Voltage Source Converter Control for Harmonic-Limited AC Demand
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Solution Overview
Problem
Maintaining AC phase voltage demands within predetermined limits is challenging, especially under extreme conditions, which can lead to unwanted conduction and harmonic generation, and unbalanced energy storage in voltage source converters.
Innovation Solution
A controller is employed to establish first and second proportion factors based on fundamental and harmonic voltage contributions, using an iterative process to accurately assess and limit the AC phase voltage demand, incorporating elliptic limiting to ensure compliance with predetermined limits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If harmonic voltage injection is used to eliminate unbalanced currents, then power quality is improved, but AC phase voltage demand may exceed predetermined limits causing unwanted conduction and harmonics
Solution Approach 1:
The controller performs preliminary assessment of the anticipated maximum resulting AC voltage demand by estimating an angle and determining proportion factors before the voltage demand is actually established. This allows the controller to predict and prevent voltage limit violations before they occur, rather than reacting after the fact.
Solution Approach 2:
The controller uses feedback by continuously monitoring the AC phase voltage reference and harmonic voltage injection, estimating their combined maximum impact, and adjusting the fundamental AC phase voltage reference magnitude accordingly to maintain the total voltage demand within predetermined limits.
2Object-affected harmful factors
If the fundamental AC phase voltage reference magnitude is limited to maintain voltage within limits, then conduction and harmonics are prevented, but accurate assessment of fundamental and harmonic contributions is required
Solution Approach 1:
The controller replaces direct measurement of the maximum resulting AC voltage demand with an estimation algorithm that calculates the anticipated maximum by combining the fundamental AC phase voltage reference and harmonic voltage injection using determined proportion factors. This computational approach substitutes complex direct measurement with a more feasible calculation method.
3Measurement precision
If an iterative process is used to establish the estimated angle, then accuracy of voltage demand assessment is improved, but processing burden on the controller increases
Solution Approach 1:
The controller employs an iterative process to establish the estimated angle, performing calculations beyond what would be strictly necessary for a simple solution. This partial excessive action ensures high accuracy in determining the maximum resulting AC voltage demand, with the iteration continuing until acceptable precision is achieved.
Data Source
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Figure 3A
AI summary
In the field of HVDC power transmission networks there is a need for an improved voltage source converter. A voltage source converter (10), for transferring power between AC and DC networks, comprises first and second DC terminals (12, 14) which are for connection to a DC network. The first and second DC terminals (12, 14) have a plurality of converter limbs (16A, 16B, 16C) extending therebetween, and each converter limb (16A, 16B, 16C) includes first and second limb portions (18A, 18B, 18C, 20A, 20B, 20C) that are separated by a corresponding AC terminal (22A, 22B, 22C), which is for connection to a respective phase (A, B, C) of an AC network. Each limb portion (18A, 18B, 18C, 20A, 20B, 20C) includes a chain-link converter (24) that extends between the associated AC terminal (22A, 22B, 22C) and a corresponding one of the first of the second DC terminals (12, 14). Each chain-link converter (24) includes a plurality of series connected chain-link modules (26), each of which has a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (24) is controllable to provide a stepped variable voltage source. The voltage source converter (10) also includes a controller (28) that is programmed to establish for each AC terminal (22A, 22B, 22C) an AC phase voltage demand vconv* that the chain-link converters (24) associated therewith are required to provide at the said corresponding AC terminal (22A, 22B, 22C). The controller (28) establishes each AC voltage demand vconv* by adding a harmonic voltage injection to a received fundamental AC phase voltage reference vconvdqpnunlimited*, and the controller (28) maintains each established AC phase voltage demand vconv* within a predetermined limit (lim) by limiting the magnitude (|vlim|) of the fundamental AC phase voltage reference vconvdqpnunlimited* based on a contribution each of the fundamental AC phase voltage reference vconvdqpnunlimited* and the harmonic voltage injection is expected to make to the resulting AC voltage demand vconv*.