6EE3A SWITCHGEAR &PROTECTION

UNIT-1

Syllabus: Static Relays: Introduction to static relays, merits and demerits.

Solid state relay or static relay is an electrical relay, in which the response is developed by electrical/ magnetic / optical or other components without mechanical movement of components. In static relay, the comparison or measurement if electrical quantities is done by a static circuit. This circuit gives an output signal for the tripping of a circuit breaker. A static relay or solid state relay employs semiconductor diodes, transistors, zener diodes, SCRs, logic gates etc. as its components. In present days, IC’s are used in place of transistors and as they are more reliable and compact. The simplified block diagram of a static relay as shown in the figure below;

As said, in static relay the comparison of the quantities is done by static electronic circuits. Rectifier converts AC to DC. Under normal conditions, “I” in the relay is zero. During I₁ – I₂ flows through the relay. This flows to transistor and pulse circuit. When there is a pulse, the transistor acts as a closed switch and opens the contacts of circuit breaker.

How a static relay works

• The output of CTs / PTs / Transducers is rectified in RECTIFIER.
• The rectified output is fed into the RELAY MEASURING UNIT.
• The output of measuring unit is then amplified in AMPLIFIER
• The amplified output is given to the OUTPUT DEVICE, which energizes the trip-coil, when the relay operates.

Advantages and Limitations of Static Relays (SR)

Advantage when compared with Electro Magnetic Relays

• Static or Solid State Relays consumes lesser power than the Electromagnetic Relays as static relays have less Burden on CT’s and PTs, compared to Electro Magnetic Relays. Static Relays power consumption is one mill watt. EMR consumes 2 Watts.
• In statics Relays, there are no moving parts and hence associated problems of arcing erosion of contacts, replacement of contacts, as in the case of EM relays, do not exists.
• There is no gravity effect on the operation of a static relay and hence found suitable to be used in ships, aircrafts etc, whereas, the EM relays are to be necessarily used in vertical position and will not work under jerks and vibrations.
• A single static relay can perform several functions like over current, under or over voltage, single phasing, short-circuit protection, by incorporating the respective functional blocks. This is not possible in electromagnetic relays.
• Static relay is very compact, whereas EM relay is not that compact. Highly superior operating characteristic and accuracy is possible with static relays, whereas in EM relays, this is not achievable. A speed of 1/2 to 1 cycle of operating time is possible with static relay, which is not possible with EM relays.
• Use of static relays in conduction with Transducers, converting even non-electrical properties like temperature, pressure etc., is possible. This is not possible in the case of EM relays.
• Static relays can think’, through its logic circuits’, where as this is not possible with EM relays’
• Programmable operations are possible with static relay. i.e., sequential instructions that direct the microprocessor in the relay to perform the specific functions.
• Effect of vibration on static relay is more or less nil. Static relay is not affected by shocks and hence suitable for earthquake – prone areas, ships, locomotive etc. EM relays are highly affected by vibrations and shocks.
• Simplified testing and servicing is possible with static relay. Defective modules can be replaced quickly. Testing of EM relays is comparatively not simple.

Limitations of Static Relays

1. Electrostatic Discharges (ESD): Semiconductor components are quite sensitive to electrostatic discharges developed by rubbing of insulating components. Hence, during manufacture of static relays, precautions are needed to avoid ESD.

2. Static relays are sensitive to voltage transients or voltage spikes caused by the operation of circuit breaker and isolator in the primary circuit of CTs and PTs. In their secondary circuit, the voltage spike can reach amplitude of 20KV even, but generally 12KV. In order to avoid this, filter circuit and usage of screen cables are adopted.

3. Static relays are temperature dependent. I.e., the characteristics of transistors, diodes used in a static relay, change with temperature variants. To avoid this, temperature compensation is provided by means of Thermistor circuits.

4. The price of static is higher than the equivalent electromagnetic relay. But multifunctional static relays provided economy of cost.

5. Total reliability of static relays depends upon the various small electronic parts.

Syllabus: Comparators: amplitude and phase comparators, duality between amplitude and phase comparators. Introduction to (a) amplitude comparators-circulating current type, phase splitting type and sampling type, (b) phase comparators-vector product type and coincidence type.

Comparator is a part of static relay, which receives two or more inputs to be compared and gives output based on the comparison.

Types of Comparator

The various types of comparators are;

1.      Amplitude Comparator

2.      Phase Comparator

3.      Hybrid Comparator

Amplitude Comparator

An amplitude comparator compares the magnitude of two input quantities irrespective of the angle between them. One of the inputs is the operating quantity and the other a restraining quantity.

When the amplitude of the operating quantity exceeds that of restraining quantity, the relay sends a tripping signal to the circuit breaker.

- The Amplitude comparator compares two vector, |A| and |B|

- Gives an output: the algebraic difference between the magnitudes |A| and |B|

- Output is +ve, if |A| > |B|

- Output is –ve, if |A| < |B|

- Output is zero, if |A| = |B|

Comparison by ratio:

- Output is >1, if |A| > |B|

- Output <1, if |A| < |B|

- Output is Zero, if |A| is zero.

Amplitude types of comparator.

1.      Integrating comparator

2.      Instantaneous comparator

3.      Sampling Comparator

Integrating Comparator

- Circulating Current Type

- Opposed Voltage Type

Circulating Current Type

It can also be used as impedance relay. Two rectifier bridges can be arranged in such a manner as shown in the figure below, to function as amplitude comparator circulating type.

The polarized relay operates when S₁>S₂ where S₁=K₁i₁ and S₂ = K₂i₂. This arrangement gives a sensitive relay whose voltage may be represented in the VI characteristic of the figure.

Opposed Voltage type

This type works with voltage input signals derived from PTs. The operation depends on the difference of the average rectified voltage (V₁-V₂).Here the rectifiers are not protected against higher currents. The relay operates when V₁ >V₂.

Instantaneous Comparator (Directing Amplitude Comparator) – Averaging Type

Here the restraining signal is rectified and smoothed completely in order to provide a level restraint.

This is then compared with the peak value of operating signal, which may or may not be rectified but is smoothened. The tripping signal is provided if the operating signal exceeds the level of the restraint. The block diagram is shown in the fig above. Since this method involves smoothening, the operation is slow. A faster method is phase splitting the wave shapes of instantaneous amplitude comparator are shown in fig below before rectification and the averaging circuit can be eliminated.

Phase Comparator

Phase comparison technique is the most widely used one for all practical directional, distance, differential and carrier relays.

If the two input signals are S₁ and S₂ the output occurs when the inputs have phase relationship lying within the specified limits.

Both the input must exist for an output to occur. The operation is independent of their magnitudes and is dependent only on their phase relationship. The figures below show that the phase comparator is simple form. The function is defined by the boundary of marginal operation and represented by the straight lines from the origin of the S-plane.

The condition of operation is β₁ < θ < β₂.

θ is the angle by which S₂ lands S₁. If β₁ = β₂ =90^o, the comparator is called cosine comparator and if β₁=0 and β₂=180^o, it is a sine comparator.

In short, a phase comparator compares two input quantities in phase angle (vertically) irrespective of the magnitude and operates if the phase angle between them is < 90^o.

There are two types of phase comparators:

1.      Vector product comparator

2.      Coincidence type phase comparator.

Vector Product comparator

This comparator recognizes the vector product or division between the two or more quantities. Thus, the output is A, B or A/B

Coincidence Comparator

Consider two signals S₁ and S₂. The period of Coincidence of S₁ and S₂ will depend on the phase difference between S₁ and S₂. The fig below shows the coincidence of S₁ and S₂ when S₂ lags S₁ by less than π/2 ie., θ.

The period of coincidence of S₁ and S₂ with a phase difference of θ is Ψ = 180^o – θ. Different techniques are used to measure the period of coincidence. Two of the important types are

1. Bloke Spike Method (Direct Phase Comparison) and

2. Coincidence type – Integrating phase comparator.

Hybrid Comparator

This kind of comparator compares both magnitude and phase of the input quantities. Hence this type is of mixed version.

In the hybrid comparator, both amplitude and phase comparators are used. Inputs are given to a phase comparator. The output of the phase comparator is given to amplitude comparator.

Static impedance relays comparing V and I are generally of Hybrid Comparator.

Level Detector

Level detector determines the level of its input with reference to a predetermined setting.

When input I exceed the level (L) and the output (O) of the level detectors exceeds and the output stage of the relays gets a triggering signal via an amplifier.

Syllabus: Static over Current Relays: Introduction to instantaneous, definite time, inverse time and directional over current relays.

Overcurrent Relay Purpose and Ratings

A relay that operates or picks up when its current exceeds a predetermined value (setting value) is called Overcurrent Relay.

Overcurrent protection protects electrical power systems against excessive currents which are caused by short circuits, ground faults, etc. Overcurrent relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.

For feeder protection, there would be more than one overcurrent relay to protect different sections of the feeder. These overcurrent relays need to coordinate with each other such that the relay nearest fault operates first.

Use time, current and a combination of both time and current are three ways to discriminate adjacent overcurrent relays.

Overcurrent Relay gives protection against:

Overcurrent includes short-circuit protection, and short circuits can be:

1.      Phase faults

2.      Earth faults

3.      Winding faults

Short-circuit currents are generally several times (5 to 20) full load current. Hence fast fault clearance is always desirable on short circuits.

Types of Overcurrent Relay

These are the types of overcurrent relay:

1.      Instantaneous Overcurrent (Define Current) Relay

2.      Define Time Overcurrent Relay

3.      Inverse Time Overcurrent Relay (IDMT Relay)

·         Moderately Inverse

·         Very Inverse Time

·         Extremely Inverse

4.      Directional overcurrent Relay

1. Instantaneous Overcurrent relay (Define Current)

Definite current relay operate instantaneously when the current reaches a predetermined value.

Instantaneous Overcurrent Relay – Definite Current

·         Operates in a definite time when current exceeds its Pick-up value.

·         Its operation criterion is only current magnitude (without time delay).

·         Operating time is constant.

·         There is no intentional time delay.

·         Coordination of definite-current relays is based on the fact that the fault current varies with the position of the fault because of the difference in the impedance between the fault and the source

·         The relay located furthest from the source operate for a low current value

·         The operating currents are progressively increased for the other relays when moving towards the source.

·         It operates in 0.1s or less

Application: This type is applied to the outgoing feeders.

2. Definite Time Overcurrent Relays

In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting value and the fault must be continuous at least a time equal to time setting of the relay.

Definite time of overcurrent relay

Modern relays may contain more than one stage of protection each stage includes each own current and time setting.

1.      For Operation of Definite Time Overcurrent Relay operating time is constant

2.      Its operation is independent of the magnitude of current above the pick-up value.

3.      It has pick-up and time dial settings, desired time delay can be set with the help of an intentional time delay mechanism.

4.      Easy to coordinate.

5.      Constant tripping time independent of in feed variation and fault location.

Drawback of Relay:

1.      The continuity in the supply cannot be maintained at the load end in the event of fault.

2.      Time lag is provided which is not desirable in on short circuits.

3.      It is difficult to co-ordinate and requires changes with the addition of load.

4.      It is not suitable for long distance transmission lines where rapid fault clearance is necessary for stability.

5.      Relay have difficulties in distinguishing between Fault currents at one point or another when fault impedances between these points are small, thus poor discrimination.

Application:

Definite time overcurrent relay is used as:

1.      Back up protection of distance relay of transmission line with time delay.

2.      Back up protection to differential relay of power transformer with time delay.

3.     Main protection to outgoing feeders and bus couplers with adjustable time delay setting.

3. Inverse Time Overcurrent Relays (IDMT Relay)

In this type of relays, operating time is inversely changed with current. So, high current will operate overcurrent relay faster than lower ones. There are standard inverse, very inverse and extremely inverse types.

Discrimination by both ‘Time’ and ‘Current’. The relay operation time is inversely proportional to the fault current.

Inverse Time relays are also referred to as Inverse Definite Minimum Time (IDMT) relay.

Inverse Definite Minimum Time (IDMT)

The operating time of an overcurrent relay can be moved up (made slower) by adjusting the ‘time dial setting’. The lowest time dial setting (fastest operating time) is generally 0.5 and the slowest is 10.

·         Operates when current exceeds its pick-up value.

·         Operating time depends on the magnitude of current.

·         It gives inverse time current characteristics at lower values of fault current and definite time characteristics at higher values

·         An inverse characteristic is obtained if the value of plug setting multiplier is below 10, for values between 10 and 20 characteristics tend towards definite time characteristics.

·         Widely used for the protection of distribution lines.

Based on the inverseness it has three different types:

Inverse types 

3.1. Normal Inverse Time Overcurrent Relay

The accuracy of the operating time may range from 5 to 7.5% of the nominal operating time as specified in the relevant norms. The uncertainty of the operating time and the necessary operating time may require a grading margin of 0.4 to 0.5 seconds.

It’s used when Fault Current is dependent on generation of fault not fault location.

Normal inverse time Overcurrent Relay is relatively small change in time per unit of change of current.

Application:

Most frequently used in utility and industrial circuits. Especially applicable where the fault magnitude is mainly dependent on the system generating capacity at the time of fault.

3.2. Very Inverse Time Overcurrent Relay

·         Gives more inverse characteristics than that of IDMT.

·         Used where there is a reduction in fault current, as the distance from source increases.

·         Particularly effective with ground faults because of their steep characteristics.

·         Suitable if there is a substantial reduction of fault current as the fault distance from the power source increases.

·         Very inverse overcurrent relays are particularly suitable if the short-circuit current drops rapidly with the distance from the substation.

·         The grading margin may be reduced to a value in the range from 0.3 to 0.4 seconds when overcurrent relays with very inverse characteristics are used.

·         Used when Fault Current is dependent on fault location.

·         Used when Fault Current independent of normal changes in generating capacity.

3.3. Extremely Inverse Time Overcurrent Relay

·         It has more inverse characteristics than that of IDMT and very inverse overcurrent relay.

·         Suitable for the protection of machines against overheating.

·         The operating time of a time overcurrent relay with an extremely inverse time-current characteristic is approximately inversely proportional to the square of the current

·         The use of extremely inverse overcurrent relays makes it possible to use a short time delay in spite of high switching-in currents.

·         Used when Fault current is dependent on fault location

·         Used when Fault current independent of normal changes in generating capacity.

Application:

·         Suitable for protection of distribution feeders with peak currents on switching in (refrigerators, pumps, water heaters and so on).

·         Particular suitable for grading and coordinates with fuses and re closes

·         For the protection of alternators, transformers. Expensive cables, etc.

3.4. Long Time Inverse Overcurrent Relay

The main application of long time overcurrent relays is as backup earth fault protection.

4. Directional Overcurrent Relays

When the power system is not radial (source on one side of the line), an overcurrent relay may not be able to provide adequate protection. This type of relay operates in on direction of current flow and blocks in the opposite direction.

Three conditions must be satisfied for its operation: current magnitude, time delay and directionality. The directionality of current flow can be identified using voltage as a reference of direction.

Application of Overcurrent Relay

Motor Protection:

·         Used against overloads and short-circuits in stator windings of motor.

·         Inverse time and instantaneous overcurrent phase and ground

·         Overcurrent relays used for motors above 1000 kW.

Transformer Protection:

·         Used only when the cost of overcurrent relays are not justified.

·         Extensively also at power-transformer locations for external-fault back-up protection.

Line Protection:

·         On some sub transmission lines where the cost of distance relaying cannot be justified.

·         primary ground-fault protection on most transmission lines where distance relays are used for phase faults.

·         For ground back-up protection on most lines having pilot relaying for primary protection.

Distribution Protection:

Overcurrent relaying is very well suited to distribution system protection for the following reasons:

·         It is basically simple and inexpensive.

·         Very often the relays do not need to be directional and hence no PT supply is required.

·         It is possible to use a set of two O/C relays for protection against inter-phase faults and a separate Overcurrent relay for ground faults.