Static Relays 6s2a3m

STATIC RELAYS

Introduction • Comparison or measurements done by static circuit • D.c polarised relay used as slave relay – Does not perform comparison or measurements – Closes s

• Thyristors used to replace electromagnetic slave relay

Reasons for developments in static relays • Better performance and characteristics • Greater standardization in manufacture • Easier in manufacture and reduction in maintenance time

Advantages of static relays • Impose low burden on CTs and PTs which in turn reduces the VA rating requirement and the size and cost of C.Ts and P.Ts • With low VA rating ,voltage levels kept at low values ,preventing CT core saturation thus improving accuracy of CT and PT. • No moving parts, thus eliminating problems like erosion, bounce,vibration and shocks,dryness in ,spring restraint and arcing.

Contd…. • Due to absence of moving parts friction ,static relays also increases the precision level of their characteristics. • The amplification facility imparts greater sensitivity. • Compact size of static relays . • Greater flexibility due to the ability to filter harmonics,differentiation,integration,and sensing of negative phase sequence component of current and voltage.

Contd… • Low resetting time for facilitating the rapid automatic reclosing of circuit breakers, which helps to achieve proper selectivity. • Provide high drop-off to pick –up ratio, low resistance over reach and low overshoot.

Limitation of static relays • Over voltage surges cause the malfunctioning of the static relay. • High voltages damage the electronic components. • Changes in temperature adversely affect the functioning of the relay. • Reliability of relays depends on the reliability of discrete electronic components . • Auxiliary power supply required for functioning

Comparison of burdens of static and electromechanical relays

Block Diagram of Static Relay

Basic block diagram of static relays

1.Converter element • Matching transformer to obtain required signal • More than one signal inputs • Measured value fed to harmonic filter • Eliminate ripples

Voltage comparison

Current comparison

Voltage comparison

2.Measuring element • Analog binary signal converter with measuring functions • Schmitt trigger circuit • Extremely fast D.C polarized relay • Acts as a level detector • Transistor common emitter connection with high input resistance and large current gain • Output –step output

Measuring element

3. Output element • Amplifies output signal from measuring element ,multiplies it. • Introduces delay if required • Binary signals processed • Gives separation between controlling and controlled circuits • Bistable or monostable multivibrators

4. Feed element • Supply power necessary for circuits • Built in auxiliary supply or station battery • Supply stabilized voltage to static circuits

Characteristic functions of protective relays 1.Summation • A static equipment like summation transformer and sequence networks used for combine a number of electrical quantities to single one

2. Single input devices

Non critical repeat function • • • • •

Produces a switching power gain of about 103 In the form of s Instantaneous devices with speed 20ms Associated with time delay function Repeat functions initiated by outputs from critical or measuring elements

Critical or measuring functions • Requires a response to an input when exceeds a prescribed limit • Fast response • Accuracy of setting • High reset ratio • Used in overcurrent, under voltage, overvoltage, differential systems • Polarised moving coil or attracted armature trype or reed relays

Fixed time or definite time functions • Necessitates a delay between input and output • Input non critical • Switching gains and multiple outputs • Can produce repeat function • Accuracy and repeatability under successive applications

Function time dependant on output • Characteristic is t=f(S n), n –negative and real • Over current and overload protection • Low set and high set instantaneous critical functions

Two input devices

Two input devices • Wide range of characteristics possible with different operating principles • Amplitude comparison • Phase comparison

Multi input devices

Multi input devices • More than two inputs • Complex characteristics possible • Amplitude and phase comparison used separately

Classification of static relays • Classified based on the type of measuring unit or comparator – Electronic relays – Transductor or magnetic amplifier relays – Rectifier bridge relays – Transistor relays – Hall effect relays – Gauss effect relays

Static relay components • • • • • • • • • •

Diodes Zener Diodes Transistors Thyristors Rectifiers Voltage rectified circuits Smoothing circuits Transistor amplifier Filter circuits logic circuits Multivibrators

• • • • • • • •

Differentiating circuits Integrating circuits Operational amplifier Level detector Time delay circuits Output circuits DC auxiliary supply Surge absorbers,/surge suppressors • Comparators.

General equation of comparators • Two input signals S1 and S2 ,derived from power system voltage and current S1  K1VL  Z R1 I L S 2  K 2VL  Z R2 I L

• K1 and K2 – constants • ZR1 and ZR2 – complex impedances

Derivation of comparator inputs

Derivation of inputs

General equation of comparators • General equation with two inputs S1  K1VL  Z R1 I L S 2  K 2VL  Z R 2 I L

• Putting in complex form •

Comparators • Comparator is the heart of the relay, decides its working characteristics. • Main principle is comparison of two operating quantities. • Quantities to be compared may be either in amplitude or in phase. • Two types of comparators – Amplitude comparator – Phase comparator.

Amplitude comparator • Compares magnitude of two input quantities

• Amplitude of operating quantity exceeds the amplitude of restraining quantity relay sends trip signal

Phase comparator • Compares two quantities in phase, irrespective of magnitudes and operates if phase angle less than or equal to 90⁰

Inputs to phase and amplitude comparison

Types of comparators

Types of comparators • • • •

Amplitude comparator Phase comparators Vector product devices Zener diode phase comparator

Amplitude comparators Electromagnetic types • Single input -- Hinged armature • Two input • Balanced beam • Polarized moving coil with two coils • Induction disc with operate and restrain electromagnets • Polarized moving iron

Amplitude comparators Static type

• Rectifier bridge – Circulating current – Opposed voltage

• Transductor type • Instantaneous (sampling) comparators

Phase comparators • Electromagnetic type – Induction disc – Induction cup

• Static type – Coincident circuit type • • • •

Direct or block spike phase comparison Phase splitting technique Integrating type phase comparator with transistor AND gate Integrating type phase comparator with rectifier type AND gate • Rectifier phase comparator

Vector product devices • Output produced proportional to the vector product of input a.c • Hall effect devices – Application as a phase comparator with sinusoidal input – Practical hall elements

• Magneto resistivity type

Amplitude comparator • Compares two inputs amplitudes. • ie, operating quantity and restraining quantity. • Three types of amplitude comparators – Integrating amplitude comparator – Instantaneous amplitude comparator – Sampling amplitude comparator

Integrating amplitude comparator • Two types – Circulating current – Voltage opposed

• Circulating current type relays assembled in two ways 1.Through rectifier bridge with a slave relay 2. Rectifier bridges with static output devices.

Integrating type rectifier bridge comparator with slave relay

Opposition of circulating current bridge

Working • Input signals are the operating and restraining current. • S0 =Ki0, Sr= Kir • Relay operates when S0> Sr • Two full wave rectifiers needed • For operating quantity and for restraining quantity. • Output of the relay applied to slave relay, which is D C polarized relay.

Contd… • Static output devices are also used for this type of relay • The circuit consists of an averaging circuit of the difference of the rectifier currents (i0-ir),along with polarity detector. • This relay operates if the average value of the output is positive.

Integrating type bridge comparator with static output device

Working of opposed voltage comparator • Works on the on the principle of averaging the difference of the rectified voltages. • Output of relay has zero torque when Vo= Vr • The bridge of this relay is not very sensitive.

Opposed voltage bridge comparator

Direct comparator • Rectification and Smoothening of output to avoid variations in voltages • 3 phase rectifier used • Greater smoothing by polyphase circuits • Gives continuous output

Rectification by phase splitting

Direct comparison

Transductor amplitude comparator

Transductor comparator • Magnetic amplifier • Three windings,primary,secondary and control windings • Output of secondary controlled by d.c • Sensitive device

Instantaneous amplitude comparator

Instantaneous amplitude comparator

Working • Similar to opposed voltage comparator. • Two types – Averaging type – Phase splitting type

• To provide a fixed restrained level, restraining signal is rectified and smoothened properly in averaging type comparator.

Contd… • To achieve the operating condition the peak of the operating signal must exceed the restraining level • Smoothing is done by capacitor ,which causes delay in operation. • Phase splitting type comparators are preferred. • Phase splitting is done before rectification

Sampling amplitude comparator • Signals before being compared are sampled either at same instant or at different instants • In sampling of one signal ,comparison of the signal proportional to its average rectified values with sampled signals is made. • Voltage signal is sampled and then compared with the average value of the current ing through the zero value.

Block diagram of sampling amplitude comparator for reactance relay

Phase comparators • Phase relation between two input signals are compared. • When phase relationship varies within certain limits output required for relay operation is obtained. • Condition for operation is -α1 ≤ ϴ ≤ α2

Phase comparators • Two types – Coincident type phase comparator – Vector product type phase comparator

Coincident type phase comparison • Similar polarities of two input signals, with a phase difference measured and compared • If α phase difference, the period of coincidence ψ= (180-α) • Criterion for operation -90⁰≤ α ≤ +90⁰

Coincidence of two sinusoidal inputs

Coincident type phase comparison • Techniques used – Block spike phase comparison – Phase splitting technique – Integrating phase comparison – Rectifier bridge phase comparison

Block spike coincident technique • The output available for different phase differences are • With spike derived at peak value -90⁰≤ α ≤ +90⁰ • At zero value 0 ≤ α ≤ 180⁰ • At any other instant ϴ ≤ α ≤ (180-ϴ)⁰

Block spike coincident technique

Working • Works on the principle of measuring the period of coincidence. • If α is the phase difference between two signals and period of coincidence is Ψ= (180- α) . • For a desired operation of α at less than +90̊ the coincidence period is greater than 90 ̊ Condition of operating angle is -90 ̊≤ α ≤ 90

Working • One input is converted into a spike and other into a square wave at the instant of ing the signal either zero value or peak value. • To get the output of coincidence of two signals,spike and square wave, the two signals are fed to an AND gate.

Phase splitting comparator • Both input signals are split into two components each. • Split by + 45 ̊ with reference to the original signal • Four components are fed to an AND gate • The AND gate gives the output at the instant when all signals are positive.

Block diagram of phase splitting comparator

Integrating phase comparator

Integrating phase comparator with phase shifting technique

Working • Measurement of period is done after integrating signals S1 and S2 • Integration is done by feeding the signals to the AND gates ,which is known as the coincident detector . • Both sine waves converted to square waves and fed to gates.

Rectifier bridge phase comparator

Working • • • •

Output across 1&2 Inputs through T1 and T2 I - gating input, function is to keep diode open Current can flow in forward and reverse direction provided reverse current less than forward current.

Analysis • Gating signal greater than twice the operating signal in r m s magnitude • I/2 flows through D1 and D2

Rectifier bridge phase comparator

Working • Used for distant relay operation. • If reverse current is less than the forward current the current can flow in both directions • A diode is acting as gate as long as it is kept open by a forward current • Operating current is kept as half the gating current.