Five steps to take (from April!) to ensure renewables comply with part of the new grid code

In this article we will explore through an example the five steps to follow to ensure compliance with the reactive power requirements that will apply once the new P.O.12.2 [1] comes into force. This is currently at the draft stage, to be approved in April, and is the document that develops for Spain the European code for the connection of generation to the grid (Regulation (EU) 631/2016 [2]). In addition, the requirements of the new P.O. 12.2 will be assessed taking into account the technical standard for conformity monitoring of electricity generation modules [3], which is also still in draft form.

The steady state reactive capacity study is defined by the two graphs in Figure 1, at maximum power (Pmax) (b) and at power below Pmax (c). The power park module must be able to operate over the whole of the polygons defined by Figure 1 at the grid connection point (RCP). The reactive limits in Figure 1 (c) in the vicinity of Pmax equal to 1 pu should coincide with those set out in Figure 1 (b). However, they do not fully coincide as they depend in turn on the voltage at the PCR. This apparent discrepancy seems to indicate that in reality these two figures are projections on a plane of a three-dimensional volume as in Figure 1 (a). This volumetric interpretation would qualify the application of Figure 1 (c) and would imply that the PCR stress should be taken into account for its analysis, as we will see in the following sections.

Step 1: Determining the type of plant

We consider a solar photovoltaic plant with an installed capacity of 100 MW of inverters and 130 MW peak power of panels, connected to the 220 kV transmission grid through three transformer substations whose ratios are 220 kV/132 kV, 132 kV/33 kV and 33 kV/0.69 kV respectively. In accordance with the provisions of the draft P.O.12.2, this plant constitutes a type D photovoltaic power plant module, given that its connection point is above 110 kV and its power output is above 50 MW. The satisfaction of at least one of the above conditions is sufficient for the power plant module to be considered as type D.

The new grid code will require certain capabilities associated with reactive power management from D-type plants. To illustrate these capabilities, an example of a study of compliance with the new grid code will be presented.

 Step 2: Reticulation and plant modelling

The first step of the study consists of the detailed modelling of the reticulation of the wind farm and the subsequent obtaining of the equivalent aggregate model of the same at the grid connection point. As can be seen in Figure 2, the elements of the same type are added, retaining their capacities and control elements such as tap changers in the trafos.

Step 3: Full load study

In the study at maximum capacity (Pmax) the maximum power at the connection point is set and the load flows are simulated by sweeping the reactive power range at each of the indicated voltages as shown in Figure 3. Note that in plants connected below 300 kV, as in this case, the extreme voltages are 1.1 and 0.9 pu. In this case the plant is able to meet the code requirement without problem.

Step 4: Part-load study

The study of reactive power below Pmax is analogous to the previous one, but making a power sweep from 0 to Pmax. From the draft of P.O.12.2 it can be seen that this operation must be carried out between 0.9 and 1.1 pu voltage, although this is something that will have to be resolved once the draft is a definitive text. As an example, and as can be seen in Figure 3, the plant complies with the grid requirements at power lower than Pmax for v=1 pu.

Step 5: Conclusions and definition of corrective action

And what would have happened if the plant had not complied with the code requirements? In such a case, actions would have to be designed to enable it to comply, otherwise its network connection could be compromised or delayed.

In the case of this plant, if the non-compliance with the grid code was associated with reactive power, solutions such as adding a capacitor bank or a Statcom could have solved the problem. If, on the other hand, the non-compliance was associated with active power, the possible solutions would be to increase the number of inverters, increasing the P/Pmax ratio, increase the voltage at the inverter terminals by controlling the ratio of one of the transformers between the PCR and the inverter or, as a last resort, reduce the declared Pmax of the installation, thus limiting its production.

Undoubtedly, the choice of the optimal solution for this case will condition the final profitability of the project, as it may require significant investments or limit its production, and it is important to have the knowledge of network codes and simulation tools to ensure that we are going down the right path.

En Norvento creemos que la nueva normativa que recoge los requisitos de potencia reactiva en el P.O.12.2 va a influir de forma notable en el dimensionamiento de las nuevas plantas de generación renovable; promotores e ingenierías han de mantenerse vigilantes sobre el texto que finalmente se apruebe si no quieren sorpresas de última hora.

1. Generation and demand-side installations: Minimum requirements for design, equipment, operation, commissioning and safety P.O.12.2. Draft version October 2018.
2. Commission Regulation (EU) 2016/631 of 14 April 2016 establishing a grid code on grid connection requirements for generators.
3. Technical standard for monitoring the conformity of electricity generation modules under EU Regulation 2016/631. Working Draft. Version 4. December 2018.