As the global energy sector transitions toward cleaner and more distributed energy systems, maintaining the stability and reliability of power networks has become increasingly challenging. One critical tool in this endeavour is the intertrip system, an advanced control mechanism designed to protect power grids from severe disturbances. This blog describes what intertrip systems are, why they are important, and how they relate to key issues such as Loss of Mains (LoM), islanding, and load management schemes.
What is an Intertrip System?
The term “intertrip” refers to a method in which the operation of protection equipment at one end of a circuit causes a signal to be transmitted to trip a circuit breaker at the remote end of the circuit. An intertrip system automatically disconnects a generator or demand from the transmission system when a specific event occurs, such as a system fault.
There are two types of Intertrips:
- Operational Intertrips are a condition of connection to the power system, typically identified during the initial connection process of a generator. These intertrips are specified within the Bilateral Connection Agreement (BCA) between the generator and the system operator, such as National Grid.
- Commercial Intertrips offer more flexibility and can be agreed upon either at the time of connection or on an ad hoc basis as system needs arise. These intertrips are negotiated bilaterally between the system operator and the generator, with payment structures that may include capability payments, arming fees, and tripping fees.
Why are Intertrip Systems Important?
Intertrip systems play a crucial role in maintaining grid stability by proactively disconnecting the weak links of the network whenever a fault or disturbance occurs. That way, it prevents cascading failures whereby an issue in one part of the grid can rapidly spread and lead to blackouts affecting many areas. Isolating problem spots rapidly, intertrip schemes prevent the mismatch between generation and demand from affecting the entire system, hence keeping the grid stable under adverse conditions.
Apart from improving grid stability, the intertrip systems serve to limit the extent of equipment damage. If there is a fault, the rapid disconnection of the affected part of the network reduces the possibility of overloading, overheating, or mechanical damage to sensitive infrastructure like transformers, circuit breakers, and generators. This will protect very expensive equipment, reducing repair costs and time loss while improving overall efficiency.
Also, intertrip systems can allow the other areas of the grid to operate normally by isolating just the faulty area. In this way, critical infrastructure such as hospitals or emergency services in the unaffected areas can still receive power. This capability is increasingly important as grids become more complex and the integration of renewable energy sources introduces greater variability and intermittency.
Loss of Mains and Islanding: A Critical Intersection
Loss of Mains (LoM) is utilised to prevent any damage when there are problems on the electricity distribution network. However, in the past, protection from LoM has often been too sensitive and has caused the unnecessary disconnection of generation when the system is still stable. This generally occurs more frequently at times of high renewable energy generation, and it is these costs of preventing such disruptions that consumers and the electricity industry have to bear.
LoM occurs when a section of the grid becomes disconnected from the main network, resulting in a localised area operating independently, a condition known as islanding. While islanding might initially appear to be a contained issue, it can introduce significant risks if not effectively managed. An islanded system lacks the stabilising influence of the larger grid, which can lead to substantial voltage and frequency fluctuations. These fluctuations can damage sensitive equipment, disrupt service, or even endanger personnel working on or near the system.
The challenge with the past LoM methods lies in striking a balance between sensitivity and reliability. Overly sensitive systems may react to minor disturbances that pose no real threat, while under-sensitive systems risk failing to detect actual islanding scenarios. This will be challenging, especially for modern grids with high renewable penetration, as such systems are highly dynamic and could further worsen conditions leading to false activations of LoM protection.
Intertrip systems are vital in addressing these LoM management challenges and preventing unintended islanding. An intertrip scheme could detect an imbalance in power or deviation in frequency and disconnect generation sources that are contributing to the problem, or reconnect the islanded section to the main grid under controlled conditions. This ensures safety and efficiency of the grid, even in scenarios where past protection mechanisms might fall short.
Challenges with Historic Outage Data
The effectiveness of LoM protection systems relies heavily on the ability to predict and accurately identify fault conditions. Here, the quality of historic outage data becomes critical. However, dependence on historic outage data can be very challenging, especially when the data is not detailed or clear. Without a proper description of why an outage occurred, it is hard to draw meaningful insights or predict future trends.
The quality of historic data directly impacts the accuracy of the intertrip assessment in many ways: it affects the very basis of risk evaluation, determination of trigger conditions, and system modelling. Without accurate and detailed historical data, there is a risk of under or overestimating risks associated with specific grid components, potentially resulting in too few or too many implementations of intertrip. Moreover, the absence of consistent data can lead to underestimating or overestimating the risks associated with specific grid components. This underscores the need for improved data collection and analytics in the power sector, enabling more accurate forecasts and robust intertrip configurations.
Accurate data is essential for intertrip assessment because it enables more precise design of intertrip schemes, ensuring they activate only when necessary and minimising disruptions to the grid. It also leads to accurate cost-benefit analyses of intertrip implementations versus other constraint management strategies. With comprehensive historical analysis, some critical points are identified in the network where intertrips can most effectively ensure system stability upon disturbances.
Intertrip Systems in Load Management Schemes
Load Management Schemes (LMS) are systems designed to manage network loading and voltages by controlling demand, generation, or network topology to maintain system stability and prevent overloads.
Intertrip systems are not limited to fault protection; they are also integral to various advanced load management schemes that go beyond the scope of mere fault protection. With increasing power network complexity, further integration of renewable energy sources, the role of an intertrip system has become essential in the balancing of real-time supply and demand.
In the context of load management, intertripping provides a dynamic mechanism to adjust grid load by selectively disconnecting non-critical consumers during peak demand periods or emergencies. For instance, large industrial loads can be temporarily disconnected to avoid system overloads so that essential services such as hospitals and public transport systems can continue uninterrupted.
This functionality is particularly important in renewable-rich grids, where the variability of solar and wind power can lead to sudden mismatches between generation and consumption. Intertrip schemes integrated with real-time monitoring and control systems will enable operators to respond quickly to these fluctuations, maintaining stability without the need for costly interventions such as fast-ramping fossil fuel generators.
Innovation in Intertrips
At a system level, intertrips can be used to manage network constraints. In 2021, NESO tendered for a Constraint Management Intertrip Service (CMIS) to be applied to boundary B6, i.e. to ease congestion on the B6 Anglo-Scottish boundary.
The system operates as follows. Once a constraint occurs, the Electricity National Control Centre (ENCC) assesses the constraint and arms the excess volume of generation that exceeds the transfer capability, provided that this does not exceed the larges permitted system loss. The User is then informed by NESO that they are armed on the CMIS. If a fault occurs on any of the monitored circuits, a fast-tripping signal either triggers opening of circuit breakers to disconnect the user, or a de-load signal reduces the user MW export within 10 seconds. The User is then notified by ENCC that they have been de-loaded or disconnected, and the ENCC investigates the cause of the intertrip and recovers the system.
In the first 10 months of operation alone, this scheme saved the consumer £80m. Once major reinforcement works are complete across this boundary, it is anticipated that the CMIS will no longer be required, and hence tender evaluation is considering the first four years of service from April 2025 to March 2029. However, the same solution is valid for other network areas experiencing similar constraints. For example, the East-Anglia operational intertripping scheme started in February 2024, earlier than anticipated.
Intertrip Systems – Conclusion
Intertrip systems are the backbone of existing power networks, important for grid stability, reliability, and efficiency. In these times of transition toward cleaner and more distributed energy systems, such advanced control mechanisms are becoming increasingly important. Key takeaways include:
- Intertrip systems automatically disconnect the generators or the load from the transmission system in case of the occurrence of one or more events predetermined by them, preventing cascading failures and thereby spreading wide-scale blackouts.
- They are essential for LoM management and prevention of unwanted islanding conditions, which might lead to dangerous voltage and frequency fluctuations.
- Intertrips are crucial in load management schemes, offering dynamic adjustments of grid load due to peak demand or emergencies.
- The effectiveness of intertrip systems relies heavily on accurate historical outage data, highlighting the need for improved data collection and analytics in the power sector.
- They can be used to great advantage to save £m to the consumer, if deployed in a widespread way.
In conclusion, intertrip systems are a key technology enabling the transition toward a resilient, flexible, and sustainable power grid. Further development and implementation will thus be integral in addressing the challenges posed by our evolving energy landscape.