The Norma Técnica de Supervisión (NTS) [1] approved on 3 November 2020 develops the technical requirements, previously established by Reglamento UE 2016/631 [2], that new generation plants to be connected to the electricity grid must comply with.
Requirement 5.7, which assesses the reactive compensation capability at and below maximum capacity, is of particular importance during the initial design phases of the generation facility. Its non-compliance could result in a derating of the maximum capacity declared by the plant (PMAX, maximum active power that a Power Generation Module can inject into the grid) or in the need to include reactive power compensation equipment such as capacitor banks or STATCOMs, or other changes in the plant design.
In the case of facilities shared by several generation plants, given the difficulties that may arise in some cases to know the details of the shared facilities up to the Point of Connection to the Grid (PCR), the NTS contemplates two methods to assess this requirement: a complete modelling (assessing the requirement at the PCR) or an alternative modelling (assessing the requirement at the Central Busbars, BC). In both cases, dummy generators are used to simulate the effect of the other MGEs sharing facilities up to the PCR.
In the case of the alternative modelling, the required range of reactive capacity in BC is larger to compensate for the non-modelled effect of transformers and lines up to the PCR, as the final objective is to ensure compliance at the PCR, making a generic approximation of the reactive consumptions of the feeder installations.
However, in this post we analyse through two examples that, from the point of view of the installation owner, it is very convenient to analyse for each case which type of modelling can be more favourable, as the results can vary quite significantly.
The NTS distinguishes between two types of cases for shared installations depending on where BC is located. The example in this post corresponds to type A as BC is located on the high side of the step-up transformer of the MGE (Figure 1).
Figure 1: Example diagram showing the location of BC (Case A and Case B) [1].
Example 1 : Shared evacuation line
In this example, the generation plant consists of 6 inverters (21.78 MW of installed power) with their respective UGE step-up transformers (30/0.66 kV), an MGE step-up transformer (400/30 kV) with on-load tap-changer (OLTC) and the evacuation line, which is shared by another identical plant shown in blue (Figure 2). The access power at PCR (PMAX) is 21 MW.
Figure 2: Single-line diagram of the system under evaluation (Example 1)
As explained above, two different modelling options are available in this case (Figure 3 and Figure 4) and different requirements apply to each: in the case of the alternative modelling up to BC, the required reactive power range is wider to compensate for the effect of the elements not modelled up to the RCP.
Figure 3 : Complete modelling for the system in Example 1
Figure 4 : Alternative modelling for the system in Example 1
To simulate the whole range of reactive power requirement of the plant in the PCR (or in BC) specified in [1], power flows are calculated by performing a sweep through all the levels of active power generation between 0 and PMAX both for the nominal voltage and for the minimum and maximum values in the PCR (0.95 and 1.05 p.u.). Likewise, another sweep is performed over the range of admissible voltages in the PCR at the maximum power of the plant. At all load flows, the reactive power output of the inverter is the maximum possible according to its working QP curves and the transformer taps are placed in the optimum position to favour the maximum reactive capacity while respecting the voltage limits at all the plant’s busbars. Thus, the inability of the plant to operate at a level of reactive power generation/consumption equal to or higher than that delimited by the requirement curves denotes non-compliance.
First of all, the results without reactive power compensation equipment for the target PMAX of 21MW are shown (Table 1). It can be seen from the graphs that the plant would not be able to certify compliance with the requirement as several operating points are within the area bounded by the requirements.
Table 1 : Example 1: Assessment of requirement 5.7 for full modelling and alternative modelling (derating=0%)
Next, the minimum PMAX derating is calculated in each case in order for the plant to comply with the applicable requirements. As can be seen in the results presented in Table 2, this value is higher in the case of the alternative modelling, as in this case the greater range required in BC penalises more than the non-modelled reactive consumption of the elements up to the PCR.
Table 2 : Example 1: Assessment of requirement 5.7 for full modelling and alternative modelling
Example 2 : Shared evacuation line and step-up transformer
In this case, the generation plant is the same as in Example 1 up to the collector network, but is connected via a step-up transformer with OLTC from MGE (132/30 kV) to a 400/132 kV transformer and the evacuation line, the latter two elements being shared by another identical plant shown in blue (Figure 5).
Figure 5 : One-line diagram of the system under evaluation (Example 2)
As in Example 1, a complete modelling (Figure 6) or an alternative modelling (identical to Example 1, Figure 4) can be chosen. Due to the presence of the shared step-up transformer, it is necessary to include a dummy MGE on the low side of the step-up transformer with a power rating equal to 
(Example II of [1]).
Figure 6 : Complete modelling for the system in Example 2
Again, the results are shown for the case where no derating is applied to the PMAX of the plant (Table 3) and, as in Example 1, with both models the requirements are not met.
Table 3 : Example 2: Assessment of requirement 5.7 for full modelling and alternative modelling (derating=0%)
In this case, when calculating the minimum derating (Table 4) it is observed that the alternative modelling is substantially lower than the full modelling, as it is not necessary to consider the 400/132 kV step-up transformer in the analysis.
Table 4 : Example 2: Assessment of requirement 5.7 for full modelling and alternative modelling
Reactive power compensation equipment
Finally, the necessary capacities of reactive power compensation equipment are calculated in order to avoid plant derating, while complying with the conditions imposed by requirement 5.7. Such equipment, installed in BC, must have adequate dynamic control to comply with requirement 5.8 for reactive power control modes [1]:
It can be seen that, depending on the connection scheme up to the PCR, one modelling or the other will be more demanding.
Conclusions
The example presented in this post shows the importance of deciding where to assess NTS, requirement 5.7, either in the PCR through full modelling or in BC through alternative modelling. In both cases different results can be obtained with economic consequences for the MGE project, either through loss of revenue by reducing PMAX or through the cost of installing larger reactive power compensation equipment.
Norvento’s Grid Studies team, among other engineering studies for the design of renewable generation plants, offers technical advice to our clients in the search for the most cost-effective solution to meet the requirements of the Grid Codes from the initial design phase.
References
- Norma Técnica de Supervisión de la Conformidad de los Módulos de Generación de Electricidad según el Reglamento UE 2016/631. Versión 2.0. 3 de noviembre de 2020.
- Reglamento (UE) 2016/631 de la Comisión, de 14 de abril de 2016, que establece un código de red sobre requisitos de conexión de generadores a la red.
Daniel Álvaro
Daniel is an industrial engineer specialising in electrical systems analysis and regulation. He is responsible for Norvento’s Grid Studies department, which develops all types of consultancy studies related to the integration of renewables into the grid. Contacta con Daniel