Protocolo de la herramienta de análisis:

Selectivity is a vital concept in protection systems. It refers to isolating the fault with the nearest circuit breaker so that the rest of the system will not be affected in case of a fault. It requires circuit breakers to react according to a hierarchy. The circuit breakers which are downstream and closer to the fault point are required to operate first. However, if the fault current is very large and the downstream circuit breaker fails to interrupt, then the upstream circuit breaker with larger capacity should operate and isolate the fault. With the introduction of DGs, the very traditionally accepted concepts of downstream and upstream circuit breakers are prone to change according to the status of the microgrid. Consider the system shown in

Figure 5.4. This network has generators connected to all branches. Through the combination of circuit breakers, various alternative network structures can be formed.

Figure 5.4. A sample microgrid

Throughout this section, relays will be referred to being closed or open as a shortcut to indicate the open/closed status of the Circuit Breakers (CBs) associated with each relay. Assume that R1, R2, R3, R4, R6 and R7 are closed whereas R5 remains open. When a fault occurs at the terminals of Load 2, then the most downstream CB will be Load 2‟s own CB

(represented by the little box) and selectivity implies that its associated relay should interrupt the fault. If Load 2‟s circuit breaker fails to achieve that in a predetermined time (delay), then the proper selectivity requires that R6 operates.

The proposed algorithm employed by the MCPU determines the network structure according to the status of critical relays. The status of a critical relay changes the structure of the network. Following this definition R1, R2, R3, R4 and R6 are all critical relays whereas Load 2‟s relay and DG1‟s relay are non-critical relays. After determining the microgrid structure,

MCPU assigns 2-pair selectivity couples. Table I shows the 2-pair selectivity data for the left branch of the example given above.

TABLE 5.1.2-PAIR SELECTIVITY METHOD DATA

Relay ID Operating Current Downstream Relay ID Downstream Relay Operating Current

R2 IrelayR2 R3 IrelayR3 R3 IrelayR3 IRDG1 IrelayIRDG1 R3 IrelayR3 IRDG2 IrelayIRDG2 R3 IrelayR3 IRLoad1 IrelayIRLoad1

The method owes its name to the fact that every relay is paired up with its downstream relay. The information provided includes; the operating current of that particular relay and the operating current of its downstream relay. Whenever a fault occurs, the relay which is closest to the fault operates to isolate the fault. If it proves to be unsuccessful in a predetermined time (i.e. time delay of the upstream relay), then its upstream relay will recognize and isolate the fault. Figure 5.5 shows the flow chart implemented by each relay for 2-pair selectivity method.

Figure 5.5. 2-pair Selectivity Method Algorithm

Let‟s consider the case in Table 5.1 the pairs are R2-R3 and R3-I.R. Should a fault occur at

isolate the fault. If unsuccessful, when the predetermined time expires (e.g. 200 msec), its upstream relay, i.e. R3 isolates the fault. In this manner, the proposed system can be implemented in simple and complex microgrids.

The value of the time delay (i.e. 200 msec) has been assigned by considering different factors. It is enforced by the stability parameters that a fault shall be cleared before a maximum time when the network becomes unstable [179]. At the same time, practical considerations for over-current relay communication and operation imply that there is a minimum time that these CBs can operate [180, 181]. Therefore, the time delay has been selected to be between these minimum and maximum time values.

To further clarify the operation of the algorithm four different structures of the system shown in Figure 5.4 are studied. These are;

1. R2, R3, R4 and R6 are closed. The middle bus is fed through R6 connection. 2. R2, R3, R4 and R5 are closed. The middle bus is fed through R5 connection.

3. R2, R3, R5 and R6 are closed. The middle bus and the right bus are fed through R5 and R6 connections, respectively.

4. R2, R4, R5 and R6 are closed. The left bus and the middle bus are fed through R5 and R6 connections, respectively.

It is required that these cases are studied by engineers and proper hierarchy of the critical relays should be determined and saved in the MCPU. While monitoring the network, MCPU will also monitor the status of critical relays. In accordance with the changes in their status, MCPU will compare the present structure with the predetermined structures saved in the memory. After determining the present structure, MCPU will retrieve the critical relay hierarchy pertaining to it and assign 2-pair selectivity. Alternatively, the graph theory-based approach developed by the authors [182] and presented in Chapter 4 can be utilized to extract

the relay hierarchy. In this fashion, the system acquires higher independence and plug-and- play concept can be realized.

For each case given above, the proper critical relay hierarchy is given in Table 5.2. As shown, these cases have independent branches in microgrid. That means critical relays in these branches are elements of discrete sets and do not belong to same 2-pair distribution. For instance, in Case 1, R3 and R4 are discrete from each other and thus they are not considered simultaneously for 2-pair distribution. R3 is considered for 1st branch whereas R4 is considered for 2nd branch.

TABLE 5.2.CRITICAL RELAY HIERARCHY FOR VARIOUS CASES Case No. 1st Branch 2nd Branch 3rd Branch

1. R2>R3>I.R* R2>R4>R6>I.R R2>R4>R7>I.R

2. R2>R3>I.R R2>R3>R5>I.R R2>R4>R7>I.R

3. R2>R3>I.R R2>R3>R5>I.R R2>R3>R5>R6>R7>I.R

4. R2>R4>R6>R5>I.R R2>R4>R6>I.R R2>R4>R7>I.R

*I.R. – Individual Relays of components (small boxes in figures) TABLE 5.3.RELAY PAIRING FOR CASE 1

Relay Hierarchy 1st branch Selectivity pairs

3 R2 2 R3 1 I.R. 2nd branch 4 R2 3 R4 2 R6 1 I.R.

Case 3 can be considered as a single branch since, due to its single line structure, 3rd branch covers 1st and 2nd branches. In these cases, none of the relays are discrete and they are all

considered in the same time for 2-pair distribution. If the network operates under Case 1 then, 1st and 2nd branches will have the 2-pair assignments shown in Table 5.3.

As shown in Figure 5.6, when R3 opens and R4 closes the structure changes from that of Case1 to that of Case 3. For this new microgrid structure, the relay hierarchy and 2-pair assignments shall be updated. The place of relays in the relay hierarchy may, also, change. R6 for instance is at 2nd level of 2nd branch in Case 1 while it is placed at 3rd level in Case 3.

(a)

(b)

As shown in Table 5.2, for Case 3, there is only one branch with 5 hierarchy levels. Therefore, 2-pair assignment for this case is as shown in Table 5.4. When Table 5.3 and Table 5.4 are compared, it becomes evident that the pairing of the relays change and they need to be updated. A reliable communication is vital for this purpose. In similar fashion, the protection system will recognize any new structure and repeat pairing process according to Table 5.2.

TABLE 5.4.RELAY PAIRING FOR CASE 3

Relay Hierarchy 1st branch Selectivity pairs

5 R2

4 R3

3 R5

2 R7

1 I.R.

If an automatic relay hierarchy detection algorithm such as in [182] is not utilized, then it has utmost importance that the network operators have their power engineers study possible network structures of the microgrid, list the proper hierarchy of the relays and prepare Table 5.2. MCPU will automatically operate according to these data.