System HILP Event Demand Disconnection (SHEDD)

Objective(s)

The objectives of the project is to design and test a new LFDD scheme to maximise its future performance as the network continues to decarbonisation, Distribution Generation (DG) integration increases, and system inertia continues to decrease.

Problem(s)

Low Frequency Demand Disconnection (LFDD) is designed to limit the fall in frequency for extreme events beyond those defined as ‘secured’ events in the SQSS and Operating Code OC6 (Demand Control) of the Grid Code.

Under exceptional circumstances where the frequency deviates for more than 60 seconds (e.g. loss of more than one large generator) there may be certain circumstances where the normal contracted frequency response may not be sufficient to maintain the system frequency between the statutory limits where the total loss of generation exceeds the amount secured for and a deficit of generation arises. In order to reduce the generation deficit (or excess in demand) to maintain stability, Distribution Network Operators (DNOs) have low frequency relays to disconnect demand (LFDD).

The current LFDD schemes do not take into account the move to decarbonize through growth of distributed generation and decreasing system inertia. This reduces the effectiveness of LFDD schemes which could potentially put the whole system at risk; while if the response results in sub-optimal economic disconnection of customers, the economic impact (and so cost to consumers) of High Impact Low Probability events will be higher than necessary.

Method(s)

The following deliverables will be explored:

• Review the year-round performance of the LFDD scheme as the uptake of Low Carbon Technologies (LCTs) increase to identify opportunities to improve its technical and commercial performance in the short and medium term.
• Consider a range of options for redesign of the current LFDD scheme to maximise its performance in the short term and for a new “smart” alternative to the LFDD scheme in the medium term.
• Assess those options for redesign, including testing within “war game” simulations to demonstrate performance under simulated HILP events.
• Identify the optimal solution for both short and medium term improvements to the LFDD scheme, and develop a plan for the way in which it could be rolled out, completed by the necessary technical specifications.

• Scope

  Frequency plays a very important role in power transmission and distribution in relation to the balance between the demand and generation requirements of the network. The maintenance of system frequency within set levels is required to maintain stability and prevent a full system collapse. Under normal operating conditions National Grid Electricity System Operator (NGESO) is obligated to maintain the system frequency between 49.8 and 50.2 Hz.

  Under exceptional circumstances (e.g. loss of a large generator) the frequency should not deviate outside the range 49.5 to 50.5Hz for more than 60 seconds. In order to achieve this, NGESO contracts frequency response to secure the power system for a number of events. There may be certain circumstances where the contracted frequency response may not be sufficient to maintain the system frequency between the statutory limits where the total loss of generation exceeds the amount secured for and a deficit of generation arises.

  In order to reduce the generation deficit (or excess in demand) to maintain stability, Distribution Network Operators (DNOs) have low frequency relays to disconnect demand (LFDD). To comply with the requirements of the Grid Code, Western Power Distribution as a DNO is obligated to install LFDD schemes. The schemes are designed to automatically disconnect at least 60% of the total DNO demand on a stage by stage basis at the time of the forecasted national electricity transmission system peak demand. The demand subject to automatic low frequency disconnection is divided into 9 predetermined discrete MW blocks which are disconnected at defined low frequency levels. Each block of demand is distributed across each license area, so far as reasonably practical, so that the demand at different Grid Supply Point (GSP) sites is reduced evenly.

  The current LFDD schemes do not take into account the growth of distributed generation and decreasing system inertia. The growth of distributed generation connected on DNO networks at voltage levels below where the LFDD relays are installed is likely to impact on the effectiveness of the scheme. If the level of distributed generation output is high when the relay is triggered, the amount of demand disconnected may be lower than expected. In addition, levels of system inertia are decreasing (e.g. due to the closure of traditional generation) along with net transmission system demand.

  This reduces the effectiveness of LFDD schemes as changes in frequency will be faster and larger. Should the frequency fall at a high rate, more than one LFDD stage could operate resulting in too much demand being disconnected. These increasing changes risk the effectiveness of LFDD, impacting security of supply, unnecessary customer interruptions and price impact. If the LFDD scheme does not deliver the demand reductions required, the whole system is at risk; while if the response results in sub-optimal economic disconnection of customers, the economic impact (and so cost to consumers) of High Impact Low Probability events will be higher than necessary. Furthermore, an ineffective LFDD scheme could increase the risk of rolling brownouts / blackouts.

  For example:

  • The LFDD scheme is simplistic in design and overestimates the demand reduction achieved by operating LFDD relays
    • Evidenced by 9 August event
    • Current approach to determining magnitude of demand reduction from each relay is simplistic
    • Unknown what volume of DG is also lost when a relay operates
  • Load shedding does not take into account the variation in Value of Lost Load (VoLL) for different customer types.
  • Vulnerable customers and safety critical loads are also not sufficiently protected by the current LFDD scheme.
  • The performance of the scheme is decreasing as the uptake of Distributed Generation increases, and system inertia falls. This degradation of performance is expected to worsen with time.
• Success criteria

  The project will be deemed a success if a proposed new LFDD design is proven successful in simulations and examined to be viable by Subject Matter Experts.

• Potential for new learning

  The learnings of the project are designed to be fit for other Network Licensees and all learnings from the project will be documented, including a detailed plan for delivery of the proposed improved process.

• Document and links

  NIA Project Registration and PEA Document

  ENA Project Closedown Report 

• Monthly updates

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