EEE-477(Protective Relays-Static Relays)

Mrs. Helena Bulbul

Deptt. Of EEE, UIU

Agenda What is a Static Relay?

History of Static Relays

Static Relay vs. Electromagnetic Relay

General block diagram of static relay

Elements used in static relay

Block diagram, electronic design and operation of Static instantaneous overcurrent relay

Block diagram, electronic design and operation of Static overcurrent definite time lag relay

Block diagram, electronic design and operation of Static inverse time overcurrent relay

What is a Static Relay? A static relay is an electrical relay in which the response or

measuring is performed by electrical/ magnetic/ optical or other components without the involvement of any mechanical motion of components.

May use some electromechanical unit in the output stages as auxiliary relay.

History of Static Relay During the invent of vaccum tube or fuses, the relay for protection purpose

were not effective.

After the invent of transistor, diode or, other semiconductor devices , the static analog relay was designed.

Digital static Relay- After that different combinational logic such as TTL, DTL etc. make the design of static relay more sophisticated. However, design using discrete components make it bulky.

After the invent of ICs, the static relay uses ICs to incorporate several functions.

Microprocessor –based or, Numerical Relay- Design of static relay using microprocessor make it more sophisticated performing several functions at a time. Involving reasoning, making decision etc. processing and monitoring several units at a time, this type of relay can decide when to operate and when not.

After the invent of optical fibre communication with thereby also became possible and leads to the development of CPMC(Combined Protection , Monitoring and Control system), EMS(Energy Management System), SCADA etc.

Static Relay vs. Electromagnetic Relay

Low power consumption- Electromechanical ~2W, static-some ‘mW’ imposing less burden on CT/PT and less VA requirement of CT/PT

Resetting time and Overshoot- As there is no mechanical motion, overtravel is absent and reset time is less.

No moving contacts- The problem of wear and tear, contact bounce etc. are absent.

No effect of gravity-As it is not responsive to gravity, can be used in vessels etc.

Performs several functions- The relay used for motor protection can protect the motor from overcurrent, over/undervoltage etc. i.e. a single relay can perform the functions of several relays.

Compactness- Occupies less space though performing several functions

Superior characteristics and faster operation

Availability of Transducer- Transducers are available to produce electrical signal.

Static Relay vs. Electromagnetic Relay Relay can think- With the use of logical circuits, it can make decisions

monitoring several inputs at a time.

On line computation and monitoring.

Repeated operation is possible

Interface to SCADA and EMS

Remote backup using carrier current communication

Effect of vibration and shock

Self-supervision- Using ‘watchdog’ software

Extension is easier

Auxiliary supply system required

Electrostatic Discharge-As comprising semiconductors, they are prone to ESD and the components may be damaged

Costly for complex design

Temperature dependance- As the set point of different semiconductor components are temperature sensitive, precise working temperature must be maintained.(-100 C to +600 C)

Sensitive to transient voltage spikes.

Disadvantages of Static Relay

Block Diagrm of Static Relay-> Input from CT/PT is rectified and fed to the static relay.-> The values are then compared against a threshold value and an output is produced accordingly.-> As the output is small, it is amplified by amplifier unit.-> The amplified unit is then fed to the output devices, the trip circuit energizes and CB responds.

Block and circuit diagram of Static Instantaneous Over-current Relay

Improved Version of Instantaneous Overcurrent Relay

Working Principle

The filter circuit R1, C1 protects the bridge circuit from transient overvoltage

C2 acts as a partial filter circuit to smooth out the output from the bridge circuit- so as to provide high speed of operation

Partially filtered voltage is limited by a limiter (zenerdiode)and is then compared against a pre-set pick up value(Again a zener diode)

If the voltage exceeds pre-set value, a signal is given to the output transistor through an amplifier.

The output transistor conducts and the breaker trip coil is energized.

Definite Time-Lag Overcurrent Relay

Working Principle Under normal operating conditions, the transistor T1 is

conducting due to the bias applied from the supply voltage Vccthrough the resistor R4.

Capacitor ‘C’ is short-circuited by the action of T1 and is not charged.

Whenever fault current exceeds the pick-up value set by the potentiometer P1, the rectified voltage is applied to the Base-Emitter junction of T1 through resistor R2 which is thereby reverse-biased and is cut-off.

Capacitor ‘C’ starts charging from the supply voltage Vcc through resistor R3 and as the voltage is constant (independent of fault current), the capacitor ‘C’ is charged to a certain voltage in a definite time which exceeds the emitter setting of T2 fixed by P2.

T2 starts conducting forcing T3 to conduct also, thereby trip coil is energized and the breaker operates.

Inverse-Time Overcurrent Relay

Working Principle

When the AC current is low, transistor Q1 is conducting, short-circuiting the capacitor ‘C’

When the input current exceeds the required setting determinedby the potentiometer P2 and the transformer secondary tap-setting, the transistor Q1 is cutoff allowing capacitor ‘C’ to chargefrom the input current.(by the voltage developed across theresistor R1) through resistor R3 and potentiometer P1.

When the voltage across the capacitor exceeds the settingdetermined by the potentiometer setting P3, Transistor Q2conducts.

Conduction of Q2 drives transistor Q3 and the T.C is energized.

P.S.M is given by the transformer secondary tap and potentiometerP2.

T.S.M setting is determined by the potentiometer P1 and P3.

Directional Static Relay

Working Principle of Static Directional(Rectifier Bridge Phase Comparator Relay A phase-shifter circuit is used to lag/lead the respective current by

desired angle for a particular fault. For example, for phase-fault, current is shifted forward by 300 and for groundfault, it is shifted backward by 450 using an R-L circuit.

Maximum output phase-angle is adjusted by a variable resistor.

Micro-processor based overcurrent Relay