Años dorados del capitalismo: Del fin de la guerra hasta los setenta

In this section overcurrent is assumed as backup feeder protection. Therefore we are concerned with the coordination between backup protections. Since generally the effect of tripping of a transformer is larger than the tripping of a line, neutral current protection should operate only when the overcurrent backup doesn’t operate. The simplest method for the coordination of neutral current protection as final feeder backup is to set the sensitivity low enough not to operate for inrush and set the timer much slower than the overcurrent’s operating time. If this is difficult because of the requirements from the point of view of the operation of the network, then coordination would be so complicated as to be difficult to find a solution which can be applied for any cases under any conditions. Nevertheless it is useful to give the general idea which can be applied for many cases in this report.

(1) In Case Only Load is Connected

Fault current flows in the three phase circuit as shown in Figure 5.7-4. As demonstrated in the figure, fault current in the faulted phase is larger than the fault current which the neutral current protection relay detects. It is usual that the neutral current protection is more sensitive than the overcurrent backup, however this difference in the measured fault currents should be taken into consideration for the margin of the sensitivity in order to realise better sensitivity coordination.

This is generally easy to do because the pick-up setting of the overcurrent backup must be larger than maximum load current. This can be explained using a sequence network model as shown in Figure 5.7-5.

The impedance of the load is so large that it can be expressed as an open circuit. The neutral current protection relay detects 3I0 and the overcurrent backup in the faulted phase detects IF which is equivalent to I1+I2+I0. It is clear that IF>3I0 in this case because all current in the positive sequence circuit and the negative sequence circuit goes through the side in which the relay is connected. The difference between 3I0 and IF varies depending on the ratio of I0 between B substation side and C substation side. It is important to ensure that the overcurrent backup operates faster than the neutral current protection for faults close to B substation.

When N transformers are connected to the B substation the current that each neutral current protection detects becomes 1/N of the total 3I0. Therefore, the setting of the neutral current protection needs to be 1/N of the result derived from the above explanation.

OC Relay

A s/s B s/s C s/s

NCP Relay Fault Current

Load L L L

Figure 5.7-4 NCP Relay and OC Relay (One-End Infeed)

Feeder Prot

～

EA

xZ1 (1-x)Z1

zero-sequence circuit negative-sequence circuit positive-sequence circuit

ZA1 ZTr1 ZTr1 ZC1

xZ2 (1-x)Z2

ZA2 ZTr2 ZTr2 ZC2

xZ0 (1-x)Z0

ZTr0 ZTr0

NCP

Feeder Prot

Feeder Prot

Figure 5.7-5 Equivalent Sequence Network Connecting NCP Relay and OC Relay (One-End Infeed)

(2) In Case There Are Generators on Both Sides

When there are generators on both sides as shown in Figure 5.7-6 the equivalent sequence network becomes Figure 5.7-7. In this case, the relationship between the magnitudes of 3I0 and IF

in the faulted phase is unknown, being determined by the fault points and by the impedance of the network including the reverse impedance behind A substation and C substation. The effect of the reverse impedance becomes large when the protected line is short and small when the protected line is long.

For example, if a total of 10 units of fault current are split into 9 and 1 units in the zero sequence circuit for B substation side and C substation side respectively and are split into 7 and 3 units in the positive and negative circuits, IF will be 23 (=9+7+7), 3I0 will be 27. Therefore 3I0 is larger than IF. However, on the contrary, if fault current is split into 1 and 9 units for B substation side and C substation side respectively in the zero sequence circuit and is split into 3 and 7 units in the positive and negative circuit, IF will be 7 (=1+3+3), 3I0 will be 3. Therefore 3I0 is smaller than IF. Therefore it is not simple to determine the sensitivity of the neutral current protection and the overcurrent protection. For coordination it is important to ensure that neutral current protection operates after the overcurrent protection even for the case that 3I0 is larger than IF.

When N transformers are connected to B substation the current that each neutral current protection detects becomes 1/N of total 3I0. Therefore the setting of the neutral current protection needs to be 1/N of the result derived from the above explanation.

OC Relay A s/s

B s/s

C s/s

NCP Relay Fault Current

Figure 5.7-6 NCP Relay and OC Relay (Both-End Infeed)

Feeder Prot

～

EA

xZ1 (1-x)Z1

zero-sequence circuit negative-sequence circuit positive-sequence circuit

ZA1 ZTr1 ZTr1 ZC1

xZ2 (1-x)Z2

ZA2 ZTr2 ZTr2 ZC2

xZ0 (1-x)Z0

ZTr0 ZTr0

NCP

Feeder Prot

Feeder Prot

～

EB

Figure 5.7-7 Equivalent Sequence Network Connecting NCP Relay and OC Relay (Both-End Infeed)

(3) In Case of Double Lines

In case of double lines as shown in Figure 5.7-8, coordination is more complicated because there is a current from/to the parallel line. When a fault is close to B substation as shown in the figure, OC Backup detects the fault current including a flow from the parallel line, which should be larger than the current which neutral current protection detects. Conversely, if the fault point is close to C substation, the fault current is split between the parallel lines so that the fault current that the overcurrent backup detects will be smaller than the fault current that the neutral current protection detects. The precise calculation is too complicated to be done by hand because many factors such as mutual impedance, reverse impedance etc. affect the result. Therefore computer simulation would be required in order to ensure that the neutral current protection operates after overcurrent even for the case that 3I is larger than I .

OC Relay

A s/s B s/s C s/s

NCP Relay Fault Current

Figure 5.7-8 NCP Relay and OC Relay (Double Line Application)

5.7.4 Basic Coordination between Earth Fault Overcurrent Protection for