Influence of connecting dispersed power generating sources to the distribution network on the proper functioning of power system protections

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In the Polish power system most of generating power plants (the so-called system power plants) are connected to the HV and EHV (220 kV and 400 kV) transmission networks. Next, HV networks are usually treated as distribution networks powered by the HV transmission networks. This results in the lack of adaptation of the power system protection automation in the distribution network to the presence of power generating sources on those (MV and HV) voltage levels.

Even more frequently, using of the DPGS, mainly wind farms, is the source of potential problems with the proper functioning of power protection automation. The basic functions vulnerable to the improper functioning in such conditions are:

• primary protection functions of lines,

• earth-fault protection functions of lines,

• restitution automation, especially auto-reclosing function,

• overload functions of lines due the application of high temperature low sag conductors and the thermal line rating,

• functions controlling an undesirable transition to the power island with the local power generation sources.

The subsequent part of this paper will focus only on the influence of the presence of the wind farms on the correctness of action of impedance criteria in distance protections.

5.1 Selected aspects of an incorrect action of the distance protections in HV lines

Distance protection provides short-circuit protection of universal application. It constitutes a basis for network protection in transmission systems and meshed distribution systems. Its mode of operation is based upon the measurement and evaluation of the short-circuit impedance, which in the typical case is proportional to the distance to the fault. They rarely use pilot lines in the 110 kV distribution network for exchange of data between the endings of lines. For the primary protection function, comparative criteria are also used. They take advantage of currents and/or phases comparisons and use of pilot communication lines. However, they are usually used in the short-length lines (Ungrad et al., 1995). The presence of the DPGS (wind farms) in the HV distribution network will affect the impedance criteria especially due to the factors listed below:

• highly changeable value of the fault current from a wind farm. For wind farms equipped with power converters, taking its reaction time for a fault, the fault current is limited by them to the value close to the nominal current after typically not more then 50 ms. So the impact of that component on the total fault current evaluated in the location of protection is relatively low.

• intermediate in-feed effect at the wind farm connection point. For protection realizing distance principles on a series of lines, this causes an incorrect fault localization both in the primary and the back-up zones,

• high dynamic changes of the wind farm generating power. Those influence the more frequent and significant fluctuations of the power flow in the distribution network. They are not only limited to the value of the load currents but also to changes of their directions. In many cases a load of high values must be transmitted. Thus, it is necessary to use wires of higher diameter or to apply high temperature low sag conductors or thermal line rating schemes (dynamically adjusting the maximum load to the seasons or the existing weather conditions). Operating and load area characteristics may overlap in these cases.

Setting distance protections for power lines

In the case of distance protections, a three-grading plan (Fig. 7) is frequently used. Additionally, there are also start-up characteristic and the optional reverse zone which reach the busbars.

System

A Substation 1

System Substation 2 g

Fig. 7. Three-grading plan of distance protection on series of lines

The following principles can be used when the digital protection terminal is located in the substation A (Fig. 7) (ziegler, 1999):

• impedance reach of the first zone is set to 90 % of the A-B line-length

tripping time t1=0 s;

impedance reach of the second zone cannot exceed the impedance reach of the first zone of protection located in the substation B

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