16-04-2013, 03:45 PM
A New Adaptive Technique for Enhancement of Zone-2 Settings of Distance Relay
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ABSTRACT
This paper presents a technique that increases the second-zone coverage of distance relays without causing overreach problems. The technique is based on the impedance seen by distance relays when faults are simulated on the reach of zone-1 relays. The approach checks the relay operation for the maximum as well as the minimum generation outputs of the power system. The proposed technique can be used for different reach setting for zone-1. Results show that it is possible to increase the coverage provided by zone-2 distance relays without causing coordination problems with the primary relays that protect the lines emanating from the remote bus. The proposed method is modified for use in an adaptive protection system. It is shown that further improvements can be achieved when settings are calculated using the proposed method and the prevailing system conditions. The proposed method and its adaptive version were applied to a part of the Power system. The simulation of the system is done by using PSCAD-EMTDC software. The simulated results are observed and compared with the conventional and proposed technique.
Introduction
It is a general practice to use high-speed primary protec-tion along with slower speed backup protection in power systems. The backup protection becomes effective only if the primary protection relays fail to isolate the fault. The coordination is achieved by providing sufficient time delays between the operations of primary and backup relays.
Several approaches are used to protect transmission lines; one of these consists of using distance relays. The zone-2 relays are set to reach a fixed percentage of the shortest line emanating from the remote bus. Coordina-tion with the zone-1 relays protecting the lines emanating from the remote bus is achieved by delaying the trip outputs. These delays are of the order of 0.4 to 0.5 s that subject the power system to substantial shock. This ap-proach intuitively considers the infeeds from sources connected to the remote buses [1].
Distance Protection and Its Settings
Distance Relaying belongs to the principle of ratio com-parison. The ratio is between voltage and current, which in turn produces impedance. The impedance is propor-tional to the distance in transmission lines, hence the distance relaying designation for the principle. This prin-ciple is primarily used for protection of high voltage transmission lines.
Under normal circumstances, the three-zone protection scheme of distance relay provides both primary and backup protection to the primary line, for all phase faults. Thus, defining the reach of all the zones along with the time delay associated with each zone can completely model a distance relay. The time delay of the zone 2 & zone 3 has to be calculated such that, all faults have to be cleared within a maximum allowable time delay. How-ever some delay in time is required to be given for the sequential breaker action to be coordinated. This time is called the minimum time delay. Generally, minimum allowable time delay is 0.3 sec. While the maximum value is 4 sec.
Zone-2 Settings by Proposed Method
The proposed technique calculates the impedances seen by the zone-2 relays when faults are on the lines emanat-ing from the remote bus. The impedances are calculated for maximum and minimum outputs of generation sour- ces. First-level contingencies are considered as well. The least apparent impedance calculated in these scenarios is used to determine the relay settings. This approach in-creases the reach of the zone-2 relays without interfering with the operation of primary relays.
Adaptive Settings of Relay
The performance of the proposed method can be further enhanced if zone-2 settings can be made adaptive to the prevailing power system conditions. This would mean that the system condition should be monitored and new zone-2 settings should be calculated in response to any changes in the operating conditions. The new settings are then communicated to the relays. The relay settings are, therefore, always attuned to the prevailing system condi-tions. This adaptive setting technique is described in this section [9,10].
Adaptive Settings Method for a Radial System
This method is primarily similar to the non-adaptive method except that impedances need not be calculated for maximum and minimum levels of generation and no contingencies need to be considered. The seen imped-ances are calculated for the existing conditions. This means that the setting for zone-2 element of distance relay in adaptive protection system is determined by cal-culating the apparent impedance seen by zone-2 relays when faults are simulated on the lines emanating from the remote bus considering the prevailing operating con-
ditions of the power system. The adaptive setting method for zone-2 relays is described with reference to the sys-tem of Figure 3 and consists of the following steps for determining the zone-2 setting of the relay RBK [11].
Conclusions
The traditional methodology for setting distance relays provides minimum backup protection to remote lines. This approach works well when distance relays are used to protect radial lines in which there are no generation sources connected to the remote buses [12,13].
The proposed technique increases the reach of the zone-2 distance relays without affecting the security of the system. This is a useful feature that improves the protection during circuit breaker failures on the lines emanating from the remote bus especially on lines that are not provided with sophisticated protection schemes. Also, some faults that would have been cleared by the zone-3 elements previously will now be cleared by zone-2 relays. The time delays in these cases are substantially reduced. The proposed technique becomes important in cases where breaker failure schemes are not used.