The dynamic response of electrical power networks

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.

previous Power Distribution

next Transient stability in power system

Power Distribution

Distribution networks may be classified as either meshed or radial. However, it is customary to operate meshed networks in radial fashion with the help of mechanically operated switches (Gönen, 1986). It is well understood that radial networks are less reliable than interconnected networks but distribution engineers have preferred them because they use simple, inexpensive protection schemes, e.g. over-current protection. Distribution engineers have traditionally argued that in meshed distribution networks operated in radial fashion, most consumers are brought back on supply a short time after the occurrence of a fault by moving the network's open points. Open-point movements are carried out by re-switching operations.

Traditional construction and operation practices served the electricity distribution industry well for nearly a century. However, the last decade has seen a marked increase in loads that are sensitive to poor quality electricity supply. Some large industrial users are critically dependent on uninterrupted electricity supply and suffer large financial losses as a result of even minor lapses in the quality of electricity supply (Hingorani, 1995).

Fig. 1.12 Three-phase unified power flow controller.

Fig. 1.13 HVDC Light systems using VSCs.

These factors coupled with the ongoing deregulation and open access electricity markets, where large consumers may shop around for competitively priced, high-quality electricity, have propelled the distribution industry into unprecedented change. On the technical front, one major development is the incorporation of power electronics controllers in the distribution system to supply electricity with high quality to selected customers. The generic, systematic solution being considered by the utility to counter the problem of interruptions and low power quality at the end-user level is known as Custom Power. This is the low voltage counterpart of the more widely known FACTS technology.

Although FACTS and custom power initiatives share the same technological base, they have different technical and economic objectives (Hingorani and Gyugyi, 2000). Flexible alternating current transmission systems controllers are aimed at the transmission level whereas Custom Power controllers are aimed at the distribution level, in particular, at the point of connection of the electricity distribution company with clients with sensitive loads and independent generators. Custom Power focuses primarily on the reliability and quality of power flows. However, voltage regulation, voltage balancing and harmonic cancellation may also benefit from this technology.

The STATCOM, the DVR and the solid state switch (SSS) are the best known Custom Power equipment. The STATCOM and the DVR both use VSCs, but the former is a shunt connected device which may include the functions of voltage control, active filtering and reactive power control. The latter is a series connected device which precisely compensates for waveform distortion and disturbances in the neighbourhood of one or more sensitive loads. Figure 1.10 shows the schematic representation of a three-phase STATCOM. Figure 1.14 shows that of a DVR and Figure 1.15 shows one phase of a three-phase thyristor-based SSS.

The STATCOM used in Custom Power applications uses PWM switching control as opposed to the fundamental frequency switching strategy preferred in FACTS applications. PWM switching is practical in Custom Power because this is a relatively low power application.

On the sustainable development front, environmentally aware consumers and government organizations are providing electricity distribution companies with a good business opportunity to supply electricity from renewable sources at a premium. The problem yet to be resolved in an interconnected system with a generation mix is how to comply with the end-user's desire for electricity from a renewable source. Clearly, a market for renewable generation has yet to be realized.

Utilization

The customers of electricity vendors may be classified as industrial, commercial and domestic (Weedy, 1987). In industrialized societies, the first group may account for as much as two fifths of total demand. Traditionally, induction motors have formed the dominant component in the vast array of electric equipment found in industry, both in terms of energy consumption and operational complexity. However, computer-assisted controllers and power electronics-based equipment, essential features in modern manufacturing processes, present the current challenge in terms of ensuring their trouble-free operation. This equipment requires to be supplied with high quality electricity.

Fig. 1.14 Three-phase dynamic voltage restorer.

Fig. 1.15 Thyristor-based solid state switch.

Some loads draw constant current from the power system and their operation may be affected by supply voltage and frequency variations. Examples of these loads are:

• induction motors
• synchronous motors
• DC motors.

Other types of loads are less susceptible to voltage and frequency variations and exhibit a constant resistance characteristic:

• incandescent lighting
• heating.

Large clusters of end user loads based on power electronics technology are capable of injecting significant harmonic currents back into the network. Examples of these are:

• colour TV sets
• microwave ovens
• energy saving lamps
• computer equipment
• industrial variable speed motor drives
• battery recharging stations.

Electric energy storage is an area of great research activity, which over the last decade has experienced some very significant breakthroughs, particularly with the use of superconductivity and hydrogen related technologies. Nevertheless, for the purpose of industrial applications it is reasonable to say that, apart from pumped hydro storage, there is very little energy storage in the system. Thus, at any time the following basic relation must be met:

Generation = Demand + Transmission Losses

Power engineers have no direct control over the electricity demand. Load shedding may be used as a last resort but this is not applicable to normal system control. It is normally carried out only under extreme pressure when serious faults or overloads persist.

previous Power Transmission

next The dynamic response of electrical power networks

Power Generation

The high demand of electricity together with the continuously variable nature, and our inability to store electricity in a significant number of calls for a diversity of production in the grid. The traditional view is that the use of different primary energy resources contributes to the continuity of supply and stable price mechanism.

Most of the electricity consumed worldwide is produced by three-phase synchronous generators (Kundur, 1994). However, three-phase induction generators will increase their production share when wind generation (Heier, 1998) becomes more widely available. Similarly, three-phase and single-phase static generators in the form of fuel cells and photovoltaic arrays should contribute significantly to global electricity production in the future.

For system analysis purposes the synchronous machine can be seen as consisting of a stationary part, i.e. armature or stator, and a moving part, the rotor, which under steady state conditions rotates at synchronous speed.

Synchronous machines are grouped into two main types, according to their rotor structure (Fitzgerald et al., 1983):

1. salient pole machines
2. round rotor machines.

Steam turbine driven generators (turbo-generators) work at high speed and have round rotors. The rotor carries a DC excited field winding. Hydro units work at low speed and have salient pole rotors. They normally have damper windings in addition to the field winding. Damper windings consist of bars placed in slots on the pole faces and connected together at both ends. In general, steam turbines contain no damper windings but the solid steel of the rotor offers a path for eddy currents, which have similar damping effects. For simulation purposes, the currents circulating in the solid steel or in the damping windings can be treated as currents circulating in two closed circuits (Kundur, 1994). Accordingly, a three-phase synchronous machine may be assumed to have three stator windings and three rotor windings. All six windings will be magnetically coupled.

Figure 1.2 shows the schematic diagram of the machine while Figure 1.3 shows the coupled circuits. The relative position of the rotor with respect to the stator is given by the angle between the rotor's direct axis and the axis of the phase A winding in the stator. In the rotor, the direct axis (d-axis) is magnetically centred in the north pole. A second axis located 90 electrical degrees behind the direct axis is called the quadrature axis (q-axis).

Fig. 1.2 Schematic diagram of a synchronous machine.

In general, three main control systems directly affect the turbine-generator set:

• the boiler's firing control
• the governor control
• the excitation system control.

Fig. 1.3 Coupled windings of a synchronous machine.

Figure 1.4 shows the interaction of these controls and the turbine-generator set. The excitation system control consists of an exciter and the AVR. The latter regulates the generator terminal voltage by controlling the amount of current supplied to the field winding by the exciter. The measured terminal voltage and the desired reference voltage are compared to produce a voltage error which is used to alter the exciter output. Generally speaking, exciters can be of two types: (1) rotating; or (2) static. Nowadays, static exciters are the preferred choice owing to their higher speed of response and smaller size. They use thyristor rectifiers to adjust the field current (Arco, 2000).

Fig. 1.4 Main controls of a generating unit.

previous General composition of the power network

next Power Transmission