A 36-bus equivalent network representing the National Electricity Transmission System of Great Britain was modelled in DIgSILENT PowerFactory. The entire system was split into 36 zones for which the relevant generation and demand were evaluated. Generators within each zone were categorised into fuel types and the demand was split into active and reactive demand. Line impedances were also calculated to represent the electrical distance between the zones.
The reduced 36 bus model is based on the (Electricity Ten Year Statements) ETYS 2013 model and the equivalent impedances of the lines between the zones have not been updated since 2013. The approach adopted to create this model was to initially segregate the transmission network into different regions. Each region was represented by a substation to which the relevant generators and loads were connected and thus all generators in the region are represented by an equivalent generator regardless of the electrical distance between them. Transmission lines were then created to represent the resulting impedances between the regions based on the ETYS 2013 model. The model can be configured for different study scenarios. It should be noted that the 36 bus model has limitations and may not reflect the current behaviour of the GB grid, due to the modelling assumptions made and the age of the data.
- Generator Parameters:
The MVA rating of each machine was calculated by assuming a power factor of 0.8. The parameters for each machine such as the synchronous reactance, transient and sub-transient reactance were derived from typical generator type data available in PowerFactory for a machine of similar fuel types. The ratings of transformers were chosen in line with the ratings of the generators.
- MW Dispatch:
The dispatch of generators has been left flexible to the user, depending on the type of study to be carried out.
The active power demand used in this model is the net demand after taking into account the contribution from embedded generation. The installed capacity of embedded generation is available for wind-based generators and other generators on an individual zone basis. The scaling factor assumed for embedded wind was 0.7 and that for the other embedded generators was around 0.5. The gross reactive power demand is used in the model, and this assumes that there is no reactive power contribution from embedded generation.
- Line Impedances:
The R, X and B parameters were obtained for all the branches and the equivalent impedance between zones were evaluated. The impedances are based on ETYS 2013 zone interconnections.
- AVR model:
The generic AVR model of type ‘avr_AVR’ was used for each synchronous generator connected to the network.
- Governor model:
The generic governor model of type ‘gov_GOV’ was used for each synchronous generator connected to the network.
- HVDC model:
The Line Commutated Converter (LCC) type HVDC has been included in the 36-bus network to represent interconnectors to the GB network.
- System Losses:
The system losses were evaluated to be around 2-3 % of the winter peak demand.
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