Static Relays 6s2a3m

STATIC RELAYS BATCH – 11 K.S.BHARATHWAJ 107114042 K.S.LAHARI

107114043

K.SREEDHAR BABU KAUSHIK RAAM A.G

107114044 107114045

RELAYS: A RECAP • A relay (protective) is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

• The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions.

• Electrical Relays can be divided into mechanical action relays called “Electromechanical Relays” and those which use semiconductor transistors, thyristors, triacs, etc, as their switching device called “Solid State Relays” or SSR’s.

WHAT ARE STATIC RELAYS ? • Solid State Relay (Static Relay) is an electrical relay in which the response is developed by electronic/magnetic/optical or other components, without mechanical motion of components.

• A static relay consists of : An input circuit that measures the value of desired property A comparator circuit that compares the measured value to a pre-set threshold An optional time delay circuit that controls the timing of the switch action after the input has reached the threshold A power supply for the static relay circuits

The static components of a static relay are shown in the figure beside. Here the relaying quantity, i.e., the output of a CT or PT of a transducer is rectified by the rectifier. The rectified output is given to a measuring unit constitute of comparators, level detectors, and logic circuits. The output is actuated when the dynamic input, i.e. the The output of relaying the measuring unit is fed to the output unit devices after it is quantity threshold amplifiedattains by the the amplifiers. The output unit activates the trip coil only when the value. relay operates. The relaying quantity such as the voltage and current is rectified and measured. When the quantity under measurement attains certain welldefined value, the output device is energized and hence, the circuit breaker trip is triggered.

• The static relay can be arranged to respond to electrical inputs. The other types of input such as heat, light, magnetic field, traveling waves, etc., can be suitably converted into equivalent analog and digital signal and then supplied to the static relay

Bridge rectifiers convert a sinusoidal voltage into full-wave rectified pulses at twice the input frequency. The problem here is that these voltage pulses start and end from zero volts which means that they will fall below the minimum turn-on voltage requirements of the SSR’s input threshold causing the output to turn “on” and “off” every half cycle. The electrical separation between the input control signal and the output load voltage is accomplished with the aid of an opto-

With no input signal applied, no load current flows through the SSR as it is effectively OFF (open-circuited) and the output terminals see the full AC supply voltage. With the application of a DC input signal, no matter which part of the sinusoidal waveform, either positive or negative the cycle is going through, due to zero-voltage switching characteristics of the SSR, the output only turns-on when the waveform crosses over the zero point. When the DC input voltage signal is removed, the output does not suddenly turn-off as once triggered into conduction, the thyristor or triac used as the switching device stays ON for the remainder of the half cycle until the load currents drops below the devices holding current, at which point it switches OFF. Thus the high dv/dt back emf’s associated with switching inductive loads in the middle of a sine wave is greatly reduced. Then the main advantages of the AC solid state relay over the electro-mechanical relay are its zero crossing function which turns ON the SSR when the AC load voltage is close to zero volts, thus suppressing any high inrush currents as the load current will always start from a point close to 0V.

ADVANTAGES 1.More accurate and fast in operation 2. No moving parts. 3.VA ratings of CT and PT is comparatively lower. 4.Low power consumption. 5.Resetting time and overshoot can be reduced. 6.Very compact. 7.Superior characteristics and accuracy. 8.Several functions can be accommodated in a static relay

DISADVANTAGES 1.Reliability cannot be predicted 2.Construction is not very robust. 3.Easily affected by surrounding interference. 4.Auxiliarty DC supply is required 5.Affected by voltage transients For integrated protection and monitoring systems programmable microprocessor controlled static relays are preferred.

COMPARISON WITH ELECTRO-MAGNETIC RELAY While the solid state relay and electro-mechanical relay are fundamentally similar in that their low voltage input is electrically isolated from the output that switches and controls a load, electro-mechanical relays have a limited life cycle, can take up a lot of room and have slower switch speeds, especially large power relays and ors. Solid state relays have no such limitations.

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Low power consumption as low as 1mW.

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No gravity effect on operation of static relays. Hence can be used in vessels, i.e., ships, aircrafts etc.

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A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays.

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Static relay is compact.

No moving s; hence associated problems of arcing, bounce, erosion, replacement of s.

Superior operating characteristics and accuracy. Static relay can think, programmable operation is possible with static relay. Effect of vibration is nil, hence can be used in earthquake-prone areas. Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers.

APPLICATIONS 1.Ultra high speed protection of EHV AC transmission lines utilizing distance protection. 2.In over current and earth fault protection schemes 3.As main element in differential relay