What is Auto-Reclosing and its schemes used in Power System?

80 to 90% of faults on transmission lines are of transient in nature, as voltage level increases, the percentage becomes low, as of insulator flash over etc.

If line is tripped and time is allowed for the fault arc to de-ionize, reclose of the circuit breaker will result in the line being successfully re-energized. Auto reclosing is carried out by auto reclose relay that may be separate or built-in protection relay. Auto reclosing helps in improvement in continuity of supply. Auto reclosing is never carried out on transformers.

For transmission lines up to 132 KV, 3 pole tripping / 3 pole closing is used for all types of faults in zone-1. For faults in zone-2 and above, auto reclosing is blocked.

Most Commonly used Auto-Reclose Scheme

For high voltage and extra high voltage lines, single pole tripping/ single pole closing, three pole tripping, three pole closing and lockout scheme is used. For phase to phase faults, three pole tripping, three pole closing and lockout is used.

Dead Time (System)

The time between the fault arc being extinguished and the circuit breaker contacts remaking.

Reclaim Time

The time following a successful closing operation, measured from the instant the auto-reclose relay closing contact make, which must elapse before the auto-reclose relay will initiate a reclosing sequence in the event of a further fault incident.

Lock-out

A feature of an auto-reclose scheme which after final tripping of the circuit breaker, prevents further auto reclosing.

High speed reclosing

If a circuit breaker is automatically reclosed with in one second after a fault trip operation, it is called high speed reclosing.

Low speed reclosing

If a circuit breaker auto-reclosing is delayed for a time in access of one second, it is called low speed reclosing.

What are Reactance and MHO Relays in Power System Protection?

Reactance Relay

These devices measure only reactance, and are not affected by variation in circuit resistance, their characteristics is a straight line parallel to to the resistance axis, with some pre-determined setting value of x. the relay will measure any value of reactance below the setting.

Like impedance relay, it is not directional and when current is flowing towards the bus bars at the end where protection is installed, it will even operate for values of x well in excess of the setting, including the reactance presented by the load.

As the zone characteristics are parallel to the R-axis, therefore any increase due to arc resistance has no effect on the performance of the relay. The relay is least effected by the arc resistance.

When a power system is operating in steady state all rotor or machine angles are constant. whenever there are sudden and large changes of power in th system, the rotor angle undergo oscillations till the system reaches a new stable state. This phenomenon is known as power swing.

During power swing the apparent impedance enters the trip region of the relay slowly, whereas during the fault it enters suddenly. The effect of power swing on reactance relays is maximum.

The relay is inherently non-directional. it needs directional element. This relay is suitable for short length lines.

 MHO Relays

The term MHO is derived from the fact that he relay characteristics when plotted on R-X diagram cuts the intersection of the x and R axis. With such a characteristics the relay measures the distance in one direction only. The characteristics of MHO relay on admittance diagram is a straight line.

A mho unit therefore combines directional action with ohmic measurement and thus combines both functions in one unit.

The characteristics are shown on the next slide. Mho relays are also known as admittance relays. The 45 degree setting is suitable for HV distribution lines 33KV or 11KV, where line angle vary between 45 and55 degree. The 60 degree setting is suitable for 66 or 132KV lines. For 220KV and above a setting of 75 degree characteristic for the line is used.

Effect of high arc resistance result in under reaching of the relay. A fault in zone 1 with high resistance may shift it i the next zone increasing the relay operation time and hence the fault clearance time. the effect is severe if the fault occurs in second half of the zone.

During power swing the apparent impedance enters the trip region of the relay slowly, whereas during the fault it enters suddenly. The effect of power swing on MHO relays is least.

The relay is inherently directional. it needs no directional element ad this relay is suitable for long transmission lines.

What are Impedance Relays and their Working Principle

The impedance relay measures the V/I ratio and the relay operates when it falls than a certain prefixed value. The tripping characteristics is simply a line on V/I diagram as stated below:

As, Z= R+jX, and if we draw the characteristics for Z having a fixed value on R & X plane,

we find it as circle with centre at origin as seen on the next figure:

In impedance relay effect of high ac resistance results in under reaching of the relay. A fault in zone 1 with high arc resistance may shift in the next zone increasing the relay operation time and hence the fault clearance time. The effect is sever if the fault occurs in the second half of the zone.

What is Power Swing?

When power system is operating in steady state, all the rotor or machine angles are constant. Whenever there are sudden and large changes of power in the system, the rotor angles undergo oscillations till the system reaches a new stable state. This phenomenon is known as power swing.

During power swing the apparent impedance enters the trip region of the relay slowly, whereas during the fault it enters suddenly The effect of power swing on impedance relays is moderate.

The relay is inherently non-directional. It needs directional element. The relay is suitable for moderate length of lines.

What are Distance Relays and their Working Operation?

A distance relay is one whose operation is based on measurement of impedance, reactance or admittance between the location of the relay and fault point.

The rapid growth in power system requires:

1. Continuity of supply

2. Good voltage regulation

3. Fast fault clearance,

To meet the above: High speed protection system along with automatic reclosing of circuit breakers has been developed and are widely used for medium, high and extra high voltage transmission lines.

Principle of Distance Relay

The impedance/reactance of transmission line is proportional to its length. A relay capable of measuring the impudence/reactance up to a given point can be used to protect a transmission line.

This designed to operate only for faults occurring between the relay location and selected point.

The relay compares the current flowing and voltage at the relay point.

  In case of fault on transmission line, the current flowing to fault point increases and the voltage at relay point decreases.

The relay sees this as fall in impedance and this is below a pre-fixed setting, the relay operates.

Distance relays are:

Double actuating quantity relays that is, one coil energized by voltage and the other coil energized by current.

The torque produced is such that when V/I reduces below a set value, the relay operates.

What is Current Error and Phase Displacement in C.T

Current Error

The error which a transformer introduces into the measurement of a current and which arises from the effect that the actual transformation ratio is not equal to the rated transformation ratio.

The current error expressed as a percentage is given by

current error %= (Kn*Is-Ip)x100/Ip

where,

Kn=rated transformation ratio.

Ip=actual primary current

Is= actual secondary current when Ip is flowing under the conditions of measurement.

Example,

Ip=100

100/5=20

(20x4.98-100)x100/100=current error= 4%

Phase Displacement

The displacement in phase between the primary and secondary current vectors, the direction of vectors being so chosen that the angle is zero for a perfect transformer.

The phase displacement is said to be positive when the secondary current vector leads the primary current vector and negative when it lags behind the primary current vector. It is usually expressed in minutes or in cent radians.

SHORT CIRCUIT CALCULATIONS IN POWER SYSTEM PART 1

The electrical energy is being sold to the consumers all over the World by either government, Semi Government, Public Limited or by Private Utilities. All of them have the same goal i.e. "Reliable, Power Supply to the Consumers"

No consumer whether domestic, commercial or industrial wants interruption in their supply. We see that in spite of the fact that neither the producers nor the consumers like interruptions, they still occur.

Interruptions are result of short circuits which can occur for many reasons and result in equipment failure. The effects of such short circuits and extend of the fault can be minimized by suitable design of the power system and related protective equipment.

Proper co-ordination of the operating and tripping characteristics is essential to achieve selective tripping of protective devices connected in series in the network and thus assure the high level of reliability of supply required by consumers.

The subject is essential for the proper selection of circuit interrupting devices ( circuit  breakers, fuses, and fuse switches load breaking devices) and the associated apparatus such as bus bars connections, current transformers, cables and boxes etc. The main source of fault power is generating plants but large contribution could be made by induction motors of larger sizes or by many induction motors of smaller sizes.

 

The faults Power supplied by induction Motors are maximum in the first half cycle of short circuit. The source of excitation however is rapidly consumed so that the fault current decays from its peak value in the first half cycle until, after say five cycles or so, it is relatively significant. The magnitude of fault current is controlled by the Combined impedances of all machines, transformers cables etc. In case of generators, transformers , synchronous motors and reactors the resistance is very small compared with its inductance and therefore can be ignored.

What is Burden, Accuracy Class and Rated Accuracy Limit Factor in CT

Accuracy Class

A designation assigned to a current transformer the errors of which remain within specified limits under prescribed conditions of use.

Burden

The value of the impedance of the secondary circuit expressed in ohms or in Volt-Amperes at the rated secondary current at the relevant power factor.

Rated Accuracy Limit Factor

The ratio of the rated accuracy limit primary current to the rated primary current.

Rated Accuracy Limit Primary Current

The value of primary current up to which the transformer will comply with the requirements for composite error.

Standard Accuracy Limit Factors

The standard accuracy limit factors are 5-10-15-20-30

Standard Accuracy Class

The standard accuracy class for protective current transformer are 5P and 10P

Composite Error

Under steady state conditions , the rms value of the difference between the instantaneous values of the primary current and instantaneous value of the actual secondary current multiplied by the rated transformer ratio.

What is Current Transformer?

Measurement of alternating current is one of the most frequent operations in electrical systems because it is necessary in determining other parameters of electrical circuits. Sample current is required for:

1. Indication instruments

2. KWh and KW meters

3. Tele metering

4. Protective relays

A current transformer is intended to operate normally with the rated current of the network flowing through the primary winding which is inserted in series in the network. The secondary winding of the current transformer is connected to measuring instruments and relays.

*CT (Current transformer) is always installed in series

*PT (Potential Transformer) is always installed in parallel

A CT is similar in construction to the single phase power transformer and also obeys the same fundamental laws. However the primary current of the CT is not controlled but is imposed on the transformer by the primary supply system.

The primary ampere turns produce a magnetic flux in the iron core which in turn induces an emf in the secondary winding. This causes a current to flow through burden connected.

For a CT,

I1N1=I2N2

or

I1/I2=N2/N1

and

V2=I2Z2

where internal drop,

E2-V2=E2-I2Z2=I2Z1 or

E2=I2(Z2+Z1)

Magnetic flux depends on E2, with variation of Z2, the flux density changes.

Where,

N1=Turns in primary winding

N2=Turns in secondary winding

I1=Current in primary winding

I2=Current in secondary winding

z2=Secondary load circuit

z1=internal impedance which consists of resistance and leakage reactance of  secondary winding

E2=voltage induced by the magnetic flux

Fi=Flux common to both windings

I0=no load current which is exiting current

In case of power transformer there is always another current, I0 known as no load current for maintaining magnetic flux and iron losses which disturbs this ideal equation and therefore I1N1=I2N2 is not fully realized. In current transformers the current flows in full through the primary winding. Hence a part of this current, called no load current is consumed to produce the required flux.

 

The voltage induced in secondary winding and the flux density of the magnetic core will depend on the secondary current and load impedance.

What is an Instrument Transformer?

Electrical current or voltage can be measured by bringing it to a suitable instrument, but in many electrical power circuits the current and voltage are both so high that it is desirable for both cost and safety to bring the circuits to the primary winding terminals of a potential transformer to measure voltage or through the primary winding of a current transformer to measure the current.

Standard secondary voltages are 100V, 115V, 120V with output ratings between 10A and 300VA, standard secondary currents are 1A, 2A and 5A.

They includes current transformers and potential (voltage) transformers.

What is Inverse Definite Minimum Time Relays (IDMT Relays)

In IDMT relays the time of operation is inversely proportional to the fault current level and the actual characteristics is a function of both “time” and “current” settings.

Each phase and ground current is separately compared with the pickup values of the inverse time over current protection elements. If a current exceeds 1.1
times the setting value, the corresponding element picks up and is signaled individually.
If the inrush restraint feature is applied, either the normal pickup signals or the corresponding inrush signals are output as long as inrush current is detected. Pickup of
a relay element is based on the RMS value of the fundamental harmonic. When the element picks up, the time delay of the trip signal is calculated using an integrated
measurement process.

The calculated time delay is dependent on the actual fault current flowing and the selected tripping characteristics. Once the time delay elapses, a trip signal is issued assuming that no inrush current is detected or inrush restraint is disabled.

If the inrush restraint feature is enabled and an inrush condition exists, no tripping takes place, but a message is recorded and displayed indicating when the over current element time delay elapses.

What is Instantaneous Over Current Protection

Over current protection includes short circuit protection. Short circuits can be phase faults, earth faults and three phase faults. Fast fault clearance is always required on short circuits. Normally short circuit currents are generally several times (5 to 20) of full load current.

In electromechanical relays fast operation was achieved by electromagnetic attraction principle. These are normally attracted armature relays. In numerical relays this is achieved through micro processor.

Whenever over current relays are used in the system, correct relay coordination is achieved either by time or by over current or by both. In discrimination by time method appropriate time interval is given by each of the relays to ensure that the breaker nearest to the fault opens first. This is known as “Differential Time Over Current Relay”.

Grading by current method relies on the fact that the fault current varies with the position of the fault. Therefore the relays controlling the various circuit breakers are set to operate the nearest one to the fault.

Both the methods, that is grading by time and grading by current have fundamental disadvantages.

In case of grading by time, the disadvantage is due to the fact that more severe faults are cleared in longest time. Current grading can be used where there is appreciable impedance between the two circuit breakers concerned.

How Extra High Voltage (EHV) System is Protected?

Mains Protection

Mains protection is the essential protection provided for protecting an equipment.

Backup Protection

As a precautionary measure, an additional protection is generally provided and is called backup protection. The primary protection is the first to act and the backup protection is the next in the line of defense. If primary protection fails, the backup protection comes in action and removes the faulty part.

For 220KV and 500KV transmission lines, two identical sets of protection are used. They are fed from two different DC supply sources. Main protection with backup protection along with all auxiliary relays makes one set of protection.

To ensure the isolation of fault, two sets of complete protection are used in EHV(Extra High Voltage) system.

What is Over Current Protection in Power Systems?

Protection against excess current was the earliest protective system developed and used by utilities. Over current protection is the protection in which the relay picks up when magnitude of current exceeds the pickup level. The over current relays are connected to the system by means of current transformer.

Over current relays are of following types:

1. Instantaneous over current protection

2. Definite time over current protection

3. IDMT (Inverse Definite Minimum Time) over current protection

4. Directional Over current protection

Over current protection has wide range of applications. The over current protection is provided for the flowing:

1. Motor Protection

For motor above 1200HP, it is used against overloads and short circuits in stator windings of motor. For small/medium size motors where cost of CT’s and relays is not justified are protected by thermal relays for overload protection and HRC fuses for short circuit.

2. Transformer Protection

For small power transformers (less than 5 MVA) over current protection is used as main protection. For bigger transformers over current protection is used as backup protection.

3. Line Protection

For 11KV feeders, over current protection is used a main protection. For 132KV lines over current protection is used as backup protection.

What is Trip Circuit Supervision in Power System Protection?

A dc circuit from station battery to protection relay trip contact and onwards to high voltage circuit breaker trip coil and back to the station battery is usually called trip circuit for the transmission line.

A continuous supervision of this circuit is necessary which ensures presence of DC supply, healthiness of complete trip circuit. Relay which performs the above functions is a trip circuit supervision relay.

The trip circuit monitor does not operate during system faults. A momentary closed tripping contact does not lead to a failure message. If, however, tripping contacts from other devices operate in parallel in the trip circuit, then the fault annunciation must be delayed. After the malfunction in the trip circuit is cleared, the fault annunciation is reset automatically after the same time period.

In modern digital relays supervision with binary inputs not only detects interruptions in the trip circuit and loss of control voltage, it also supervises the response of the circuit breaker using the position of the circuit breaker auxiliary contacts.

What is Role of DC Supply in Power System Protection?

IN high voltage substations, an independent source of supply is necessary for actuating the protective gear, operation of trip relays, release of trip order to the circuit breakers, interlocking, annunciation, audible alarm, remote position indication and other similar purposes.

The supply is particularly important for trip circuits. A reliable source of supply is necessary which can perform the above functions at the time of system faults. The choice is to select one of the following two systems:

1. AC supply fro station auxiliary transformer.

2. DC supply from independent source that is from storage batteries.

If we use AC supply from the station transformer, it can go to very low voltage levels in case of faults very close to the station. Therefore this source is not reliable and can not be used for station protective relays.

An independent supply from DC storage batteries is ideal for protective relays and switchgear trip circuit. This supply is not affected by the system faults. Lead acid accumulators or Alkaline accumulators can be used for this purpose. Lead acid is most commonly used. Complete DC equipment for a substation may be divided into three parts that is, storage batteries, charging equipment and distribution board.

It is utmost important to monitor that dc supply circuits and trip circuits remain always healthy and dc supply is available at points designed in the scheme, for this purpose trip circuit supervision relays are used.

Relays in Power System Protection

A relay is a device which makes a measurement or receives a controlling signal in consequence of which it produces a sudden pre-determined changes in one or more electrical output circuits.

A protection relay is a relay which responds to abnormal conditions in an electrical power system and controls a circuit breaker so as to isolate the faulty section of the system with the minimum interruption to the service.

Electromechanical relays

Electromechanical relay is a conventional relay in which the measurement is performed by moveable parts. The operation of such relays is based upon the following effects of electrical current:

1. Electro-magnetic attraction

2. Electro-magnetic induction

3. Thermal effect (Heat generation, some electromechanical relays respond to gas pressure generated due to heat of arc like Buchholz relay)

Static Relays

The expansion and growing complexity of modern power system required high performance and sophisticated characteristics from protection relays. This was possible with the use of semiconductors and other components in static relays.

Numerical Relays

The Numerical relays purely work on mathematical solution of different equations. They are micro processor based relays. Tripping decisions are not made by any measuring elements but are done by micro computers who continuously calculate and monitor the system data. Main advantages of Numerical Relays are:

1. Highly economical

2. Continuous self monitoring

3. More availability

4. Less work at panel fabrication

5. High flexibility in use

6. Memory

7. Possibility of remote control

8. Possibility of downloading/uploading information to computers

Important requirements of relays are:

1. Reliability

The protection relay should be reliable. It should not  fail to operate when faults and abnormal conditions, to which it is meant to respond appear. At some times it should not perform false operation.

2. Selectivity

Protection is arranged in zones, which should cover the power system completely, leaving no part unprotected.

When a fault occurs the protection is required to select and trip only the nearest circuit breakers. This is also known as “Discriminate tripping”.

A Proper coordination of the operating and tripping characteristic is essential to achieve selective tripping of protective device connected in series in the network and thus assure high level of reliability of supply required by the consumers.

Today main challenge for protection engineer is to design a very intelligent protection scheme which in case of fault would isolate the minimum part of the network. Sometime a fault on one point causes tripping of unrelated breakers resulting in unwanted tripping.

Protection engineer needs to analyze this and requires sufficient data from affected circuit to recommend the remedial steps to have better security of the system.

3. Sensitivity

The relay should have sufficient sensitivity so that it picks up under minimum fault conditions within its own protective zone. It should operate for faults at farthest end of zone under conditions of minimum generation.

4. Fast

The relay should clear the faults in shortest possible time. However, speed must not be obtained at the cost of selectivity.

5. Stability

Ability of relay to withstand the changes outside its zone.

What is the purpose of Power System Protection

The power system protection can not prevent system faults, but it can limit the damage caused by short circuits while protecting people and equipment from damage. Goals are to selectively clearing faults in milliseconds and protecting equipments from overload conditions. The damages are due to short circuits in a system, abnormal conditions in the system and majorly human mistakes.

The basic purpose of installation of protective relays are:

1. To protect very expensive equipment like transformers, generators, motors, cables and other installed components.

2. To ensure the interruption of supply to minimum number of consumers in case of system fault.

3. To minimize the production losses of the factories.

4. To avoid major shutdowns.

Since the protection system limits the damages/losses and keeps the system running therefore it is considered a vital importance for the system.

What is Soft Starter and How it Works?

Technically a soft starter is any device that will reduce the torque delivered to the power train. Mechanically, this can be a clutch, fluid drive, magnetic coupling, short coupling or any of the variety of devices that allow the motor to start-up across the line while slowly applying the shaft torque to the load to avoid “torque shock”. Electrically it can be any system that reduces the torque by virtue of reduced voltage or a change in motor connection.

Changing the motor terminal voltage reduces the torque because the motor output torque varies by the square of applied voltage. So if 50 percent voltage is applied to a motor, it will produce  25 percent of its available torque at that point.

Reduced Voltage starting can be accomplished in several ways as well, A common method is to use an auto transformer that drops the motor voltage during starting, then is switched out so that the motor gets full voltage when running. This method is called reduced voltage auto transformer starting. Similar to this are reactor and primary resistor starters which drop the voltage through those devices as well. All of the above technologies can be and often are referred to as “Soft Start” devices, but more recently this terminology has come to usually mean one specific type, the solid state reduced voltage starter.

The solid state reduced voltage starter(SSRV) uses high speed switching devices call SCRs(silicon controlled rectifiers) to switch on for only a portion of each half of the sine-wave line power. By doing so, the RMS voltage getting to the motor is reduced proportionally by the amount of time the switch is delayed. So if the SCR is not allowed to be conducting until the sine wave is already 1/2 over with, the output RMS will be 1/2 of the line voltage. By moving the gate point further back in the sine wave, the RMS voltage is increased until the SCR is being gated at zero crossing point and the motor is getting full line voltage. The speed at which the SCR gating is backed up is called the Ramp Time, and can typically be anywhere from a fraction of a second to 60 seconds. Although longer times are technically possible, most AC motors applications will not allow this because the increased current caused by the reduced voltage will begin to exceed the thermal safety limits of the motor itself, particularly the rotor. In addition the ramp time can be over ridden by a current limit setting, which determines the motor current through feedback sensors and stops the gate advancements in order to maintain the particular current setting. This feature is useful when the power system has limited delivery capabilities, such as weak utility lines or portable generators.

Finally once the motor is at full voltage the SCR firing becomes unnecessary and it is often beneficial to use a bypass contactor to shut power around SCRs. SCRs are not perfect conductors, and will reject approx 1.5 Watt of heat per running load ampere per phase. So on a  phase 100A motor, the SCRs will be rejecting 450W of heat into the enclosure continuously. A bypass contactor is a good way of avoiding that heat buildup without introducing dust, moisture or other contaminants into the enclosure.