During ED2 SPEN intend to remove 79 legacy UG secondary substations as part of modernisation work. The current proposed baseline solution consists of removing the existing equipment, relocating to the nearest convenient land, and replacing with a standard over ground enclosure.
This project aims to eliminate the identified risk of relocation site suitability/availability. Studies will be carried out to assess the feasibility of combining an innovative transformer design with power electronics.
Innovating the transformer design allows installation at the existing location resulting in potential land cost savings, reduced excavation disruptions/CO2 emissions and protection of green space.
Adoption of power electronics could provide a greater capacity to accommodate new LCTs, a more efficient network through phase/power balancing reducing costs for customers.
Benefits
The project is expecting to generate learning that can be disseminated to all relevant network licensees in the area of underground substation management and best practice in asset management. This learning will be shared with the GB licensees via conferences and best practice fora.
Further benefits would be found from the CBA that this research will generate.
The findings from this research could also be used as an alternative to standard secondary substations
Learnings
Outcomes
This project has investigated the hybrid transformer solutions with integrated power electronics devices to replace the traditional transformers for an underground substation. The proposed Hybrid Transformer solutions consist of slim Low Frequency Transformer (LFT) with integrated PEDs. Based on the Stage 1 outcomes of this project, Stage 2 report focused on developing the practical requirements of the proposed Hybrid Transformer, including protection design and optimised dimension estimation.
The protection requirements for the Hybrid Transformers were overviewed under various fault scenarios. For external grid faults, the PED in the Hybrid Transformer needs to be protected from damage. For internal faults of the PED, failsafe operation of the PED is required so that the functionality of the LFT is not disrupted. Future work on protection is needed to intrinsically guarantee the fail-safe operation of the hybrid transformers in the case of failures of the proposed protection mechanisms for the solution to be applied to the network.
The solutions which were identified and assessed during the project did not provide the final answer for our challenges due to identified safety and operational issues. Therefore, we were not able to update our policy documents or CBA accordingly to provide clear guidance to design and operation engineers.
Lessons Learnt
Following our research we have identified the following challenges which would to be considered and addressed before proposing future projects that target underground substation replacement:
The application of compact transformers (in their existing design) in close proximity to public paths may introduce safety risks to the public, as high voltage terminals may be exposed as a result of vandalism or accidental damage. Also, there is a possibility of transformer oil pressure overflow to the public space. Those key safety issues would need to be addressed (by redesign, careful site selection, or other protective measures) in similar future projects. The concept of a sidewalk transformer with a small capacity can be promising to compensate the supply capacity lost when decommissioning an underground transformer.
LV STATCOMs can be an effective solution for overhead line circuits; however, our power system studies on the LV network and engagements with a number of potential suppliers suggested that STATCOMs may not be suitable for voltage regulations in cable LV circuits.
Considering the growing limitations for securing space for new substations, especially in city centres or close to built-up area where load is concentrated, we believe there is a value in developing a new underground substation/transformer design. The new design needs to specifically address the impact of environment (humidity, etc.) and safe access for operation and maintenance.
