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Static relay

Need for static relays: A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic and other components without mechanical motion. The solid-state components used are transistors, diodes, resistors, capacitors and so on. Static circuits accomplish the function of comparison and measurement. A relay using combination of both static and electro-magnetic units is also called a static relay provided that static units accomplish the response. In static relays, the measurement is performed by electronic, magnetic, optical or other components without mechanical motion. Additional electro-mechanical relay units may be employed in output stage as auxiliary relays. A protective system is formed by static relays and electro-mechanical auxiliary relays. Essential components of static relays: The essential components of static relays are shown in fig. Rectifier rectifies the relaying quantity i.e., the output from a CT or PT or a transducer. The rectified output is supplied to a measuring unit comprising of comparators, level detectors, filters, logic circuits. The output is actuated when the dynamic input (i.e., the relaying quantity) attains the threshold value. This output of the measuring unit is amplified by amplifier and fed to the output unit device, which is usually an electro-magnetic one. The output unit energizes the trip coil only when relay operates.

Block diagram of a static Relay Comparison of static relays with electromagnetic relays: The conventional electromagnetic relays are robust and quite reliable, but are required to work under differential forces under fault conditions. This leads to delicate small contact gaps, special bearing systems, special clutch assembles and pose several manufacturing difficulties. These relays require current and voltage transformers with high burden and are bulky in size too. The advantages of the static relays are : i) The moving parts and the contacts are greatly reduced : the only moving parts are those of the actual tripping circuit.

ii) The volt-ampere burden on the instrument transformer is reduced to a very low value which permits the use of linear coupler in place of current transformer, making it cheaper as well as solving the difficulty of the D.C. component of the fault current. iii) A high degree of accuracy and speed of operation. iv) Low power consumption. v) Resetting time and overshoots can be reduced. vi) Static relays are very compact. vii) Static relays have superior characteristic and accuracy. viii) Simplified testing and servicing is possible. ix) Several functions can be accommodated in a static relay. With all these advantages, in some installations static relays could not find popularity because of the following reasons: (i) Auxiliary D.C. supply is needed constituting a constant loss of power. (ii) Reliability is unpredictable. (iii) Susceptable to voltage transients. (iv) It is not very robust in construction and easily affected by surrounding interference. Classification of Static Relay: i) Electronic Relays

ii) Transductor (Magnetic relays)

iii) Rectifier bridge

iv) Transistor relays

v) Hall effect Relays

vi) Gauss effect Relays

i) Electronic Relays: These were the first to be developed in the series of static relays. The components used were electronic valves for measuring unit. In spite of the advantages of fast operation, low maintenance, low burden on CTs and PTs, absence of mechanical inertia and bouncing contacts, they suffered inherently from the requirements of short life, large power consumption. These relays could not meet practical requirements and hence never reached the commercial stages. ii) Transductor (Magnetic relays): A transductor comprises essentially a magnetic core carrying two groups of windings usually known as operating windings and control windings. Each group may comprise only one winding but if there is more than one winding in a group all those windings are magnetically linked. On the other hand the windings of the different groups are not magnetically linked. The control windings are energized with dc and the operating windings with ac. A transductor operates so as to present a variable impedance being varied by the current flowing through the control winding.

If tansductor is employed as amplitude comparator its sensitivity is limited due to the sensitivity of the slave relay in its output circuit. On the otherhand if it is used as a phase comparator to achieve higher sensitivity it will depend o an external ac supply which is sometimes difficult to arrange. The transductor relays are mechanically very simple and are quite reliable. Due to smoothing and rectifying a signal, a delay is introduced because of the time constant of the smoothing circuit and hence the relays are slow in operation and, therefore, are discarded for protection applications. iii) Rectifier bridge: Such relays became popular because of development of semiconductor diodes. This relay consists of two rectifier bridges and a moving coil polarized moving iron relay. The most common are relay comparators based on rectifier bridges, which can e arraged as either amplitude or phase comparators. iv) Transistor relays: Transistor relays are the most widely used static relays. In fact when we talk of static relays we generally mean transistor relays. Transistor which acts like a triode can overcome most of the limitations posed by the electronic valves and thus has made possible to develop the electronic relays more commonly known as static relays. The fact that a transistor can be employed both as an amplifying device and a switching device, makes it suitable for achieving any functional characteristic. v) Hall effect relays: When a conductor is kept perpendicular to the magnetic field and a direct current is passed through it, it results in an electric field perpendicular to the directions of both the magnetic field and current with a magnitude proportional to the product if the magnetic field strength and current. The voltage so developed is very small and it is difficult to detect it. This phenomenon is called the Hall effect. vi) Gauss Effect Relays: The resistivity of some metals and semiconductors are low temperature changes when exposed to the magnetic field. This phenomenon is called the magneto-resistivity or Gauss effect. This effect depends upon the ratio of depth to width and increase with the increase in the ratio. Electronic circuits commonly used in Static Relays: (i) Auxiliary d.c. voltage supply: Usually a d.c. to d.c. convertor is used if rating 200 V d.c. to 50 V d.c. The converters of sufficient ratings to supply the d.c.' power requirement of several static relays. (ii) Full wave bridge rectifier: A four diode bridge is usually used for filtering the input relaying quantity from a.c. to d.c. Ripply is made very low by adding smoothing circuits. (iii) Smoothing circuit. This comprises of RC or RL circuits in order to smoothen the output of the Rectifier. (iv) Voltage stabilization circuitry: Usually zener diode is used for this purpose. It stabilizes the output voltage of the rectifier over a wide range of current.

(v) Time delay circuit: These circuits are used for introducing very short delay (order of microseconds) in the protection systems. Mostly RC circuits are used for this purpose. Time delay elements are necessary for certain static relays and are used between level detectors and amplifiers. The delay of a few microseconds can be obtained by the help of delay line shown in fig.1 The resonant circuit of Fig. 2 can be used for providing a time delay of the order of a few milliseconds. The condenser in this circuit charges to its peak value through the LC circuit and then discharges through the diode to give a delay equal to the time period corresponding to resonant frequency. The variable resistance can be used to give a variable time delay.

Fig 1

Fig 2 Delay timer using SCR can be used for providing delays up to a few seconds. The resonant delay circuit of Fig.2 is used to trigger the SCR by means of a UJT. When the SCR conducts, it energizes the coil circuit of the auxiliary telephone type relay to terminate the timing period. vi) Level Detectors: For providing overload protection a certain level must be fixed at which the relay should be set to operate. The value of the capacitor C in the circuitry of Fig. 3 fixes the bias to transistors T2. Whenever the bias crosses a certain limit high currents flow in the relay (which also may be completely transistorized) causing it to trip.

Fig 3 Capacitor level detector Multivibrator Introduction A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices crosscoupled by resistor and capacitors. Multivibrators are classified according to the number of steady (stable) states of the circuit. A steady state exists when circuit operation is essentially constant that is, one transistor remains in conduction and the other remains cut off until an external signal is applied. The three types of multivibrators are the Astable, Monostable, and Bistable .

Fig. Monostable connection diagram Fig. Bistable connection diagram

Fig. Bistaable output curves Fig. Monostable output curves Monostable Multivibrators Monostable Multivibrators have only one stable state (hence there name: "Mono"), and they deliver a single output pulse when it is triggered externally only returning back to its first original and stable state after a period of time determined by the time constant of the RC coupled circuit. Monostable Multivibrators or "One-Shot Multivibrators" are used to generate a single output pulse of a specified width, either "High" or "Low" when a suitable external trigger signal or pulse T is applied. This trigger signal initiates a timing cycle which causes the output of the monostable to change its state at the start of the timing cycle and remains in this second state, which is determined by the time constant of the Capacitor, C and the Resistor, R until it resets or returns itself back to its original (stable) state. It will remain in this stable state indefinitely until another input pulse or signal is received. Then, Monostable Multivibrators have only one stable state and go through a full cycle in response to a single triggering input pulse.

Monostable Multivibrators can produce a very short pulse or a much longer rectangular shaped waveform whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon the RC time constant of the feedback components used. This RC time constant may be varied with time to produce a series of pulses which have a controlled fixed time delay in relation to the original trigger pulse as shown below . Bistable Multivibrators: Bistable Multivibrators are another type of two state device similar to the Monostable Multivibrator tutorial but the difference this time is that both states are stable . As Bistable Multivibrators have two stable states they are more commonly known as Flip-flops for use in sequential type circuits. Bistable Multivibrators are two state non-regenerative devices and in each state one of the transistors is cut-off while the other transistor is in saturation , this means that the bistable circuit is capable of remaining indefinitely in either stable state. To change over from one state to the other the circuit requires a suitable trigger pulse and to go through a full cycle , two triggering pulses, one for each stage are required. The Bistable Multivibrator circuit above is stable in both states, either with one transistor "OFF" and the other "ON" or with the first transistor "ON" and the second "OFF". Switching between the two states is achieved by applying a trigger pulse which in turn will cause the "ON" transistor to turn "OFF". The circuit will switch sequentially by applying a pulse to each base in turn and this is achieved from a single input trigger pulse using a biased diodes as a steering circuit . Equally, we could remove the diodes, capacitors and feedback resistors and apply individual trigger pulses directly to the transistor Bases. The Bistable Multivibrators output is dependent upon the application of two individual trigger pulses. So Monostable Multivibrators can produce a very short output pulse or a much longer rectangular shaped output whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon a second trigger pulse as shown below. Astable Multivibrator Regenerative switching circuits such as Astable Multivibrators are the most commonly used type of relaxation oscillator as they produce a constant square wave output waveform as well as their simplicity, reliability and ease of construction . Astable Multivibrators switch continuously between their two unstable states at a constant repetition rate without the need for any external triggering. Then, Astable Multivibrators have NO stable states and are therefore also known as Free-running Oscillators that produce a continuous square waveform from their output or outputs, (two outputs no inputs). The basic transistor circuit for Astable Multivibrators produces a square wave output from a pair of grounded Emitter cross-coupled transistors. Both transistors either NPN or PNP , in the multivibrator are biased for linear operation and are operated as Common Emitter Amplifiers with 100 % positive feedback. This results in one stage conducting "fully-ON" (Saturation) while the other is switched "fully-OFF" (cut-off) giving a very high level of mutual amplification

between the two transistors. Conduction is transferred from one stage to the other by the discharging action of a capacitor through a resistor as shown below .

Fig. Astable multivibrator connection diagram Fig. Astable output curves Astable Multivibrators can produce two very short square wave output waveforms from each transistor or a much longer rectangular shaped output either symmetrical or non-symmetrical depending upon the time constant of the RC network as shown below . Logic gates Digital systems are said to be constructed by using logic gates. These gates are the AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates. The basic operations are described below with the aid of truth tables. AND gate : The AND gate is an electronic circuit that gives a high output (1) only if all its inputs are high. A dot (.) is used to show the AND operation i.e. A.B. Bear in mind that this dot is sometimes omitted i.e. AB OR gate: The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR operation. NOT gate : The NOT gate is an electronic circuit that produces an inverted version of the input at its output. It is also known as an inverter. If the input variable is A, the inverted output is known as NOT A. This is also shown as A', or A with a bar over the top, as shown at the outputs. The diagrams below show two ways that the NAND logic gate can be configured to produce a NOT gate. It can also be done using NOR logic gates in the same way. NAND gate : This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate. The outputs of all NAND gates are high if any of the inputs are low. The symbol is an AND gate with a small circle on the output. The small circle represents inversion. NOR gate : This is a NOT-OR gate which is equal to an OR gate followed by a NOT gate. The outputs of all NOR gates are low if any of the inputs are high. The symbol is an OR gate with a small circle on the output. The small circle represents inversion.

EXOR gate: The 'Exclusive-OR' gate is a circuit which will give a high output if either, but not both, of its two inputs are high. An encircled plus sign ( ) is used to show the EOR operation. EXNOR gate: The 'Exclusive-NOR' gate circuit does the opposite to the EOR gate. It will give a low output if either, but not both, of its two inputs are high. The symbol is an EXOR gate with a small circle on the output. The small circle represents inversion. The NAND and NOR gates are called universal functions since with either one the AND and OR functions and NOT can be generated. Note: A function in sum of products form can be implemented using NAND gates by replacing all AND and OR gates by NAND gates. A function in product of sums form can be implemented using NOR gates by replacing all AND and OR gates by NOR gates.

Table 1: Logic gate symbols

Table 2 : Logic gates representation using the Truth table

Analog Circuits used in Static Relay: Operational amplifiers, presently available in IC form, can be used in circuits for addition subtraction, integration differentiation and other combinations. Their functions are described below in brief. (a) Use of operational amplifier as inverter is shown fig.

Fig: Op. Amplifier as an inverter

Fig: Non-inverting mode of Op. Amp.

The output is fed back to the negative polarity of the input through the feedback resistance RF. Here, V0 =

 

x V1

; Also    provided (RF / R1)

The negative feedback provides higher input voltage without saturation makes the gain independent of open circuit gain and produces inverting closed loop gain. (b) Non-inverting amplifier: If the feedback is applied at the negative polarity of the input as shown in Fig., output voltage is the non-inverted form of the input voltage. Here, V0 =

  x V1 

when R2 = 0, output voltage V0 tends to infinity.

(c) Voltage follower: ln this mode, output of the Op.Amp is feedback to the negative terminal of the input as shown in Fig. 12.33. Here, V0 = VI i.e., the output voltage follows the input.

Fig. Voltage follower circuit Fig. Adder circuit by Op. Amp.

(d) Adder Circuitry. Two voltages V1 and V2 are added through the Op. Amp. as shown in fig. 

Here, V0 =  󰇣





         󰇤

when RF, R1 and R2 are equal V0 =  (V1 + V2) (e) Subtraction Circuitry: Two voltages V1 and V2 can be subtracted by an Op. Amp. Circuitry shown in fig.     Here, V0 = 󰇣 1 3 󰇤x 󰇣 4 󰇤 x V2   2  x V1 1 1 2 4

When R1, R2, R3 and R4 are equal V0 = V2 – V1 (f) Integrator Circuitry: A RC circuit, as shown in Fig. can be used to integrate the input voltage of the Op. Amp. with respect to time. Here, V0 = 







     

Fig. Subtraction circuitry

Fig. Op. Amp. Based integrator circuit

(g) Differentiator Circuitry: In case a differential function is added in the input of an Op. Amp. In inverting mode, the output is V0 =  RC

 

This is the differential mode of operation. (h) Detector. Operational amplifier can also be used as zero detector and level detector circuit. Comparators can also be fabricated utilizing operational amplifiers. Function generators are important elements in static relays when different forms of functions are required to be generated. The forms of functions may be step function, ramp function, square wave, saw tooth wave function, sine wave function etc.

Comparators: The function of a protective relay is to sense any abnormal condition in the system and send a signal to the breaker which in turn isolates the faulty section of the feeder from the healthy one. The relay does all this by comparing two quantities either in amplitude or in phase. The amplitude or phase relation depends on the conditions of the system and for a predetermined value of this relation, indicative of a particular type and location of fault, the relay operates. Except in relays, such as over-current relays, where only one electrical quantity overcomes a mechanical quantity such as the restraint from a spring, u...