Protection relay also called the protective relay is a device designed to trip a breaker when any fault is detected Firstly protective relays were electromagnetic devices, counting on coils performing on moving parts to supply detection of abnormal operating situations like over-voltage over-current, reverse power flow, over-frequency, and under-frequency. Now a day’s protection relay or protective relay is a smart device that accepts inputs, compares them to set points, and generates outputs. Inputs can be voltage, current, resistance, or temperature. The output may involve visual feedback within the sort of indicator lights and/or a digital display, communications, control warnings, alarms, and turning the power off and on. The main features of a good protective relaying are its reliability, simplicity, sensitivity, speed, and economy. A diagram showing the concept of a protection relay is given below.
Protection relays can either be electromechanical or electronic/microprocessor-based. Electromechanical protecting relays are an out-dated technology consisting of mechanical parts that require routine calibration to stay within the intended tolerances. The microprocessor or electronic relays use digital technology to provide fast, reliable, accurate, and repeatable outputs. The advantages of using an electronic or microprocessor-based relay instead of an electromechanical design provide numerous benefits including improved accuracy, reduced maintenance, additional functions, smaller space requirements, and lifecycle costs.
These inputs can be collected in multiple ways. In some cases, the wires in the field can directly be connected to the relay. In some other applications, additional devices are required to convert the measured parameters in a form that can be processed by the relay. These additional devices can be potential transformers, current transformers, tension couplers, RTDs or some other devices.
Many protection relays have variable settings. The user can program the desired settings (pick-up levels etc) that allow the relay to make a decision. The relay compares the incoming inputs to these settings and responds accordingly.
Once the inputs are connected and the settings are pre-set; the relay compares these values and makes a decision. Depending on the requirement, different types of relays are available for different functions.
The relay has numerous ways to communicate that a decision has been made. Normally the relay will operate a switch (relay contact) to show that input has surpassed a setting, or the relay can provide warning through visual feedback such as a meter or LED. One advantage of electronic or microprocessor relays is its ability to communicate with a network or a PLC.
Different types of relay
1) The electrical protective relay can be broad, classified into two categories
• Electromagnetic Relay
• Static Relay
• On the basis of the operating principle and construction, the relay may be classified such as the electromagnetic attraction type, electromagnetic induction type, electrodynamic type, electro-thermal type, moving coil type, physics electric type, and static relays.
2) The most common types of protective relays based on applications may be classified as –
• Overcurrent, Overvoltage, and Overpower Relay – This relay operates when the voltage, current or power rises above a specified value.
• Under voltage, Undercurrent, and under power Relay – These types of relays operate when the voltage, current or power falls below a specified value.
• Directional relay or Reverse Current Relay – These relays operate when applied current assumes a specified phase shift on the supply voltage and the relay is compensated for the drop in voltage.
• Directional relay or Reverse Power Relay – These types of relays operates when the applied voltage and current assume specified space displacement and no compensation is allowed for a drop in voltage.
• Differential Relay – These relays operate when some indicated phase of magnitude difference between the two or more electrical quantities occurs.
• Distance Relay – operation depends on the ratio of the voltage to the current.
3) Electronic Relay
The electronic relay is a sort of electronic switch that opens or closes the circuit contacts by using electronic components without using any mechanical operation. In these relays, the current carrier pilot relaying technic is used for the protection of the transmission line.
Advantages of Electronic Relay
They require low maintenance.
The relay has fast response time.
They decrease the burden on the instrument transformer.
Protection Relay Testing
Since the protective relays play an important role in the prevention of hazard to domestic and plant equipment, they should be provided first-line maintenance attention. A relay may only need to function for 0.15 seconds in its whole lifetime. But failure to properly work as intended can result in extensive harm, extended power outages, and loss of life. NETA (InterNational Electrical Testing Association) reports says 12% Failure Rates on Protection Relays Testing. An inclusive testing program should simulate the fault and normal operating conditions of the relay. Acceptance testing, of relays, commissioning and startup includes control power tests, potential transformer, and current transformer tests, and any other equipment testing accompanying the protective relay. Routine preventive maintenance testing is also required.
Tests are different for each type of relay and are determined by the manufacturer’s recommendations and special application considerations.
NETA procedures include:
• Inspection for mechanical problems
• Pickup on each operating element
• Timing at three points on the curve
• Target and seal-in operation
• Special tests for restraint, directional overcurrent, and other special operating elements
• Insulation tests
• Actual breaker trip test
• Peripheral and auxiliary device test
• Function test