The behaviour of a power system in dynamic fault states is much more complicated for the reason of the presence of dispersed power sources than when only the conventional ones are in existence. This is a direct consequence of such factors as the technical construction of driving units, different types of generators, the method of connection to the distribution network, regulators and control units, the presence of fault ride-through function as well as a wide range of the generating power determined by e.g. the weather conditions. Taking the level of fault current as the division criteria, the following classification of dispersed power sources can be suggested:
• sources generating a constant fault current on a much higher level than the nominal current (mainly sources with synchronous generators),
• sources generating a constant fault current close to the nominal current (units with DFIG generators or units connected by the power converters with the fault ride-through function),
• sources not designed for operation in faulty conditions (sources with asynchronous generators or units with power converters without the fault ride-through function).
Sources with synchronous generators are capable of generating a constant fault current of higher level than the nominal one. This ability is connected with the excitation unit which is employed and with the voltage regulator. Synchronous generators with an electromechanical excitation unit are capable of holding up a three-phase fault current of the level of three times or higher than the nominal current for a few seconds. For the electronic (static) excitation units, in the case of a close three-phase fault, it is dropping to zero after the disappearance of transients. This is due to the little value of voltage on the output of the generator during a close three-phase fault.
For asynchronous generators, the course of a three-phase current on its outputs is only limited by the fault impedance. The fault current drops to zero in about (0,2 4 0,3) s. The maximum impulse current is close to the inrush current during the motor start-up of the generator (Lubosny, 2003). The value of such a current for typical machines is five times higher than the nominal current. This property makes it possible to limit the influence of such sources only on the initial value of the fault current and value of the impulse current. The construction and parameters of the power converters in the power output circuit determine the level of fault current for such dispersed power sources. Depending on the construction, they generate a constant fault current on the level of its nominal current or are immediately cut off from the distribution network after a detection of a fault. If the latter is the case, only a current impulse is generated just after the beginning of a fault. A common characteristic of dispersed sources cooperating with the power system is the fact that they can achieve local stability. Some of the construction features (power converters) and regulatory capabilities (reactive power, frequency regulation) make the dispersed power generation sources units highly capable of maintaining the stability in the local network area during the faulty conditions (Lubosny, 2003).
Dynamic states analyses must take into consideration the fact that present wind turbines are characterized by much higher resistance to faults (voltage dips) to be found in the power system than the conventional power sources based on the synchronous generators. A very important and useful feature of some wind turbines equipped with power converters, is the fact that they can operate in a higher frequency range (43 4 57 Hz) than in conventional sources (47 4- 53 Hz) (Ungrad et al., 1995).
Dispersed generation may have a positive influence on the stability of the local network structures: dispersed source - distribution network during the faults. Whether or not it can be well exploited, depends on the proper functioning of the power system protection automation dedicated to the distribution network and dispersed power generation sources.
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