Transformers

What is over-excitation protection?

Over-excitation protection detects impermissible high magnetic flux density in the iron core of power transformers which can occur in case of increase in voltage and/or decrease in frequency. Flux density above the rated value saturates the iron core which may result in power transformer overheating due to large iron losses. Over-excitation protection processes the voltage to frequency ratio (V/f) in relation to their nominal values.

How to select fuses with right size for transformer protection?

The rated current of protection fuses in primary circuit of transformers shall be selected about two times of transformer rated current considering transformer inrush current.

How can the maximum efficiency of a transformer be achieved?

The efficiency of a transformer depends on its load. The smallest load power factor leads to the lower efficiency. The point that maximum efficiency is achieved the copper dissipation and iron dissipation are equal in transformer.

Is it possible to have a negative voltage drop at transformer output?

The voltage drop at transformer output depends on the load condition. The voltage drop on transformer would be higher if the connected load is inductive. Because in this case the voltage phase angle across the load will be same as the phase angle of voltage drop caused by the transformer winding resistance and leakage reactance.

The voltage drop will be less in ohmic loads compared to inductive loads. The difference will be more for higher load currents.

The voltage on transformer output will increase (negative voltage drop) for capacitance loads. In this case the induction caused by transformer leakage reactance will be in series with capacitance of the load and they will compensate each other.

The maximum voltage drop at transformer output occurs when the ratio of ohmic part of the load to the inductive part of the load is equal to the ratio of transformer winding resistance to its leakage resistance.

Is it possible to implement a three phase Yy transformer for one phase application?

The three phase Yy transformers can be used to supply one phase load. But the output power is restricted to 10% of transformer nominated power.

Using Transformers in Parallel

To make two transformers parallel they should have same

  • Voltage
  • Vector group (phase change between primary and secondary)
  • Percentage impedance

Although the nominated power can be different, it is recommended the ratio of the nominated powers be less than three times.

The percentage impedance of smaller transformer shall not be less than that of bigger transformer, but it is acceptable for 10% higher.

If the percentage impedance of transformers be equal they will share the load based on their nominated power. But if they do not have equal percentage impedance, the load between them will be shared based on inverse of their percentage impedance.

What is the percentage impedance of a transformer?

The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage. It is also the percentage of the normal terminal voltage at on side required to circulate full-load current under short circuit conditions on other side. The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions.

It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions. By way of example, consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary side is:

2 MVA x 100/5 = 40 MVA

and from this figure the equivalent primary and secondary fault currents can be calculated. The figure calculated above is a maximum. In practice, the actual fault level will be reduced by the source impedance, the impedance of cables and overhead lines between the transformer and the fault, and the fault impedance itself.

A transformer with lower impedance will lead to a higher fault level (and vice versa) but lower voltage drop during normal operation.

What types of instruments are used for transformer protection/measurements?

The main instruments used for transformer protection are as below.

Giber Box
Giber box is used to measure the transformer winding temperature. For each current value, there is a constant temperature difference between the winding and the oil in which the winding is immersed. Therefore it is possible to measure the winding temperature by measuring the current and the oil temperature.

The current of winding is measured by 2A CT (Current Transformer). The output current will be implemented to a resistance with the value of R Ω. The temperature raised from resistance (which is equals to RI2) is physically implemented to the oil. Therefore the final measured temperature would be oil temperature plus a temperature cause by winding current. By selecting a right value for resistance (R) the correct winding temperature could be achieved.

Buchholz Relay
In the field of electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.

Depending on the model, the relay has multiple methods to detect a failing transformer. On a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float switch in the relay is used to initiate an alarm signal. Depending on design, a second float may also serve to detect slow oil leaks.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage. Buchholz relays have a test port to allow the accumulated gas to be withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as overheating or arcing, whereas air found in the relay may only indicate low oil level or a leak.

Buchholz relays have been applied to large power transformers at least since the 1940s. The relay was first developed by Max Buchholz (1875–1956) in 1921.

Jonson Relay
Jonson relay is same as Buchholz relay but in smaller size and is used to protect the on-line tap changer (OLTC) and is located in the pass of OLTC oil.

What type of transformer is recommended for earth transformer?

Usually transformers with Y type of windings are not used as earth transformer. Transformers with Z type of windings distribute the fault current between phases. Therefore they are more suitable to be used as earth transformers.

Why it is recommended to keep a transformer which is not in used powered?

Keeping a transformer, which is not in used, powered brings some hazards into account. For example someone could try to change the tap of off-load tap changer neglecting that the transformer is powered. Ignoring these hazards, it is recommended to keep the transformer powered if it will not be online or in use for a long period.

There are some tests must be performed before powering a transformer and bring it online. If you keep the transformer powered all the time, then it can be brought online immediately without carrying out the tests. Other benefit is that keeping transformer powered will keep its oil warm and will prevent oil to reach its pour point.

What are different types of tap changers?

There are three different types of tap changers.

  • On-line tap changer
  • Off-line tap changer
  • Off-circuit tap changer

Off-load tab changer must be connected to the secondary of transformer and on star winding in order to have the minimum voltage. Always a current exists in the primary side of transformer to generating the electromagnetic field in the core. According to this current it is not possible to install the off-load tap changer on the primary side of transformers. If the tap changer must be installed in primary side, then the off-circuit or on-line tap changer must be provided.

Off-circuit tab changer can be installed in either primary or secondary as the taps can be changed while there is no current or voltage on transformer.

On-line tap changer can be operated online during the transformer operation.

Why core of transformer is permitted to be earthed just in one point?

It is important to connect the core to the transformer body just in one point for earthing. This will avoid forming a circuit for eddy currents and reduce the core dissipation. The temperature inside the core of transformer can be reached to 130ºC in which the oil can be broken up. But the outside temperature of the core never reaches to this point

What is transformer neutral circulating current?

Portion of fault current that flows back to the transformer neutral point via the metallic part and/or the earthing system without ever discharging into ground is called transformer neutral circulating current (AS 2067).

What is transformer primary time constant?

Time required for the DC component present in the short circuit primary winding current, following a sudden change in operating conditions, to decrease to 1/e, that is 0.368 of its initial value, the machine running at rated speed (AS 2067) is called transformer primary time constant.

What is your recommendation for electrical protection of distribution transformers?

Distribution transformers shall be controlled and protected on the primary side by either of the following.

Circuit breakers in conjunction with phase short circuit and earth fault protective relays

Phase short circuit protection should be by means of Invers Definite Mean Time (IDMT) over current protection set to detect secondary side faults. Instantaneous over current protection shall be provided and set to detect primary side faults only. Primary side earth fault protection shall be by a residual current relay, set to achieve minimum fault clearance time.

Fused contactors (incorporating a mechanical latch mechanism) in conjunction with overcurrent and earth fault protective relays

Phase short circuit protection should be by IDMT overcurrent relay set to detect secondary side faults and by mean of the main fuse for primary side faults. Primary side earth fault protection should be by a relay connected to a core balance CT. The earth fault relay may require a short time delay to permit proper co-ordination between the fuses and the contactor.

Additionally, transformers which supply a HV system shall be provided by biased differential protection and restricted earth fault protection. The transformer primary and secondary cables shall be included within the protected zone.

Unrestricted earth fault protection should be provided on the (earthed) star connected windings of distribution transformers. This shall be achieved by means of a relay which shall be energized by a CT placed in the neutral-earth connection of the power transformer secondary winding. The primary current rating of the CT supplying this relay shall correspond to the nominal current of the transformer secondary or to the current as limited by resistance earthing.

Top-down inter-tripping shall be provided such that the tripping of the primary breaker will result in opening of the transformer secondary circuit breaker. Interlocking shall also be provided such that closing of the primary circuit breaker is not permitted if the secondary circuit breaker is not open.

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