Find out more about Grid Forming Control and both the opportunities and Challenges it presents.
Battery energy storage systems (BESS) and other types of inverter based resources (IBRs) that have connected to the electricity network in the past, mostly employ a type of control called grid following. This type of control is effective when inverters connect to a strong network, characterised by how much the current from the inverter impacts the voltage at the point of connection (PoC). However, as more generation is connected to the network via inverters, the strength of the network decreases and the Grid becomes more susceptible to instability. This has led to the development of a new type of control, commonly called Grid Forming Control.
What is Grid Forming Control?
The objective of grid forming control is to emulate some of the characteristics of synchronous generators, that have been the primary source of electricity for the past century. The benefits of the synchronous generator is that its physical characteristics help maintain a stable operation of the power system, but that capability is not inherent to technologies connected via inverters. To understand why grid following inverters do not have these characteristics, we need to look first at how they operate.
Traditional grid following control can achieve specified active and reactive power output at their PoC by measuring the voltage and phase angle of the network and injecting a current that corresponds to that set point. This control is currently used by most inverter-connected technologies, such as BESS. The grid following method works well with strong networks, where the injection of current does not significantly impact the voltage at the PoC.
Grid forming control employs a different strategy. It aims to control the voltage and phase angle at its terminal or the PoC, which is particularly effective in weak networks where the voltage at the PoC is more sensitive to changes in current.
Why Does Grid Strength Matter for Grid Forming Control?
A traditional measure of system strength is the short circuit level at a particular node, which determines the amount of current flowing into a fault at a specific terminal. Therefore, each node in the power system where BESS or other inverter based technologies connect have different system strengths. The effectiveness of grid forming control is dependent on the grid strength at the PoC so it is a vital point to consider. To understand how this impacts the operation of grid forming control let’s look at the relationship between current and voltage for a connected power plant.
If we model the external grid as a three phase voltage source connected at the PoC the system strength can be represented by the equivalent impedance, i.e. a resistance and inductance, which gives the following relationship:
Now the characteristic of strong grids is that the equivalent impedance, , is low so the current is sensitive to changes in the PoC voltage. This can destabilize the system unless the response is slowed down.
For this reason the system strength is a key consideration for grid forming control. Even though stable operation can be achieved by slowing the controller down the grid forming plant must still be able to meet relevant grid code requirements. This might indicate that grid forming control could have trouble connecting to the network, but to understand why it can provide a vital alternative to the traditional grid following control the evolution of system strength needs to be taken into account.
Future of the GB Power System
One of the key metrics for system strength has been the short circuit level (SCL), which details how much current flows to a fault at a particular bus. This is influenced by how much current flows from generators connected to the system to the fault. Synchronous generators provide high short circuit currents which maintains the SCL across the network. However, IBRs like BESS, wind and PV have limited overcurrent availability due to current limits of the inverters. As the penetration of these technologies has increased the SCL across the network has decreased, and will continue to do so in the near future. The figures below show how the SCL across the network is expected to evolve during the next years.
As the SCL of the system decreases it can increase the chances of grid following control to become unstable. On the other hand, grid forming control benefits from this reduction in SCL and its employment allows more IBRs to connect to the network.
Reduction in SCL is not the only change that increased penetration of IBRs brings. Another important factor to ensure stable operation of the power system is availability of inertia.
What is Inertia and why is it Important?
In the UK the power system is operated at a frequency of 50 Hz. The frequency in the power system is determined by the rotational speed of the machines connected to it and if they maintain a constant speed the frequency will not change. If any generation is lost the other synchronous generators will convert the kinetic energy of the machine into electricity, which results in the frequency falling. If the frequency falls too sharply or goes too low it can cause generation or demand to trip to shield equipment. Synchronous generators ensure that these types of events do not cause blackouts by providing inertia.
Inertia is therefore a measure of the kinetic energy stored in the rotational masses connected to the power system and is important for the resiliency of the network. Figure 4 below shows the forecasted national inertia in the UK from the year 2021 to 2031.
This reduction in national inertia is mostly attributed to more IBRs connecting to the network, that do not have the inherent capability to provide inertia like synchronous generators. To combat the effects of decreasing system inertia some types of grid forming control can provide what is called “virtual” inertia. This type of control emulates the characteristics of synchronous generators to keep the frequency from falling too quickly.
Market Opportunities for Grid Forming Plants
As seen the SCL and inertia in the UK system is expected to continue to decrease in the upcoming years. To ensure reliable operation of the electricity network NESO has developed a competitive stability market, with the aim to acquire stability services. Previously the only mechanism to increase the stability of the network was the balancing market but the incorporation of the stability market is expected to decrease cost of managing the network substantially. These new markets also provide opportunities for grid forming plants and can provide an alternative revenue stream for technologies like BESS.
Three different types of stability markets have proposed by NESO, where the aim is to procure stability services across different timescales. A summary of each market is provided below:
- Long-term (Y-4) Stability Market: NESO has launched the first long-term (Y-4) tender where they aim to procure stability services from 2029 onwards. The intention is to acquire services for a ten year delivery period until 2039.
- Mid-term (Y-1) Stability Market: NESO has already completed the first allocation for the mid-term (Y-1) stability market, and the second tender process is ongoing. This market procures stability services a year in advance of delivery, and the delivery period is one year.
- Short-term (D-1) Stability Market: This stability market has not been implemented and is still undergoing development and industry engagement. It would allow NESO to balance their stability requirements across the system closer to real time by procuring the services the day before delivery.
Participation in these markets can provide opportunities for technologies like BESS, but there are also certain requirements that need to be met. A disconnection of a generator causing the frequency of the system to fall is a random event that could occur at any time so the BESS must limit its power output so it can deliver the service. This can impact its participation in the wholesale and balancing markets. An alternative option to enable BESS to provide inertia response is to oversize the inverter capacity to limit the impact on participation in other markets.
With new market opportunities and changing characteristics of the power system understanding how grid forming control operates is vital. At Blake Clough Consulting, we can help developer complete the necessary studies for future stability tenders, demonstrate the necessary characteristics for grid code compliance, and assess the value of participating in the stability market.