Load Flow & Reactive Power Capability Studies

Generating stations connected to the UK transmission system are required to comply with specific reactive power requirements, as outlined in the Engineering Recommendation G5/4 (ER G5/4) of the Energy Networks Association (ENA). These requirements are designed to ensure that generating stations can provide the necessary reactive power support to maintain voltage stability on the grid.

The latest issue of the Engineering Recommendation EREC G99 (Issue 1 – Amendment 9, 03 October 2022) requirements for the connection of generation equipment [2] categorizes the plant types (Power Park Module (PPM), as per the definitions below:

>>? Type A - A Power Generating Module (PGM) with a connection point below 110 kV and a registered capacity of 0.8 kW or greater but less than 1 MW.

>> Type B - A Power Generating Module with a connection point below 110 kV and registered capacity of 1 MW or greater but less than 10 MW.

>> Type C - A Power Generating Module with a connection point below 110 kV and a registered capacity of 10 MW or greater but less than 50 MW.

>> Type D - A Power Generating Module with a connection point at or greater than 110 kV, and/or with a registered capacity of 50 MW or greater.

As per ECC.6.1.4.1 users connected between 110 kV and 300 kV are required to maintain continuous operation between 0.90 p.u. to 1.10 p.u. of the nominal voltage at the POC. Based on the Grid Code [2] compliance requirement for Type C & D Power Park Module (PPM) with POC voltage above 33 kV, the plant should be capable of satisfying the reactive power requirements as shown in Figure ECC6.3.2.4 (a). This defines the extreme operating conditions which are assessed in this study, and are shown as points H, B, F, and D in Fig 4-1. Taking these points into consideration, the studies usually undertaken at some critical points of operation of the plant at Registered Capacity and Minimum Stable Operating Level (0.2 p.u. of active power as per Figure ECC6.3.2.4 (c)). These operating points are indicated in Fig. 4.2 by the points H1, B1, H2, B2, D1, F1, D2, and F2, and are defined as:

>> Point H1: Operating at registered capacity with -0.33 p.u. reactive power and POC voltage of 1.10 p.u.

>> Point B1: Operating at registered capacity with -0.33 p.u. reactive power and POC voltage of 0.97 p.u.

>> Point D1: Operating at registered capacity with +0.33 p.u. reactive power and POC voltage of 0.90 p.u.

>> Point F1: Operating at registered capacity with +0.33 p.u. reactive power and POC voltage of 1.03 p.u.

>> Point H2: Operating at 20% of registered capacity with -0.12 p.u. reactive power and POC voltage of 1.10 p.u.

>> Point B2: Operating at 20% of registered capacity with -0.12 p.u. reactive power and POC voltage of 0.97 p.u.

>> Point D2: Operating at 20% of registered capacity with +0.33 p.u. reactive power and POC voltage of 0.90 p.u.

>> Point F2: Operating at 20% of registered capacity with +0.33 p.u. reactive power and POC voltage of 1.03 p.u.

PQ and QV Curves of Solar & BESS Plant

PQ and QV curves are essential tools for analyzing the reactive power capability of a solar and battery energy storage system (BESS) plant. These curves provide valuable insights into the plant's ability to inject or absorb reactive power, which is crucial for maintaining voltage stability on the grid.

PQ Curve

The PQ curve, also known as the power-reactive power curve, represents the relationship between real power (P) and reactive power (Q) that the plant can deliver at a given voltage level. The curve typically exhibits a U-shaped or oval shape, with the plant's maximum reactive power capability occurring at a specific real power output.

PQ Curve of Solar & BESS Plant

The PQ curve provides several important pieces of information about the plant's reactive power capability:

·?????? The maximum reactive power (Q) that the plant can inject or absorb

·?????? The real power (P) at which the maximum reactive power occurs

·?????? The range of real power over which the plant can provide reactive power support

QV Curve

The QV curve, also known as the voltage-reactive power curve, represents the relationship between voltage (V) and reactive power (Q) that the plant can deliver at a given real power output. The curve typically exhibits a negative slope, indicating that the plant can provide more reactive power support at lower voltages.

Figure 3: PQ curve of a typical type D Hybrid Power Plant (Solar+ BESS) of 100 MW Export and 60 MW import

QV Curve of Solar & BESS Plant

The QV curve provides several important pieces of information about the plant's ability to regulate voltage:

·?????? The range of voltages over which the plant can provide reactive power support

·?????? The amount of reactive power that the plant can inject or absorb at specific voltage levels

·?????? The plant's ability to maintain voltage stability during voltage fluctuations

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Figure 4: VQ curve of a typical type D Hybrid Power Plant (Solar+ BESS) of 100 MW Export and 60 MW import

Typical Graphs

PQ and QV curves for a solar and BESS plant typically exhibit the following characteristics:

·?????? The PQ curve is U-shaped or oval-shaped,?with the maximum reactive power capability occurring at a specific real power output.

·?????? The QV curve has a negative slope,?indicating that the plant can provide more reactive power support at lower voltages.

·?????? The PQ and QV curves are influenced by the size and configuration of the solar array and BESS,?as well as the inverter control settings.

Applications

PQ and QV curves are used for a variety of applications, including:

·?????? Planning and design of solar and BESS plants:?The curves help ensure that the plant has sufficient reactive power capability to meet the needs of the grid.

·?????? Operation of solar and BESS plants:?The curves are used to optimize the plant's reactive power output and maintain voltage stability.

·?????? Troubleshooting of solar and BESS plants:?The curves can be used to identify and resolve issues with the plant's reactive power capability.

Conclusion

PQ and QV curves are essential tools for understanding and managing the reactive power capabilities of solar and BESS plants. These curves provide valuable insights into the plant's ability to inject or absorb reactive power, which is crucial for maintaining voltage stability on the grid.

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