Dealing with unintentional islanding - a complex issue in developing decentralized energy resources which directly relates to the safety of people and assets
03 HYB24-40
Presented by: Maxime Velay
EDF SEI (Systèmes Electriques Insulaires) which operates French isolated power systems in Overseas Departments and Territories, is facing significant challenges in achieving the decarbonization of the energy mix. These power systems vary in size, from low-voltage (LV) and medium-voltage (MV) microgrids supplying isolated areas, such as in French Guiana or in the Cirque de Mafate in La Réunion, to high-voltage (HV) insular grids with peak loads ranging from 150 MW to 500 MW (French Guiana, Martinique, Guadeloupe, Réunion, and Corsica).
This target implies to develop different renewable energy resources and storage capacities, and significant capacity of both solar and wind farms are expected to be connected to distribution networks and interfaced with power electronics.
This shift has several implications for network adaptation, with one key challenge being the robustness of the protection scheme responsible for ensuring the safety of people and assets.
This paper specifically addresses the unintentional islanding of MV feeders, which is one of the main issues often mentioned in the literature. Unintentional islanding occurs when a portion of the network such as a feeder is disconnected from the upstream but remains energized due to a fortuitous consumption-generation balance between loads and DGs. To prevent these situations, DGs are equipped with interface protection relays monitoring voltage and frequency. Direct transfer trip systems can also be used to disconnect DGs.
First, network interface protection setting values must meet conflicting objectives. Voltage and frequency ranges need to be broad to minimize unwanted disconnections of DGs during HV and MV faults or large consumption-generation unbalances at the system level, and yet be narrow to mitigate the risks of unintentional islanding. In the same vein, while Grid Forming control of the inverters at MV voltage level is strongly recommended to ensure system frequency stability, it may raise the risk of unintentional islanding.
Also, in the absence of a fault on the MV feeder, this situation is to be cleared but does not pose an immediate risk to people and assets. However, in the presence of a fault, if the fault remains energized after the substation breaker opens, it must be cleared. If the fault affects an accessible and grounded facility, the fault duration must comply with the requirements regarding touch voltage limits. For example, it is necessary to ensure the safety of pedestrians near facilities such as MV/LV substations.
In this context, unintentional islanding is a major concern for EDF-SEI and EDF R&D. The focus is on finding solutions that harmonize with safety of people and assets, ensure supply quality and power system stability, and maintain economic sustainability.
The paper's first section will detail the topology of French networks, earthing systems and protection schemes. It will also outline the measures implemented to keep touch voltages within specified limits.
The second section will delve into the issues raised by DGs, ranging from the importance of limiting untimely disconnections to ensure stability, to unintentional islanding and its impacts on the safety of people and assets.
The third section will propose solutions, accompanied by an analysis of their advantages and disadvantages, among which increasing outage times for clients, and unwanted disconnection of DGs. This panel of solutions will help SEI to build the most adapted strategy to tackle this issue.
This target implies to develop different renewable energy resources and storage capacities, and significant capacity of both solar and wind farms are expected to be connected to distribution networks and interfaced with power electronics.
This shift has several implications for network adaptation, with one key challenge being the robustness of the protection scheme responsible for ensuring the safety of people and assets.
This paper specifically addresses the unintentional islanding of MV feeders, which is one of the main issues often mentioned in the literature. Unintentional islanding occurs when a portion of the network such as a feeder is disconnected from the upstream but remains energized due to a fortuitous consumption-generation balance between loads and DGs. To prevent these situations, DGs are equipped with interface protection relays monitoring voltage and frequency. Direct transfer trip systems can also be used to disconnect DGs.
First, network interface protection setting values must meet conflicting objectives. Voltage and frequency ranges need to be broad to minimize unwanted disconnections of DGs during HV and MV faults or large consumption-generation unbalances at the system level, and yet be narrow to mitigate the risks of unintentional islanding. In the same vein, while Grid Forming control of the inverters at MV voltage level is strongly recommended to ensure system frequency stability, it may raise the risk of unintentional islanding.
Also, in the absence of a fault on the MV feeder, this situation is to be cleared but does not pose an immediate risk to people and assets. However, in the presence of a fault, if the fault remains energized after the substation breaker opens, it must be cleared. If the fault affects an accessible and grounded facility, the fault duration must comply with the requirements regarding touch voltage limits. For example, it is necessary to ensure the safety of pedestrians near facilities such as MV/LV substations.
In this context, unintentional islanding is a major concern for EDF-SEI and EDF R&D. The focus is on finding solutions that harmonize with safety of people and assets, ensure supply quality and power system stability, and maintain economic sustainability.
The paper's first section will detail the topology of French networks, earthing systems and protection schemes. It will also outline the measures implemented to keep touch voltages within specified limits.
The second section will delve into the issues raised by DGs, ranging from the importance of limiting untimely disconnections to ensure stability, to unintentional islanding and its impacts on the safety of people and assets.
The third section will propose solutions, accompanied by an analysis of their advantages and disadvantages, among which increasing outage times for clients, and unwanted disconnection of DGs. This panel of solutions will help SEI to build the most adapted strategy to tackle this issue.