Protective Relays and Zones of Protection
Protective relays are devices that are connected to instrument transformers to receive input signals and to circuit breakers to issue control commands for opening or closing. In some instances, the relays are also connected to the communication channels to exchange information with other relays. The electronic relays always require a power supply, which is commonly provided through a connection to the station dc battery. Often, relays are connected to some auxiliary monitoring and control equipment to allow for coordination with other similar equipment and supervision by the operators. In the high-voltage power systems, relays are located in substations and, most frequently, in a control house. The connections to the instrument transformers and circuit breakers located in the substation switchyard are done through standard wiring originating from the substation switchyard and terminating in the control house.
To achieve effective protection solutions, the entire relaying problem is built around the concept of relaying zones. The zone is defined to include the power system component that has to be protected and include the circuit breakers needed to disconnect the component from the rest of the system. A typical allocation of protective relaying zones for the power system shown below:
Several points regarding the zone selection and allocation are important. First, the zones are selected to ensure that a multiple usage of the breakers associated with each power system component is achieved. Each circuit breaker may be serving at least two protection functions. This enables separation of the neighboring components in the case either one is faulted. At the same time, the number of breakers used to connect the components is minimized. By overlapping at least two zones of protection around each circuit breaker, it is important to make sure that there is no part of the power system left unprotected. This includes the short connection between circuit breakers or between circuit breakers and busses. Such an overlap is only possible if instrument transformers exist on both sides of the breaker, which is the case with so-called dead-tank breakers that have current transformers located in the breaker bushings.
Another important notion of the zones is to define a backup coverage. This is typical for the transmission line protection where multiple zones of protection are used for different sections of the transmission line. Figure 9.5 shows how the zones may be selected in case three zones of protection are used by each relay. The zones of protection are selected by determining the settings of the relay reach and the time associated with relay operation.
Each zone of protection is set to cover specific length of the transmission line, which is termed the relay reach. Typical selection of the zones in the transmission line protection is to cover 80 to 90% of the line in zone 1, 120–130% in zone 2, and 240–250% in zone 3. This protection is selected by locating a relay at a given line terminal and determining the length corresponding to the relay coverage as a percentage of the line length between the relay terminal and adjacent relay terminals. When doing this, the selected direction is down the transmission line starting from the terminal where the relay is located. The length of the transmission line originating from the location of the relay and ending at the next terminal is assumed to be 100%. The meaning of 120% is that the entire transmission line is covered as well as the additional 20% of the line originating from the adjacent terminal. The times of operation associated with zones are different: zone 1 operation is instantaneous, zone 2 is delayed to allow zone 1 relays to operate first, and zone 3 times allow the corresponding relays closer to the fault to operate first in either the zone 1 or zone 2. With this time-step approach selected for different zones of protection, the relays closest to the fault are allowed to operate first. If they fail to operate, the relays located at the remote terminals, that “see” the same fault in zone 2, will still disconnect the failed component. If zone 2 relay operation fails, relays located further away from the faulted line will operate next with the zone 3 settings. The advantage of this approach is a redundant coverage of each line section. They are also covered with multiple relay zones of the relay located on the adjacent lines, ensuring that the faulted component will be eventually removed even if the relay closest to the fault fails. The disadvantage is that each time a backup relay operates, a larger section of the system is removed from service because the relays operating in zone 2 (sometimes) or zone 3 (always) are connected to the circuit breakers that are remote from the ends of the transmission line experiencing the fault. In addition, the time to remove faulted sections from service increases as the zone coverage responsible for the relay action increases due to the time delays associated with the zone 2 and zone 3 settings.