Transformer protection -- part 1 [Industrial Electrical Power Systems]

Winding polarity

A transformer consists of two windings viz., primary and secondary coupled to a common magnetic core. International standards define the polarity of the primary and secondary windings sharing the same magnetic circuit as follows.

If the core flux induces an instantaneous emf from a low-number terminal to a high number terminal in one winding, then the direction of induced emf in all other windings linked by that flux will also be from a low-number terminal to a high-number terminal. In the following sketch the induced emf on primary winding E_p is from A1 to A2 in the A phase when a primary voltage V is applied across A as shown.

The secondary emf Es is also from a1 to a2 in secondary a phase. From the laws of induction, it will be seen that the current flow in the windings is in the opposite direction.

++++ Principle of operation of a transformer.

Transformer connections

Transformer windings can be connected either in a star (Y) or delta (D) configuration, bearing in mind that each phase will be displaced 120° from the other.

++++ show the three windings of a three-phase core type transformer.

This shows the primary connected in delta while the secondary windings are connected in star. The vectorial representation of primary and secondary voltages are also indicated.

++++ Physical connection of delta (D) or star (Y) configuration.

++++ Vectorial representation of delta and star configuration.

Depending on the method chosen for the primary and the secondary, a phase-shift can take place between the corresponding phases in the primary and secondary voltages of a transformer.

++++ Phase shift of transformer

Clock-face numbers are used to represent phase shifts, the highest voltage winding being used as the reference. A 360° shift corresponds to a full 12h of a clock with each 30° shift being represented by 1 h. For example, 30° corresponds to 1 o'clock position, 150° shift corresponds to 5 o'clock position and 330 (or -30)° shift corresponds to 11 o'clock position.

The vector grouping and phase shift can then be expressed using a simple code. The primary winding connection is represented by capital letter while small letter represents the secondary connection. The 'N' means the primary neutral has been brought out.

For example:

YNd1= Primary winding connected in star with neutral brought out.

Secondary winding connected in delta.

Phase shift of secondary 30° from 12 to 1 o'clock compared to primary phase angle.

A knowledge of the primary connections and polarities of the windings enables connections of the CT secondary leads to be correctly determined, which are very important for sensing the fault currents, the basic need for correct protection.

Transformer magnetizing characteristics

When a transformer is energized, it follows the classic magnetization curve given. For efficiency reasons transformers are generally operated near to the 'knee point' of the magnetic characteristic. Any increase above the rated terminal voltage tends to cause core saturation and therefore demands an excessive increase in magnetization current.

No load current at twice normal flux; Normal no load current; Normal flux; Twice normal flux

++++ Transformer magnetizing characteristics.

In-rush current

Under normal steady-state conditions, the magnetizing current required to produce the necessary flux is relatively small, usually less than 1% of full load current.

++++ Steady-state conditions

However, if the transformer is energized at a voltage zero then the flux demand during the first half voltage cycle can be as high as twice the normal maximum flux. This causes an excessive unidirectional current to flow, referred to as the magnetizing in-rush current.

++++ Illustration of magnetizing in-rush current.

An analysis of this waveform will show that it contains a high proportion of second harmonic and will last for several cycles. Residual flux can increase the current still further, the peak value attained being of the order of 2.8 times the normal value if there is 80% reminisce present at switch-on.

As the magnetizing characteristic is non-linear, the envelope of this transient in-rush current is not strictly exponential. In some cases, it has been observed to be still changing up to 30 min after switching on. It’s therefore important to be aware of this transient phenomenon when considering differential protection of transformers, which will be discussed later.

++++ Typical transient current-rush when switching in a transformer at instant when E=O 15.5 Neutral grounding:

It’s important that the neutral of a power system be grounded otherwise this could 'float' all over with respect to true ground, thereby stressing the insulation above its design capability. This is normally done at the power transformer as it provides a convenient access to the neutral point.

On HV systems (i.e. 6 kV and above), it’s a common practice to effectively ground the primary neutral by means of a solid copper, in which case the system is referred to as an effectively grounded system.

++++ Grounding of the neutral

This has the advantage that when an ground fault occurs on one phase, the two healthy phases remain at phase-to-neutral voltage above ground. This allows insulation of the transformer windings to be graded towards the neutral point, resulting in a significant saving in cost. All other primary plant need only phase-to-neutral insulation and surge arrestors in particular need only be rated for 80% line-to-line voltage. This provides an enormous saving in capital expenditure and explains why Eskom's HV system are invariably solidly grounded.

The disadvantage is that when an ground fault occurs an extremely high current flows (approximately equal to three-phase fault current), stressing the HV windings both electromagnetically and thermally. The forces and heat being proportional to the current squared. Grounding of the LV system neutral can be achieved.

++++ Grounding of the LV system

It will be noted that the LV system is impedance and/or resistance grounded. This allows the ground fault current to be controlled to manageable levels, normally of the order of the transformer full load current, typically 300 A. Here, the transformer does not get a shock on the occurrence of each ground fault; however, the phase conductors now rise to line potential above ground during the period of the ground fault.

++++ Phase diagram illustrating phase conductors rising to phase voltage on fault-- Phase-to- ground insulation of all items of primary plant must therefore withstand line-to line voltage.

On-load tap changers

On-load tap changers are very necessary to maintain a constant voltage on the LV terminals of the transformer for varying load conditions.

This is achieved by providing taps, generally on the HV winding because of the lower current levels. The tap changer changes the turns ratio between primary and secondary, thereby maintaining a nominal LV voltage within a specific tolerance. A typical range of taps would be +15 to -5% giving an overall range of 20%. The tap changer is usually mounted in a separate compartment to the main tank with a barrier board in between. This sometimes has a vent between the two to equalize the pressures.

Diverter switch (M-B4-B); Selector switch; Resistors

++++ On-load tap changer, Selector switch operation, Point of no return, Run down, Diverter switch

++++ Top changer range of operations

Mismatch of current transformers

Current transformers are provided on the HV and LV sides of a power transformer for protection purposes. If we consider a nominal 132/11 kV 10 MVA transformer, the HV and LV full load currents would be 43.8A 525A.

++++ Nominal 132/11 kV 10 MVA transformer

A ratio of 50/1 A would most likely be chosen for the HV current transformers, as it’s not possible to obtain fractions of a turn. 525/1 could be achieved comfortably for the LV current transformers. We therefore have a mismatch of current transformer ratios.

Furthermore, it’s more than likely that the HV CTs will be supplied by a different manufacturer than the LV CTs. There is therefore no guarantee that the magnetization curves will be the same, so adding to the mismatch.