Electrical power systems aim to provide a reliable service to all consumers and should be designed to cope with a wide range of normal, i.e. expected, operating conditions, such as:
- connection and disconnection of both large and small loads in any part of the network
- connection and disconnection of generating units to meet system demand
- scheduled topology changes in the transmission system.
They must also cope with a range of abnormal operating conditions resulting from faulty connections in the network, such as sudden loss of generation, phase conductors falling to the ground and phase conductors coming into direct contact with each other.
The ensuing transient phenomena that follow both planned and unplanned events bring the network into dynamic operation. In practice, the system load and the generation are changing continuously and the electrical network is never in a truly steady state condition, but in a perpetual dynamic state. The dynamic performance of the network exhibits a very different behavior within different time frames because of the diversity of its components (de Mello, 1975):
- rotating machinery
- transmission lines and cables
- power transformers
- power electronics based controllers
- protective equipment
- special controls.
The various plant components respond differently to the same stimulus. Accordingly, it is necessary to simplify, as much as is practicable, the representation of the plant components which are not relevant to the phenomena under study and to represent in sufficient detail the plant components which are essential to the study being taken. A general formulation and analysis of the electrical power network is complex because electrical, mechanical and thermal effects are interrelated.
For dynamic analysis purposes the power network has traditionally been subdivided as follows (Anderson and Fouad, 1977):
- synchronous generator and excitation system
- turbine-governor and automatic generation control
- boiler control
- transmission network
- loads.
The importance of the study, the time scales for which the study is intended and the time constants of the plant components are some of the factors which influence model selection (de Mello, 1975). Figure 1.16 gives a classification of power systems' dynamic phenomena.
Fig. 1.16 Classification of power systems' dynamic studies. Studies involving over-voltages due to lightning and switching operations require a detailed representation of the transmission system and the electrical properties of the generators, with particular attention paid to the capacitive effects of transmission lines, cables, generators and transformers. Over very short time scales the mechanical parameters of the generators and most controls can be ignored because they have no time to react to these very fast events, which take place in the time scale 10-7s ≤ t ≤ 10-2 s.
On the other hand, the long term dynamics associated with load frequency control and load shedding involve the dynamic response of the boiler and turbine-governor set and do not require a detailed representation of the transmission system because at the time scales 10-1s < t < 103s, the electrical transient has already died out. However, a thorough representation of the turbine governor and boiler controls is essential if meaningful conclusions are to be obtained. The mechanical behavior of the generators has to be represented in some detail because mechanical transients take much longer to die out than electrical transients.
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