Protective relays are low-voltage low-current intelligent devices which detect the fault by measuring the current and/or voltage of the system as reflected at the secondary side of specially designed current and voltage transformers (see Fig. 2). The measured phase currents and/or voltages generally have sufficient information to feed a relaying algorithm (or mechanism) which determines the breaker disconnection. Relays are configurable and settable, so it is possible to use the same kind of line protective relay to protect different types of lines; however, there will be different relay settings for each particular application. These relay or protection settings are calculated according to a well established set of criteria.

The protection system has to be: (a)sensitive, to detect all faults; (b)fast, to minimize the damage that faults may cause; (c)secure, to avoid operation for non-fault or out of reach disturbances; (d)selective, to strictly disconnect the minimum amount of service to clear the fault; and (e)dependable, to ensure a reliability close to 100%. When a short circuit occurs at some point in the power system, one or more protective relays in the neighborhood of the fault detect the fault; however, only those relays that accomplish conditions (a) to (d) should be allowed to disconnect their corresponding circuit breakers (see Fig. 3). A protection system that simultaneously accomplishes all these characteristics is said to be coordinated. Some relays, called backup relays, are set in a way that their circuit breakers clear a fault in case there is a failure in the operation of the relays originally intended to clear that fault. This system backup type of design redounds in reliability.

Protection Coordination

When all the relay designs, schemes and configurations of a protection system are defined, the process of calculating the relay settings in order to make such a system coordinated, as defined in the previous paragraph, is commonly called protection coordination. Engineers perform coordination studies to assure that the protection system works according to the aforementioned philosophy. Protection coordination -or relay coordination- studies are traditionally carried out using short circuit simulation programs and relay simulation programs. In traditional methods, the relay settings are calculated using rules of thumb based on engineers' experience: faults are simulated over critical points of the system and the relays' behavior is observed for each fault. If no coordination is achieved at one attempt, the process must be repeated as many times as needed until the protection system works according to the coordination criteria. Engineers use simulation programs (for example, the protection modules of PSS^®SINCAL used at Siemens PTI) as a support in the iteration process. In some cases, due to the complexity of the system, the volume of calculations involved is enormous; consequently, engineers cannot find a satisfactory coordination solution for all the relay settings. There are some (not many) automatic coordination programs that help in this task [2]. Among other characteristics, good automatic coordination programs have to be able to receive practical inputs from the protection engineer in order to adapt to any particular criteria and experience.

Figure 2 - Protection Hardware

A. Simplified scheme of line protection
B. Engineers "set" the relays
C. One-line diagram of (A)

As any power system changes in short, medium and long term, engineers have to re-calculate the optimum relay settings for each new power system configuration. In other words, coordination studies have to be performed on a regular basis to guarantee a proper system performance. Relay coordination and all aspects related to protective relaying represent a permanent source of work for the protection engineering specialists.

Download document as .pdf