Methods to Improve System Stability

It is obvious that it is desirable that a power system can withstand as many disturbances as possible without becoming unstable. However, it is realised that it is not possible to design a system that can cope with all conceivable contingencies, so one has to restrict the considered disturbances in the design of the system. Usually, one considers only the most frequent faults in the system. It is also important to consider the consequence of a fault or disturbance when designing a system. Generally it can be said that the more investments that can be made in a system, the more robust the system can be made. One is thus faced with a technical-economical optimisation problem that is very complex. Furthermore, there are a number of parameters that are both very important in the optimisation process and at the same time very hard to quantify. Such a parameter is the value that consumers put on uninterruptable supply of electric power of good quality. This optimisation process has gained a lot of interest during the last few years due to the liberalisation (de-regulation) of the power market taking place all over the world. Theoretically the optimum value of reliability of power supply is when the marginal increase in the value perceived by the customers is equal to the cost of the investment made to achieve this increase in reliability.

Practically it is of course impossible to deduce this optimum, and one has to judge by different methods if the consumers are satisfied with the reliability of the system, and if not if they are willing to pay more to improve it. To have some more clear and practical rules for system design a number of deterministic rules have been used over the years. One such rule is the so called (N − 1)-rule, which says that a system should be able to supply customer loads with any component, generator, line, etc., out of service. The disconnection of the component should preceded by a fault and the transients triggered by this fault should be considered. A more conservative approach would be to apply an (N − 2)-criterion, which would result in a more reliable but more expensive system. Modern methods apply probabilistic tools. In many of these approaches one considers both the probability of a fault and its consequences. By assigning a measure, i.e. a number, for this combination of fault probability and consequence a risk based security analysis can be made. Some of these aspects are also discussed in chapter 14. This section will be concluded by a discussion of different ways to improve the angular stability in power systems. The stabilising effect of the different methods can be verified by studying Figures 11.3 and 11.9. Which method to apply in a given situation depends on a large number of parameters and must be determined from case to case, usually after extensive studies. In some cases, one or several methods can be excluded, e.g. the construction of new lines since the needed permission cannot be granted from the authorities. The most common methods to increase system angular stability are:

• Increase of the inertia constant of the generators. This makes the rotors more difficult to accelerate in connection with faults and the risk for losing synchronism is reduced. In most cases this is a very expensive means and only in special cases it can be applied, e.g. by installing a flywheel on a small hydro unit.

• Increase of system voltage. This increases Pe,max and for given power Pm the stability margin is increased.

• Reduction of the transfer reactance Xe. This will also increase Pe,max as in the previous case. This can be achieved by constructing parallel lines, or by installing series capacitors on existing lines or new lines. By installing series capacitors the effective reactance of the line is reduced. This method has been used extensively over the years, e.g. in the Swedish system.

• Installation of fast protections and fast breakers. In this way the time with a fault connected can be reduced and thereby the time during which the generator rotors are accelerated. The ability for the system

to decelerate the rotor swings is increased. Another way is to use automatic re-closure after the fault is cleared, see Appendix A.

• Implementation of fast valving in steam turbines. By fast control of the mechanical power during and after a fault, the acceleration of the rotors can be decreased. It cannot be applied to nuclear power plants by security reasons. The method has not been used to any larger extent, since it is claimed to impose large thermal and mechanical stresses on valves, turbines, etc.

• Installation of braking resistors. These are shunt resistors that are connected by breakers fast after a fault close to a critical generator. The electric load of the critical machine increases and the risk for losing synchronism is reduced.

• Stability control of controllable devices such as High Voltage Direct Current (HVDC), controllable series capacitors, controllable reactive shunt compensation (SVC), etc. These devices are usually too expensive to install just for stabilising the system, but when they are installed, the cost to use their controllability for stabilisation is usually marginal.