DOS CONSTITUCIONES DEUDORAS DE CÁDIZ: DOS SICILIAS (1821) Y PORTUGAL (1822)

TheT ENDtime delay is started subsequent to the protection pickup. The fault loop is determined now. The loop impedance components are compared with the limit values of the zones previously set. The tripping is executed if the impedance is within its zone during the expiration of the corresponding time stage. For the first Z1 zone and also for the Z1B overreach zone, the time delays will in most cases be zero or at least very short, i.e. the tripping is executed as soon as it is certain that the fault is within this zone.

The Z1B overreach stage can be switched effective from outside, via a binary input.

For the Z2 zone which may reach into the network, a time delay is selected overreaching the first stage of the power system protection.

R1=Z1 X2=Z2 X1b=Z1b

X1=Z1

R2=Z2 R1b=Z1b

R X

Impedance Protection (ANSI 21)

A drop-out can only be caused by a drop-out of the overcurrent pickup and not by leaving the tripping polygon.

Figure 2-61 illustrates the logic diagram of the impedance protection.

Figure 2-61 Logic Diagram of the Impedance Protection

3306 ZONE Z1

see Figure 2-120 in Section 2.38.1.4

2.17.2 Setting Hints

General The machine impedance detection is only effective and accessible if it was previously set within the framework of project configuration (Section 2.2) at address0133, IMPEDANCE PROT.=Enabled. SetDisabled if the function is not required.

Address3301 IMPEDANCE PROT.is used to switch the functionONandOFF, or to block only the trip command (Block Relay).

Pickup The maximum load current during operation is the most important criterion to be considered when setting the overcurrent pickup. A pickup by an overload must be excluded in any case! For this reason, the3302 IMP I>pick-up value must be set above the maximum (over) load current to be expected. Recommended setting: 1.2 to 1.5 times the nominal machine current. The pickup logic corresponds to the logic of the UMZ I> definite time-overcurrent protection.

If the pickup is derived from the generator terminals and if, for this reason, the short circuit can fall below the pickup value (address3302) due to the collapsing voltage, the undervoltage seal-in feature is used, i.e. the address3303 U< SEAL-INis switchedON.

The undervoltage seal-in featureU<(address3304) is set to a value below the lowest phase-to-phase voltage occurring during operation, e.g. toU<= 75 % to 80 % of the nominal voltage. The seal-in time (address3305 T-SEAL-IN) must exceed the maximum fault clearance time in a back-up case (recommended setting: address 3312 T END+ 1 s).

Impedance Stages As illustrated in figure 2-60, the protection has the following characteristics which may be set independently:

− 1st zone (Z1 instantaneous zone) with the setting parameters ZONE Z1 Reactance = reach,

T-Z1 = 0 or short delay, if required.

− Z1B overreach zone, externally controlled via binary input, with the setting parameters

ZONE Z1B Reactance = reach,

T-Z1B T1B = 0 or short delay, if required.

− 2nd zone (Z2 zone) with the setting parameters ZONE Z2 Reactance = reach,

ZONE2 T2 The user must select a value for T2 which is situated above the grading time of the network protection.

− Non-directional final stage with the setting parameter

T END The user must select a value for T END which ensures that the 2nd or 3rd stage of the series-connected power system distance protection is overreached.

As the user can proceed on the assumption that the impedance protection measuring influences the unit transformer, it must be ensured that the parameterization selection sufficiently considers the transformer control range.

Therefore,ZONE Z1is normally set to a reach of approx. 70 % of the protected zone (i.e. about 70 % of the transformer reactance), with no or only a small delay (i.e.T-Z1

= 0.00 s to 0.50 s). In this case, the protective relay will switch off faults on this distance

Impedance Protection (ANSI 21)

with its operating time or with a slight time delay (undelayed tripping). A 0.1 s time delay is preferred.

ForZONE Z2the reach could be set to about 100 % of the transformer reactance, or in addition to a network impedance. The correspondingZONE2 T2time stage is to be selected in a way that it overreaches the network protective relays of the following lines. TheT ENDtime is the last back-up time.

The following formula is generally valid for the primary impedance (with limiting to the unit transformer):

with k_R Zone reach of zone Z1 in [%]

u_sc Relative transformer short-circuit voltage [%]

S_N Rated transformer power [MVA]

U_N Machine-side rated transformer voltage [kV]

The values determined such must be converted for the secondary side of current and voltage transformers. In general:

The nominal current of the protective relay (= secondary nominal current of the current transformer) is automatically considered by the relay. You have already

communicated the transformation ratios of the current and voltage transformers to the relay by entering the nominal transformer quantities (see Section 2.3).

Example:

Transformer data:

u_sc= 7 % S_N= 5.3 MVA U_N= 6.3 kV

Transformation ratios:

Current transformer transformation ratio = 500 A/1 A

This results to a 70 % reach for zone 1:

Z_prim k_R 100--- u_sc

100--- U_N² S_N

---⋅ ⋅

=

Z_secondary CT transformation ratio VT transformation ratio

---⋅Z_primary

=

6.3 kV 3

--- 100 V 3

---⁄ VT transformation ratio =

Z1_prim 70 100--- 7

100--- 6.3² ---5.3

⋅ ⋅ 0.3669Ω

= =

Consequently, the secondary side setting value of zone 1 at address3306 ZONE Z1 is:

Note: The following ratio would result from the connection of a 5 A device to a 5 A transformer:

The following primary reactance results for a 100 % reach for zone 2:

The following secondary side setting value of zone 2 results at address3310 ZONE Z2:

Figure 2-62 Time Grading for Machine Impedance Protection – Example

Z1B Overreach Zone

The Z1B overreach zone (address3308 ZONE Z1B) is an externally controlled stage which does not influence the Z1 normal stage. Consequently, there is no changeover, but the overreach zone is switched effective or ineffective, depending on the position of the high-voltage side circuit breaker.

Z1_secondary 500 A 1 A⁄ 6.3 kV 100 V⁄

---⋅0.3669Ω 2.91 Ω

= =

Z1_secondary 500 A 5 A⁄ 6.3 kV 100 V⁄

---⋅0.3669Ω 0.58 Ω

= =

Z2_prim 100 100--- 7

100--- 6.3² ---5.3

⋅ ⋅ 0.5242Ω

= =

Z2_secondary 500 A 1 A⁄ 6.3 kV 100 V⁄

---⋅0.5242Ω 4.16 Ω

= =

Z<

t T3

Z1 Z (X₁, R₁) T1

T2

Z1B (X_1B, R_1B) Z2 (X₂, R₂)

Internal consumption

BI X₁=0.70*X_1Transf

X₁=X_1Transf

G

Impedance Protection (ANSI 21)

The Z1B zone is usually switched effective with an open high-voltage circuit breaker.

In this case, every impedance protection pickup can only be due to a fault in the protection zone of the block, as the power system is disconnected from the block.

Consequently, the undelayed tripping zone can be extended to 100 % to 120 % of the protection zone without any loss of selectivity.

The Z1B zone is activated via a binary input controlled by the circuit breaker auxiliary contact (see figure 2-62). The overreach zone is assigned to an individual3309 T-Z1Btime delay.

Final Stage In case of short circuits outside the Z1 and Z2 zones, the relay functions as a time-delayed overcurrent protection. Its nondirectional final timeT ENDis selected in a way that its time value overreaches the second or third stage of the series-connected network distance protection.