How a Zig-Zag Transformer Will Benefit the Electrical System at Your Campus Facility

An enhanced grounding scheme not only helps protect your people and your electrical equipment, as per the NEC, but it also helps maintain sequential tripping, ensure maximum service continuity during faults, and minimize nuisance tripping and disturbances. An enhanced grounding scheme contains system faults within a localized area of the electrical distribution system.

Such schemes are often found in specialized power distribution systems where reliability and protection are of the utmost importance. For a campus type facility, such as a major hospital complex or a large infectious disease medical research facility, it is important for a ground fault to be contained exclusively within the localized area. This prevents interruption of the entire campus during either utility or emergency generator operation. For such vital facilities, you should invest in a Zig-Zag transformer for enhanced protection, safety, and reliability.

Grounding and Bonding Requirements of the NEC

The National Electrical Code (NEC) defines the mandatory requirements of grounding and bonding in electrical systems. The code requirements are intended to ensure that an installation provides appropriate personnel protection and prevents electrical fires. It provides guidelines on how to achieve the desired level of protection, techniques for establishing an effective current return path, approaches to mitigate line surges on electrical equipment, and devices that can be used to achieve a properly grounded electrical system.

How to benefit from a Zig-Zag Transformer for Large-Scale Campus Distribution

If you are responsible for a large research campus where reliable power is a must, a Zig-Zag transformer is a worthwhile investment to exceed the requirements of the NEC.  Due to the amount of power consumed and the distances it must travel as part of the underground distribution system, emergency generators are often rated for medium voltage and are supplied with high resistance grounding. When normal power is supplied to the campus by the local utility company, a solid grounding reference is available. However, when the utility is not in service, the solid system grounding is not present during generator operation.  This presents two problems. First, the normal zero-sequence current returning from all those energized cables may overwhelm the high resistance ground system when only one or two generators are online.  Secondly, the ground-fault current varies with the number of generators that are online and may pose downstream protection or coordination issues.

The application of a Zig-Zag transformer solves both problems simultaneously. It can derive a single point low impedance ground on the emergency service bus that will contribute the same amount of ground-fault current regardless of how many generators are online. Furthermore, it can be designed to give the same ground-fault current magnitudes as when the utility is in service with its normal grounding provisions; thereby allowing for easy coordination of all downstream protective devices. A breaker or a switching device can also allow the Zig-Zag transformer to be switched in or out of the system as needed. Finally, the transformer can be designed to take continuous zero-sequence return currents with less size and cost impact than with a medium voltage class grounding resistor alone.

What is a Zig-Zag Transformer?

A Zig-Zag transformer is a transformer with a specialized winding configuration that is used for establishing a grounding reference point. When a conductive or metallic surface becomes electrically energized during a ground fault condition, it can induce a surface voltage to cause an electrocution hazard. Proper grounding helps eliminate electrical shock hazards, prevent electrical fires, avoid costly electrical equipment damage, and establish an effective return path to the power source.  A Zig-Zag transformer is commonly used as a grounding (earthing) means to establish an enhanced protection scheme. The Zig-Zag winding is used in an interconnected fashion as an autotransformer or it can be a winding within a multi-winding transformer. This special configuration allows for phase shifts and voltage ranges that may not be practical with an extended delta winding. Its properties are also useful in the rectifier and industrial drive applications because of its abilities to cancel the triplen harmonic content of winding currents similar to a delta transformer (but with a grounding point).

How a Zig-Zag Transformer Works

Under a Zig-Zag transformer approach, each winding is split into two equal halves. The first half of the winding is on a given core, and the second half is on a separate core. This pattern is repeated for each of the other phases and their respective cores. With the neutral point connected to the ground and when balanced voltages are applied to the phases, the system remains passive. The windings are configured so that induced voltages cancel each other where the phase current is essentially zero establishing the transformer as a very high impedance to positive and negative sequence voltages. However, when an unbalanced state occurs, such as for a ground fault, the windings offer a very low impedance path for a return to the source of zero sequence current. The current entering the neutral point is split evenly into three currents and returns to the respective phases. Additionally, the impedance of the unit can be adjusted to set the maximum value of the ground-fault current, or the transformer can be used with a ground resistor to fix a value throughout a medium voltage system.

To ensure the effective operation of your complex electrical power system, you should analyze your electrical system and specify the appropriate grounding methodology. We make specific recommendations to improve reliability which can save you money over the life of your installation.  If you would like to know more, feel free to call us at (240) 582-3900 or email us at info@helioselectric.net

To learn about our other unique solutions that can enhance your electrical facilities, click here.