Transformer Protection

 

1-  Transformer malfunctions

Most of the conditions of malfunctions that occur to power transformers can be determined as follows:

a) Ground Faults:

   A failure in the transformer coil leads to the presence of currents dependent on the source, the impedance connected between the break-even point between the transformer and the ground,  the leakage impedance of the transformer, the location of the failure in the coils, as well as the effect of the coil connections on the value of the fault current.  Zg=o) means that if the break-even point is directly grounded, the value of the fault current will be affected by the leak reactor.

It is clear that the value of the leaky reactor depends on the location of the fault itself, i.e. the leakage reactor decreases the closer the fault is to the breakeven point. As a result, the fault current increases whenever the fault is close to the breakeven point.  Figure (32) shows a comparison between the general changes in the fault current and the fault location in the case of connecting the coils in  the form of a star in the form of a Y. In the case of connecting  the coils in a delta form, the level of the fault current will be lower than the fault current in the case of a star connection, with the actual value of the current affected by the grounding method used in the power system.

As for the phase fault currents, they are often lower in the case of connecting the coils in a triangular shape due to the high impedance of the failure, and this factor is taken into account when designing the protection system for this coil.

 

b) Adapter core malfunctions

This type of fault is the result of the collapse of the insulation and leads to the flow of eddy current, causing a rise in temperature, which can reach a value sufficient to destroy the coil.

 

Figure (32): Change in the current of the ground fault with the location of the fault

 

 

c) Faults inside the windings

This type occurs as a result of an accidental spark in the coils that occurs due to sudden changes in the line voltage and the occurrence of a contact failure in a few coil rolls will produce high currents in the loops that have had a contact failure, but the terminal currents will be small.

 

d) Face-to-face malfunctions

This type is rare but leads to the occurrence of high-value currents that are similar to ground faults.

 

e) Tank malfunctions

This malfunction leads to a loss of oil which reduces insulation and also leads to an abnormal rise in temperature.

 

In addition to the conditions of failures that occur inside the transformer itself, there are some external factors that occur in natural conditions that lead to stresses on the transformer, and these conditions include:

1.    Increased load: which leads to an increase in the loss of I2 R resistance  and the associated increase in temperature.

2.    System malfunctions: Similar but sometimes much more serious effects occur than those caused by overload

3.    High voltage: It is often the result of sudden transient changes or increased voltage, causing stress in insulation and an increase in overflow.

4.    Operating the system at a lower frequency: It leads to an increase in overflow, causing a loss of heart associated with an increase in temperature.

 

2-  Magnetic and thermal phenomena

When the transformer is turned on at any point of the source voltage wave, the maximum values of the overflow in the core will depend on the residual magnetism and also on the moment of operation, and the maximum value of the overflow is higher than its value in the stable state and is limited by the saturation of the core, and the magnetization current needed to produce the overflow of the heart is 8 to 10 times the maximum value of the full load current and has no equivalent in the secondary coil, and this phenomenon is called magnetizing inrush current . It appears as an internal malfunction. The highest current flow occurs if the transformer is connected to the network when the source voltage is zero. Recognizing this, it is imperative when designing differential relays that they do not operate in the case of flux magnetization current and use a number of methods that rely mainly on the harmonic properties of the flow current to prevent the relay from operating during high flow currents.

By placing a heat-sensitive unit inside the transformer tank, it is able to protect the transformer from overheating due to heating.  Surge relays are used as additional protection with a higher time delay than the main protection relay is set to.  Limited protection for ground faults is used in the case of connecting the coils by  the star method Y, and this method is shown in Figure (2). Where the sum of the phase currents is equal to the current of the break-even point, and therefore the relay does not respond to the faults inside the coils.

 

3- Differential prevention

It is the basic way to protect transformers, taking into account some points, which are:

1. Conversion Ratio: The rated values of the current transformer must correspond to the rated currents of the transformer coils to which it is connected

2. As a result of the difference in the angular phase of 30o between the connected coils  in the star direction Y and the trigonometric side, and the fact that the zero sequence components in the star direction Y do not appear at the ends of the trigonometric side, the current transformers are connected in the star shape Y of the connected coil in  the shape of the triangle and connected in the shape of the star Y.

Figure (3) shows the differential protection system applied to a Y delta transformer  . Figure (4) shows the details of the differential protection system for a Y / Delta/Y triplex transformer, and when the current transformers are connected to the delta shape, the secondary rated values should be reduced by 1/Ö3 times the secondary rated values of the Y star transformers.

1-   There is some permissibility when changing the branch point  using the movement restriction files, which creates a bias, and the bias file must be chosen so that its effect exceeds the highest relative deviation. .

Figure (2): Ground Protection System Specific (Restricted) for the Y Star File

 

Figure (3): Differential protection system for the (Y/ transformer)

 

Figure (4): Differential protection system for a three-coil transformer