In electricity transmission, earthing stabilizes the electrical system, helping to prevent damage from earth fault currents. This article describes the benefits of neutral earthing transformers for electricity transmission.

Neutral earthing transformers (NETs) are typically found in 3-phase power systems, which form most electrical networks (see Figure 1). Many systems have delta connections, which means that a closed loop is formed, and there is no neutral terminal.

Figure 1: Cressall produces systems that include dry type NETs and resistors to suit a variety of applications, which have power ratings ranging from ten seconds to continuous. Courtesy: Cressall Resistors

Without a neutral, an electrical system remains ungrounded, which risks destabilizing the entire system if subjected to an earth fault. In a worst-case scenario, this could result in dangerous transient overvoltages and extreme damage to equipment. So, how does an NET mitigate these risks?

Preventing problems

The primary function of an NET is to provide a neutral, and from this a single electrical line linked to earth. Using the earth gives a way to add protective current transformers and relay to the system to detect earth fault current to open upstream circuit breakers, which can be used to clear any potential faults without negatively impacting operations. NETs create an artificial neutral through one of two possible configurations: a zig-zag connection or a star-delta arrangement.

Although these configurations are different, they perform the same function: to provide a neutral point from which the electrical system can be grounded. A grounded system is significantly safer than an ungrounded system. However, for optimum safety, the transformer should be used in conjunction with a neutral earthing resistor (NER), which adds resistance to the circuit to limit the fault current to a known level.

Courtesy: Cressall Resistors

An NER is placed in the neutral line of the NET so that, in the event of a fault, excessively high currents do not travel down the neutral line. The NER absorbs fault currents and safely dissipates them as heat. This ensures that protective relaying equipment remains operational, while preventing thermal damage to the system’s components.

Monitoring change

NETs also facilitate electrical system operation monitoring. Since faults flow through the NER, a type of instrument transformer known as a current transformer (CT) can be placed before the resistor to allow instruments to monitor changes.

The CT provides a ratioed signal, scaling the large values of current present in the system down to protective measuring instruments that can be easily read. They are found at all generating stations, electrical substations and in power distribution networks.

By continuously measuring and monitoring the operations of the power system, the CT allows operators to identify small changes in voltage, current or function. This helps them plan maintenance and enhance system availability, which in turn reduces downtime and ensures a consistent power output.

Be it a smaller commercial electricity production or a gigantic offshore wind farm, NETs can be essential to mitigating faults and keeping power systems safe when grounded.

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