STAR-DELTA STARTER

It is simple mechanical switch connects the stator to the supply in star on ‘start’ position. When he motor reaches near synchronous speed, the handle is taken to ‘run’ position. The stator connections area then changed to delta. Thus at start 1/√3 times live voltage is applied per phase and the currents area correspondingly reduced. In ‘run’ position, the rated line voltage is restored per phase. The starter also has mechanical locking arrangement, no volt, and over load releases. When the motor is over loaded the over load relay operates and NC contact is opened and contactors are opened and motor stops and hence protected from over loading.

STARTER FOR SLIP RING MOTOR

Rotor Resistance Starter: Three equal resistances in the form of a circular arrangement are connected in the rotor circuit. ?These are cut out progressively as three variable contacts move over the studs as the handle is moved from ‘start’ to ‘run’ position. The rotor resistance is maximum (the current minimum) at ‘start’ and progressively reduced till ‘run’  position. A switch is provided which disconnects the motor phase from supply if the stator handle is not brought back to ‘start’ position. This prevents starting the motor on ‘Run’ position. Usual protections are provided against no voltage and over load on stator side switch by over load relay.

STARTER FOR BOTH TYPES OF MOTORS

Push button/Direct On-line Starter: The starter has a magnetic coil ‘A’ wound on the attracted armature type of relay. When a ‘ON’ button is pressed it gets energized and the armature ‘B’ is attracted. The set of contacts ‘C’ in the contactor are made ‘ON’. The auxiliary contact at the bottom enables the coil to remain in the circuit even if the button ‘ON’ is released. The motor is thus started through the bimetallic strips ‘D’. If the supply fails the coil releases the armature due to lack of excitation and gravitational pull and the motor is disconnected from supply. If the motor is over loaded the strip ‘D’ bend downwards to release ‘NO’ contact is made open and in both cases the motor stops as the armature is released due to incomplete circuit of the coil ‘A’. The contact E can be ‘reset’ through ‘OFF’ button before restoring the motor.

SINGLE PHASING PREVENTER

Main parts of circuit: (i) Control coil, (ii) Negative sequence filter, (iii) Contactors, (iv) Normally closed NC contacts, (v) Normally open NO contact, (vi) C.T.S., (vii) Thermal relay.

The preventive circuit is connected in secondary’s of C.T. and R.Y.B. lines act as primaries of the CT. The negative sequence filter is shown connected. The output of this filter is fed to a leel detector. This sends the tripping command to the starter and NC contact gets opened and contactors get opened, stopping the motor as it is disconnected from the supply R.Y.B. This type of preventer is generally used for small/medium capacity motors.

      EXERCISE

1.    State the probable faults in case of a 3-phase induction motor.

2.    Explain how 3-phase induction motor shows abnormal behaviors in case of the following faults:

a) Change in supply frequencies, b) Single phasing, c) Over loading

d) Crawling/Cogging,  e) Unbalanced voltage.

3.    suggest the proper protective scheme for the motor protection under the following faults:

a) Single phasing, b) Over loading, c) Reverse phasing, d) Faults in stator,

e) faults in rotor,  f) Unbalanced supply voltage.

4.    Explain D.O.L. starter and state how it trips in over load conditions?

5.    Explain by drawing a schematic diagram the working of single phase preventer.

6.    Which protections are suggested for:

a) Small motors, b) Medium HP motors and c) High Hp motors for short circuit faults.

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       PROTECTION OF BUSBARS AND TRANSMISSION LINE            CHAPTER - 7

 

INTRODUCTION

Bus bars are important parts in power system and industrial switchgear. Bus bars form important link between incoming and outgoing circuits. The fault level at bus bars is very high, since it is connected to a larger part of a power system. The fault on bus bar may cause disconnection of supply to major portion of power system.

The bus bar area for he protection purpose includes the bus bar, circuit breakers and isolating switches. When fault occurs within the bus bar area, the faulty section must be immediately isolated in shortest possible time (50 msec.).

Lack of proper protection scheme may cause a major shut down which is not desired.

ABNORMALITIES AND FAULTS

As said earlier, the fault level at bus bars is very high. The bus bar protection is designed to operate for two types of fault:

a)    Earth fault or line to ground fault.

b)    Phase to phase fault.

The reasons for occurrence of fault are as follows:

1.    Failure of support insulators.

2.    Flash over due to damaged insulator.

3.    Flash over across insulators due to over voltage.

4.    Mechanical damage, earthquake.

Following are the methods of bus bar protection.

1.    Bus bar protection by over current relays.

2.    Bus bar protection by differential protection.

3.    Fault bus protection.

4.    Frame leakage protection.

The earth fault or line to ground fault is common fault and phase to phase fault occurs very rarely.

BUSBAR PROTECTION

Bus bar Protection by Over Current Relays

                                   

The CTs are connected in two lines and CT secondary is connected to trip coil of over current relay R₁ and R₂.

Under normal working conditions, all CB’s contacts are closed. When, fault occurs on any line say line 2. The CT secondary current increases to a sufficient value that sufficiently energizes the relay coil and R₂ operates. The CB of that line operates.

Bus bar Protection by Differential Protection

                                   

The incoming line is connected to bus bar which is supplying load to line 1 and 2. Two CTs are connected in line 1 and 2. The secondary’s of these CTs are connected in parallel. The incoming CT secondary is also in parallel with them. All CTs are identical i.e. rating, ration is same. A protective relay is connected across the CT secondary’s as shown.

Under normal working conditions no current flows through relay coil. This is because sum of incoming current entering the bus is equal to sum of currents leaving the bus.

But on occurrence of fault, differential current flows through relay coil. The relay operates the incoming circuit breaker and then the circuit breaker in line 1 and 2. Thus, faulty section is isolated.

Fault Bus Protection

                                   

It is generally preferred for indoor installations. An earthed bus is provided which is made to run along the conductor. This earthed bus is also known as fault bus.

With this type of arrangement, the fault current is made to flow through the fault bus. This fault current and location of fault is detected by suitable arrangement.

The bus bars are supported on insulators, are installed on the earthed bus or fault bus. The fault bus is earthed properly and a CT is inserted in the eathing connection. The trip coil is connected across CT secondary.

Under normal working conditions no current flows through the trip coil when earth fault develops, the current flows through the earthed connection and CT secondary current is circulated which energizes the trip coil and trip circuit contacts are closed. The circuit breaker receives the trip signal and faulty part is isolated.

Frame Leakage Protection

This protection scheme is preferred for indoor substation. Figure shows the arrangement.

                                   

The bus bar and circuit breaker are installed inside metallic casing. The metallic casing is earthed properly.

When earth fault occurs, leakage current is produced, it passes to the earth connection. CT secondary current increases, causing trip coil to close the contacts and trip signal is given to CB. The respective CB then operates and isolates the faulty part.

TRANSMISSION LINE PROTECTION

The transmission lines have large length, and it is exposed to different atmospheric conditions. So chances of occurrence of fault are more.

The requirements of transmission line protection scheme are:

1.    In the event of short circuit in any sections, the CB closest to that fault should open and other CBs should be in closed position.

2.    If CB nearest to fault fails to open, the back up protection CB should operate.

3.    The relay operating time should as short as possible.

The differential protection scheme can satisfy above conditions. The transmission line can be divided into number of sections. Each section can be protected by differential protection scheme. But since length of transmission line is large, such scheme becomes uneconomical so following methods are used.

Distance protection is applied where time lag cannot be permitted.

1.    Time graded over current protection.

2.    Current graded over current protection.

3.    Distance protection.

Time graded over current protection is used as main protection for distribution lines and back up for main lines where main protection is of distance type.

Current graded protection is used where a time lag can be permitted and instantaneous operation is not necessary.

Distance protection is applied where time lag cannot be permitted.

Time Graded Over Current Protection

The line or feeder is divided into number of sections. Over current relays are provided for each section. On occurrence of fault in any section, the respective relay operates and gives trip signal to CB. The  CB then opens and isolates the faulty part.

The figure shows time graded over current protection scheme.

                       

The entire feeder is sectionalized (i.e. AB, BC, CD). Definite time relays are used for individual section which gives trip signal to respective CB.

Relay R₁ has operating time of 0.5 second, for R₂ it is 1 sec and so on. If fault occurs in section CD, that section will be isolated in 0.5 sec by R₁ and CB₁. Remaining relays will not operate because their operating time is higher.

If R₁ and CB₁ fails to operate even after 0.5 seconds, R2 operates after 1 second causing CB2 to open. Thus acts as back up protection. In the similar way if R₂ and CB₂ fails to operate in 1 second, R₃ and CB₃ will operate.

Drawbacks:

1.    If feeder is having large length or more number of sections, the operating time near supply side point is more. For our case it will be 2 second which is appreciable.

2.    If a fault say line to ground fault occurs in the section near the transformer, or source, in that cause amount of fault current will be more because line impedance is less.

Ideally, the operating time should be less for more amount of fault current. If feeder is having large length and number of sections are more, this condition can not be satisfied.

So time graded principle is applicable where impedance between substation is low i.e. feeder length is less.

Note: In addition to relay operating time, the CB operating time should also be considered. In most of the cases it is necessary to limit the maximum tripping time to 2 seconds.

Current Graded Over Current Protection

The drawback of time graded system is that relay near the source operates at last and the fault current level is high near the source.

The relays used for this scheme are high speed instantaneous over current relays. The operating time for every relay is same.

Consider a feeder which is divided into sections A,B,C.

                                   

The current setting for a relay corresponds to the fault current level for the respective section. Relay R₂ should operate for faults between B and C. But it should not operate for faults beyond C. Similarly, relay R₁ should operate for faults between A and B. The relay R₃ should operate for faults beyond C.

But above operation is difficult to obtain because of following reasons:

1.    Relay R₁ is not able to differentiate between faults very close to B i.e. neither side of B. If fault in section BC is very close to B, the relay R₁ considers that the fault is in section AB and it operates, which is not desired.

This happens because there is a very little difference in fault currents if a fault occurs at the end of section AB or in the beginning of section BC.

2.    It is difficult to evaluate value of fault current because the conditions may be different for respective faults.

3.    During fault, transients are produced and relay performance is affected.

The current graded system is used where impedance between substations or sections is sufficiently large to create a marginal difference in fault current.

Combination of current and time grading: This is a popular scheme for protection of distribution lines. IDMT relays are used for such scheme. These have combined features of current and time grading. IDMT relays have current as well as time setting arrangements.  The current setting of relay is made according to fault current level of a particular section to be protected. The relays are set to pick up progressively at higher current levels, towards the source. Time setting is also done in a progressively increasing order towards the source.

Pilot Wire Protection

It uses differential protection principle i.e. under normal working conditions the current entering one end of line is equal to that leaving the other end. If fault develops, the balance is disturbed and the difference of incoming and outgoing current flows through the relay coil and trip signal is produced that operates the CB and thus faulty part is isolated.

Pilot wire protection methods: Following methods are used:

1.    Mertz   - Price voltage balance system

2.    Translay scheme.

                       

1.    Mertz – Price voltage balance system: Figure shows arrangement for this scheme.

To identical  CTs are placed at both ends of the line. The CT secondary’s are connected in phase opposition i.e. their secondary voltages oppose each other. Relay coils are connected in CT secondary’s as shown.

Under normal conditions, current entering from A is equal to current leaving from B. The CT secondary voltages are equal in magnitude and opposite in phase, so no current flows through the relay coils.

When fault occurs between A and B, this causes CT₁ secondary current to increase compared to CT₂, so voltages of CT secondary’s are no longer equal. Circulating current flows in CT secondary circuit through pilot wires and relay coils. The relays R₁ and R₂ operate causing operation of CB₁ and CB₂. This faulty part is isolated.

Advantages:

1.    This schemed can be used for ring system and parallel feeder system

2.    Provides instantaneous protection for ground faults.

Disadvantages

1.    CTs should be identical

2.    If pilot wire breaks, malfunctioning occurs.

3.    It is expensive.

4.    Not suitable for very long lines.

5.    It can not be used beyond 33 kV.

This scheme can be used for 3 phase system also. In that case total 6b CTs and 6 relays will be connected in the three phase line.

2.    Tran-slay Scheme: It uses over current induction relays. The CT secondary’s are   connected to the windings which are placed on central limb of the induction relay.

                                   

Each induction relay has two electromagnets E-shaped and C-shaped.

E-shaped electromagnet central limb carries winding A or A’ which are connected in CT secondary’s. The connections are made such that there is submission transformer action. All the CTs are identical.

The central limb also carries another winding BB’ just below AA’. This BB’ windings and CC’ windings are connected in series through pilot wires and in phase opposition i.e. their voltages act in opposite direction.

Operation: Under normal working conditions CT secondary currents are equal. These currents produce flux that links with B, B’ and e.m.f. is induced in these coils. Since B and B’ are in phase opposition, there is no current flowing in the circuit of these windings i.e. no current flows through C and C’. So there is no torque produced on the disc.

When fault occurs, the voltages induced in B and B’  are no longer the same, corresponding difference current flows through C and C’. And due to induction action a torque is produced on the disc, the disc rotates and trip circuit contacts are closed. The respective CBs operate such that faulty section is isolated.

Advantages:

1.    It is economical because only two pilot wires are used.

2.    Normal CTs can be used.

Distance Protection

The time graded system, pilot wire system are not suitable for protection of high voltage transmission lines having a large length.

The time graded system has a drawback of long operating time and pilot wire sysstem is expensive.

Distance protection scheme uses impedance relay. The relay operation is based on the impedance (for distance) between the relay and point of fault. This scheme of protection does not require any pilot wire. Figure shows arrangement for distance protection scheme.

                       

The voltage element of impedance receives supply from PT secondary and current element receives supply from CT secondary.

The protection zone of line is between A and b. under normal working conditions, the impedance of line is ZL. The impedence relay is so designed that, it operates only when line impedance becomes less than ZL.

When fault occurs between A and B (say F1), the impedance of line becomes less than ZL and impedance relay operates, trip signal is produced that operates CB and line is disconnected.

If fault occurs beyond C (say F2), the impedance in that case is greater than ZL, hence relay does not operate. The sectiojn beyond C will be protected by another protection scheme in that section.

Advantages:

1.    No need of pilot wires.

2.    Less expensive.

3.    Operation is simple.

4.    Three zone protection scheme can be employed to increase reliability.

EXERCISE

1.    What are abnormalities and fault in bus-bars?

2.    What are the reasons for occurrence of fault in bus-bar?

3.    Explain frame leakage protection with a neat sketch.

4.    What are the requirements of transmission line protection?

5.    What are the drawbacks of time graded over current protection?

6.    What accurate operation is not possible in current graded over current protection scheme?

7.    Explain translay scheme for protection of transmission line.

8.    What are the advantages and disadvantages of Mertz price voltage balance system?