Análisis prueba piloto dos

Selectivity is a well-known protection concept which means isolating the fault with the nearest relay in an effort to minimize its effect on the rest of the system (grid) [168, 169]. This requires that in case of a fault, the relays should react according to a certain hierarchy. In conventional protection systems designed for passive networks, the relays which are downstream and closer to the fault point are required to operate first. However, if the fault current is very large and downstream relays are not capable of interrupting it, then other relays with larger capacities are expected to operate and isolate the fault.

That being said, implementation of selectivity is not that straightforward with the introduction of DGs. The very concepts of downstream and upstream relays are prone to change according to the status of the microgrid. The operating mode, i.e. grid-connected or

islanded-mode, changing network structure with alternative paths and new deployments are some of the factors that would alter the selectivity parameters.

Consider the system shown in Figure 4.6. In this network, all branches have generation and load, and various alternative network structures can be formed through the combination of CB connections.

Figure 4.6. A sample microgrid

As first case, assume that the circuit breakers CB1, CB2, CB3, CB4, CB6 and CB7 are closed whereas CB5 remains open, as shown below in Figure 4.7. When a fault occurs at the terminals of Load 2, then the most downstream relay will be Load 2‟s own relay (represented

by the little box) and selectivity implies that it should interrupt the connection. If Load 2‟s relay fails to achieve that in a predetermined time (delay), then the proper sequence for the selective operation should be CB6, CB4 and finally CB2. In similar fashion, should a fault occur at the terminals of Load 3, the proper selective operation requires the following sequence: Load 3‟s relay, CB7, CB4 and CB2.

Figure 4.7. The network structure when CB5 is open and CB4 is closed (1st Case)

If CB4 is disconnected for any reason, for example maintenance or breakdown, in order to keep the integrity of the network, CB5 closes. The line between Load 1 and Load 2 (protected by CB5) has therefore been added to form a loop structure when necessary and protect the microgrid against contingencies and failures. When CB5 loses, the structure change from case 1 to case 2 is shown in Figure 4.8.

As shown, now in Figure 4.8, there is only one branch for the power flow instead of two. For this new microgrid structure all selective levels, time steps and time delay calculations shall be repeated. Following the same examples should a fault occur at Load 2 or Load 3, the proper relay hierarchies are; Load2‟s relay, CB5, CB3, CB2 and Load 3‟s relay, CB6, CB5, CB3, CB2, respectively.

One exceptional case arises when a fault occurs at upper levels of the microgrid. In this case, fault should be isolated by the closest relay at the higher level and this requires the inversion of the hierarchy structure. For example, if a fault occurs outside the microgrid, i.e. in the utility grid, the microgrid should be isolated from the system by opening CB2. In this case, the interfacing relay CB2, which is at the top of the selectivity pyramid, is required to operate first. This can be managed by detecting the reverse flow of the fault current and assigning two different time delays to the relays, one for upstream faults and the other for downstream faults.

The above mentioned factors require that the selectivity hierarchy of the relays should be dynamic and updated frequently. An algorithm should be employed which determines the network structure whenever the status of a critical relay is changed. A critical relay refers to a relay the status of which changes the structure of the network. Following this definition relays of CB2, CB3, CB4, CB5 and CB6 are all critical relays whereas Load 2‟s relay, DG1‟s

relay are non-critical relays.

After determining the microgrid structure and its relay hierarchy, suitable time delays can be appointed easily. Let tmax be the maximum time length allowed by the grid code before the

fault is cleared in the system. This value is divided into the number of selective levels, n, in the relay hierarchy and the base time delay is found as in (4.1).

n

t

t

The time delay for each relay is calculated by multiplying this base time delay with the selective level, s, of that particular relay. When calculated with (4.2), the relay at the highest level will be in nthselective level and its time delay will be the largest possible time delay, i.e. tmax. relay relay

S

n

t

t

Alternatively, 2-pair selectivity approach can be assumed by the protection system where only two relays are paired up for back-up protection. Therefore, each relay is supposed to monitor its immediate downstream relay and clear the fault if the downstream CB fails. In this case the time delay of the relay can be selected from a large window bounded by the smallest possible time delay dictated by relay capabilities, i.e. tmin, and tmax.

(4.3) The conceptual design depicted in Chapter 3 necessitates that in order to update the operating currents of the relays; some sort of communication of data is required. This will also help detect the direction of fault currents and thus isolate the fault properly. Worthy to note, the developed system is not dependent on any communication network and can work on a wide range of alternatives. The delay occurring over the communication lines is called latency and represented as tcommunication. For proper operation, this latency shall be taken into account and

deducted from the calculated relay time delays.

The final relay time delay, tassigned, which considers the communications delays, can be

calculated as shown in (4.4).