Voltage Source Converter Independent Power Control
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Solution Overview
Problem
Conventional voltage source converters fail to maintain independent control of active and reactive power in challenging AC network conditions, such as weak or unbalanced AC networks, leading to loss of synchronisation and integrity of the dq reference frame.
Innovation Solution
A voltage source converter with a controller that independently determines active and reactive power exchanges using separate control blocks and a current reference generator based on positive and negative voltage sequence components in a stationary α-β reference frame, eliminating the need for phase locked loops and allowing operation in weak AC networks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a phase locked loop is used to maintain synchronisation in a dq reference frame, then synchronisation is maintained under normal AC network conditions, but the system fails under weak or unbalanced AC network conditions
Solution Approach 1:
The invention extracts and eliminates the phase locked loop component from the control system. By removing the PLL, the system no longer suffers from its limitations in weak or unbalanced network conditions. The control method directly processes voltage and current measurements in the dq reference frame without requiring phase angle estimation, thereby solving the reliability problem under challenging conditions.
Solution Approach 2:
The invention replaces the mechanical phase locked loop system with a direct control approach. Instead of using a PLL to estimate phase angles and maintain synchronisation, the system directly controls the dq reference frame quantities. This substitution eliminates the intermediate estimation step that fails under weak network conditions, providing more reliable and direct control.
2Ease of operation
If a dq reference frame with phase locked loop is used, then independent control of active and reactive power is achieved under normal conditions, but independence is lost under weak or unbalanced AC network conditions
Solution Approach 1:
The invention removes the phase locked loop that causes the loss of control independence. By eliminating the PLL's phase angle estimation mechanism, the system avoids the interaction between d and q components that occurs under weak or unbalanced conditions. The direct control of dq quantities maintains independence without requiring accurate phase synchronization.
Solution Approach 2:
The invention segments the control into independent active power and reactive power control channels. The active power control adjusts the d-component current reference, while the reactive power control adjusts the q-component current reference. This segmentation ensures that each control objective can be achieved independently without interference, even when the AC network is weak or unbalanced.
Data Source
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AI summary
In the field of high voltage direct current (HVDC) power transmission networks, a voltage source converter (10) comprises: first and second DC terminals (24, 26) for connection to a DC electrical network (16); a plurality of single-phase limbs (28), each single-phase limb (28) including a phase element (30), each phase element (30) including at least one switching element (32) configured to interconnect a DC voltage and an AC voltage, an AC side (34) of each phase element (30) being connectable to a respective phase (a, b, c) of a multi-phase AC electrical network (14), each single-phase limb (28) being connected between the first and second DC terminals (24, 26); and a controller (36) configured to determine independently of one another an amount of active power (Pref) that the voltage source converter (10) should exchange with the AC electrical network (14) and an amount of reactive power (Qref) that the voltage source converter (10) should exchange with the AC electrical network (14), the controller (36) being further configured to establish a respective phase current reference (IrefA, IrefB, IrefC) for each single-phase limb (28) which is independent of the or each other respective phase current reference (IrefA, IrefB, IrefC) and which defines the current (Ia, Ib, Ic) each single-phase limb (28) is required to draw from or pass to a corresponding phase (a, b, c) of the AC electrical network (14) to effect the determined active power (Pref) and reactive power (Qref) exchanges with the AC electrical network (14).