Thesis
Control and operation of Solid-State Transformer (SST) in distribution network
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2023
- Thesis identifier
- T16762
- Person Identifier (Local)
- 201761274
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Distribution network has been confronting with many challenges, such as reverse power flow, power quality issue and DC integration due to the increasing penetration of renewable energy. To tackle these issues, solid-state transformer (SST) has recently been considered as a promising solution for the future distribution network. Compared with the line-frequency transformer (LFT), SST can not only feature full controllability of voltage and power in the LV network, but also provide the potential DC connectivity at the middle stage and fault isolation capability between MV and LV networks. To ensure that SST operates under grid-forming and grid-connected operation without large control mode switch transient, a universal control scheme using a grid forming structure is proposed where SST can operate when the LV side operates either as an isolated network or grid connected to the adjacent network (through the conventional normal opened point (CNOP)). To address the coupling issue between active and reactive power with the grid forming control design when the SST LV side is grid connected, an additional compensation control using active and reactive power feedforward compensation to dq-axis current components of the DC/AC converter is proposed. The active and reactive power fluctuation due to the coupling is significantly reduced without affecting system response. Based on the control scheme above, SST-based distribution network can benefit distributed generation (DG) consumed locally. To avoid unnecessary converter blocking and increase the reliability of SST, the postfault performance of SST under grid-forming and grid-connected operation is compared with the LFT in different network configurations. The fault current limiting of SST reduces the fault current contribution, and SST can isolate fault current from faulty side to healthy side. In order to prevent SST from unnecessary blocking due to large MVDC voltage fluctuation caused by active power unbalance between MV and LV sides during the fault and improve postfault recovery, an active power limiting control of the SST is proposed where the MVDC voltage variation directly set the d-axis voltage (grid-forming operation) or the active power output reference (grid-connected operation) of the LV side converter. The MVDC voltage variation can be reduced within 10%, and SST can remain operative during fault, which achieve fast post-fault recovery after fault is cleared. Finally, considering the topology configuration of SST converter, the AC and DC networks can be co-ordinately operated via the SST. The impact of introducing DC network to SST-based distribution network on the operation and control of the SST is investigated. The types of DC network integration are reviewed and discussed. A coordinated control of the SST and the DC network is proposed for optimising the operation of the SST-based hybrid AC/DC distribution network. With linking MVDC voltage with power output of the DC network, the SST enables the DC network to provide the needed power to ensure the power supply to the load side autonomously. And the adaptive control scheme can regulate the power output of the connected ESD without communication. In addition, the DC fault characterisation and protection requirement of the proposed system are investigated. Fast SSCBs allow the immediate isolation of the faulty DC section without the need for converter (TAB) blocking, which in turn minimizing the disruption for the healthy networks. Meanwhile, the slower DCCBs, e.g., HCBs, will result in fault current contribution from the MV side for a short period, which can affect the operation of the healthy network. The effectiveness of SST control schemes, including basic control, grid forming control, additional compensation control, coordinated control are all validated in MATLAB/Simulink.
- Advisor / supervisor
- Xu, Lie
- Resource Type
- DOI
Relations
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PDF of thesis T16762 | 2023-11-06 | Public | Download |