Z2 a 09 ZAB 09ZBC

tripping time should be one step higher than the first one t2=At s from the range of (0.3^0.5) s. Typically for the digital protections and fast switches, a delay of 0.3 s is taken;

impedance reach of the third zone is maximum 90% of the second zone of the shortest line outgoing from the subsubstation B:

For the selectivity condition, tripping time for this zone cannot by shorter than t3=2At s. Improper fault elimination due to the low fault current value

As mentioned before, when the fault current flowing from the DPGS is close to the nominal current, in most of cases overcurrent and distance criteria are difficult or even impossible to apply for the proper fault elimination (Pradhan & Geza, 2007). Figure 8 presents sample t. = 2 At s t = At s t = 0 s

courses of the rms value of voltage U, current I, active and reactive power (P and Q) when there are voltage dips caused by faults in the network. The recordings are from a wind turbine equipped with a 2 MW generator with a fault ride-through function (Datasheet, Vestas). This function permits wind farm operation during voltage dips, which is generally required for wind farms connected to the HV networks.

Fig. 8. Courses of electric quantities for Vestas V80 wind turbine of 2 MW: a) voltage dip to 0.6 Un, b) voltage dip to 0.15 Un (Datasheet, Vestas)

Analyzing the course of the current presented in Fig. 8, it can be observed that it is close to the nominal value and in fact independent a of voltage dip. Basing on the technical data it is possible to approximate ti time, when the steady-state current will be close to the nominal value (Fig. 9).

UG [p.u.]> 1

IG Ip.u.] . 1 -

l\

t

1 1

Ilm_G [p.U. 1-

\

t

_

0

2 fa t

Fig. 9. Linear approximation of current and voltage values for the wind turbine with DFIG generator during voltage dips: Ug - voltage on generator outputs, Ig - current on generator outputs, iim_G - generator reactive current, ti «50 ms, Î3-Î2 «100 ms

Fig. 10. Course of the wind turbine reactive current

Fig. 10. Course of the wind turbine reactive current

The negative influence of the low value steady current from the wind farm is cumulating especially when the distribution network is operating in the open configuration (Fig. 11).

System A

System B

System B

Fig. 11. Wind farm in the distribution network operating in the open configuration

The selected wind turbine is the one most frequently used in the Polish power grid. The impulse current at the beginning of the fault is reduced to the value of the nominal current after 50 ms. Additionally, the current has the capacitance character and is only dependent on the stator star/delta connection. This current has the nominal value for delta connection (high rotation speed of turbine) and nominal value divided by V3 for the star connection as presented in Fig. 9.

Reaction of protection automation systems in this configuration can be estimated comparing the fault current to the pick-up currents of protections. For a three-phase fault at point F (Fig. 11) the steady fault current flowing through the wind farm cannot exceed the nominal current of the line. The steady fault current of the single wind turbine of PN=2 MW (SN=2.04 MW) is Ik = ING = 10.7 A at the HV side (delta stator connection). However initial fault current Ik is 3,3 times higher than the nominal current (Ik = 35.31 A ).It must be emphasized that the number of working wind turbines at the moment of a fault is not predictable. This of course depends on weather conditions or the network operator's requirements. All these influence a variable fault current flowing from a wind farm. In many cases there is a starting function of the distance protection in the form of a start-up current at the level of 20% of the nominal current of the protected line. Taking 600 A as the typical line nominal current, even several wind turbines working simultaneously are not able to exceed the pick-up value both in the initial and the steady state fault conditions. When the impedance function is used for the pick-up of the distance protection, the occurrence of high inaccuracy and fluctuations of measuring impedance parameters are expected, especially in the transient states from the initial to steady fault conditions.

The following considerations will present a potential vulnerability of the power system distribution networks to the improper (missing) operation of power line protections with connected wind farms. In such situations, when there is a low fault current flow from a wind farm, even using the alternative comparison criteria will not result in the improvement of its operation. It is because of the pick-up value which is generally set at (1,2 4 1,5) IN. To minimize the negative consequences of functioning of power system protection automation in HV network operating in an open configuration with connected wind farms, the following instructions should be taken:

• limiting the generated power and/or turning off the wind farm in the case of a radial connection of the wind farm with the power system. In this case, as a result of planned or fault switch-offs, low fault WF current occurs,

• applying distance protection terminals equipped with the weak end infeed logic on all of the series of HV lines, on which the wind farm is connected. The consequences are building up the fast teletransmission network and relatively high investment costs,

• using banks of settings, configuring adaptive distance protection for variant operation of the network structure causing different fault current flows. When the HV distribution network is operating in a close configuration, the fault currents considerably exceed the nominal currents of power network elements. In the radial configuration, the fault current which flows from the local power source will be under the nominal value.

Selected factors influencing improper fault location of the distance protections of lines

In the case of modifying the network structure by inserting additional power sources, i.e. wind farms, the intermediate in-feeds occur. This effect is the source of impedance paths measurement errors, especially when a wind farm is connected in a three-terminal configuration. Figure 12a shows the network structure and Fig. 12b a short-circuit equivalent scheme for three-phase faults on the M-F segment. Without considering the measuring transformers, voltage Up in the station A is:

On the other hand current Ip measured by the protection in the initial time of fault is the fault current Ia flowing in the segment A-M. Thus the evaluated impedance is:

where:

Up - positive sequence voltage component on the primary side of voltage transformers at point A,

Ip- positive sequence current component on the primary side of current transformers at point A,

Ia - fault current flowing from system A, Iwf - fault current flowing from WF, Zam - impedance of the AM segment, Zmf - impedance of the MF segment, kif - intermediate in-feed factor.

System A

B System

Fig. 12. Teed feeders configuration a) general scheme, b) equivalent short-circuit scheme. It is evident that estimated from (5) impedance is influenced by error AZ:

The error level is dependent on the quotient of fault current IZ from system A and power source WF (wind farm). Next the error is always positive so the impedance reaches of the operating characteristics are shorter. Evaluating the error level from the impedance of the equivalent short-circuit:

7wf + 7wfm

Equation (7) shows the significant impact on the error level of short-circuit powers (impedances of power sources), location of faults (ZAM, ZFWM ) and types of faults. Minimizing possible errors in the evaluation of impedance can be achieved by modifying the reaches of operating characteristics covering the WF location point. Thus the reaches of the second and the third zone of protection located at point A (Fig. 7) are:

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