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“Scheme II”

5.2.1 Definition of “Scheme I” and “Scheme II”

A fault clearance relay should detect an internal fault within its protective zone. This protection zone is decided by the principle of the protection, the location of the CT, and the setting values for the relay which detects an internal fault at the local end. It is also necessary to take into consideration whether the relay has directional characteristics. In addition, the maximum value of the operating time should be less than the withstand capability of the apparatus and the operation limit in the power system.

Proper coordination between protection relays is required to clear the fault and reduce equipment damage and to minimize outage area. All protection relays applied in the power system should operate systematically according to good coordination for a power system fault, or subsequent to any main protection which fails to operate, or even when the breaker didn't open. In coordination between protection relays, the protective zone of a relay could overlap with that of another adjacent

zone relay. By good coordination, all relays can operate so as to become the minimum outage area against any fault. When the protective zone of a relay overlaps with that of another relay, it is necessary to establish correct time coordination and/or sensitivity coordination.

Although protection relays are classified into a fault clearance relay and SPS, coordination is needed when the operating time, the detection time against an event, or the protective zone overlaps with that of another relay.

Fault clearance relays may have clearly defined protection zones such as a current differential relay based on the location of the CTs or interaction of two relays and hence is called Unit/Restricted Protection or "Scheme I" type protection.

Relays where the extent of the protective zone is determined by the setting value like a distance relay or an overcurrent relay are called Unrestricted Protection or "Scheme II" type protection.

The result classified according to the protection system is shown in Table 5.2-1. This classification is classified from the necessity for the time coordination or sensitivity coordination between the relay of the self-section and that of the adjoining section. As a result, the former is the group that adds a directional comparison scheme to unit protection defined in IEV 448 and the latter is the group that excludes directional comparison scheme from non-unit protection.

Table 5.2-1 Typical Examples of “Scheme I” and “Scheme II”

Protection Scheme

Current differential

(Percentage current differential High-impedance current differential Low-impedance current differential)

Necessity of coordination with the

next zone protection

Unnecessary Phase comparison

Directional comparison (POP,PUTT,BOP)

Transverse differential Distance

Ground directional Overcurrent

Ground overcurrent Ground overvoltage

Necessary Scheme I

Scheme II Classification

If the power system is protected only by Type I Unit Protection. , since the protective zone of each relay is clear, only one relay which detects the fault inside its zone will operate. Therefore, no additional coordination by time or sensitivity is necessary. However, in this case, when a protection relay or a breaker does not operate correctly to clear the fault there is no inherent backup mechanism and hence additional relays are required.

Unrestricted Type II schemes have the fault coverage zone determined by the setting of the relay.

The Relay A in Figure 5.2-1 is a relay which is classified into "Scheme II" such as a distance relay.

For example, if the setting of the Relay A is set to only partially cover the line as for Case 1 , a fault

the substation or “behind” the relay inside the substation or on another line. Furthermore, as described in Chapter 3.3, system conditions may influence the sensitivity of the relay.

The overlapping of protective zones causes the necessity of coordination between the relays as shown in Figure 5.2-1, which is described below. Figure 5.2-1 shows the transmission line protected by a distance relay under the conditions of two different settings or as the case may be for Zone 1 and Zone2 of the distance relay.

In Case 1 the relay setting is relative high which limits the zone of coverage to the near end of the line where the fault value is above the setting.

In Case 2, the relay has a lower setting and hence has an extended zone of fault coverage beyond the far end substation. Consequently the time delay setting on this relay must be coordinated with the operating time of the relays at the far end substation which must be allowed to clear the fault at the far end substation first in order to minimise the extent of power system outage.

Figure 5.2-1 Change of Protective Zone for “Scheme II” by Change of Setting Value

5.2.2 Coordination of Fault Clearance Relays

The necessity of coordination between fault clearance relays as mentioned above is determined by the protection system characteristic (unit vs. non-unit schemes) applied to the power system. If the power system is protected by "Scheme I" type relays it is clearly distinguishable between internal faults which must be detected and external faults which must not cause operation. However, for protection functions within the "Scheme II" group, a protective zone may overlap another depending on the setting values and power system operation conditions. Where "Scheme I" is applied as the main protection for each equipment and "Scheme II" is applied as the backup protection for each equipment, necessity of coordination between the relays is shown in Table 5.2-2.

According to this, coordination is needed between the main protection and the backup protection of the local terminal, between the main protection of the near end and the backup protection of the adjacent section and between the backup protection of the near end and the backup protection of the

Ry

Protective Zone of relay A – Case 1

Relay A Setting value

of Relay A Fault value

Case 1 Case 2

F Protective Zone of relay A – Case 2

adjacent section. These examples are described along with the setting criteria issues and approaches for each application in this report.

Table 5.2-2 Example of Coordination between Fault Clearance Relays

Line Protection

Busbar Protection

Transformer Protection

(Scheme I)Main Backup (Scheme II)

(Scheme I)Main Backup (Scheme II)

Transformer Protection (Scheme I)Main Backup

(Scheme II)

CoordinationTime (Scheme I)Main

Backup (Scheme II) ProtectionLine

(Scheme I)Main Backup (Scheme II)

Unnecessary Time Coordination Time Coordination Sensitive Coordination

Unnecessary Time Coordination Time Coordination Sensitive Coordination CoordinationTime

Unnecessary CoordinationTime

Time Coordination Time Coordination Sensitive Coordination Unnecessary Time Coordination CoordinationTime Time Coordination Sensitive Coordination

<Note> Main is of Unit-type and Backup is of Non-unit type schemes.