INTRODUCTION

High speed may be of more importance than the potential damage to the equipment for designing protection systems for transmission lines.  But in the case of power transformers, high cost of repair or replacement, and the possibility of fire or violent failure makes “limiting the damage” a major objective.  These and other philosophical issues are discussed in clause 5 of the guide. 

Insulation failure leading to shorting of a few turns of a transformer winding results in high circulating current in the shorted turns but this does not result in any significant change in currents at the terminals of the transformer. This suggests a need for a protection system that is sensitive enough to detect low-current faults and operates at high speed to limit the damage.  Types of failures experienced in transformers and the consequences of those failures are discussed in clause 6.  The issue of minimum and maximum fault currents is addressed in clause 7.  It is also shown that a CT provided at the transformer terminals for providing current to a differential relay may saturate during a system fault while no fault current may be flowing in the transformer windings.

There is no standard way to protect all transformers. Economic considerations may have a major impact on the selection of the protection scheme and the need for an interrupting device.

IEEE guide, C37.91TM addresses all these issues starting with field data of statistics for types of failures experienced in transformers to help the readers in making considered decision on selecting protection systems in their applications.  Some topics discussed in the guide are briefly presented in the following sections.

Magnetizing Inrush

The issue of magnetizing inrush current is also discussed in clause 7. When a transformer is energized, inrush currents that are several times the full load current flow in to the transformer even if the secondary windings of the transformer are not connected to a load.  The levels of these currents depend on the parameters of the power system and the magnetic properties of the transformer core. If one winding of a transformer is connected to a power system and the other windings are not, an abrupt change of voltage connected to the transformer will cause magnetizing inrush currents to flow in to the transformer.  Other transients can also create similar inrush currents of large magnitudes in one of the windings of a transformer as is explained in clause 7.4.2 of the guide. The magnetizing inrush current has two distinct features that separates an inrush from load currents and fault currents. They are:

• Inrush currents have substantial even-harmonic components in them.
• There is always a low-current region that is greater than one-quarter period of the magnetizing-current waveform as shown in Figure 1.