Transformers represent a significant asset in any utility delivery system. It has been estimated that for every watt of generated power, there are about 3.5 watts of transformer capacity. In other words, the power flows through approximately 3 to 4 transformers before it reaches the load. The larger transformers are most critical to the operation of the delivery system. The failure of a generator step-up transformer will put a power plant out of commission until the transformer is either repaired or a spare transformer is installed. The transmission transformers are almost as critical. There are two critical phenomena which have been shown to affect the reliability of these transformers. One is the static electrification (SE) of the dielectric system in large, high voltage transformers. The other is geomagnetically induced currents (GIC), which can affect all transformers in directly-grounded transmission systems.

Indeed, there are a number of transformer failures associated with both of these phenomena. The majority of the failures have been associated with generator step up transformers and most of the documented failures have been the result of SE.

Because of the plant size, the larger generator step-up transformers are typically single-phase units. There are three units in the bank and, possibly, one spare transformer per plant. However, both SE and GIC impact all of the units at the same time. This is referred to as a "common mode" stress, i.e., more than one of the units can fail at the same time.

Such failures have occurred.

In this situation, there are not sufficient spare units to bring the plant back into operation and the result is a prolonged outage of the plant. If this were to occur in a 1000 MW nuclear plant, the financial impact to the utility would be substantial. Therefore, from the risk assessment point of view, we are dealing with a relatively low probability failure mode and a large financial penalty, should failure occur.

To date, SE research has shown that all forced cooled transformers are likely to experience some degree of SE. There are approximately two dozen relatively well documented failures. Most of these failures have been a result of utilizing the shell form designed transformer.

Based on present knowledge of SE, there are probably a number of failures for which the SE contribution has remained undetected, but could have contributed to failures. The research has not yet yielded a full understanding of how the transformers actually fail from static charge buildup. Static charging tendency is strongly correlated with temperature and the dynamics of moisture exchange between the paper dielectric system and the insulating oil. The risk for SE failure appears to be highest in the first 24 to 48 hours after the startup of a new or reconditioned transformer or after a long outage, such as those experienced in nuclear power plants in conjunction with a refueling cycle. Thus, high risk transformers are more readily identifiable.

GIC affects generator transformers because they are typically located at the ends of longer lines. Thus, they serve as entry/exit points for the currents resulting from disturbances of the earth's magnetic field by changes in the solar wind. The transmission lines act like shunts for the magnetically induced currents flowing in the earth's surface. While these currents will flow into the grounded neutrals of transformers, the high voltage transformers experience more GIC than those of the lower voltage system because the dc resistance in the high voltage system is very low. The GIC will saturate all but the three-phase, three-leg core type transformers, which leads to increased reactive power flows and high harmonic levels in the transmission system. Stray flux heating of the transformers can lead to transformer damage but there is also a risk for a voltage collapse of the power system and loss of capacitor banks as a result of the high harmonic current flows. Thus, GIC is a more diffused problem, which makes it more difficult to assess the risk for damage to individual pieces of equipment. However, the generator transformers may be the items to monitor more closely because of the cost of a plant outage.

Research has also shown that the generators may experience rotor heating for which the conventional protective relaying systems may be inadequate.

Fortunately, high GIC levels are rare, so the stress on the system is infrequent. However, the closer to the thermal limits the transmission system is pushed, the more vulnerable to GIC it becomes. Research has not yet yielded information which is generally useful for assessing the risk of GIC damage to any individual power system, individual plant, or transformer.

The SUNBURST and SE Monitoring Systems process very similar information. The combination of the two would provide the critical information necessary for a transformer operations management/monitoring tool. The loading of the transformer is required in both systems. Oil temperatures need to be monitored for both GIC and SE purposes, too. The neutral current sensor for GIC and the SE/partial discharge sensor for SE are the unique components that now differentiate the two systems. A transformer monitoring system would be equipped with both of these sensors and in addition, an on-line gas-in-oil analyzer. That should provide information about incipient dielectric failures from the electrical partial discharge detector and longer term thermal degradation resulting from overheating of parts inside the tank. The static charge detector could be included with a very low incremental cost, as well as the GIC detector. Since moisture sensing is highly desirable for SE monitoring, such a detector should be part of the system. The only addition necessary for transformer overload management is a software program to provide the real time dynamic loading limit.

This is illustrated in the following figure.