Staying grounded

source: Electronics Specifier article

Resistor manufacturer Cressall has developed a cost-effective range of neutral earthing resistors (NERs) designed to help generator manufacturers and end users around the world protect distributed generation systems. The NERs have been specifically designed to be supplied with multiple vacuum contactors, for applications requiring the earthing of multiple parallel-connected generators.
NERs are used in medium-voltage AC distribution networks to limit the current that would flow through the neutral point of a generator in the event of an earth fault. The resistors limit fault currents to a value high enough to be detected but low enough to prevent damage to switchgear or generators.

Cressall’s switched NERs have been designed as a cost-effective method of providing a single resistance earth for generators that are connected in parallel.

When generators are connected in parallel, there are typically three options for earthing – using a single fixed earthing point, earthing all of the neutrals or using a single earthing point through individually switched neutrals. Using a Cressall switched NER gives the user the flexibility to select which generator is to be earthed. Because there is only one earth path, protection relay coordination is straightforward and the risk of damage due to circulating currents is reduced.

Applications for Cressall’s NERs range from backup supplies for airports and hospitals to generation for small towns and industrial centres that are too far from the main grid.

“As the price of energy increases, so does the requirement for alternative sources,” explains Martin Nicholls, Sales Director, Cressall. “For example, in remote applications it is not cost effective to build and maintain power transmission lines over long distances. Therefore, remote diesel or natural gas generators are used, operating independently off the normal grid. These effectively mimic a normal power station, only smaller.

“We have been supplying switched NERs for standby and combined heat and power projects for more than 20 years; however the demand for cost effective solutions for multiple point earthing resistors led us to introduce this modular range. Previously when earthing of multiple generators was needed, the main option was to source fully specified metal clad switchgear and package it with a stand-alone resistor, which makes the project large and very expensive. By creating and developing a standard range, Cressall has improved the response time to customers, shortened delivery times, lowered cost and improved quality.”