Learnings

Outcomes

2023/2024

The key outcomes of the project are: 

Dynamic models for voltage control systems that mimic a variety of substation layouts including supergrid transformers, on-load tap changers (OLTCs), 13/33 kV transformers, and other pre-existing tertiary equipment (reactors and capacitors) were developed. 

·       Initial voltage simulation results confirmed that device performance aligns with expectations, which will be further validated with substation monitoring data. 

·       A dynamic transformer thermal model that considers three windings, dual-temperature cooling modes and tap changers was developed. 

·       Initial thermal modelling results showed that tertiary winding temperature indicator (WTI) will dominate the transformer cooling control when BESS is in active use. 

 
2024/2025

The key outcomes are as follows:

·       Quasi-dynamic simulations provide inaccurate quantification of control performance. RMS simulations are necessary to capture dynamics associated with OLTC control.

·       The behaviour of OLTC tapping of parallel SGTs is significantly influenced by the connection of tertiary connected equipment. The presence of tertiary connected reactors can create a permanent circulating current, leading to constant SGT tap stagger of two taps (which is the maximum permitted). This can then mask the variations in circulating current caused by tertiary connected BESS, resulting in no effective change in SGT tap operations caused by the BESS. Conversely, where the reactor is not connected, the BESS injections can lead to significant additional SGT tapping due to variations in circulating current. 

·       A coupled multi-component transformer CFD model has been developed, which can investigate thermal performance of multiple windings with different cooling modes and at unbalanced loading levels.

·       Variation of loading levels on the main windings have noticeable effects on the thermal profile of tertiary winding. The Winding Temperature Indicator (WTI) setting of the tertiary winding may need to account for this effect.      

Recommendations for further work

No recommendations for further work at this stage.

Lessons Learnt

2024/2025

Learnings are as follows: 

• Network model reduction could result in some unusual model artefacts which make model manipulation difficult and may limit scope of studies. It is important to consider this at the start of future projects.
• A complete 3D transformer CFD model considering multiple windings and oil tank is not practical due to the huge demand of computational resource. A hybrid model (e.g. combined 3D and 2D) is realistic and should be considered in future projects if needed.  
   

Dissemination
2024/2025

• Presentation at the CIGRE UK SC A2-D1 Annual Laision meeting on ”Development of Thermal Models for Grid Transformers with Tertiary Windings Connected to BESS”. The University of Manchester, 28th November 2024.  
• Presentation at the CIGRE UK SC-A2 & NGN Workshop on “Optimal Control Strategies for Parallel Operation of Supergrid Transformers”. The University of Manchester, 7th March 2025.