This guide outlines the considerations necessary for the installation of neutral earthing resistors or reactors (NERs), including resonant reactances, in power systems with a phase-to-earth voltage below 20.5kV. The aim is to restrict the magnitude of phase-to-earth fault currents to manage the Earth Potential Rise (EPR) and induced voltages in adjacent telecommunication systems to safe levels.
This Guide does not exempt owners and operators of electric power lines from adhering to the Electricity Regulations or any other statutory Acts or Regulations.
The guide does not cover all the requirements for systems operating above phase-to-earth voltages of 20.5kV, as these systems might include equipment with graded insulation or be designed for lower basic insulation levels (BIL) than those required for the use of a Neutral Earthing Resistor (NER).
The effect of insulation ageing and consequent reduction of basic insulation level has not been taken into account in this guide.
Impedance Earthing Techniques
Two primary methods are commonly used when implementing impedance earthing techniques: one Neutral Earthing Resistor (NER) per transformer unit, and one NER per substation serving one or more transformer units. With one NER per transformer unit, the maximum 11 kV earth fault current is determined by the number of transformers operating in parallel, as the associated NERs are also effectively connected in parallel.
Refer to Figure 2 (a), (b), (c), (d).
When a single NER is utilized for all transformers at a substation, the maximum 11 kV earth fault current typically does not depend on the number of transformers and is primarily governed by the value of the NER.
In a 33/11 kV substation with a single NER linked to multiple transformers, it has been demonstrated that for NER values exceeding roughly 3-4 ohms, the available earth fault current approaches a constant value for an earth fault near the substation, regardless of the number of transformers operating in parallel.
The table below details the advantages and disadvantages of each type of Neutral Earthing Resistor (NER) connection.
One NER per transformer unit | One NER per substation serving one or more transformer unit(s) |
Maintenance is simplified as each transformer or resistor can be individually removed from service when necessary. | Consideration of switching and isolation arrangements is required to ensure safe working conditions on units that remain in service. |
Increasing the number of Neutral Earthing Resistors (NERs) per substation leads to higher expenses. | Having only one Neutral Earthing Resistor (NER) per substation results in the lowest cost and is also more suitable in situations where space is limited. |
Higher levels of earth fault current are observed when multiple transformers are connected in parallel. | Lower levels of earth fault current are observed. The Earth Fault Current is not influenced by the number of transformers in operation. It is essentially a fixed level of earth fault current. |
The choice between the two cases is mainly determined by the choice of NER. Early applications of neutral resistance involved liquid resistors which require regular routine maintenance. Where these units are associated with non-dual-rated transformers, one resistor per transformer has generally been used.
The alternative of using a single resistor per substation has been implemented by power authorities in the UK, as well as in Christchurch, New Zealand, and Victoria, Australia. In situations where the Neutral Earthing Resistor (NER) requires minimal maintenance and can be temporarily disconnected without disrupting the power supply, a single NER proves to be an effective solution.
Installation of Neutral Earthing Resistors (NERs) necessitates bypassing and isolating mechanisms due to testing and maintenance requirements.
The preferred solution is to interlock the NER isolation and bypass switches in such a way as to make it impossible to accidentally disconnect the neutral point from earth.
While Neutral Earthing Resistors (NERs) will function at or near earth potential for the majority of their lifespan, they must be regarded as high-voltage equipment concerning insulation rating, testing, and maintenance. The selected connection configuration should facilitate regular servicing or repair of the NER following its commissioning.
Installation of Neutral Earthing Resistors (NERs)
Two different scenarios emerge based on whether a Neutral Earthing Resistor (NER) is retrofitted onto an existing transformer or if a new installation is being designed with impedance earthing included.
Retrofit of an NER
In retrofitting scenarios, space constraints can be considerable. During a phase-to-earth fault, the Neutral Earthing Resistor (NER) and its associated connection apparatus may experience voltages that reach at least the standard phase-to-neutral system voltage, such as 6350 volts in an 11 kV system.
Consequently, Neutral Earthing Resistors (NERs) and associated equipment must be designed and constructed to the same standards as normal High Voltage (HV) equipment, ensuring that proper clearances are maintained.
New Installations incorporating NER’s
In a new installation, space constraints may affect the choice of NER used. Should liquid resistors be employed, depending on the climate, a sheltered environment might be necessary to minimize standing heat losses. Conversely, other resistor types could require enhanced airflow to achieve an appropriate heat dissipation rating.
Document: | Guide for Application of Neutral Earthing Resistors (NERs) in Substations by The New Zealand Committee for the Co-ordination of Power and Telecommunication Systems Inc. |
Format: | |
Size: | 1.6 MB |
Pages: | 98 |
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