Voltage Source Converter Delta-Connection Voltage Balancing
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Solution Overview
Problem
Existing three-phase electric power networks face challenges in maintaining balance of total direct voltage across phase legs due to faults or disturbances, which can cause inefficiencies and disturbances if not properly compensated.
Innovation Solution
A control unit calculates and adds a zero-sequence current in the delta-connection circuit of a Voltage Source Converter to balance the total direct voltage across phase legs without additional current exchange with the network, using a defined differential mode vector to determine the necessary current reference for semiconductor device control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Voltage Source Converter is used to exchange power with the three-phase electric power network, then reactive power compensation and network stabilization are improved, but unbalance of total direct voltage across phase legs occurs due to faults or disturbances
Solution Approach 1:
The control unit continuously monitors the total direct voltage across each phase leg and automatically adjusts the zero-sequence current reference based on detected unbalance, creating a closed-loop feedback system that restores voltage balance without manual intervention
Solution Approach 2:
A zero-sequence current component is introduced as an intermediary control variable that circulates within the delta-connected converter to balance the total direct voltage across phase legs without exchanging current with the external power network
2Stability of the object's composition
If zero-sequence current is added to balance phase leg voltages, then voltage balance is restored, but additional current exchange with the network may occur causing disturbances
Solution Approach 1:
The zero-sequence current is confined to circulate locally within the delta-connected converter phases rather than being exchanged with the external power network, achieving voltage balance without propagating disturbances to the network
Solution Approach 2:
The voltage balancing function is extracted from the network interface and implemented independently within the converter's internal delta-connection, allowing balance restoration without affecting the external power network
Data Source
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AI summary
An arrangement for exchanging power with a three-phase electric power network comprises a Voltage Source Converter (5) having three phase legs (A-C) with each a series connection of switching cells. The three phase legs are interconnected by forming a delta-connection. The arrangement also comprises a control unit (19) configured to calculate a value for amplitude and phase position for a zero-sequence current for which, when circulated in the delta-connection circuit of said three phase legs, the balance of the total direct voltage of each of said three phase legs (A-C) with respect to the other two phase legs is restored will there be an unbalance and control the semiconductor devices of switching cells of the phase legs to add such a zero-sequence current to the currents of each phase leg of the converter.