation in supply frequency beyond permissible limits also affects the performance of electric motors.

4. The extensive use of synchronous clocks establishes a strong requirement for maintaining supply frequency constant in order to have correct timing from such clocks.

When the load on a generator or a group of generators increases, the rotors slow down resulting in reduction in frequency. However, the governors adjust the input so as to bring the frequency to the original level. This control of frequency by the action of governors is called the primary control. The action of the governors is automatic. A drop in speed due to increased load causes governor action so as to increase the input (admit more steam into the turbine in case of steam power plants) and thus output.

In the event of loss of load or sudden change in load, the governor controls the speed of generators. However, frequency control by governors alone is not adequate and ‘secondary control’ is necessary. In secondary control, the loading on different plants is changed according to the instructions of the central load despatcher.

Load and frequency control of interconnected generators introduces problems which are relatively simple in a system having one or two generating stations but are more difficult and complex in large interconnected systems with many stations scattered over a wide area.

Methods commonly employed for load- frequency control in interconnected power systems are explained below:

Very small isolated generating stations can have manual control of frequency. The governors adjust the input to bring the frequency within permissible limits.

1. Flat Frequency Control:

Consider two stations A and B operating in parallel and interconnected by a tie-line.

The frequency of the system is maintained constant by regulating only one station A and without any regulation of station B. If the load at either station A, station B or both changes, the generation at station A is changed to maintain a balance between generation and load on the system. This type of frequency control is called flat frequency control.

The drawback of this method of frequency control is that station A must have enough capacity to absorb the load variations for the entire system. Secondly the tie-line between the two stations would have to absorb all load variations at station B as the generation at station B is maintained constant. These may make the operation of generator at station A uneconomical and could result in certain limitations in the operation of the system.

2. Parallel Frequency Control:

In this method of frequency control both stations A and B would be regulated simultaneously to maintain frequency constant. By this method, the load swings are shared by both stations A and B and therefore, the swings on each station and on the line would be reduced.

3. Flat Tie-Line Control:

In this method of frequency control the increase in load of an area is met by increasing the generation in that area and thus power flow in the tie-line is kept constant irrespective of load demands. This method is used when a small system and a large system are interconnected through a tie-line. The large system maintains the system frequency constant while the small system is controlled to keep the tie-line power constant.

This method is not suitable when two or more large systems are interconnected as in such cases and with this type of control the tie-line power and frequency deviation have a tendency to swing back and forth (in addition to the swings at tie-line natural frequency) following a load change. The control equipment used consists of the frequency controller only at the larger system and tie-line power controller- recorder at the smaller system.

4. Tie-Line Load Bias Control:

This is the modification of (1) and (3) in that the system is allowed to follow its normal regulating characteristic directed toward holding normal frequency. This is the most widely used method on large interconnections. All power systems assist in regulating frequency and tie-line power flow regardless of where from the frequency variation originates.

The amount of assistance given by any one system is controlled by simple adjustment of the control equipment. The control equipment consists of load frequency controller and tie-line load recorder-controller. The tie-line instrument biases the frequency controller till a desired relationship between tie-line loading and system frequency is had.

Automatic Load-Frequency Control:

In modern large interconnected systems, manual operation is not feasible as continuous watch on the frequency and loadings of various generators and tie-lines has to be kept by the operators and they have to continuously make adjustments. The manual controls are sluggish and involve inherent human time lags. Obviously automatic control is more efficient method of load- frequency control.

The problem of automatic load-frequency control resolves itself into:

(i) The measurement of a quantity,

(ii) Interpretation of the measurement in terms of deviation from a control point, and

(iii) The application of corrections to restore the measured quantity to its normal value.

In some cases more than one measurement is needed for proper operation of the control equipment. In one of these control systems, developed by Leeds and Northrup Co, both load and frequency are measured to give various types of combined load-frequency control.

The loads on various generators, stations, and systems are measured through the summation of various thermal- converter millivolt outputs and frequency is measured by a frequency-bridge type instrument. As the system frequency changes, the bridge circuit is rebalanced by the instrument movement which positions a slide wire used for transmission of a direct voltage. All these data’s are fed into master controller; it is able to detect the need for more or less generation and to send impulses to the different stations calling for load increase or decrease.

By the use of area requirement, proportional load control, the equipment is able to call for changes at the several generating stations such that they each, in effect, supply the load of their respective areas, thereby causing a minimum of power flow over the tie feeders from one station to another.

Within each generating station it is possible to select the units that will be employed for regulation and for adjustment of the percentage of the required load change that is placed on each machine.

The following three types of area control may be used, one at a time, as selected by the system operator.

Flat frequency control varies the power input to the prime mover in order to correct the system frequency to the pre­determined value.

Flat tie-line control varies the power input to the prime mover so that the tie-line load is corrected to a predetermined schedule. In this method another system is required to maintain frequency constant.

Tie-line bias control is a modification of the above two types of area control and has been found to work very satisfactorily on power systems where a large number of generators and stations are required to be kept under control. If the system frequency variation is more than a predetermined value, say 1/4-1/2 Hz, the control can be made to change from automatic to “manual” automatically and sound an alarm, so the system operator can correct the faulty condition.