The electrical grids, like other infrastructures that society relies on, are exposed to faults and outages due to natural events, ageing and degradation of assets, human errors, and physical and cyber-attacks.
Faults can result in the supply interruption and damage to equipment. Faults fall into three categories: permanent, temporary or intermittent. Permanent faults are characterised if it remains and operation is impossible without repairing equipment. Temporary faults clear automatically without a power outage, while intermittent faults occur at intervals.
Most of the faults in overhead lines are temporary in comparison with underground cables. Moreover, faults can happen in one or multiple phases where the majority of faults are single-phase to ground faults, also known as ‘earth faults’ and ‘ground faults’.
Fault management is the scheme and infrastructure put in place by grid operators to minimise the impact of faults on the continuity of supply and the grid equipment.
In medium and low voltage distribution grids, fault management functionalities are grouped as following: i) protection systems, ii) fault identification systems and iii) isolation, repair and restoration systems.
The protection system is first to step in after a fault occurs. The protection system is responsible for separating the faulty zone very quickly from the rest of the grid, maintaining the power grid stable.
Protection equipment includes:
i) Measurement sensors, including voltage and current transformers
ii) Intelligent decision-making units, i.e. relays
iii) Switches with the capability to interrupt fault current e.g., circuit breakers and reclosers.
The protection systems are quite expensive and only used at the strategic points. Principles and designs can vary for every grid component and at different voltage levels. The protection selectivity and back-up are essential to achieve acceptable performance.
After isolation of a faulty zone, the fault identification system intervenes to identify the defective section and decreases the duration of supply interruption. The required equipment and efforts are:
i) Measurement sensors, including voltage and current transformers
ii) Intelligent processing unit, i.e. fault detector
iii) Try-and-error manoeuvres in switches, either by field personnel or telecommand, in coordination with the control centre and grid operator. After the identification of the faulty section, the exact location of the fault is identified by signal injection equipment.
The third step is to isolate the faulty section and reconnect the power supply to as many customers as possible. Once the fault is repaired, the power supply can be restored to all customers. This part of the intervention is performed by field personnel in coordination with the control centre and operator.
The performance and effectiveness of the fault management scheme for reducing the supply interruption duration highly depends on the fault identification approach. Depending on the method and equipment used, the length of the fault identification process may vary from seconds to hours. Hence, it is important to improve the fault identification process and deploy suitable approach and equipment to identify the faulty section as quickly as possible.
The majority of faults in distribution grids are currently identified with reactive approaches. A small portion of distribution grids is only equipped with technology that allows proactive fault identification. Several distribution grid operators are moving towards proactive fault identification as a requirement to decrease the supply interruption duration.