Solid-State Transformer Control for Decoupled Energy Balancing
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Solution Overview
Problem
Conventional control architectures for solid state transformers (SSTs) face challenges in managing multiple state variables, leading to interference between converters, requiring complex voltage balancing and power sharing mechanisms, and necessitating large capacitors and filtering, which complicates the control system.
Innovation Solution
A fully decoupled control system for SSTs, comprising a stored energy controller, power flow controller, and energy balancing controllers, each with independent control objectives, operates based on energy and power flow hierarchies rather than physical paths, eliminating the need for voltage balancing and power sharing, and actively removing voltage ripples.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a conventional control architecture based on actual power path is used, then the control system can follow the physical structure, but interference between converters occurs and voltage balancing becomes complex
Solution Approach 1:
The control system is segmented into three independent controllers: stored energy controller for AC-to-DC stage, power flow controller for DC-to-AC stage, and energy balancing controller for DC-to-DC stage. Each controller manages specific state variables independently, eliminating the need for complex voltage balancing algorithms and current sharing mechanisms required in conventional coupled control architectures.
Solution Approach 2:
Energy and power flow hierarchies are introduced as intermediary control layers that decouple the control of different converter stages. Instead of directly controlling voltages and currents in a coupled manner, the system uses energy balance equations as intermediaries to coordinate the operation of AC-to-DC, DC-to-DC, and DC-to-AC converters independently.
2Stability of the object's composition
If voltage balancing algorithm and current sharing mechanism are implemented, then capacitor voltages can be balanced, but controller coupling increases system complexity
Solution Approach 1:
The control functions are segmented and assigned to separate controllers: stored energy controller manages total energy in capacitors, power flow controller manages power transfer, and energy balancing controller manages energy distribution in DC-to-DC converters. This segmentation eliminates the need for coupled voltage balancing algorithms and current sharing mechanisms, as each controller independently manages its designated function without interfering with others.
Solution Approach 2:
Each converter stage is equipped with its own dedicated controller that autonomously manages its operation based on energy balance principles. The AC-to-DC converter independently controls stored energy, the DC-to-DC converters independently balance energy, and the DC-to-AC converter independently controls power flow, eliminating the need for inter-controller communication and coordination mechanisms.
3Stability of the object's composition
If capacitor voltages are kept constant with conventional control, then voltage stability is maintained, but capacitor size increases due to second-order harmonic ripple
Solution Approach 1:
The control approach changes from directly regulating capacitor voltages to regulating energy balance and power flow. By controlling the stored energy in capacitors rather than maintaining constant voltages, the system naturally suppresses second-order harmonic ripples. The stored energy controller adjusts the AC-to-DC converter to maintain optimal energy levels, which inherently filters out voltage ripples without requiring larger capacitors.
Solution Approach 2:
The system replaces passive voltage filtering (which would require larger capacitors) with active energy-based control. Instead of relying on the physical size of capacitors to filter second-order harmonic ripples, the stored energy controller actively manages energy flow to prevent ripple propagation, substituting mechanical filtering with control system intelligence.
4Stability of the object's composition
If filtering mechanism is added to remove voltage ripple, then voltage quality improves, but system complexity and hardware cost increase
Solution Approach 1:
The system replaces passive hardware filtering mechanisms with active control-based ripple suppression. The stored energy controller and energy balancing controller work together to actively manage energy flow and prevent voltage ripple propagation through the system. This control-based approach eliminates the need for additional passive filter components, reducing both hardware complexity and cost while achieving superior voltage quality.
Solution Approach 2:
The control system inherently suppresses voltage ripples through its energy balance control mechanism without requiring separate filtering subsystems. The stored energy controller automatically adjusts power flow to prevent ripple generation, and the energy balancing controller distributes energy to maintain voltage stability, making the control system itself serve the filtering function.
Data Source
AI summary
A decoupled system for controlling a solid state transformer (SST), the SST comprising an AC-to-DC stage, a DC-to-AC stage, and a DC-to-DC stage, the DC-to-DC stage comprising one or more DC-to-DC converters. The system comprises a stored energy controller coupled to the AC-to-DC stage, the energy controller configured to control the total amount of stored energy within the capacitors of the SST; a power flow controller coupled to the DC-to-AC stage, the power flow controller configured to control power flow in the SST; and one or more energy balancing controllers each coupled to a corresponding DC-to-DC converter, each energy balancing controller configured to balance energy in the corresponding DC-to-DC converter.