differntial protection (1)

Power System Protection

Rizwan KhanUniversity of Engineering and Technology, Lahore

1

Transformer Protection

2Rizwan Khan (Lecturer - UET LHE)

Introduction

Overcurrent Protection

Percentage Differential Protection

Causes of False Differential Currents

Supervised Differential Relays

Three Phase Transformer Protection

Contents of Transformer Protection


Volts per Hertz Protection

Non Electrical Protection

Protection Systems for Transformers

Summary

4

Contents of Transformer Protection

Rizwan Khan (Lecturer - UET LHE)

Transformer Faults Statistics


The inherent characteristics of power transformers introduce a

number of unique problems that are not present in the protection

of transmission lines, generators, motors or other power system

apparatus.

In general, a transformer may be protected by fuses, overcurrent

relays, differential relays and pressure relays, and can be

monitored for incipient trouble with the help of winding

temperature measurements, and chemical analysis of the gas

above the insulating oil.

Introduction


Transformer Size

Location and Function

Voltage

Connection and Design

Factors of Transformer Protection


Transformer Size

9

Transformer Size Protection

2500kVAFuses

Thermal Overload Relays

2500-5000kVAFuses

Overcurrent Relays

5000-10000kVA

Fuses

Overcurrent Relays

Ordinary Differential Relays

Above 10MVA

Fuses

Overcurrent Relays

Percentage Differential Relays

Temperature & Pressure Relays

If the transformer is an integral part of the bulk power system,

it will probably require more sophisticated relays in terms of

design and redundancy.

If it is a distribution station step down transformer, a single

differential relay and overcurrent backup will usually sufficient.

If the transformer is near a generation source, the high X/R ratio

of the fault path will require harmonic restraint relays to

accommodate the higher magnetic inrush currents.

Location and Function


Generally, higher voltages demand more sophisticated and costly

protective devices, due to the effect of a delayed fault clearing on

the system performance, and the high cost of transformer repair.

The protection schemes will vary considerably between auto

transformers, and two or three winding transformers. The

winding connection of a three phase transformer, whether delta or

wye, will make a difference in the protection scheme chosen.

Also important are the presence of tertiary windings, type of

grounding used, tap changers or phase-shifting windings.

Voltage, Connection and Design

11

Fuses

Time Delay Overcurrent Relays

Instantaneous Relays

Overcurrent Protection


Clearly, the fuse interrupting capability must exceed the maximum

short-circuit current that the fuse will be called upon to interrupt.

The continuous rating of the fuse must exceed the maximum

transformer load.

Typically, the fuse rating should be greater than 150% of the

maximum load.

It is also clear that the transformer magnetizing current should not

cause damage to the fuse.

Fuse


The minimum melt characteristic of the fuse must coordinate with

(i.e. should be well separated from) the protective devices on the

low side of the power transformer. In considering the

coordination, the ambient temperature, prior loading and reclosing

adjustment factors should be taken into account.

The speed ratio of the fuse is dened as the ratio between the

minimum melt current values at two widely separated times: for

example, 0.1 and 100 s. It is desirable to have a fuse with as high a

speed ratio as possible.

Fuse


Fuse

16

Protection against excessive overload, or persisting external fault,

is provided by time-delay overcurrent relays.

The pickup setting is usually 115% of the maximum overload

acceptable.

The time-delay overcurrent relays must coordinate with the low-

side protective devices. These may include low-voltage bus relays

for phase-to-phase faults, phase-directional relays on parallel

transformers and the breaker failure relay timers on the low-

voltage breakers.

Time Delayed/Instantaneous O/C Relays


Differential Protection


For a normal power transformer

If we use current transformers having turns ratios of 1 : n1 and 1 :

n2 on the primary and the secondary side respectively, under

normal conditions the currents in the secondary windings of the

current transformers are related by

20



If we select the CTs appropriately, we may make N1n1 = N2n2, and

then, for a normal transformer, i1s = i2s. However, if an internal

fault develops, this condition is no longer satisfied, and the

difference of ils and i2s becomes much larger; in fact, it is

proportional to the fault current. The differential current provides a

highly sensitive measure of the fault current.

21



Mismatching in CT Ratings

Mismatching in Errors of Transformations

Tap Changer of Transformer

22

Practical Issues in Differential Protection


First, it may not be possible to obtain the CT ratios on the primary

and the secondary side which will satisfy the condition N1n1 =

N2n2, as we must select CTs with standard ratios.

A somewhat less desirable procedure is to use auxiliary CTs to

achieve the same goal.

In any case, even with these adjustments, there remains some

residual ratio mismatch, which leads to a small differential current

id during normal conditions.

23



Second, the errors of transformation of the two CTs may differ

from each other, thus leading to significant differential current

when there is normal load flow, or an external fault.

Finally, if the power transformer is equipped with a tap changer, it

will introduce a main transformer ratio change when the taps are

changed.

24



In a percentage differential relay, the differential current must exceed a

fixed percentage of the through current in the transformer. The through

current is defined as the average of the primary and the secondary

currents:

The current ir is known as the restraint current a name that comes from

the electromechanical relay design, where this current produced a

restraint torque on the moving disc25

Percentage Differential Protection

26

Percentage Differential Relay Characteristics


Magnetizing Inrush Current during Energization

Harmonic Content of the Inrush Current

Magnetizing Inrush during Fault Removal

Sympathetic Inrush

Transformer Over-excitation

CT Saturation 28

Causes of False Differential Currents


Consider the energization of an unloaded power transformer. As

the switch is closed, the source voltage is applied to the

transformer, and a magnetizing current is drawn from the source.

Let the source voltages and flux linkages are,

29



30



In reality, the magnetizing inductance of the transformer is

nonlinear.

As the flux linkages go above the saturation knee point, a much

larger current is drawn from the source. The magnitude of this

current is determined by the slope of the magnetizing

characteristic in the saturated region, and by the leakage

inductance of the transformer.

It is obvious that magnetizing inrush currents of the order of fault

currents are possible.31



It should be clear that in most modern transformers very large

inrush currents are possible, depending upon the instant of

energization, and the remnant flux in the transformer core. Since

the inrush current flows only in the primary and not in the

secondary winding of the transformer, it is clear that it produces a

differential current which is 200% of the restraining current, and

would cause a false operation.

32



The false operation of a percentage differential relay for a

transformer is prevented by taking advantage of the fact that the

inrush current is rich in harmonic components.

While the fault current is a pure fundamental frequency

component (except for a possible decaying DC component).

33

Harmonics Content of the Inrush Current


When a fault external to, but near, the transformer is removed by

the appropriate circuit breaker, the conditions inside the

transformer core are quite similar to those during magnetization of

the transformer.

As the voltage applied to the transformer windings jumps from a

low pre-fault value to the normal (or larger) post-fault value, the

flux linkages in the transformer core are once again forced to

change from a low pre-fault value to a value close to normal.

34

Magnetizing Inrush Current during Fault Removal


Depending upon the instant at which the fault is removed, the

transition may force a DC offset on the flux linkages, and primary

current waveforms similar to those encountered during

energization would result.

It should be noted that as there is no remnant flux in the core

during this process; the inrush is in general smaller than that

during the transformer energization.

35

Magnetizing Inrush Current during Fault Removal


36

Sympathetic Inrush


The phenomenon which causes inrush to flow in a previously

energized transformer, when a parallel bank is energized, is known

as the sympathetic inrush.

The current waveforms of a typical sympathetic inrush

phenomenon are illustrated in next slide.

37

Sympathetic Inrush


38

Sympathetic Inrush

During load rejection and certain other operating conditions, a

transformer may be subjected to a steady-state overvoltage at its

nominal frequency.

During over-excitation, the transformer flux remains symmetric,

but goes into saturation for equal periods in the positive and the

negative half-periods of the waveform, due to the rich contents of

the harmonics.

39

Transformer Over-excitation


For certain external faults, where the fault currents are large, it is

likely that one of the CTs may saturate, this may directly effect the

operation of both ordinary and percentage differential relay.

40

CT Saturation


To desensitize the differential relay when the transformer is

energized.

Introduction of the Concept of Voltage Supervision

Harmonics Characterization as Supervised Differential Relay

42

How to Avoid False Differential Currents


One of the earliest ideas was to desensitize the differential relay when

the transformer is energized. Thus, the less sensitive relay would not see

the inrush current, thus avoiding a false trip.

However, this is a poor practice, as it is precisely during the initial

energization of the transformer, when the transformer is first energized,

or some repair work on the transformer may have been completed, that

the transformer is in need of protection. This is to ensure that the repair

work has been successfully completed, and no maintenance tools

inadvertently left inside or around the transformer.

43

Desensitize the Differential Relay


It may be expected that during the inrush conditions, the

transformer voltage would be close to normal, while during faults,

the voltage would be much less. Thus, an under-voltage relay may

be used to supervise the differential relay.

In general, this type of voltage supervision is not preferred, as the

under-voltage relay tends to be slow, and consequently the entire

protection becomes slower.

44

Voltage Supervision


The method currently in use on large transformers is based upon using

the harmonic characterization of the inrush and over-excitation currents.

The differential current is almost purely sinusoidal when the transformer

has an internal fault, whereas it is full of harmonics when the

magnetizing inrush current is present, or when the transformer is

overexcited.

Thus, the differential current is filtered with filters tuned to an

appropriate set of harmonics, and the output of the filters is used to

restrain the differential relay.45

Harmonics Characterization

46

Supervised Differential Relays

The major difference between three-phase transformer protection and

that of three single-phase transformers is the necessity to deal with the

effect of a wyedelta transformation.

Under normal load conditions, the currents in the primary and secondary

windings are in phase, but the line currents on the wye and delta sides of

the three-phase transformer are out of phase by 30.

The difficulty is resolved by connecting the current transformers in such

a manner that they undo the effect of the wyedelta phase shift produced

by the main transformer.48



The current transformers on the wye side of the power transformer

are connected in delta, and the current transformers on the delta

side of the power transformer are connected in wye.

In addition to the phasing consideration discussed above, it is also

necessary to adjust the turns ratios of the CTs so that the delta

connection on the wye side of the power transformer produces

relay currents that are numerically matched with the relay currents

produced by the wye-connected CTs.

49



50


Consider the three-winding transformer shown in Figure 8.13. One

winding is assumed to be delta connected, while the other two are

assumed to be wye connected. The CTs must of course be

connected in wye on the delta side and in delta on the wye side of

the power transformer.

This will ensure that the phase shifts created in the currents of the

power transformer are compensated by the CTs, so that the

secondary currents are once again in phase.

51

Multi Winding Transformer Protection


52

Multi-winding Transformer Protection


It is interesting to note that under certain conditions a two-winding

differential relay can be used to protect a three-winding

transformer. If the transformer is connected to a source only on

one side, the other two winding CTs could be paralleled to

produce a net secondary current, which can then be used in a two-

winding protection scheme.

53

Multi Winding Transformer Protection


The regulating transformers may regulate the turns ratio or the

phase shift between the primary and the secondary windings.

The regulating transformers usually consist of two transformers:

one to provide the magnetizing current for the transformers and

the other to provide the variable turns ratio or the variable phase

shift.

In either case, the percentage differential relay is not suitable for

the protection of these transformers. Often, a sudden pressure

relay (SPR) provides the most sensitive protection.54

Regulating Transformers Protection

Transformer cores are normally subjected to flux levels

approaching the knee point in their magnetizing characteristic.

If the core flux should exceed the saturation level, the flux patterns

in the core and the surrounding structure would change, and

significant flux levels may be reached in the transformer tank and

other structural members. As these are not laminated, very high

eddy currents are likely to result, producing severe damage to the

transformer.

56

Voltage/Hertz Protection


As the flux is proportional to the voltage, and inversely

proportional to the operating frequency, the significant relaying

quantity is the ratio of the per unit voltage to the per unit

frequency. This is known as volts/hertz protection.

This protection is specially needed in the case of unit-connected

generator transformers. If the turbinegenerator is shut down with

the voltage regulator in service, the volts/hertz limit of the

transformers (and indeed of generators as well) could be easily

exceeded.57



Similar conditions could also be reached by load rejection with

voltage regulators disconnected, or in manual position, or with

faulty instrumentation in the regulator circuits.

The volts/hertz capability of transformers is specified by

manufacturers.

Many volts/hertz relays have two settings, a lower setting for

alarm and a higher setting which may be used for tripping.

58



59



When a fault occurs inside an oil-filled transformer tank, the fault

arc produces gases, which create pressure waves inside the oil. In

the conservator type of tank construction, which is more

common in Europe, the pressure wave created in the oil is detected

by a pressure vane in the pipe which connects the transformer tank

with the conservator.

The movement of the vane is detected by a micro-switch, which

can be used to sound an alarm, or trip the transformer. This type of

a relay is known as a Buchholz relay.61

Pressure Devices

In the USA, the more common transformer construction is of the

tank type with a gas cushion at the top of the tank. In such a

transformer, the pressure wave is detected by a SPR mounted on

the side of the transformer. The mechanical sensor consists of a

bellows and a pressure equalizer, which together is insensitive to

slow changes of pressure, for example those caused by thermal or

loading changes in the transformer.

However, a pressure wave created by a fault is detected by the

relay, and can be used to trip or alarm.62

Pressure Devices

Faults on the bushings, and connecting leads, do not create an arc

in the insulating oil or gas, and must be protected by differential

relays.

63

Pressure Devices


Some devices simply measure the oil temperature, usually the top

oil.

Other devices use a combination of current, by placing a small

search coil around a lead, and oil temperature to measure the total

effect of load and ambient temperature.

The critical temperature is referred to as the hot-spot

temperature, and is the highest temperature that will occur

somewhere in the winding.

64

Temperature Devices


The temperature devices actuate alarms to a central dispatching

office, to alert the operators, who can either remotely unload the

transformer by opening the circuit breaker, or can dispatch an

operator to the station.

The hot-spot sensors are also commonly used to start and stop

cooling fans and pumps.

65

Temperature Devices


Parallel Transformer Banks

Tapped Transformer Banks

Substation Design

Station Service

Generator Station Design

67

Protection Systems for Transformers


It is not uncommon when two transformers are connected in

parallel with no circuit breakers used to separate them that a single

differential relay is used for the protection of both banks.

However, the problem of sympathetic inrush presents difficulties

to the common differential relay, when one of the transformers is

energized with its switch while the other transformer is in service.

The sympathetic inrush is now likely to trip the two transformers.

The problem can be dealt with satisfactorily by the use of separate

differential relays.68

Parallel Transformer Banks


Connecting a transformer directly to the high-voltage transmission

line is a practical and relatively inexpensive way of providing

service to an isolated industrial or residential load.

In the next slides, complete protection schemes are introduced for

the protection of lines and for the protection of tapped transformer,

in different scenarios.

69



70


Full complement of circuit breakers for transformer protection

71


Direct connection of transformer and transmission lines - shows a variation in which

the line circuit breakers are not used. In this configuration a line fault will remove the

transformer by opening the remote terminals.

72


Use of motor-operated air break switches at a transformer tap

Since a power transformer interconnects two or more voltage

levels, its location requires special consideration in the design of a

substation and the protection of all of the elements within it.

Usually the various voltage levels are contained within separate

areas, each with its own bus configuration and associated

equipment and separated by considerable distances.

Proceeding Figures show 345 kV/138 kV station with different

protection schemes.

73

Substation Design


74

Substation Design

Substation transformer with a full complement of circuit breakers

75

Substation Design

MOAB connection of a transformer to a 345 kV bus

Most power transformers are built with a delta tertiary winding to

provide a path for third harmonics and to help stabilize the neutral.

Although the capacity of the tertiary need not be very large, it is a

good source for the station auxiliary equipment such as circuit

breaker air compressors, oil pumps, battery chargers, etc.

If the load taken off the tertiary is large enough to upset the

transformer differential, it must be connected into the differential

CTs, as shown in Figure on next slide.

76

Station Service


77

Station Service

Protection of generating station service transformer

The most common configuration used presently for large

generating plants is a unit-connected system. In this system, the

generator, generator step up transformer (GSU), the unit auxiliary

transformer (UAT) and the reserve or start up auxiliary

transformer (RAT) are all connected as a unit and their protection

and controls are interrelated.

Figure on next slide, shows a typical design with the associated

zones of protection.

78

Generator Station Design


79Generating station transformers and their protection systems

References

Stanley H. Horowiz, Arun G. Phadke, Power System

Relaying, 3rd Edition, Chapter No. 8

THANKS FOR YOUR TIME81

differntial protection (1)

Documents

sophisticated relays

hertz protection

protection schemes

fuse rating

false differential currents

harmonic restraint relays

bulk power system

overcurrent backup