Static Relays Static relays are those in which the designed response is developed by electronic or magnetic means without mechanical motion. The designation ‘static relay’ covers the electronic relays of both the analog and digital designs. The analog relays refer to electronic circuits with discrete devices like transistors, diodes, etc., which were adopted in the initial stages. The digital designs incorporate integrated chips, microprocessors, etc., which had been developed subsequently. Most modern overcurrent relays are of the digital type. The main objective of using static relays is to improve the sensitivity, speed and reliability of a protection system by removing the delicate mechanical parts that can be subject to wear due to vibration, dust and corrosion.

Static Comparators as Relays Comparison vs Computation An over-current relay compares the magnitude of the current in its current coil with a set value and operates if the current is more than the set value. A directional relay compares phase angle of the measured quantity (i.e. current) with a reference phasor (i.e.voltage) and operates if this phase angle exceeds a predetermined threshold. All the relays perfom some or the other kind of comparison. Thus, at the heart of any relay, is always a comparator. Historically, these comparators were implemented using electromechanical technology. But eventually the electromechanical relays gave way to the solid-state relays.

The comparators helped in avoiding direct numerical computation. The relays based on comparators were found to be quite simple and robust. This gave rise to the technology of protective relays built around comparators. The comparator-based relays are very attractive because of their inherent simplicity and low cost. However, they suffer from the drawback that (since a comparator-based relay essentially gives a go-no-go type of decision) the fault cannot be precisely located. The comparators can be classified into two types; those based on comparison of amplitude and those based on comparison of phase angle. It is possible to synthesize a variety of relays using static comparators.

Numerical Relays Microprocessor-based relay, works on numbers representing instantaneous values of the signals. Hence, they are named numerical relay. Other popular nomenclatures for such relays are digital relay,computer-based relay or microprocessor-based relay. In numerical relays, the software, runs in the background and which actually runs the relay. What distinguishes one numerical relay from the other generally is the software. Conventional relay performs comparison only . The numerical relay does not have any such limitation because of its ability to perform real-time computation. Existlng relaying concept can be implemented using the numerical technique. The possibilities of developing a new numerical relay are almost endless and there is very little standardization.

Block Diagram of Numerical Relay

The signals from the CTs and VTs are first passed through a low-pass filter, which has to be an analogue type of filter, because digital processing can only take place after the frequency spectrum of the signal is properly shaped. Next, the analogue signal is sampled and held constant during the time the value is converted to digital form. The range of frequencies that can be handled by the analogue-to-digital converter (ADC) without the sample and hold (S/H) circuit is extremely low

The sampled and held value is passed on to the ADC through a multiplexer so as to accommodate a large number of input signals The sample and hold circuit and the ADC work under the control of the microprocessor and communicate with it wlth the help of control signals such as the end-of conversion signal issued by the ADC. The ADC passes on the digital representation of the instantaneous value of the signal to the microprocessor via an input port. The output of the ADC may be 4, 8, 12, 16, or 32 bits wide or even wider. The wider the output of the ADC, the greater its resolution.

The incoming digital values from the ADC are stored in the RAM of the microprocessor and processed by the relay software in accordance with an underlying relaying algorithm. The microprocessor issues the trip signal on one of the bits of its output port which is then suitably processed so as to make it compatible with the trip coil of the CB. The microprocessor can also be used to communicate with other relays or another supervisory computer, if so desired. The relaying program or the relay software, which resides in the EPROM, can only be upgraded or modified by authorized personnel. Thus, new features and functionalities can be added to an existing relay by upgrading its software. A numerical relay can be made to run a program which periodically performs a self diagnostic test and issues an alarm signal if any discrepancy is noticed. Other features like a watch-dog timer can also be implemented, which issues an alarm if the microprocessor does not reset it, periodically, within a stipulated time of a few milliseconds. This gives an increased user confidence and improves the reliability of the relay.

Intelligent Electronic Device’ (IED) The functions of a typical IED can be classified into 5 main areas, namely protection, control, monitoring, metering and communications.

Protection related functions of IED Non-directional and directional three-phase Directional and non directional overcurrent earth fault protection • Phase discontinuity protection • Three-phase overvoltage protection • Residual overvoltage protection • Three-phase under voltage protection • Three-phase transformer inrush/motor start-up current detector • Auto-reclosure function • Under frequency protection • Over frequency protection etc.

Control related functions of IED • Local and remote control (open/close commands for circuit breakers, isolators, etc.) Control sequencing • Bay level interlocking of the controlled devices Status information Information of alarm channels HMI panel on device.

Metering functions of IED • Three-phase currents • Neutral current • Three-phase voltages • Residual voltage • Frequency • Active power • Reactive power • Power factor • Energy • Harmonics • Transient disturbance recorder

Communication functions of an IED Communication capability of an IED is one of the most important aspects of modern electrical and protection systems, and is the one aspect that clearly separates the different manufacturers’ devices from one another regarding their level of functionality. IED has the capability of extensive communications directly to a SCADA system.