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System Frequency & Voltage
38 EEE8044: Fundamentals
Section 4
Control of Network Frequency and Voltage
Introduction
Consumers require an electrical supply at constant voltage and 代写留学生论文frequency. Both quantitiesmust be controlled within tight limits, or the customer must be disconnected for safetyreasons. In general, as consumer demand increases there is a tendency for the systemvoltage and frequency to fall. This change in voltage and frequency must be detected at the
power stations and counteracted by the generator controls.
In this section we will look at the need for maintaining the real power balance in thesystem and examine the relationship between this and the control of system frequency. Wewill also describe some of the popular methods used for line voltage control.
Learning Outcomes
On completing this section you will be able to:
• Describe the relationship between load and frequency in a power system.
• Summarise the methods used to control network voltage levels.
Time
You will need about 2 hours for this section.
Resources
Access to the Internet, calculator, pen and paper.
S4: System Frequency & Voltage
39 EEE8044: Fundamentals
4.1 Matching of Supply and Demand
The problem of maintaining the frequency constant at 50 Hz is the most basic taskconfronting the power system operator and is closely interwoven with the problem ofkeeping the balance of real power in the network. First, however, we will explore what wemean by the term load and examine some of its characteristics.In an electrical network, many types of customers or end users consume power at allvoltage levels. The load on a system, or on any piece of equipment within the system, is
the sum of the electrical demand of all the users supplied by that system or piece ofequipment.
All individual loads, whether they are three-phase large industrial loads (such as a big 2.5MW pump in a water pumping station or a 6 MW gas compressor in an oil refinery) orsingle-phase small domestic loads (like a kettle or vacuum cleaner), are random in nature.
However, because of the very large number of individual loads, the total load will varyfrom season to season, day to day, and even from hour to hour, in a smooth and highlypredictable manner. Diagrams of load as a function of time are called load curves. Atypical winter load curve showing a daily load cycle is given in Fig.4.1.This means that the power system operator can usually predict with a high degree ofaccuracy what the load would be from one day to the next and thus allocate the appropriatepower generation.
0
20
40
00:00
04:00
08:00
12:00
16:00
20:00
00:00
Time
Demand (GW)
peak
average
Fig. 4.1 Winter daily variation in consumer demand.
S4: System Frequency & Voltage
40 EEE8044: Fundamentals
It is not possible to store electrical energy economically on a scale significant in the
context of national demand. Therefore:
Power Generated = Power Consumed + Losses
Hence, there must be:
􀂃 sufficient installed and available capacity to meet peak demand: requires prediction of
future demand, careful scheduling of maintenance
􀂃 reserve capacity (spinning reserve) to meet fast changes in demand: requires short-term
forecasting of demand using weather forecasts, TV schedules, previous experience
4.2 Interconnected Systems
Interconnection of many generating