Case study of fault mitigation measures in 132kV electrical network of South Africa

Fault current limiting devices (FCLDs)

Utilities are governed by standard in terms of their system availability and quality of power. These standards are becoming key factors in determining pricing and network investment. It is thus important to fully understand the introduction of new equipment in the network. It is important that volt drop, harmonics and ferroresonance are studied in the network when fault current limiting devices (FCLD) is be considered.

Volt drop and Regulation

One of the first criteria that is checked in any network is the voltage at various junctions and customers. Fault current limiting devices (FCLDs) must have low impedance in order to reduce the voltage drop across it during normal operation. This would reduce the energy losses and be a strong motivation factor in the selection process of FCL mitigation measure. This is the key factor in the choice of passive devices while active ones will generally not have this problem.

The low or no energy consumption and better voltage quality of active devices will be weighed against the high costs of the device and installation. FCLD location will have an effect on the voltage regulation in the network.

During a fault, fault current limiting devices (FCLD) can cause voltage sags in other part of the system. Feeders downstream to the FCLD will be effected more than those adjacent to it.

Ferroresonance

This is a phenomenon that occur in networks with non-linear inductance properties. This could lead to overvoltage and overcurrent in the network.

The response characteristics of the FCLD has to be examined at the voltage using actual test results. No ferroresonance will occur if the FCLD displays linear inductance characteristics. If not, a capacitor will have to be used in series with the reactor.

Impacts of Fault Current Limitation on Existing Protection Schemes

Protection systems play one of the most important roles in power networks by ensuring that equipment only experience stresses within their designed limits. In order to ensure that the criteria is met, protection co-ordination settings will ensure that equipment deliver the necessary power under normal conditions.

When fault current limiting devices (FCLs) change in a network the protection grading would thus have to change accordingly. The effects of fault current limiters have to be fully investigated and their effects on the network quantified.

In a protection scheme, the pickup processing and the co-ordination can be influenced for different types of protection schemes and investigates the effect of the FCLD being located within the zone of protection or outside. Protection schemes may have to change once fault currents limiters have been installed.

A study has to be carried out to determine the effect on the existing protection schemes and what changes, if any would have to take place.

Fault Current Limiter Location

Fault current limiting devices (FCLDs) can be used at various locations within a network as shown in the Figure 1. Position A is at the busbar that supplies the transformers at the primary voltage.

In the HV context, at the 275 kV voltage level, it is a rare occurrence to experience high fault current limiting devices (FCLs). It is likely to occur in a highly inter-connected 275 kV network with large generation capacity being supplied by short lines. Installing FCLDs at position B is avoided due to the losses volt drop that would be experienced. The most optimal position is between the bus bars at position C.

The bus-section is operated open and the FCLD is used when the bus-section has to be closed. D shows the FCLD on used to supply out of the SS, which might be a portion of the network where high FLCs are a problem.

Analysis of Fault Levels in HV networks

The accuracy of fault levels is dependent on the model parameters used. When using simulation packages, the results obtained give an approximation of the network behaviour in the event of a fault level. The introduction of new plant, generation and the growing trends of embedded generation have shown to increase fault current levels.

The pre-fault voltage level has great impact on the calculated fault level. The following factor affects the fault voltage level:

• Operating conditions of generators (power factor)
• Tap changer positions of transformers
• Network impedances of lines and cables
• The estimated load demand on the network at the time of the study