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1 Session Twelve: Fault Reduction Strategy using neutral Earth Resistor Electrical Safety & Power system protection Forum 1 Session Twelve: Fault Reduction Strategy using neutral Earth Resistor (NER) Installation Cheng Lee Lead Engineer, Peracon Co-author : Frans Cloete, Peracon Abstract Fault level reduction is an ongoing challenge in a growing electrical network. With generation and network capacity continually being added to the system , the fault levels in various locations of a distribution network will soon approach their maximum allowable design level. For electrical power distribution companies there are a number of business drivers that force the implementation of fault level reduction schemes. While the general operation of neutral Earthing Resistors (NER) for fault level reduction is well understood, optimisation of the design under various network configurations to yield maximum benefit requires detailed study.

2 This paper presents the studies carried out in assessing the impact of neutral Earthing Impedances of varying sizes for phase-ground faults on a generic network with three different configurations: Delta-Star Transformer configuration; Star-Star Transformer configuration; and Bus Tie CB Open (Delta-star Transformer configuration) The impact of Transient Recovery Overvoltage (TRV) on selected zone substation configuration resulting from the installation of NERs is also presented. 1. INTRODUCTION Fault level reduction is an ongoing task on a growing electrical network. With increasing generation and network capacity added to the system , the fault levels in various locations of a distribution network will approach their maximum allowable design level. For electrical power distribution companies, there are a number of business drivers that result in the implementation of fault level reduction schemes.

3 These business drivers are: Maintain the existing equipment within design ratings to avoid expensive upgrades. Bushfire risk mitigation. Quality of Supply Improvement. Safety and reliability. In particular, the quality of supply improvement generates a key driver for the electrically sensitive LV industrial and commercial customers, and the HV customers with step down transformers. Session Twelve: Fault Reduction Strategy using neutral Earth Resistor Electrical Safety & Power system protection Forum 2 The present 66/22kV Systems under studied are solidly earthed ( without NER installed), which results in higher earth fault currents under fault condition. With the expanding network, the situation is quickly worsened because the existing switchgear and other equipment are not rated for the higher fault level. These are shown graphically and mathematically as follows.

4 Without the NER, it can be seen that the Ifault will be naturally high. A commonly accepted and cost efficient approach to reduce the single phase to ground fault level is to install a neutral earthing resistor on the transformer neutral (as shown below). During the fault, the NER will form part of the positive, negative and zero sequence circuits, providing a mean to control the Ifault, consequently a higher NER value will result in a lower fault current. The simplified sequence circuits for the symmetrical components to represent the unbalance condition under single phase to ground fault are demonstrated graphically as follows: NER Vs Earth Potential Rise 22kV 66kV RL NER Re2 (Earth Resistance) Re1 (Earth Resistance) Ifault 22kV 66kV NER Positive Sequence I1 Z+ V1 Negative Sequence I2 Z- V2 Zero Sequence I0 Z0 V0 I1 = I2 = I0 Session Twelve: Fault Reduction Strategy using neutral Earth Resistor Electrical Safety & Power system protection Forum 3 For sequence currents, For phase currents, where Z0 = 3 x ZNER While the general operation of neutral Earthing Resistors for fault level reduction is well understood, optimisation of the design under various network configurations to yield maximum benefit requires detailed study.

5 The studies involved assessing the impact of neutral Earthing Impedances of varying sizes for phase-ground faults on a generic network with three different configurations: Delta-Star Transformer configuration; Star-Star Transformer configuration; and Bus Tie CB Open (Delta-star Transformer configuration) The impact on Transient Recovery Overvoltage (TRV) resulting from the installation of NERs has also been studied to help determine eventual NER selection. As the use of NERs to reduce the fault level is a widely accepted approach in the electrical industry. This paper aims to provide an insight into the NER effect on the network and to present the outcome of the study and simulation results. 2. METHODOLOGY The modeling and computation associated with fault and Temporary Overvoltage (TOV) calculation was carried out using the load flow software.

6 The work associated with Transient Recovery Overvoltage (TRV) was carried out using ATP software, a widely used EMTP software for the analysis of electrical transient phenomenon. The study has been performed by modeling the generic network (see Figure 4) in the load flow program, and modifying it to reflect the required transformer configuration and additional bus tie circuit breakers. Load flow studies were then conducted for each configuration. The NER fault studies (for the range of NER values) were conducted according to IEC methodology for phase-ground faults at selected locations. The voltages and fault currents were recorded and tabulated for various transformer configurations with the 22kV bus tie CB both open and closed. From these results, voltage rises on healthy phases for each study were calculated and graphed at both the 22kV zone substation bus level and at the 415V LV customer level for each transformer configuration.

7 The effects of the Session Twelve: Fault Reduction Strategy using neutral Earth Resistor Electrical Safety & Power system protection Forum 4 NER on the LV customer level within the generic network were investigated because the impact on customer voltages is one of the major benefits of neutral earthed impedance installations. It should be noted that all voltages specified herein are phase to ground voltages. When the neutral earthing resistor value is zero, the phase to neutral voltage is equivalent to the phase to ground voltage , solidly earthed. The transient recovery overvoltage (TRV) was separately investigated using ATP. The TRV results simulated based on the ATP models were compared with accepted TRV ratings for 24kV circuit breaker specified in the Australian Standard (AS-62271-100), both in terms of the levels of TRV and the rate of rise of the recovery voltage .

8 Figure 1 shows a typical response of the phase voltages following the clearing of a phase to ground fault. Figure 1 Typical phase voltage response during fault clearance The ratings of relevance in this study are the TRV rating and the short-time power frequency-withstand voltage rating of the 22kV circuit breaker. The first refers to the peak TRV value the circuit breaker can tolerate during mechanical breaking and the second is relevant to the TOV, the breaker s tolerance to withstand phase voltage displacement during the fault. (f ile STA_020805_R2_w ; x-var t) v:SECA v:SECB v:SECC [s]-40-200204060[kV]Typical ATP OutputTRV generated when clearing fault TOV during fault conditions Session Twelve: Fault Reduction Strategy using neutral Earth Resistor Electrical Safety & Power system protection Forum 5 3. voltage DISPLACEMENT CAUSED BY NER In the event of a single phase to earth fault, the fault current flows through the star-point of a delta-star connected transformer and then through the NER as shown in Figure 2, causing a rise in the voltage of the neutral point.

9 Figure 2 Flow of Fault Current through NER As a result, the neutral phase voltage is displaced and causes other phases (relative to it) to shift as well. This is also the fundamental cause of temporary overvoltage (TOV). See Figure 3. The unbalance voltage also provides a means of detecting earth faults and is a technique many relays employ. Figure 3 Vector Diagram - Demonstration of voltage Displacement 4. KEY FINDINGS FROM THE GENERIC NETWORK STUDY A generic network as shown in Figure 4 was modified to include a 22kV/415V delta-star to ground transformer connected at an arbitrary location on Feeder 2. Simulation studies were then carried out to observe the effect of phase to ground faults and neutral earthing impedances at the low voltage 415V customer level as it was identified that quality of supply is one of the key business drivers, particularly for industrial and commercial customers.

10 The key findings related to the impact of the NER with different transformer configurations, bus tie CB states, and fault types were as follows. Regardless of transformer configuration, opening the bus tie CB reduces the impact of all types of faults but reliability and quality of supply issues remain for feeders on the affected bus section. NERs have no impact on phase to phase type fault mitigation but it would help to reduce phase to phase faults developing from phase to ground fault. NERs maintain voltage supply at the 415V level but do cause 22kV phase voltages to rise. However, NERs do not have any reliability or voltage depression issues associated with opening the bus tie CB. At the 415V low voltage customer level, neutral earthing resistors would continue to assist distribution companies to meet their fault level mitigation 66kV (Pri) a c b voltage displacement resulting from NER TOV 22kV (Sec) NER If Displaced voltage Session Twelve: Fault Reduction Strategy using neutral Earth Resistor Electrical Safety & Power system protection Forum 6 and quality of supply objectives regardless of 66/22kV transformer configurations and bus tie CB status.