Distribution Transformer Protection.

A distribution transformer is the type of transformer that performs the final voltage transformation in a distribution grid. They are normally rated less than 200 MVA and the output is usually 3~ 415V/1~ 240V or whatever the service voltage is in the customer's premises.

Basic Operating Principle

 A transformer operates according to Faradays Law of Induction. Faraday’s law states that a current will be induced in a conductor which is exposed to a changing magnetic field.

 An electric current produces a magnetic field around it. In an AC circuit, the magnetic field produced is constantly changing as a result of current that oscillates back and forth.

The primary and the secondary coils are wrapped together around a common “iron core”. This soft iron core is made up of individual laminations stacked together to help reduce the core’s magnetic losses.

When alternating current is introduced on the primary coil, the coil develops a changing magnetic field. That magnetic field travels through the transformer core to the secondary coil.

Air has high reluctance therefore the magnetic field generated will flow through the material with the least reluctance. The transformer core provides the low reluctance flux path to channel the magnetic flux to the secondary coil. To a large extent it also confines the magnetic field generated to the transformer core.

The changing magnetic field cuts through the windings of the secondary coil and this will induce voltage in the secondary coil. When an external circuit is connected to the secondary coil, current will flow.

 

Transformer Design

 The main parts of a distribution transformer are the HV primary side, the LV secondary side, the windings and core, Oil tank and radiator.

The winding and core assembly are immersed inside the oil tank and bushings come out of the tank for the HV and LV termination.

Typical Image of a 1.5MVA Distribution Transformer.

Transformer Protection.

Transformers like the one above (Below 5 MVA) typically have protection in the event of Earth Fault, Overcurrent, And Internal faults.

Internal faults could occur when the transformer's windings get too hot. If this happens, the insulating varnish used on the windings fails, which can lead to a short between the windings. During the regular operation of the transformer heat is produced due to coil and core losses. When the load on the transformer is increased more heat is generated and if the transformer is loaded above nameplate capacity, even more heat is generated.

To dissipate this heat, the transformer windings are immersed into mineral oil for cooling as well as insulation. Heat generated in the windings is lost to the oil and heat from the oil is lost to the environment via the cooling fins of the transformer.

We cannot consistently assume that the transformer will always be loaded as per its nameplate specifications and maintained properly. In some cases, the transformer oil may become contaminated, creating a conductive path from the transformer windings to the transformer frame (resulting in an Earth Fault). Additionally, the transformer could be overloaded for extended periods, leading to insulation failure and internal shorts. To mitigate these risks, protective devices are installed in and around the transformer to minimize the extent of damage in the event of a fault.

Most transformers I worked with have the following installed on them.

1) Oil Level Indicator (OLI)

2) Oil Temperature Indicator (OTI)

3) Winding temperature Indicator (WTI)

4) Sudden Pressure Relay (ANSI 63) or Bucholz Relay (ANSI 80)

5) Pressure-Vacuum Gauge

And the following installed around them.

1) Primary Side Circuit Breaker or fuse + Isolator

2) Secondary Side Circuit breaker or fuses.

3) Lock out relay (ANSI 86) - receives trip signal from sudden pressure relay, differential protection setup etc. and sends trip command to circuit breaker.

1) Oil Level Indicator (OLI)

As its name suggests, its function is to provide the technician a sense of the oil level in the tank. Oil leaks over time will cause a shortage of oil in a transformer. This can lead to overheating, reduced insulation capabilities and an increased risk of failure. The level gauge is usually marked 25°C around halfway between Max and Min and most people don't know what to make of it so let me explain.

As the temperature of the transformer goes up, the oil expands about 1% for every 10°C temp rise. The pointer should be on 25°C at ambient temperature i.e. when the transformer oil is cold, and the transformer is not loaded.

At the 25°C mark, the float at the back of the oil gage would be lying flat (horizontally) on top of the oil because the level indicator is often installed at the top of the tank and at the level where the oil is meant to be filled up to, and it raises and lowers with the liquid level thanks to a float at the rear of the oil level indicator.

 As the transformer is loaded the temperature rises and the oil expands this means the oil level in the tank rises, lifting the float and the pointer should move above the 25°C mark.

If the transformer is operational and the OTI shows a temperature above ambient temperature the OLI pointer should be above the 25°C mark. If it is on or below the 25°C mark, there is oil shortage.

In simple words the 25°C level is the proper oil level at ambient temperature ad as long as the transformer is operational the pointer should be above this mark.

2) Oil Temperature Indicator (OTI)

This measures the temperature of the top oil using a sensor bulb (probe). The sensor bulb is fitted at the top of the oil because due to convection, the top is the location where the oil should be at the hottest.

An extremely hot oil could indicate a problem in the windings and an alarm or trip signal could be sent to the protection system.

3) Winding Temperature Indicator (WTI)

 The overall temperature of the oil changes very slowly as it is an excellent insulator and has a large thermal mass therefore for oil temperature alone is not a sufficient indicator of what is going on in the winding.

The just like oil temp indicator, the winding temp indicator has a sensor bulb in immersed into the transformer oil near the top of the oil. The winding temperature indicator does not really detect the winding's temperature, it creates a simulation of the temperature of the winding to give a more accurate idea of what is going on in the windings.  This is achieved by heating the sensor bulb pocket(probe). This heater coil is fed from a current transformer by a current proportionate to the current flowing through the transformer winding.

4)Sudden Pressure Relay (ANSI 63)

        A sudden pressure relay is a protection device used to detect internal transformer faults by watching for rapid pressure changes inside the transformer oil tank. The Relay consists of three main parts:  pressure sensing bellows, a micro switch and a pressure equalizing orifice.

The sudden pressure relay is mounted on top the transformer. When the transformer is operating normally the pressure in the tank is remains steady, but when an internal fault, such as a shorted winding, occurs the accompanying arcing generates a significant amount of heat. This heat breaks down the oil into combustible gases that rises to the top rapidly increasing the pressure within the tank.

 Consequently, a pressure actuated switch housed inside the Sudden-Pressure relay sends a trip signal to the lock-out relay (ANSI 86) and the lock out relay in turn sends a signal to open the circuit breakers on the transformer primary and secondary.

  However, for slow changes in pressure due to loading and ambient temperature change there is a pressure equalizing orifice also housed inside the sudden pressure relay that gradually equalizes the pressures between the Relay and the transformer gas space to avoid nuisance trips.

5) Pressure-Vacuum Guage

This is installed to directly reflect the pressure changes caused by the oil temperature changes inside the transformers. The pressure-vacuum gauge indicates whether the gas blanket in the tank is under positive or negative pressure. The pressure will vary depending on the transformer temperature.

A sealed-tank unit should have a slight positive or a slight negative pressure. If the pressure gauge consistently records zero pressure under all loading and temperature conditions, this is an indication that the transformer tank has a leak, allowing it to ‘breathe ‘.