TERTIARY-WINDING OVERCURRENT PROTECTION BASIC INFORMATION

The tertiary winding of an autotransformer, or three-winding transformer, is usually of much smaller kVA rating than the main windings. Therefore, fuses or overcurrent relays set to protect the main windings offer almost no protection to tertiaries.

During external system ground faults, tertiary windings may carry very heavy currents. Hence, to guard against failure of the primary protection for external ground faults, separate tertiary overcurrent protection may be desirable.

The method selected for protecting the tertiary generally depends on whether or not the tertiary is used to carry load. If the tertiary does not carry load, protection can be provided by a single overcurrent relay connected to a CT in series with one winding of the Δ.

This relay will sense system grounds as well as phase faults in the tertiary or in its leads. When tertiary windings are connected by cables, the overcurrent protection provided to the tertiary winding should account for the thermal withstand of the cables.

Alarming and tripping as a result of a prolonged unbalance condition or load tap changer malfunction should prevent damage to cables. If the tertiary is used to carry load, partial protection can be provided by a single overcurrent relay supplied by three CTs, one in each winding of the Δ and connected in parallel to the relay.

This connection provides only zero sequence overload protection and does not protect for positive and negative sequence overload current. In this case, the relay will operate for system ground but will not operate for phase faults in the tertiary or its leads.

Where deemed necessary, separate relaying such as differential type should be provided for protection for phase faults in the tertiary or its leads. The setting of the tertiary overcurrent relay can normally be based on considerations similar to those in line time overcurrent.

However, if the tertiary does not carry load, or if load is to be carried and the three CT, zero sequence connection is used, the associated overcurrent relay can be set below the rating of the tertiary winding. This relay should still be set to coordinate with other system relays.

TYPES OF POWER TRANSFORMER FAULTS BASIC INFORMATION

The electrical windings and the magnetic core in a transformer are subject to a number of different forces during operation, for example

a)Expansion and contraction due to thermal cycling

b)Vibration

c)Local heating due to magnetic flux

d)Impact forces due to through fault current

e)Excessive heating to to overloading or inadequate cooling

These forces can cause deterioration and failure of the winding electrical insulation. Table below summarizes failure statistics for a broad range of transformer failure causes reported by a group of U.S. utilities over a period of years.

This guide deals primarily with the application of electrical relays to detect the fault current that results from an insulation failure. The current a relay can expect to see as a result of various types of winding insulation failures.

The detection systems that monitor other transformer parameters can be used to indicate an incipient electrical fault. Prompt response to these indicators may help avoid a serious fault. For example

a)Temperature monitors for winding or oil temperature are typically used to initiate an alarm requiring investigation by maintenance staff.

b)Gas detection relays can detect the evolution of gases within the transformer oil. Analysis of the gas composition indicates the mechanism that caused the formation of the gas; e.g., acetylene can be caused by electrical arcing, other gases are caused by corona and thermal degradation of the cellulose insulation.

The gas detection relays may be used to trip or alarm depending on utility practice. Generally, gas analysis is performed on samples of the oil, which are collected periodically. Alternatively, a continuous gas analyzer is available to allow on-line detection of insulation system degradation.

c)Sudden-pressure relays respond to the pressure waves in the transformer oil caused by the gas evolution associated with arcing.

d)Oil level detectors sense the oil level in the tank and are used to alarm for minor reductions in oil level and trip for severe reductions.

POWER TRANSFORMER RELAYING PHILOSOPHY AND ECONOMIC CONSIDERATIONS

Protective relaying is applied to components of a power system for the following reasons:

a)Separate the faulted equipment from the remainder of the system so that the system can continue to function

b)Limit damage to the faulted equipment

c)Minimize the possibility of fire

d)Minimize hazards to personnel

e)Minimize the risk of damage to adjacent high voltage apparatus

In protecting some components, particularly high-voltage transmission lines, the limiting of damage becomes a by-product of the system protection function of the relay. However, since the cost of repairing faulty transformers may be great and since high-speed, highly sensitive protective devices can reduce damage and therefore repair cost, relays should be considered for protecting transformers also, particularly in the larger sizes.

Faults internal to a transformer quite often involve a magnitude of fault current that is low relative to the transformer base rating. This indicates a need for high sensitivity and high speed to ensure good protection. There is no one standard way to protect all transformers, or even identical transformers that are applied differently.

Most installations require individual engineering analysis to determine the best and most cost-effective scheme. Usually more than one scheme is technically feasible, and the alternatives offer varying degrees of sensitivity, speed, and selectivity.

The plan selected should balance the best combination of these factors against the overall economics of the situation while holding to a minimum

a)Cost of repairing damage

b)Cost of lost production

c)Adverse effects on the balance of the system

d)The spread of damage to adjacent equipment

e)The period of vulnerability of the damaged equipment

In protecting transformers, backup protection needs to be considered. The failure of a relay or breaker during a transformer fault may cause such extensive damage to the transformer that its repair would not be practical.

When the fault is not cleared by the transformer protection, remote line relays or other protective relays may operate. Part of the evaluation of the type of protection applied to a transformer should include how the system integrity may be affected by such a failure.

In this determination, since rare but costly failures are involved, a diversity of opinion on the degree of protection required by transformers might be expected among those familiar with power system relay engineering.

The major economic consideration is not ordinarily the fault detection equipment but the isolation devices. Circuit breakers often cannot be justified on the basis of transformer protection alone.

At least as much weight should be given to the service requirements, the operating philosophy, and system design philosophy as to the protection of the transformer. Evaluations of the risks involved and the cost-effectiveness of the protection are necessary to avoid going to extremes. Such considerations involve the art rather than the science of protective relaying. 

OVERCURRENT PROTECTION OF POWER TRANSFORMER BASIC INFORMATION

Effects of Overcurrent.
A transformer may be subjected to overcurrents ranging from just in excess of nameplate rating to as much as 10 or 20 times rating. Currents up to about twice rating normally result from overload conditions on the system, while higher currents are a consequence of system faults.

When such overcurrents are of extended duration, they may produce either mechanical or thermal damage in a transformer, or possibly both. At current levels near the maximum design capability (worst-case through fault), mechanical effects from electromagnetically generated forces are of primary concern.

The pulsating forces tend to loosen the coils, conductors may be deformed or displaced, and insulation may be damaged. Lower levels of current principally produce thermal heating, with consequences as described later on loading practices. For all current levels, the extent of the damage is increased with time duration.

Protective Devices. 
Whatever the cause, magnitude, or duration of the overcurrent, it is desirable that some component of the system recognize the abnormal condition and initiate action to protect the transformer. Fuses and protective relays are two forms of protective devices in common use.

A fuse consists of a fusible conducting link which will be destroyed after it is subjected to an overcurrent for some period of time, thus opening the circuit. Typically, fuses are employed to protect distribution transformers and small power transformers up to 5000 to 10,000 kVA.

Traditional relays are electromagnetic devices which operate on a reduced current derived from a current transformer in the main transformer line to close or open control contacts, which can initiate the operation of a circuit breaker in the transformer line circuit. Relays are used to protect all medium and large power transformers.

Coordination.
All protective devices, such as fuses and relays, have a defined operating characteristic in the current-time domain. This characteristic should be properly coordinated with the current-carrying capability of the transformer to avoid damage from prolonged overloads or through faults.

Transformer capability is defined in general terms in a guide document, ANSI/IEEE C57.109, Transformer Through Fault Current Duration Guide. The format of the transformer capability curves is shown in Fig. 10-35.

The solid curve, A, defines the thermal capability for all ratings, while the dashed curves, B (appropriate to the specific transformer impedance), define mechanical capability. For proper coordination on any power transformer, the protective-device characteristic should fall below both the mechanical and thermal portions of the transformer capability curve.

(See ANSI/ IEEE C57.10-38 for details of application.)

DIFFERENT TYPES OF TRANSFORMER PROTECTION BASIC INFORMATION

The protection of the transformer is as important a part of the application as the rating values on the transformer. Entire texts are devoted to the subject of transformer protection.

When investigating a failure, one should collect all the protection-scheme application and confirm that the operation of any tripping function was correct.

Surge Arresters

Surge arrester protective level must be coordinated with the BIL of the transformer. Their purpose, to state what may seem obvious, is to protect the transformer from impulse voltages and high-frequency transients.

Surge arresters do not eliminate voltage transients. They clip the voltages to a level that the transformer insulation system is designed to tolerate. However, repeated impulse voltages can have a harmful effect on the transformer insulation.

Overcurrent Protection

Overcurrent devices must adequately protect the transformer from short circuits. Properly applied, the time–current characteristic of the device should coordinate with that of the transformer.

These characteristics are described in IEEE C57.109-1993, Guide for Liquid-Immersed Transformer Through-Fault Duration. Overcurrent devices may be as simple as power fuses or more complex overcurrent relays.

Modern overcurrent relays contain recording capability that may contain valuable information on the fault being investigated.

Differential Protection

Differential relays, if applied, should be coordinated with the short-circuit current available, the transformer turns ratio and connection, and the current transformers employed in the differential scheme.

If differential relays have operated correctly, a fault occurred within the protected zone. One must determine if the protected zone includes only the transformer, or if other devices, such as buswork or circuit breakers, might have faulted.

TRANSFORMER FUSING FACTOR BASIC AND TUTORIALS

TRANSFORMER FUSING FACTOR BASIC INFORMATION
What Is Transformer Fusing Factor?


The "fusing factor" is used to determine the K, or T fuse link rating that will strike a suitable balance between operation on secondary fault currents and operation on expected overload currents, such as motor starting currents.

It is obtained by using a rule of thumb such as one of the following: (The current obtained by the selected rule of thumb becomes the "fusing factor.")

1. 1.5 times the rated full-load current of the transformer (Generally used on transformers 25 kva and larger where motor starting currents are not the controlling factor)

2. 2.0 times the rated full-load transformer current

3. 2.4 times the rated full-load transformer current (This rule is frequently expressed as, “1 ampere per kva rating of transformers at 2400 volts, ½ ampere per kva at 4800 volts, and 1/3 ampere per kva at 6900 to 7600 volts.”)

4. 3.0 (or above) times the rated full-load transformer current.

Example:
If the selected rule of thumb is 2.4 times rated full-load current, the system voltage is 4800 volts and the transformer is rated 50 kva, what fuse link should be used?

Rated full-load current = 50,000 / 4800 = 10.4 amperes (see “Load Current Tables” on pages 98 and 99).
2.4X10.4 = 24.9 amperes. Use a fusing factor of 25

Suggested fuse link from table: 15K or 15T.

INRUSH CURRENT CONSIDERATION FOR TRANSFORMERS BASIC INFORMATION

POWER TRANSFORMER INRUSH CURRENT CONSIDERATION

What Are The Inrush Current Consideration For Power Transformers?

Two distinctly different definitions for inrush current have been offered because one definition cannot serve all the purposes where inrush current is of interest.

Inrush Current:

is the maximum root-mean-square or average current value, determined for a specified interval, resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum primary turns tap available and its rated voltage.

Peak Inrush Current:

is the peak instantaneous current value resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum turns primary tap and rated voltage.

Magnetic and thermal cut-out devices usually are not responsive to one-half cycle of energy regardless of magnitude, hence two or more half cycles are needed to define the trip-out characteristics. Furthermore, these devices are not responsive to peak values, but rather to energy content. (I2 t) becomes the parameter of interest, using root-meansquare current values for fusing characteristics.

Relays and magnetic cut-outs are responsive to the average current value. Therefore, when inrush current is cited it should be made clear which of the two values (root mean square) (average) is indicated.

It should be noted that the inrush current of a transformer is seldom the same value as the steady-state exciting current, but is typically larger and decays to steady state after several cycles, depending on the condition of the core, the instantaneous value of applied voltage, etc.

It is important to consider this asymmetry of inrush current in the design and use of transformers and particularly in the specification of protective devices for the transformer. Maximum inrush current values occur when a transformer core that has an existing maximum residual flux is switched on at zero instantaneous voltage so the residual flux and the instantaneous magnetizing flux are additive.

Circuits are available using silicon controlled rectifier switching to cause this to happen deliberately. Alternately, random switch on twenty or more times will usually produce a near maximum value for a single-phase transformer.

It may take more times for a three-phase transformer unless all three lines are monitored. For the measurement of root-mean-square or average current it is necessary to use an adequate X-axis spread or chart speed so that curve area per cycle can be measured.

Peak inrush current values are of interest in connection with contact welding problems and with devices sensitive to instantaneous current magnitude. The measurement of true inrush current with any degree of accuracy can be very difficult because of the usual nonavailability of zero source impedance power lines for larger systems.

This problem can best be circumvented when the installed source capacity is known and specified in terms of impedance and phase angle, and rated capacity.

These values can then be used in test or computation to determine the installed inrush characteristics of a system which, of course, is the final value of interest. When inrush current values are presented for conditions other than essentially zero source impedance, the actual source impedance values applicable to the data should also be given.

EFFECTS OF SHORT CIRCUITS ON TRANSFORMERS BASICS AND TUTORIALS

EFFECTS OF SHORT CIRCUITS ON TRANSFORMERS BASIC
What Are The Effects Of Short Circuits On Transformers?


Transformers are susceptible to damage by secondary short-circuit currents having magnitudes that can be many times rated load current. The damage results from the following effects:

• The I 2R losses in the winding conductors are increased by the square of the current. This increases the temperature rise of the windings.

Because protective devices limit the duration of short circuits (as opposed to overloads), the temperature rise of the winding can be calculated by dividing the total energy released by the I 2R losses by the thermal capacity of the conductor.

• The short-circuit currents exclude flux in the core and increase stray flux around the core. This stray flux induces currents in metallic parts other than the winding conductors, which can be damaged thermally.

• A short circuit applied to the secondary circuit of an autotransformer can substantially increase the voltage across the series winding and across the common winding through induction.

This not only presents the possibility of damaging the winding insulation by overvoltage, but will also drive the core into saturation and significantly increase core losses with potential damaging effects from temperature.


• Bushings and tap changers have current ratings that are usually only marginally greater than the rated load of the transformer.

Since fault currents are many times rated currents and these components have short thermal time constants, they can be seriously overloaded and thermally damaged.

• Stray flux in the vicinity of current-carrying conductors produces mechanical forces on the conductors. When a short circuit is applied to a transformer, there is a significant increase in stray flux, resulting in greater mechanical forces on the windings, leads, bushings, and all other current-carrying components.

These components, especially the windings, must be braced to withstand these forces.

A good transformer design must take all of the above effects into account to minimize the risk of damage and assure a long service life.

BUCHHOLZ RELAY OR POWER TRANSFORMER BASICS AND TUTORIALS

BUCHHOLZ RELAY FOR POWER TRANSFORMER BASIC INFORMATION

What Are Buchholz Relay? How Buccholz Relay Works?


The Buchholz Relay (Gas Relay) is designed to protect equipments submerged in insulating liquid, by means of supervision of the oil abse nce or abnormal flow, and abnormal gassing caused by the equipment. Buchholz relay is usually fitted on transformers provided with an expansion tank for the insulating liquid.

Buchholz Relay

Buchholz relay is capable to accurately detect, for example, the following problems: Leakage of insulating liquid, short - circuit inside the equipment causing a great displacement of insulating liquid, inside gassing due to intermittent or continuous failures occurring inside the equipment.

Buchholz relay is usually installed between the main tank and the oil expansion tank of the transformer.

Buchholz relay housing is made of cast iron, having two flanged openings and two sight glasses showing a graduated scale of gas volume. There are two inside floats, being that the upper float is fo rced to move downwards (this also happens in case of oil leakage).

On the other hand, in case an excessive gassing causes an oil ci rculation through the relay, the lower float reacts, even before the gas reaches the relay. In both cases, the floats make contacts when they are displaced.

The Buchholz Relay has a device for the inside float testing and locking. To check for proper operation of the relay contacts, when it is installed in the transformer, proceed as follows:

Alarm:
· Connect an Ohmmeter to terminal s + C - D. It should indicate an open circuit.

· Remove the testing device plug and introduce it upside down into the device, lowering it as much as possible in all of its length. The Ohmmeter should indicate a closed circuit.

Shutdown
· Connect an Ohmmeter to terminals + A - B. It should indicate an open circuit.

· Remove the testing device plug and introduce it upside down into the device, lowering it as much as possible in all of its length. The Ohmmeter should indicate a closed circuit.

Before supplying po wer to the transformer, the following items should be checked:

· Remove the lid of the relay -testing device.

· Remove the float -locking pin from the inside of the testing device. Both floats should be free to move.

· Replace the cover of the relay -testing device.

· Purge the air from the relay by means of the 1/8” air valve located on the relay lid.

· Check the relay for possible leakage that might have occurred during the installation on the transformer and fix it.

· Check the relay for proper fitting wi th regards to the oil float direction, which arrow should be pointing towards the transformer’s oil expansion tank.

If the alarm sounds without turning off the transformer, it is necessary to turn it off immediately and then test the gas removed from the inside of the relay. In this case, the origin of the failure can be assessed according to the gas testing result, i.e.:

· Combustible gas (contents of acetylene): In this case there must be a failure to be repaired on the electrical part;

· Non-combustible gas (without acetylene) : in this case, it means there is pure air. The transformer can be turned on again without danger after the air is bled out from the relay. When the alarm sounds repeatedly, it indicates that air is penetrating into the transformer. Tur n it off and repair the failure.

· No gassing (the gas level inside the relay is getting lower and an amount of air is being drawn through the open valve), in this case, the oil level is too low, possibly due to a leakage. Top up with oil until the control level and carry out the air tightness essay.

The transformer is turned off without a previous alarm. In this case, the transformer must have been thermally overloaded. Turn it on again after a considerable time interval for cooling. The failure can be found atcthe short-circuit contact in the protection relay system.

The alarm sounds and the transformer is shutdown immediately before or after the alarm sounds. In this case, one of the above mentioned failures must be the cause. Make the gas testing and proceed as described above.

ATTENTION!
Float locking device for transport purpose and testing of contacts : After installing the relay, remove the insert used to lock the floats.

Operation: To test the contacts, press the internal part with the lid pin. The contacts should actuate automatically. If everything is properly working, close the device again in order to prevent any leakage. Now the relay is ready to be put in operation.


NOTE: The insert is used for transport purposes only.