Parallel PCS Angle Synchronization for Seamless Grid Switching
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Solution Overview
Problem
During on/off-grid switching in energy storage systems, the angles of output voltages of power conversion systems (PCSs) tend to vary greatly, leading to a large cross current between PCSs, which affects the stability of power supply to the local load.
Innovation Solution
The energy storage system synchronizes the angles of output voltages of PCSs by adjusting them to a common reference angle obtained based on the angular frequency of the power grid voltage when the system is on-grid, ensuring that all PCSs switch to the same reference angle during on/off-grid switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If each PCS independently detects power grid voltage and independently performs on/off-grid switching, then the switching operation is simple and autonomous, but the angles of output voltages of multiple PCSs vary greatly causing large cross current
Solution Approach 1:
The patent applies equipotentiality by making all PCSs switch to the same reference angle during on/off-grid transition. The controller adjusts the output voltage angle of each PCS to a common reference angle, ensuring all PCSs operate at the same electrical potential reference point, thereby eliminating phase differences and cross currents while maintaining autonomous operation.
2Adaptability or versatility
If PCSs switch angles of output voltages to different values during on/off-grid switching, then each PCS can independently respond to grid exceptions, but phases of output voltages become different causing large cross current
Solution Approach 1:
The patent introduces a common reference angle as an intermediary that mediates between the independent response capability of each PCS and the need for synchronized operation. Each PCS independently detects grid exceptions and switches to this shared reference angle, allowing autonomous response while maintaining phase coherence and preventing cross current generation.
3Device complexity
If PCSs perform on/off-grid switching without communication, then the system complexity is reduced, but seamless switching cannot be implemented affecting power supply to load
Solution Approach 1:
The patent implements self-service by enabling each PCS to autonomously detect grid exceptions, independently calculate the common reference angle from local voltage measurements, and automatically switch its output voltage angle to match others. This self-sufficient approach achieves seamless switching without requiring complex communication infrastructure between PCSs.
Data Source
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AI summary
This application discloses an energy storage system, an on/off-grid switching method, and a power conversion system. The energy storage system includes at least two power conversion systems PCSs, output terminals of the at least two PCSs are connected in parallel to connect to an alternating current power grid, and input terminals of the at least two PCSs are connected to an energy storage power supply. A first PCS of the at least two PCSs includes a voltage detection circuit, a power conversion circuit, and a controller. The first PCS is any one of the at least two PCSs. The voltage detection circuit detects a power grid voltage at a parallel connection point of the output terminals of the at least two PCSs. When determining, based on the power grid voltage at the parallel connection point, that islanding occurs in the alternating current power grid, the controller adjusts an angle of an output voltage of the first PCS to a reference angle, so that angles of output voltages of the at least two PCSs are the same. The reference angle is obtained by the first PCS based on an angular frequency of the power grid voltage when the at least two PCSs are on-grid. During on/off-grid switching, angles of the output voltages of the PCSs are the same, thereby suppressing a cross current.