M

Real values Evaluated values onnection point

20 40 60 80

13 8

Distance protection ZB

Distance protection ZB

R

al values

/aluated values

Ev

connecti

on point

Distance protection ZC

Distance protection ZC

Fig. 20. Divergences between the evaluated and expected values of the amplitude of impedance for protections in substations A, B and C

ZA

N

ZC

/

connec

ion poin

t

/

Fig. 20. Divergences between the evaluated and expected values of the amplitude of impedance for protections in substations A, B and C

Analyzing courses in Fig. 20, it can be observed that the highest inaccuracy in the amplitude of impedance evaluation concerns protections in substation C. The divergences between evaluated and expected values are rising along with the distance from the measuring point to the location of fault. It is characteristic that in substations A and B these divergences are at least one class lower than for substation C. This is the consequence of a significant

disproportion of the short-circuit powers of systems A and B in relation to the nominal power of WF.

On the other hand, for the fault in the C-M segment of line the evaluation error of an impedance fault loop is rising for distance protections in substations A and B. For distance protection in substation B a relative error is 53 % at fault point located 4 km from the busbars of substation C. For distance of 2 km from station C the error exceeds 86 % of the real impedance to the location of a fault (Lubosny, 2003).

Example 2

The network as in Figure 17 is operating with variable generating power of WF from 100 % to 10 % of the nominal power. The connection point is at 10 % of the line LA-B length. A simulated fault is located at 90 % of the LA-B length.

Table 3 shows the initial fault currents and error levels of estimated impedance components of distance protections in stations A and C. Changes of WF generating power PWF influence the miscalculations both for protections in station A and C. However, what is essential is the level of error. For protection in station A the maximum error level is 20 % and can be corrected by the modification of reactance setting by 2 Q (when the reactance of the line LAB is 12 Q). This error is dropping with the lowering of the WF generated power (Table 3).

WF power

IkA

I kc

ÖR( A)%

ÖX (A)%

ÖR(C)%

ÖX(C)%

Pwf

%Pwfn

[MW]

[%]

[kA]

[%]

[%]

[%]

[%]

[%]

60

100

2.362

0.481

18.101

18.101

453.286

453.286

54

90

2.374

0.453

16.962

16.962

483.749

483.749

48

80

2.386

0.422

15.721

15.721

521.910

521.910

42

70

2.401

0.388

14.364

14.364

571.213

571.213

36

60

2.416

0.35

12.877

12.877

637.187

637.187

30

50

2.433

0.308

11.253

11.253

729.171

729.171

24

40

2.454

0.261

9.454

9.454

867.905

867.905

18

30

2.474

0.208

7.473

7.473

1097.929

1097.929

12

20

2.499

0.148

5.264

5.264

1558.628

1558.628

6

10

2.527

0.079

2.779

2.779

2952.678

2952.678

Table 3. Initial fault currents and relative error levels of impedance estimation for protections in substations A and C in relation to the WF generated power

Table 3. Initial fault currents and relative error levels of impedance estimation for protections in substations A and C in relation to the WF generated power

For protection in substation C the error level is rising with the lowering of WF generated power. Moreover the level of this error is several times higher than for protection in station A. The impedance correction should be AR=92.124 Q and AX=307.078 Q. For the impedance of Lcb segment ZLCB=(3.48+j11.6) Q such correction is practically impossible. With this correction the impedance reach of operating characteristics of distance protections in substation C will be deeply in systems A and B. Figure 21 shows the course of error level of estimated resistance and reactance in protections located in the substations A and C in relation to the WF generated power.

When the duration of a fault is so long that the control units of WF are coming into action, the error level of impedance components evaluation for protections in the station C is still rising. This is the consequence of the reduction of WF participation in the total fault current.

Figure 22 shows the change of the quotient of steady fault currents flowing from substations A and C in relation to WF generated power PWF.

Fig. 21. Impedance components estimation errors in relation to WF generated power for protections a) in substation A, b) in substation C

Fig. 21. Impedance components estimation errors in relation to WF generated power for protections a) in substation A, b) in substation C

Quotient of short-circuit powers of sources A and C

Quotient of short-circuit powers of sources A and C

Fig. 22. Change of the quotient of steady fault currents flowing from sources B and C in relation of WF generated power

Fig. 22. Change of the quotient of steady fault currents flowing from sources B and C in relation of WF generated power

Example 3

Once again the network is operating as in Figure 17. There are quasi-steady conditions, WF is generating the nominal power of 60 MW, the fault point is at 90 % of the LA-B length. The changing parameter is the location of WF connection point. It is changing from 3 to 24 km from substation A.

Also for these conditions a higher influence of WF connection point location on the proper functioning of power protections can be observed in substation C than in substations A and B. The further the connection point is away from substation A, the lower are the error levels of estimated impedance components in substations A and C. It is the consequence of the rise of WF participation in the initial fault current (Table 4). The error levels for protections in substation A are almost together, whereas in substation C they are many times lower than in the case of a change in the WF generated power. If the fault time is so long that the control units of WF will come into action, limiting the WF fault current, the error level for protections in substation C will rise more. This is due to the quotient ~A(u)/~c(u) which is leading to the rise of estimation error AZc= ZMF ~A(u) .

Figure 23 shows the course of error of reactance estimation for the initial and steady fault current for impedances evaluated by the algorithms implemented in protection in substation C.

WF connection point location

Ia

Ic

Ic/Ia

Ia/Ic

AR(a)

AX(a)

AR(C)

AX(C)

[km]

[kA]

[kA]

[-]

[-]

[Q]

[Q]

[Q]

[Q]

3

2.362

0.481

0.204

4.911

0.586

1.955

14.143

47.142

6

2.371

0.525

0.221

4.516

0.558

1.860

11.381

37.936

9

2.385

0.57

0.239

4.184

0.516

1.721

9.038

30.126

12

2.402

0.617

0.257

3.893

0.462

1.541

7.007

23.358

15

2.424

0.6652

0.274

3.644

0.395

1.317

5.247

17.491

18

2.45

0.716

0.292

3.422

0.316

1.052

3.696

12.318

21

2.48

0.769

0.310

3.225

0.223

0.744

2.322

7.740

24

2.518

0.825

0.328

3.052

0.118

0.393

1.099

3.663

Table 4. Values and quotients of the initial fault currents flowing from sources A and C, and the error levels of impedance components estimation in relation to the WF connection point location

Table 4. Values and quotients of the initial fault currents flowing from sources A and C, and the error levels of impedance components estimation in relation to the WF connection point location

Error levels of reactance estimation for protection in substation C

Error levels of reactance estimation for protection in substation C

WF connection point [km]

Initial fault current — —Steady fault current

WF connection point [km]

Initial fault current — —Steady fault current

Fig. 23. Error level of the reactance estimation for distance protection in substation C in relation of WF connection point

Taking the network structure shown in Fig. 24, according to distance protection principles, the reach of the first zone should be set at 90 % of the protected line length. But in this case, if the first zone is not to reach the busbars of the surrounding substations, the maximum reactance settings should not exceed:

For distance protection in substation A: X1A < (1.2 + 0.8) = 2 Q

For distance protection in substation B: X1g < (10.8 + 0.8) = 11.6 Q

For distance protection in substation C: X1C < (1.2 + 0.8) = 2 Q

With these settings most of the faults on segment LMB will not be switched off with the self-time of the first zone of protection in substation A. This leads to the following switching-off sequence. The protection in substation B will switch off the fault immediately. The network will operate in configuration with two sources A and C. If the fault has to be switched off with the time At, the reaches of second zones of protections in substations A and C have to include the fault location. So their reach must extend deeply into the system A and the WF structure. Such a solution will produce serious problems with the selectivity of functioning of power protection automation.

Taking advantage of the in-feed factor kif also leads to a significant extension of these zones, especially for protection in substation C. Due to the highly changeable value of this factor in relation to the WF generated power and the location of connection, what will be efficient is only adaptive modified settings, according to the operating conditions identified in real time.

System A

System B

Fig. 24. Simplified impedance scheme of the network structure from the Figure 17

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