What is a protective relay?
Protective relay systems are an important aspect of the electrical engineering involved in all power systems, such as high voltage systems, in order to ensure safety and productivity. Simply put, a protective relay is a device used to isolate an element that is not working normally, when the abnormality is detected. The abnormalities that are being constantly tested for through ongoing electrical quantity measurements are conditions relating to voltage, frequency, current, and phase angle. These measurements would qualify as a cause for alarm when they are over or under their expected measurement values. If the protective relay system detects an unexpected quantity, the relay goes through a process to trip the circuit breaker in order to cut off the irregular part of the power system. The response time once a fault has been detected is as low as a few thousandths of a second in advanced relay systems. Various standards dictate the required response time for a protective relay. In large power systems, it takes time to find the exact location of the fault after the relay system has tripped the circuit breaker in order to cut off the fault, however with advancing technology it is common to flag these systems in order to quickly find which system has been triggered to locate the fault.
What types of protective relay systems are there?
Talking about all of the types of protective relay systems could be multiple more blog posts in itself, however in this intro to protective relays, we will not go that in depth into the types, and will instead just provide a brief description of some of the more popular types. Protective relay systems can be broadly classified into three categories: types according to construction, types according to function, and types according to power source.
Types of protective relays according to construction:
The first type of relay as broken down by construction are electromechanical relays which can be further broken down, however for the sake of simplicity, electromechanical relays can be described as an electrically operated switch. This switch operates from four main components: an electromagnet, an armature, a spring, and a set of electrical contacts. Changes in electrical current cause movement, or a switch, in the system which leads to the trip in the circuit breaker.
Induction disk over current relays are another type of relay which would be classified as an electromagnetic relay. They get their names due to using the induction principle in order to operate. This principle causes electromagnets to rotate a disk which in turn operates a contact. When the system reaches over or undercurrent, the disk rotation changes and the contacts will reach each other causing a trip.
The other three types of relays as defined by their construction are static, digital – which is a newer type, and numerical – which is almost identical to digital other than specific technical points.
Types of protective relays according to function:
When talking about relays according to function, this is specific to their protective features. These different types are defined on the relays by their ANSI numbers and include all of the following: Over current relay, inverse definite maximum time, distance relay/impedance relay, current differential protection scheme, directional relay, and synchronism check.
Types of protective relays according to power source:
The last way to classify relays is based on their power source. A self powered relay will reduce costs of having to have a separate power supply as in the other types, and runs by energy that it gets from the protected circuit. Auxiliary powered relays can run as either AC or DC supplies that are either in the form of an external power supply or a battery. Dual powered relays are more reliable that auxiliary powered relays, as they have multiple batteries or chargers that can be on standby as back up.
Protective Relay Testing
In order to ensure that a protective relay is working properly and does not need to be replaced, protective relay testing should be another aspect of regularly scheduled maintenance. Protective relay testing can find flaws in your protective relay that may prevent it from detecting the small changes in the power system. Due to the various types of protective relays, it is important to be able to distinguish what type you have and then run protective relay testing accordingly.