Why is both RTD temperature and thermal overload protection needed?

Although both methods give protection against overload and thermal rise, both have own pros and cons. Especially with the important objects, both methods are used because they complement each other.

RTD basics

A fundamental physical property of a metal is that its resistivity changes with temperature. For many metals, this relationship is quite linear over wide temperature ranges making them ideal for measuring temperatures. A Resistance Temperature Detector (RTD) is a resistor designed to measure temperature using the known resistance vs. temperature relationship of metals. An RTD element is the actual temperature-sensing unit.

RTD’s are used to measure temperatures of protected device windings or shaft bearings. A rise in temperature may indicate overloading of the protected device, or the beginning of a fault.

Thermal overload protection basics

Thermal overload protection is based on a mathematical model of the thermal behavior of the protected device (motor etc.) The protection relay continuously measures the phase current amplitudes and calculates the thermal image. The thermal model might consist of one or more time-constant, separate protection for stator and rotor etc.

Why both are needed?

Although both methods give protection against overload and thermal rise, both have own pros and cons. Especially with the important objects, both methods are used because they complement each other.

RTDThermal overload protection
Resistors must be located in the protected object (isolation, wiring, etc)Requires only the phase current measurement (current transformers)
Temperature measured in certain spots, typically only in the statorMore overall (average) thermal image, can have separate images for stator and rotor
Measures actual (true) temperatureDoes not recognize thermal rise caused by reduced cooling (dirt or cooling system failure)
Slow response for rapid changes in loadBetter (faster) operation in start-ups and heavy overload (might include hot spot behavior modeling)
Can be used for temperature measuring of bearings etc.Does not see bearing etc. faults unless fault causes rise in the phase current amplitude

Protection relay codes

Introduction:

Numerical codes are sometimes used when defining the protection relay, whereas in other cases symbols are used. The numerical codes refer to the IEEE C37-2 Standard, whereas the symbols refer to the IEC Standards. In the definition of the symbols the IEC Standards have not detailed all the symbols to be used and so in practice, one still uses the codes mentioned in C37-2.

ANSI Protetion Relay Codes:

An extract of the numerical codes is given below, as given in the C37-2 Standard relative to protection systems. The description is a summary of what is given in the Standard:

– 2 starting timer;
– 21 distance relay (impedance);
– 24 overfluxing relay (volts per hertz);
– 25 synchronizer or synchronism verifier;
– 26 apparatus for temperature control;
– 27 undervoltage relay;
– 32 directional power relay;
– 37 undercurrent or under-power relay;
– 40 loss of field relay;
– 46 negative sequence relay or for current balance by
means of current measurement;
– 47 cyclic sequence relay by means of voltage measure-
ment;
– 48 incomplete sequence relay;
– 49 thermal relay for transformers or machines;
– 50 instantaneous overcurrent relay;
– 51 overcurrent relay with inverse time;
– 55 power factor control relay;
– 59 overvoltage relay;
– 60 voltage balance relay;
– 62 stop timer;
– 63 pressure sensor;
– 64 relay to identify ground faults (not used for networks
with grounded neutral);
– 66 apparatus which detects a certain number of opera-
tions;
– 67 directional overcurrent relay for alternating current;
– 68 locking relay (for example to prevent reclosing after
loss of step);
– 74 alarm relay;
– 76 overcurrent relay for direct current;
– 78 loss of step relay or for measurement of phase angle;
– 79 reclosing relay for alternating current;
– 81 frequency relay;
– 82 reclosing relay for direct current;
– 83 automatic changeover relay of for selective control;

– 85 pilot wire relay;
– 86 look-out relay;
– 87 differential relay;
– 90 regulator device;
– 91 directional voltage relay;
– 92 directional power voltage relay;
– 94 trip relay.

A Little Explanation:

The meaning of the codes used most frequently is given in
more detail since they are often the cause of misinterpretations
and misunderstandings.

code 48: is a little known code which is, however, now commonly used to indicate the protection against prolonged motor starts. Sometimes it is confused with the protection called 51LR (‘locked rotor’ overcurrent). There are two codes to be used to indicate the protections which serve to control motor starting and locked rotor: 48 for the starting phase (prolonged starting) and 51LR for the locked rotor (when the motor is already running);

– code 50: for the Standard this is an overcurrent protection of instantaneous type. The definition of instantaneous relay was valid for the electromechanical, now the various thresholds of the overcurrent relays always have the possibility of introducing a delay. In common practice it is considered to be the overcurrent protection which identifies strong currents typical of short-circuit;

– code 51: for the Standard, this is an overcurrent protection of the dependent (inverse) time type. The definition of a relay with inverse time is typical of American tradition. In common practice code, 51 is used both for overcurrent relay with dependent (inverse) time characteristic and with independent (definite) time characteristic. In general, it is considered the overcurrent protection which identifies weak currents typical of an overload or of short circuits with high fault impedance.

Further clarifications are necessary in defining the numerical codes to be used for the protection against ground faults. The C37-2 Standard only specifies a code to be used for ground faults: 64, but specifies that this code cannot be used for the protections connected to the CT secondary in grounded networks where code 51 must be used with suffixes N or G. In defining the N and G suffixes, the C37-2 Standard is very clear and they are used as follows:

– N when the protection is connected by means of transducers which measure the phase parameters and the vectorial sum of the parameter to be measured (current or voltage) is sent to the relay. This connection is generally called residual connection (Holmgreen);

– G when the protection is connected directly to the secondary of a transducer (CT or VT) which measures the homopolar parameter directly (current or voltage);

Therefore it is correct to use the following definitions for protection against ground fault:

– 51G for the overcurrent protection connected to the secondary of a ring CT which measures the ground current;

– 51G for the homopolar overcurrent protection connected to the secondary of a CT positioned on the grounding of the machine (star point generator or transformer);

– 51N for the homopolar overcurrent protection connected with residual connection to three phase CTs;

– 59N for the homopolar overvoltage protection con- nected on the vectorial sum of the three phase VTs (open delta – residual voltage);

– 59G homopolar overvoltage protection connected to the VT secondary positioned on the machine grounding (star point generator or transformer);

– 64 only applicable in networks with isolated neutral both for overcurrent and overvoltage protection.

Apart from the N and G suffixes, sometimes other suffixes are added to indicate the application of the protection in detail.
For example:

– G generator (for example 87G differential protection for generator);
– T transformer (for example 87T differential protection for transformer);
– M motor (for example 87M differential protection for motor);
– P pilot (for example 87P differential protection with pilot wire);
– S stator (for example 51S overcurrent stator);
– LR motor protection against locked running rotor (51LR);
– BF failed opening circuit-breaker 50 BF (BF = breaker failure);
– R used for different applications:
– reactance (for example 87R differential protection);
– undervoltage to indicate residual voltage (27R);
– rotor of a synchronous machine (64R ground rotor);
– V associated with the overcurrent protection (51) it indicates that there is voltage control or voltage restraint (51V);
– t indicates that the protection is timed (for example 50t protection against overcurrent short-circuit with delay added).

All About Buchholz Relay

Whenever a fault in the transformer develops slowly, heat is produced locally, which begins to decompose solid of liquid-insulated materials and thus to produce inflammable gas and oil flow. This phenomenon has been used in the gas protection relay or popularly known as Buchholz relay. This relay is applicable only to the so-called conservator type transformer in which the transformer tank is completely filled with oil, and a pipe connects the transformer tank to an auxiliary tank or ” Conservator” which acts as an expansion chamber. The figure is shown as Buchholz relay connected into the pipe leading to the conservator tank and arrange to detect gas produced in the transformer tank. As the gas accumulates for a minor fault the oil level falls and, with it a float ‘F’ which operates a mercury switch sounding an alarm. When a more serious fault occurs within the transformer during which intense heating takes place, an intense liberation of gases results. These gases rush towards the conservator and create a rise in pressure in the transformer tank due to which the oil is forced through the connecting pipe to the conservator. The oil flow develops a force on the lower float shown as “V” in the figure and overtrips it causing it contacts to complete the trip circuit of the transformer breaker. Operation of the upper float indicates an incipient fault and that of the lower float a serious fault.

Figure: Buchholz Relay
Buchholz Relay
Buchholz Relay

Buchholz relay Operation : Certain Precautions:

The Buchholz relay may become operative not only during faults within the transformer. For instance, when oil is added to a transformer, air may get in together with oil, accumulate under the relay cover and thus cause false operation of the gas relay. For this reason when the ‘Gas’ alarm signal is energized the operators must take a sample of the gas from the relay, for which purpose a special clock is provided. Gases due to faults always have color and an odor and are inflammable.

The lower float may also falsely operate if the oil velocity in the connection pipe through not due to internal faults, is sufficient to trip over the float. This can occur in the event of an external short circuit when overcurrents flowing through the windings over-heat the copper and the oil and cause the oil to expand. If mal-operation of Buchholz relay due to overloads or external short circuits is experienced it may be necessary that the lower float is adjusted for operation for still higher velocities.

In installing these relays the following requirements should be fulfilled.

a) The conductor connecting the contacts to the terminals on the cover must have paper insulation, as rubber insulation may be damaged by the oil.

Trip Circuit Connection for Power Transformer
Trip Circuit Connection for Power Transformer

b) The floats must be tested for air tightness by for example, submerging them in hot oil to create a surplus pressure in them.
c) The relay cover and the connection pipe should have a slope of 1.5 to 3 percent and not have any protruding surface to ensure unrestricted passage of the gases into the conservator.

Electrical Grounding System Explained

Introduction:

Power systems grounding is probably the most misunderstood element of any power systems design. Historically, the method of system grounding selected for various electrical system settings, e.g., industrial, commercial, etc., dates back to the early part of this century when only two methods were considered: solid grounded and ungrounded. Solid grounding with its advantage of high fault levels to drive protective devices had equally significant disadvantages such as dangers posed by arcs in hazardous areas. Also, the issue of service continuity of critical loads pointed away from this grounding method. The perception that ungrounded systems provide service continuity, at least through the first ground fault, strongly suggested ungrounded systems. In more recent times, however, well accepted, if not misapplied grounding techniques utilizing resistance or reactance, have provided the power systems engineer with other alternatives.

In order to establish a common perspective, some definitions and
short explanations of terms must be presented.

Ungrounded system:

A system, circuit, or apparatus without an intentional connection to ground, except through potential indicating or measuring devices or other very high-impedance devices. Note that although called ungrounded, this type of system is in reality coupled to ground through the distributed capacitance of its phase windings and conductors. In absence of a ground fault, the neutral of an ungrounded system under reasonably balanced load conditions will usually be held there by the balanced electrostatic capacitance between each phase conductor and ground.

Grounded system

A system of conductors in which at least one conductor or point (usually the middle wire or neutral point of a transformer or generator winding ) is intentionally grounded, either solidly or through an impedance

Solidly Groundeded System

Connected directly through an adequate ground connection in which no impedance has been intentionally inserted.


Solidly Grounded System

Solidly grounded systems are by far the most common found in industrial/commercial power systems today.

For line-to-neutral loads to be applied, the neutral point of the wye-connected source must be solidly grounded for the system to function properly and safely. If the system is not solidly grounded, the neutral point of the system would “float” with respect to ground as a function of load subjecting the line-to-neutral loads to voltag unbalances and instability.

To ensure that these systems are safe, the NEC requires that equipment grounding conductors (bare or green insulated) must extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain a very low impedance to ground faults so that a relatively high fault current will flow, thus ensuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage. It also greatly reduces shock hazard risk to personnel.

The logic behind requiring systems with less than 150 V to ground to be solidly grounded, is that studies, laboratory experiments, and case histories have shown that it takes about 150 V across a gap in low-voltage systems, to sustain an arc. With less than 150 V, the arc is generally self healing and rarely continues. Solid grounding in this case provides equipment and personnel safety, permits the application of economical line-to-neutral loads, and in the case of a “solid” ground fault, ensures prompt actuation of phase protective devices—assuming the equipment grounding function (green insulated or bare connector in the same raceway) is intact. The historical incidence of sustained ground faults is so low in 120/208 V or 120/240 V systems that the NEC has not found it necessary to require separate system ground fault protection.

Resistance grouning

Grounded through an impedance, the principal element of which is resistance.


Resistance grounding

For large electrical systems where there is high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact, resistance grounding has been selected. A resistor is connected from the system neutral point to ground and generally sized to permit only 200 A to 1200 A of ground fault current to flow. Enough current must flow such that protective devices can detect the faulted circuit and trip it off-line but not so much current as to create major damage at the fault point. Because the grounding impedance is in the form of resistance, any transient overvoltages are quickly damped out and the whole transient overvoltage phenomena is no longer applicable

Reactance grounding

Grounded through an impedance, the principal element of which is inductance.

Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available ground fault from something near the maximum three-phase short-circuit capacity (thousands of amperes) to a relatively low value (200 to 800 A). However, experience and studies have indicated that this inductive reactance to ground resonates with the system shunt capacitance to the ground under arcing ground fault conditions and creates very high transient overvoltages on the system. The mechanism under which this occurs is very similar to that discussed under the ungrounded system characteristics. To control the transient overvoltages, studies have shown that the design must permit at least 60% of the three-phase to short-circuit current to flow underground fault conditions, for example, 6000 A grounding reactor for a system having 10,000 A three-phase short-circuit capacity available. Due to the high magnitude of ground fault current required to control transient overvoltages, inductance grounding is rarely used within the industry.

High resistance grounding

This is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 A or less. High resistance grounding accomplishes two things. The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage is done at the fault point. This means that the faulted circuit need not be tripped offline when the fault first occurs. It also means that once a fault does occur, you don’t know where the fault is located. In this respect, it performs just like an ungrounded system.

Grounding transformers

So far this paper has discussed system grounding where the neutral point of the source has been readily available. But what does one do when the neutral point is not available? As the ungrounded system problems became more apparent to industry, they recognized that it was to their advantage to ground their delta connected systems. Some took the approach of purposely grounding one phase. Although somewhat effective for the transient overvoltage criterion, it leaves the system with the continuous line-to-line overvoltage condition and the multiple fault (line-to-line) problems mentioned previously.

The best way to ground an ungrounded delta system (existing or new) is to derive a neutral point through grounding transformers. This may be accomplished in one of two ways as shown in Figure a, high resistance grounding is accomplished through three auxiliary transformers connected wye-broken delta. The resistor inserted in the “broken delta” leg is reflected the primary underground fault conditions and limits the current to a nominal value as dictated by its design. Under any system condition other than ground faults, the three secondary voltages add vectorially to zero. With zero voltage across the resistor, no current flows and the grounding resistor does not impact the system. However, underground fault conditions, one of the three voltages is shorted out and the voltage across the resistor now is no longer zero. Under these conditions, the resistor is now in the circuit and currently does flow with the effect of limiting the primary current to the design value. Also, sensing the voltage drop across the resistor (device 59G) can be used to signal an alarm advising that a ground fault has occurred. The three lights across each individual transformer will constitute a version of the normal ground detection scheme currently employed on ungrounded systems.

High resistance grounding can also be achieved alternatively by a zig-zag grounding transformer as shown in Figure b. The scheme in Figure a uses the flux in the transformer’s iron core to produce secondary voltages with their respective phase relationships as described previously. With the zig-zag transformer, the windings are connected in a zig-zag fashion such that the flux in the iron is vectorially summed opposed to vectorially summing the secondary voltages. Consequently, it behaves on the system just as the three auxiliary transformers do. It appears “transparent” to the system except under ground fault conditions. The resistor makes it resistance grounded. In both of these cases, either approach accomplishes the same end. Therefore, selection should be based on space, weight, size, and/ or economics as applied to the system in question. Although high resistance examples are shown, other variations are available for higher voltage systems.

In terms of grounding there are two types of grounding system available. They are Neutral grounding system and Equipment grounding system.

Neutral Grounding System:

The term Grounding or Earthing refers to the connecting of a conductor to earth. The neutral points of the generator and transformer are deliberately connected to the earth. In 3 phase a.c. systems the earthing is provided at each voltage level. If a neutral point is not available, a special Earthing Transformer is installed to obtain the neutral point for the purpose of earthing. Neutral points of star connected VTs and CTs are earthed.

The neutral earthing has several advantages such as :


— Freedom from persistent arcing grounds. The capacitance between the line and earth gets charged from supply voltage. During the flashover, the capacitance get discharged to the earth. The supply voltage charges it again. Such alternate charging and discharging produces repeated arcs called Arcing Grounds. The neutral grounding eliminates the problem of ‘arcing grounds’.

— The neutral grounding stabilizes the neutral point. The voltages of healthy phases with respect to neutral are stabilized by neutral earthing.

-The neutral earthing is useful in discharging over-voltages due to lightning to the earth.

— Simplfied design of earth fault protection.

— The grounded systems require relatively lower insulation levels as compareed with ungrounded systems.

The modern power systems are 3 phase ac systems with grounded neutrals

The Equipment Grounding System:

The Equipment Grounding System refers to the grounding of non-current carrying metal parts to earth. It is used for safety of personnel. It is a metal part is grounded, its voltage with respect to earth does not rise to a dangerously high value and the danger of a severe shock to personnel is avoided.

All About Induction Machines

Introduction

Induction machines, also called asynchronous machines, can be used as generators or motors. Induction machines can be either of one or three-phase construction. The following discussion focuses on the three-phase machines and their properties. The maximum of the applications for three-phase induction machines is with power ranges varying from a few kilowatts to a few hun-dred kilowatts with rated voltages below 1 kV. The range can be extended roughly up to 20 MW with rated voltages up to 15 kV. The simple, robust and low loss construction of an induction machine has contributed to its wide-spread success in different applications.

squirrel cage motor
squirrel cage motor
wound winding rotor motor with slip rings
wound winding rotor motor with slip rings

The stator has basically the same construction as with synchronous machines. It is fed by three-phase alternating current providing rotating flux. This flux rotates at synchronous speed.

The rotor is a three-phase short-circuited winding. This winding can be a normal wound wind-ing or it can be done by casting aluminum “cage” windings into the slots in a laminated iron rotor construction. In the earlier case, it is referred to as “wound rotor machine,” and in the latter case, it is referred to as “squirrel cage rotor machine.”

The rotating stator field induces a flux in the rotor windings. Since the rotor windings are short-circuited, the induced flux will create a current in the rotor. This current will produce a flux of its own, opposing the flux that created it. As a result, there will develop a torque on the machine shaft. When the developed torque is higher than the resisting load torque, the machine starts to rotate as a motor.

During operation, under no-load conditions, the speed of the rotor is very close to the syn-chronous speed, thus the currents induced in the rotor will have a low frequency. The rotor currents will have a frequency corresponding to the rotating speed difference between stator field and rotor (shaft). This difference is referred to as slip. Normally, the slip is stated in relation to the synchronous speed i.e. relative slip (s).

When the slip is positive, the machine is working as a motor, and when it is negative, the machine is working as a generator. The slip is often given as percentage value s%.

The name “induction machine” comes from the fact that the induced voltage in the rotor wind-ings is due to the rotating stator field, whereas the term “asynchronous machine” refers to the fact that the rotor is always rotating at a different speed than the stator field.

The performance of an induction machine can be studied based on a one-phase equivalent circuit.

Equivalent circuit of an induction machine
Equivalent circuit of an induction machine

The reduced rotor current flowing through the slip-related rotor resistance component de-scribes actually the mechanical power developed at the rotor shaft at each operation point. Also the rotor leakage reactance value is depending on the slip since the rotor current frequency is depending on the slip. Under light load conditions, the significance of the rotor leakage reactance is negligible, but the situation will change as the load, and slip increases.

Torque-versus-slip characteristic of a three-phase induction motor

From the below figure, it can be seen that when the load torque requirement rises, the slip increases until a point of equilibrium is found. The torque can be increased until the breakdown torque point is reached, and sliding on the left-hand side of this point with a constant-load torque would mean stopping of the motor. It can be also noted that the starting torque is much less than the maximum, breakdown, torque. Also, the rated torque of the motor is below the maximum, at least by a relation of 1.6

Torque as a function of slip with a three-phase induction motor
Torque as a function of slip with a three-phase induction motor

Increasing the rotor resistance increases the starting torque, and this lowers the torque curve gradient between no-load and breakdown torque points. On the other hand, the rotor re-sistance does not affect the maximum, breakdown, torque value.

This phenomenon is utilized with induction motors having wound winding-type rotor with slip rings. External variable resistance is connected to the non-short-circuited rotor windings using the slip rings. By adjusting the external resistance, it is possible to increase the starting torque, lower the starting current and within certain limits to control the motor speed. The drawbacks are the losses in the external resistor, limited speed adjustment range, speed varia-tions with variable torque (load) and more complex structure of the motor.

Power Factor Correction: Methods Explained

The main means for the generation of reactive power
are:
• synchronous alternators;
• synchronous compensators (SC);
• static var compensators (SVC);
• banks of static capacitors

Synchronous alternators

Synchronous alternators are the main machines used for the generation of electrical energy. They are intended to supply electrical power to the final loads through transmission and distribution systems. Besides, without
going into technical details, by acting on the excitation of alternators, it is possible to vary the value of the generated voltage and consequently to regulate the injections of reactive power into the network, so that the voltage profiles of the system can be improved and the losses due to the Joule effect along the lines can be reduced.

Synchronous compensators

They are synchronous motors running no-load in synchronism with the network and having the only function to absorb the reactive power in excess (under-excited operation) or to supply the missing one (over-excited operation).

Under-excited synchronous compensator
Over-excited synchronous compensator

E : e.m.f. induced in the stator phases
V : phase voltage imposed by the network to the alternator terminals
I : stator current
Xs : stator reactance

These devices are used mainly in definite nodes of the power transmission and sub-transmission network for the regulation of voltages and of reactive power flows. The use of synchronous compensators in power distribution networks is not favorable from an economic point of view because of their high installation and maintenance costs.

Static var compensators

The considerable development of power electronics is encouraging the replacement of synchronous compensators with static systems for the control of the reactive power such as for example TSC (thyristor switched capacitors) and TCR (thyristor controlled reactors). These are an electronic version of the reactive power compensation systems based on electromechanical components in which, however, the switching of the various capacitors is not carried out through the opening and closing of
suitable contactors, but through the control carried out by couples of antiparallel thyristors.

Basic diagram of a TCR Basic diagram of a TSC

TSC allow a step-by-step control of the reactive power delivered by groups of capacitors, whereas with TCR a continuous control of the reactive power drawn by the inductors is possible.

By coupling a TSC with a TCR it is possible to obtain a continuous modulated regulation of the delivered/drawn reactive power.

From the point of view of applications, these devices are used above all in high and very high voltage networks.

Static Capacitor Bank

A capacitor is a passive dipole consisting of two conducting surfaces called plates, isolated from one another by a dielectric material.

The system thus obtained is impregnated to prevent the penetration of humidity or of gas pockets which could cause electrical discharges.

The last generation capacitors are dry-type and undergo a specific treatment which improves their electrical characteristics. Using dry-type capacitors there is no risk of pollution because of the incidental leak of the impregnating substance.
According to the geometry of the metal plates, it is possible to have:
• plane capacitors;
• cylindrical capacitors;
• spherical capacitors.


Power Factor Improvement (PFI) Panel

The main parameters which characterize a capacitor are:

  • Rated capacitance Cn: the value obtained from the rated values of power, voltage, and frequency of the capacitor;
  • Rated power Qn: the reactive power for which the capacitor has been designed;
  • Rated voltage Un: the r.m.s. value of the alternating voltage for which the capacitor has been designed;
  • Rated frequency fn: the frequency for which the capacitor has been designed.

When an alternating voltage is applied across the plates,
the capacitor is subjected to charge and discharge cycles, during which it stores reactive energy (capacitor charge) and injects such energy into the circuit to which it is connected (capacitor discharge).

Such energy is given by the following relation:

where:
• C is the capacitance;
• U is the voltage applied to the terminals of the capacitor.
Because of their capability of storing and delivering energy, capacitors are used as a basic element for the realization of PF correction banks (for all voltage levels) and of static devices for the regulation of reactive power. Particularly, the PFI capacitors used for low voltage, applications are constituted by single-phase components of metalized polypropylene film and can be of the self-healing type. In these capacitors, the dielectric part damaged by a discharge is capable of self-restoring; in fact, when such situations occur, the part of the polypropylene film affected by the discharge evaporates due to the thermal effect caused by the discharge itself, thus restoring the damaged part.

Switchgear and Protection Short Question Answer

INTRODUCTION

Switchgear and protection is one of the most important topic in electrical and power engineering. All the electrical machines are very expensive assets for our electrical grid network, e.g: motor, generator, transformer. So knowledge on switchgear and protection is must.

In this post we will look some common questions on switchgear and protection. This question are widely used in job interviews too. So it will be a nice to collection of short questions and answers on this topic.

1. What are the functions of protective relays

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.

2. Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed , thecircuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

3. Define protected zone.

Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

4. What are unit system and non unit system?

A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.

5. What is primary protection?

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.

6. What is back up protection?

Is the second line of defence , which operates if the primary protection fails to activate within a definite time delay.

7. Name the different kinds of over current relays.

Induction type non-directional over current relay,Induction type directional over current relay & current differential relay.

8. Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

9. Define operating time of a relay.

It is defined as the time period extendind from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

10. Define resetting time of a relay.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

11. What are over and under current relays?

Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type non-directional/directional overcurrent relay, differential  overcurrent  relay)whereas  undercurrent  relays  are  those  which  operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

12. Mention any two applications of differential relay.

Protection of generator & generator transformer unit; protection of large motors and busbars .

13. What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.

OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

14. What is the need of relay coordination?

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

15. Mention the short comings of Merz Price scheme of protection applied to a power transformer.

In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition.

16. What are the various faults to which a turbo alternator is likely to be subjected?

Failure of steam supply; failure of speed; overcurrent; over voltage; unbalanced loading; stator winding fault .

17. What is an under frequency relay?

An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.

18. Define the term pilot with reference to power line protection.

Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

19. Mention any two disadvantage of carrier current scheme for transmission line only.

The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response time of band pass filter; capacitance phase-shift of the transmission line

20. What are the features of directional relay?

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.

21. What are the causes of over speed and how alternators are protected from it?

Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

22. What are the main types of stator winding faults?

Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.

23. Give the limitations of Merz Price protection.

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault.

24. What are the uses of Buchholz’s relay?

Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion transformers with a rating over 750KVA.

APPARATUS PROTECTION

25. What are the types of graded used in line of radial relay feeder?

Definite time relay and inverse-definite time relay.

26. What are the various faults that would affect an alternator?

(a) Stator faults

1. Phase to phase faults

2. Phase to earth faults

3. Inter turn faults

(b)       1. Earth faults

2. Fault between turns

3. Loss of excitation due to fuel failure

(c)       1. Over speed

2. Loss of drive

3. Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing

(d)       1. Fault on lines

2. Fault on busbars

27. Why neutral resistor is added between neutral and earth of an alternator?

In order to limit the flow of current through neutral and earth a resistor is introduced between them.

28. What is the backup protection available for an alternator?

Overcurrent and earth fault protection is the backup protections.

29. What are faults associated with an alternator?

(a) External fault or through fault

(b) Internal fault

  1. Short circuit in transformer winding and connectio
  2. Incipient or slow developing faults

30. What are the main safety devices available with transformer?

Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator .

31. What are the limitations of Buchholz relay?

(a) Only fault below the oil level are detected.

(b) Mercury switch setting should be very accurate, otherwise even for vibration, there can be a false operation.

(c) The relay is of slow operating type, which is unsatisfactory.

32. What are the problems arising in differential protection in power transformer and how are they overcome?

1. Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.

2. Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.

3. Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.

4. Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary.

CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.

33. What is REF relay?

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

34. What is over fluxing protection in transformer?

If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.

35. Why busbar protection is needed?

(a) Fault level at busbar is high

b) The stability of the system is affected by the faults in the bus zone.

(c) A fault in the bus bar causes interruption of supply to a large portion of the system network.

36. What are the merits of carrier current protection?

Fast operation, auto re-closing possible, easy discrimination of simultaneous faults .

37. What are the errors in CT?

(a) Ratio error

Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100

The value of transformation ratio is not equal to the turns ratio. (b) Phase angle error:

Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]

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38. What is field suppression?

When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself.

Hence  the  field  circuit  breaker  is  opened  and  stored  energy  in  the  field  winding  is discharged through another resistor. This method is known as field suppression.

39. What are the causes of bus zone faults?

_ Failure of support insulator resulting in earth fault

_ Flashover across support insulator during over voltage

_ Heavily polluted insulator causing flashover

_ Earthquake, mechanical damage etc.

40. What are the problems in bus zone differential protection?

_ Large number of circuits, different current levels for different circuits for external faults.

_ Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error.

_ Sectionalizing of the bus makes circuit complicated.

_ Setting of relays need a change with large load changes.

41. What is static relay?

It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device.

42. What is power swing?

During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves originating in load regions and traveling towards relay characteristics.

43. What is a programmable relay?

A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit.

44. What is CPMC?

It is combined protection, monitoring and control system incorporated in the static system.

45. What are the advantages of static relay over electromagnetic relay?

Low power consumption as low as 1mW

No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts

No gravity effect on operation of static relays. Hence can be used in

vessels ie, ships, aircrafts etc.

A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional

blocks. This is not possible in electromagnetic relays

Static relay is compact

Superior operating characteristics and accuracy

Static relay can think , programmable operation is possible with static relay

Effect of vibration is nil, hence can be used in earthquake-prone areas Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers.

THEORY OF CIRCUIT INTERRUPTION

46. What is resistance switching?

It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current.

47. What do you mean by current chopping?

When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

48. What are the methods of capacitive switching?

• Opening of single capacitor bank

• Closing of one capacitor bank against another

49. What is an arc?

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition.

50. Give the two methods of arc interruption?

High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished _ Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac.

51. What is restriking voltage?

It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished.

52. What is meant by recovery voltage?

The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage.

53. What is RRRV?

It is the rate of rise of restriking voltage, expressed in volts per microsecond. It is closely associated with natural frequency of oscillation.

CIRCUIT BREAKERS

54. What is circuit breaker?

It  is  a  piece  of  equipment  used  to  break  a  circuit  automatically  under  fault conditions. It breaks a circuit either manually or by remote control under normal conditions and under fault conditions.

55. Write the classification of circuit breakers based on the medium used for arc extinction?

_ Air break circuit breaker

_ Oil circuit breaker

_ Minimum oil circuit breaker

_ Air blast circuit breaker

_ SF6 circuit breaker

_ Vacuum circuit breaker

56. What is the main problem of the circuit breaker?

When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem.

57. What are demerits of MOCB?

_ Short contact life

_ Frequent maintenance

_ Possibility of explosion

_ Larger arcing time for small currents

_ Prone to restricts

58. What are the advantages of oil as arc quenching medium?

• It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties

• It acts as an insulator and permits smaller clearance between line conductors and earthed components

59. What are the hazards imposed by oil when it is used as an arc quenching medium?

There is a risk of fire since it is inflammable. It may form an explosive mixture with arc. So oil is preferred as an arc quenching medium.

60. What are the advantages of MOCB over a bulk oil circuit breaker?

• It requires lesser quantity of oil

• It requires smaller space

• There is a reduced risk of fire

• Maintenance problem are reduced

61. What are the disadvantages of MOCB over a bulk oil circuit breaker?

The degree of carbonization is increased due to smaller quantity of oil o There is difficulty of removing the gases from the contact space in time o The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization.

62. What are the types of air blast circuit breaker?

_ Arial-blast type

_ Cross blast

_ Radial-blast

63. What are the advantages of air blast circuit breaker over oil circuit breaker?

o The risk of fire is diminished

o The arcing time is very small due to rapid buildup of dielectric strength between contacts

o The arcing products are completely removed by the blast whereas oil deteriorates with successive operations

64. What are the demerits of using oil as an arc quenching medium?

• The air has relatively inferior arc quenching properties

• The air blast circuit breakers are very sensitive to variations in the rate of rise of restriking voltage

• Maintenance is required for the compression plant which supplies the air blast

65. What is meant by electro negativity of SF6 gas?

SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro negativity of SF6 gas.

66. What are the characteristic of SF6 gas?

It has good dielectric strength and excellent arc quenching property. It is inert, non- toxic, noninflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil.

67. Write the classifications of test conducted on circuit breakers.

_ Type test

_ Routine test

_ Reliability test

_ Commissioning test

68. What are the indirect methods of circuit breaker testing?

Unit test

  • Synthetic test
  • Substitution testing
  • Compensation testing o Capacitance testing

69. What are the advantages of synthetic testing methods?

• The breaker can be tested for desired transient recovery voltage and RRRV.

• Both test current and test voltage can be independently varied. This gives flexibility to the test

• The method is simple

• With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be tested.

70. How does the over voltage surge affect the power system?

The over voltage of the power system leads to insulation breakdown of the equipment’s. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line.

71. What is pick up value?

It is the minimum current in the relay coil at which the relay starts to operate.

72. Define target.

It is the indicator used for showing the operation of the relay.

73. Define reach.

It is the distance upto which the relay will cover for protection.

74. Define blocking.

It means preventing the relay from tripping due to its own characteristics or due to additional relays.

75. Define a over current relay.

Relay which operates when the current ia a line exceeds a predetermined value.

76. Define an under current relay?

Relays   which   operates   whenever   the   current   in   a   circuit   drops   below   a predetermined value.

77. Mention any 2 applications of differential relays.

Protection of generator and generator-transformer unit: protection of large motors and bus bars

78.Mention the various tests carried out in a circuit breaker at HV labs.

Short circuit tests, Synthetic tests& direct tests.

78. Mention the advantages of field tests.

The circuit breaker is tested under actual conditions like those that occur in the network. Special occasions like breaking of charging currents of long lines ,very short line faults ,interruption of small inductive currents etc… can be tested by direct testing only.

79. State the disadvantages of field tests.

The circuit breaker can be tested at only a given rated voltage and network capacity. The necessity to interrupt the normal services and to test only at light load conditions.

Extra inconvenience and expenses in installation of controlling and measuring equipment in the field.

80. Define composite testing of a circuit breaker.

In this method the breaker is first tested for its rated breaking capacity at a reduced voltage and afterwards for rated voltage at a low current. This method does not give a proper estimate of the breaker performance.

81. State the various types of earthing.

Solid  earthing,  resistance  earthing  ,  reactance  Earthing  ,  voltage  transformer

Earthing and zig-zag transformer earthing.

82. What are arcing grounds?

The presence of inductive and capacitive currents in the isolated neutral system leads to formation of arcs called as arcing grounds.

83. What is arc suppression coil?

A method of reactance grounding used to suppress the arc due to arcing grounds.

84. State the significance of single line to ground fault.

In single line to ground fault all the sequence networks are connected in series. All the sequence currents are equal and the fault current magnitude is three times its sequence currents.

85. What are symmetrical components?

It  is  a  mathematical  tool  to  resolve  unbalanced  components  into  balanced components.

86. State the three sequence components.

Positive sequence components, negative sequence components and zero sequence components.

87. Define positive sequence component.

has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and having the phase sequence as original vectors.

88. Define zero sequence component.

They has 3 vectors having equal magnitudes and displaced from each other by an angle zero degees.

89. State the significance of double line fault.

It has no zero sequence component and the positive and negative sequence networks are connected in parallel.

90. Define negative sequence component.

It has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and has the phase sequence in opposite to its original phasors.

91. State the different types of faults.

Symmetrical faults and unsymmetrical faults and open conductor faults.

92. State the various types of unsymmetrical faults.

Line to ground ,line to line and double line to ground faults

93. Mention the withstanding current in our human body.

9mA

94. State the different types of circuit breakers.

Air ,oil,vacuum circuit breakers.

95. Define per unit value.

It is defined as the ratio of actual value to its base value.

96. Mention the inductance value of the peterson’s coil.

                          L=1/3wc2

97. Define single line diagram.

Representation of various power system components in a single line is defined as single line diagram.

98. Differentiate between a fuse and a circuit breaker.

Fuse is a low current interrupting device. It is a copper or an aluminium wire.Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.

99. How direct tests are conducted in circuit breakers?

Using a short circuit generator as the source.

Using the power utility system or network as the source.

100. What is dielectric test of a circuit breaker?

It consists of overvoltage withstand test of power frequency lightning and impulse voltages.Testa are done for both internal and external insulation with switch in both open and closed conditions.

Philosophy of Protection System Settings

The objectives  of the protection system are:

–  limit damage to people and to the plant;

–  permit different  service conditions;

–  guarantee maximum service continuity for the plant not affected  by faults;

–  activate the automatisms provided.

The characteristics of the protection system of an electric networks are:

–   Dependence: it can be called on to work after either a short or long period after installation. In any case, it must work when it is called on to operate;

–   Safety: it must not operate when is not required (it must not operate during transients). It must allow the various service conditions and activate the automatism provided;

–   Selectivity: it must operate only and when necessary, guaranteeing maximum service continuity with minimum disconnection of the network;

–   Speed: represented by the minimum fault time and by damage to the machinery;

–   Simplicity: measured by the number of pieces of equipment needed to protect the network;

–   Economy: assessed as the cost of the protection system in relation to the cost of malfunctioning.

The protection system is the ensemble of the instrument transformers and the relays with adequate settings. The relay is only one of the components making up the protection system. Selection of  the type of function and of the functions required to adequately protect a machine or a plant must be made on the basis of:

–   Standards;

–   interface with the external network;

–   acceptable risk (consequences of the fault);

–   short-circuit currents (maximum and minimum);

–   status of the neutral;

–   presence of self-production in a plant;

–   coordination with the existing system;

–   configurations and network running criteria;

–   practices.

The aim is to achieve the best technical economic compromise which allows adequate protection against “faults” with “significant” probability and to verify that the investment is commensurate with the importance of the plant.

The electric protections are of different types and have different applications:

–   zone protections (e.g. differential or with impedance);

–   machine protections (e.g. reverse power);

–   selective protections (e.g. over current);

–   non-selective protections (e.g. Under voltage, frequency);

–   protections in support (e.g. fuses, overcurrent, under voltage);

–   interface protections (e.g. under voltage protections; under/over and rate of change of frequency; overcurrent for a disconnection between the plant network and the utility network);

–   protections for making automatism

(e.g. synchronism check).

The criterion which is followed when the setting of a protection is calculated is to efficaciously protect the machine or plant and then look for trip selectivity. Trip selectivity means isolating the smallest area of a plant in the case of a fault in the shortest time possible (selectivity) and then to ensure a reserve (back-up) in the case of failure of the primary protection. There are various different selectivity criteria which can be used in plants.

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Electrical characteristics

(Peak) making current:

The peak value of the first major loop of the current in one pole of a
switching device during the transient period following the initiation of current during a
making operation.


Peak current:

The peak value of the first major loop of current during the transient period
following initiation.


Breaking current:

Current in one pole of a switching device at the instant of initiation of
an arc during a breaking process.


Breaking capacity:

Value of the prospective breaking current that a circuit-breaker or load switch can break at a given voltage under prescribed conditions for application
and performance; e.g. overhead line (charging current) breaking capacity.


Short-line fault:

Short circuit on an overhead line at a short but not negligible distance
from the terminals of the circuit-breaker.


Out of phase (making or breaking) capacity:

Making or breaking capacity for which the specified conditions for use and behavior include the loss or the lack of synchronism between the parts of an electrical system on either side of the circuit breaker.


Applied voltage:

The voltage between the terminals of a circuit-breaker pole immediately
before making the current.


Recovery voltage:

Voltage occurring between the terminals of a circuit-breaker pole
after interruption of the current.


Opening time:

The interval of time between application of the auxiliary power to the opening release of a switching device and the separation of the contacts in all three
poles.

Closing time:

The interval of time between application of the auxiliary power to the closing circuit of a switching device and the contact touch in all poles.

Break time:

interval of time between the beginning of the opening time of a switching
device and the end of the arcing time.

Make time:

the interval of time between application of the auxiliary power to the closing circuit of a switching device and the instant in which the current begins to flow in the
main circuit.

Rated value:

value of a characteristic quantity used to define the operating conditions
for which a switching device is designed and built and which must be verified by the
manufacturer.

Rated normal current:

the current that the main circuit of a switching device can
continuously carry under specified conditions for use and behaviour. See below for
standardized values.

Rated short-time withstand current:

current that a switching device in a closed position can carry during a specified short time under prescribed conditions for use and behaviour. See below for standardized values.

Rated voltage:

The upper limit of the highest voltage of the network for which a switching device is rated. See below for standardized values.

Three Phase Transformers Basics

A three-phase transformer is used to generate and transmit a large amount of power. Three-phase step up and step down transformers are basically used in various fields in the power system network. A transformer can be formed by joining three separate single-phase winding in a suitable way. Such an arrangement is called 3-phase bank of the transformer. Otherwise, a single 3 phase transformer is one where the cores and winding of all three phases are combined in a single structure.

Advantage of a three-phase unit transformer

  • A three-phase unit transformer has the following advantages over the 3 single-phase transformer of same KVA rating such as,
  • It takes less space
  • It is lighter, smaller and cheaper compared to the other one.
  • Only three terminals are needed rather than six terminals in a single 3 phase transformer.
  • The installation cost for the three-phase transformer is much lower than the other one.

Advantage of a transformer bank or three single-phase transformer

  • A transformer bank or three single-phase transformer has the following advantages over a 3 phase transformer of same KVA rating such as,
  • A transformer bank may be provided with higher KVA rating to supply an imbalanced load.
  • When a transformer is damaged in a transformer bank then remaining two transformers may be used as open Δ (delta) connection.
  • Transformer tank is less costly compared to a single-phase transformer.
  • The transportation of a single-phase transformer is more convenient

But generally, we use a three-phase transformer rather than a transformer tank.

 

Construction of three-phase transformer

Construction of the magnetic core of a three-phase transformer may be understood by considering three single-phase transformer positioned at 1200  to each other. If three-phase sinusoidal voltage is applied to the winding then the flux produced φR, φS, φT will also be sinusoidal and balanced. If the three legs of the transformer carrying these fluxes are merged, the total flux in the merged leg will be zero.so these legs can be removed as it carries no flux. Usually, the structure is not convenient to build so the structure with the tree limbs in the same plane is used. Here the constructions of core transformer are shown in the figure.

 

Here the constructions of core transformer are shown in the figure.

 

 

 Conditions of Three-Phase transformer in Parallel

  • There are some conditions which must be fulfilled before connecting two 3-phase transformers in parallel. These are
  • The polarities of the transformers must be the same.
  • The primary and secondary voltage rating should be identical.
  • Impedance is inversely proportional to the KVA ratings.
  • Identical X/R ratio in the transformer impedance.
  • The phase sequence of transformers must be the same.
  • The phase shift between primary and secondary voltage must be the same for all the transformers.