Electrical-Engineering-portal.com-Special Considerations When Selecting Transformer Protection

7/27/2019 Electrical-Engineering-portal.com-Special Considerations When Selecting Transformer Protection

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Oil po wer transforme rs 11/0.42kV 2000kVA

Edvard

Special considerations when selecting transformer protection

Current TransformersCurrent transf ormerratio selection and performance require special attention when applying transf ormer

protection. Unique factors associated with transf ormers, including its winding ratios , magnetizing inrush

current, and the presence of winding taps or load tap changers, are sources of dif f iculties in engineering a

dependable and secure protection scheme for the transf ormer.

Errors resulting f rom CT saturation and load-tap-changers are particularly critical f or dif f erential protection

schemes where the currents f rom more than one set o f CTs are compared. To compensate f or the

saturation=mismatch errors, overcurrent relays must be set to operate above these errors.

CT Current Mismatch

Under normal, non- f ault conditions, a t ransformerdif f erential relay should ideally have identical currents in th

secondaries o f all current transf ormers connected to the relay so that no current would f low in its o perating

coil. It is dif f icult, however, to match current t ransf ormer ratios exactly to t he transf ormer winding ratios. This

task becomes imposs ible with the presence of transf ormer of f -load and on-load taps or load tap changers

that change the voltage ratios of the transformer windings depending on system voltage and transformer

loading.
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The highest secondary current mismatch between all current t ransf ormers connected in the dif f erential schem

must be calculated when selecting the relay operating set ting. If time delayed overcurrent protection is used,

the t ime delay sett ing must also be based on the same consideration. The mismatch calculation should be

performed for maximum load and through-fault conditions.

CT Saturation

CT saturation could have a negative impact on t he ability o f the t ransformer protection to operate f or interna

f aults (dependability) and not to operate f or external faults (security).

For internal faults, dependability of the harmonic restraint type relays could be negatively af f ected if current

harmonics generated in the CT secondary circuit due to CT saturation are high enough to res train the relay.

With a saturated CT, 2nd and 3rd harmonics predominate initially, but the even harmonics gradually disappear

with the decay of the DC component o f the f ault current. The relay may then operate eventually when the

restraining harmonic component is reduced. These relays usually include an instantaneous overcurrent eleme

that is not restrained by harmonics, but is set very high (typically 20 times t ransf ormer rating). This element

may operate on severe internal faults.

For external faults, security of the differentially connected transformer protection may be jeopardized if the

current t ransf ormers unequal saturation is severe enough to produce error current above the relay sett ing.Relays equipped with restraint windings in each current transformer circuit would be more secure.

The security problem is particularly critical when the current t ransf ormers are connected to bus breakers rathe

than the transf ormer itself . External faults in this case could be of very high magnitude as they are not limited

by the t ransf ormer impedance.

Magnetizing Inrush (Initial, Recovery, Sympathetic)

Initial

When a transf ormer is energized af ter being de-energized, a transient magnetizing or exciting current that ma

reach instantaneous peaks of up to 30 t imes f ull load current may f low.

This can cause operation of overcurrent or diff erential relays protecting the transf ormer. The magnetizing

current f lows in only one winding, thus it will appear to a dif f erentially connected relay as an internal f ault.

Techniques used to prevent dif f erential relays f rom operating on inrush include detection of current harmonic

and zero current periods, both being characteristics of the magnetizing inrush current. The former takes

advantage of the presence of harmonics, especially the second harmonic, in the magnetizing inrush current to

restrain the relay f rom operation. The latter dif f erentiates between the f ault and inrush currents by measuringthe zero current periods, which will be much longer f or the inrush than f or t he fault current.

Recovery Inrush

A magnet izing inrush current can also f low if a voltage dip is f ollowed by recovery to normal voltage.

Typically, this occurs upon removal of an external f ault. The magnetizing inrush is usually less severe in this

case than in initial energizat ion as the transf ormer was not totally de-energized prior to voltage recovery.

Sympathet ic Inrush
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A magnet izing inrush current can f low in an energized t ransf ormer when a nearby transf ormer is energized.

The of f set inrush current o f the bank being energized will f ind a parallel path in the energized bank. Again, the

magnitude is usually less than t he case o f initial inrush. Bot h the recovery and sympathetic inrush phenomena

suggest that restraining the transf ormer protection on magnetizing inrush current is required at all times, not

only when switching the transf ormer in service af ter a period o f de-energization.

Primary-Secondary Phase-Shift

For t ransf ormers with standard delta-wye connections, the currents on t he delta and wye sides will have

a308phase shif t relative to each other. Current transf ormers used for t raditional dif f erential relays must

be connected in wye-delta (opposite o f the transf ormer winding connections) to compensate f or t he

transformer phase shift.

Phase correction is of ten internally provided in microprocessor transf ormer protection relays via

so f tware virtual interposing CTs f or each transf ormer winding and, as with the ratio correction, will

depend upon the selected conf iguration f or the restrained inputs. This allows t he primary current

transf ormers to all be connected in wye.

Turn-to-Turn Faults

Fault currents resulting f rom a turn- to -t urn f ault have low magnitudes and are hard to detect. Typically, the

f ault will have to evolve and af f ect a good port ion of the winding or arc over to other parts of the transf orme

before being detected by overcurrent or differential protection relays.

For early detection, reliance is usually made on devices that can measure the resulting accumulation o f gas o r

changes in pressure inside the transf ormer tank.

Through Faults

Through f aults could have an impact on bo th the transf ormer and its pro tection scheme. Depending on their

severity, f requency, and duration, t hrough f ault currents can cause mechanical transf ormer damage, even

though the f ault is somewhat limited by the transf ormer impedance.

For t ransf ormer dif f erential protection, current transf ormer mismatch and saturation could produce operat ing

currents on through f aults. This must be taken into consideration when selecting the scheme, current

transf ormer ratio, relay sensitivity, and operat ing time. Dif f erential protection schemes equipped with

restraining windings of f er better security for these through faults.

Backup Protection

Backup protection, typically overcurrent or impedance relays applied to one or both sides of the t ransf ormer,

perform two f unctions. One f unction is to backup the primary protection, mos t likely a dif f erential relay, and

operate in event of its f ailure to t rip.


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The second f unction is protection f or thermal or mechanical damage to the transf ormer. Protection that can

detect these external faults and operate in time to prevent t ransf ormer damage should be considered. The

protection must be set to operate befo re the through-f ault withstand capability of the transf ormer is reached

If , because of its large size or importance, only dif f erential protection is applied to a transf ormer, clearing of

external faults before t ransf ormer damage can occur by other protective devices must be ensured.

Resource: Power System Protection Arun Phadke, Virginia Polytechnic Institute

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Electrical-Engineering-portal.com-Special Considerations When Selecting Transformer Protection