Solid State Transformer Voltage Balancing Control
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Solution Overview
Problem
Conventional Solid State Transformer (SST) systems face challenges in balancing High Voltage (HV) side DC bus capacitor voltage, particularly at start-up and light load conditions, due to imperfect power switching operations and circuit parameter variations, leading to unbalanced voltage and potential overvoltage issues.
Innovation Solution
A dual voltage and current loop control method is implemented, which includes dividing the dual active bridge (DAB) power reference by the Low Voltage (LV) side DC link voltage to determine module-specific current references, and adjusting these references based on capacitor voltage deviations to minimize HV side DC bus voltage unbalance, while also linearizing the DAB feedback control using non-linear compensation functions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional SST systems operate at start-up and light load conditions, then the system can power on and maintain basic function, but the HV side DC bus capacitor voltage becomes unbalanced due to imperfect power switching operations and circuit parameter variations
Solution Approach 1:
The patent implements a voltage balancing control method that is specifically activated during start-up and light load conditions to preemptively correct voltage imbalances before they lead to overvoltage issues. The control method calculates voltage deviations of individual DC bus capacitors from the average voltage and adjusts the output voltage of H-bridge modules accordingly, ensuring voltage balance is maintained from the beginning of operation and during transitions.
Solution Approach 2:
The patent employs a feedback control mechanism where the actual voltage deviations of DC bus capacitors are continuously monitored and fed back to the control system. The control method uses these feedback signals to dynamically adjust the output voltage of each H-bridge module, creating a closed-loop system that automatically corrects voltage imbalances without requiring manual intervention or complex circuit modifications.
2Reliability
If chopper discharging circuit is used for capacitor over voltage protection, then overvoltage protection is provided, but the chopper circuit should not be activated in normal power regulation and requires additional circuit complexity
Solution Approach 1:
The patent implements a voltage balancing control method that enables the SST system to self-regulate and maintain voltage balance within normal operating ranges during start-up and light load conditions. By preventing voltage deviations before they reach overvoltage thresholds, the system eliminates the need to activate chopper discharging circuits, allowing the protection circuit to remain in standby without affecting normal power regulation operation.
3Reliability
If module level voltage balancing control is implemented, then DC bus voltage variation is minimized, but reactive current requirements increase
Solution Approach 1:
The patent implements a voltage balancing control method that dynamically adjusts the output voltage of H-bridge modules based on real-time voltage deviation measurements. By changing the voltage parameter adaptively rather than using fixed compensation values, the system achieves effective voltage balancing while minimizing unnecessary reactive current flow. The control method calculates optimal voltage adjustments based on actual voltage deviations, ensuring reactive current is only used when necessary to correct imbalances.
Data Source
AI summary
This invention relates to a method of controlling a Solid State Transformer (SST). The method comprises dividing a dual active bridge (DAB) power reference on a Low Voltage (LV) side Direct Current (DC) link voltage to obtain a total DAB reference current; dividing the total DAB current reference by N number of DAB modules to obtain a reference current for each DAB module; in response to SST being in a normal power regulation, determining a deviation of a capacitor voltage at the HV side of each of a k DAB modules with reference from an average voltage from the HV DC bus; determining a current adjustment signal for each of the k DAB modules based on the total DAB reference current and a corresponding deviation of the capacitor voltage of each of the k DAB modules; adding a saturated current adjustment signal together with the reference current for each of the k DAB modules to obtain a total current reference for each of the k DAB modules; and subtracting a sum of the total current reference of each of the k DAB modules from the total DAB reference current to determine a total current reference for an Nth DAB module.