Grid Code Compliance Issues Caused by LVRT are a critical challenge when connecting generation plants to the GB Grid, particularly under weak grid conditions. Adherence to the GB Grid Code demands that plants demonstrate their ability to meet strict reactive power requirements within tight timeframes, but triggering the LVRT response often disrupts this compliance.

Introduction to Grid Code Compliance

In order to connect embedded or distributed generation to the GB Grid, adherence to the strict requirements outlined in the GB Grid Code is essential. These requirements cover various aspects of generation behaviour, ensuring that plants can connect to the grid securely and contribute effectively to the energy mix. Prior to connection. a series of simulation studies, as outlined in Section ECP.A.3.1, must be performed to demonstrate compliance. At Blake Clough we have performed these simulations many times across various technology types and we find that it is the same few scenarios that generate the majority of issues. ECP.A.3.4.1 (v) is one such problematic scenario, which is described as:

“a dynamic time series simulation study result of a sufficiently large negative step in System voltage to cause a change in Reactive Power from the maximum leading value to the maximum lagging value at Rated MW.”

The plant is expected to change its operating point from maximum leading (inductive reactive power) to maximum lagging (capacitive reactive power) within 2 seconds of a voltage drop.

Why LVRT Threshold Can Cause Non Compliance

What is the problem in demonstrating Grid Code compliance? The key challenge lies in the demanding nature of the required response. The system must shift from importing reactive power to exporting it within a very short timeframe, placing significant stress on the system, particularly the inverters. This problem becomes more severe when the generation plant is connected to a weak grid. A weak grid is characterised by a lower short-circuit current, high voltage sensitivity and reduced stability. When a voltage event occurs on the grid, e.g. a voltage step change reduction in voltage, the plant must transition its reactive power output from leading to its maximum lagging state, and the voltage across the inverters may fall below the Low Voltage Ride Through (LVRT) threshold. This, in turn, triggers the LVRT response.  When LVRT is triggered, the inverter control mode changes and the plant is unable to achieve the required 90% reactive power output within the specified 2-second window, resulting in non-compliance with the Grid Code.

Examples of the Response

The example below is based on a generic Western Electricity Coordinating Council (WECC) model operating with a 4% droop. The results show that the voltage at the Point Of Connection (POC) and the inverter terminal fall below the 0.9pu threshold and thus would typically trigger the LVRT response (in the first plot, the LVRT is disabled to demonstrate the response whereas in the second plot LVRT is enabled).

How Do I Achieve Grid Code Compliance Issues Caused by LVRT?

So, what can be done to achieve compliance under these circumstances? The obvious solution is to lower the LVRT threshold; however, this risks creating non-compliance with the Fast Fault Current Injection (FFCI) requirements of the grid code. There are also other tools including adjusting tap-settings or controller settings, such as lower the QV-droop, which can help with avoiding triggering LVRT and achieving compliance. However, it seems as though the intention of this study case is to assess the ability of the plant to transition from leading to lagging within the required time-frame rather than the need to ride through such a large voltage step. Therefore, these changes to the control systems appear to be an unintended consequence of this simulation scenarios and is potentially resulting in non-ideal performance purely for the sake of passing this test. A more sensible approach would be to permit the lowering of the LVRT threshold just for the purposes of demonstrating compliance for this scenario, but as it stands there is no guidance on dealing with this issue.

Our Thoughts on Grid Code Compliance Issues Caused by LVRT

The voltage response simulation scenario, as described in ECP.A.3.4.1 (v), can be particularly challenging for plants connecting to weak grids as there is a risk of triggering LVRT. This seems to be an unintended consequence and is resulting in delays in the compliance process and/or abnormal LVRT settings needing to be applied.

As we see significant changes across the network driven by the increased penetration of inverter based resources, we expect to see more grid code compliance issues arising, with amendments to the Grid Code and Engineering Recommendations likely to be needed as the mix of technologies continues to change.