Anti-Parallel HVDC Converters for Reactive Power Control
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Solution Overview
Problem
Line-commutated converters used in high-voltage direct current (HVDC) transmission systems face limitations in independent control of reactive and active power, making them inefficient for reactive power compensation without altering active power flow, and they require complex and costly equipment to manage AC voltage changes.
Innovation Solution
A method and converter station design where two line-commutated converters are connected in an anti-parallel circuit to the same pole of the HVDC link, allowing one to operate as a rectifier and the other as an inverter, with active power specifications controlling reactive power exchange without affecting overall active power flow, enabling independent reactive power control similar to self-commutated converters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If line-commutated converters are used for HVDC transmission, then active power transmission is achieved, but independent reactive power control is not possible without changing active power flow
Solution Approach 1:
The converter station is divided into two separate line-commutated converters operating in anti-parallel, where each converter can independently control reactive power while maintaining the same active power flow, thus achieving independent reactive power control without requiring a single complex converter design
Solution Approach 2:
The anti-parallel converter configuration allows the same equipment to perform multiple functions: both active power transmission and independent reactive power compensation, eliminating the need for separate equipment and reducing overall system complexity
2Ease of operation
If reactive power exchange is changed by modulating firing angle, then reactive power control is achieved, but AC voltage and DC voltage change causing complex equipment design
Solution Approach 1:
By separating the reactive power control function to a dedicated converter operating in anti-parallel, the firing angle modulation affects only that converter's reactive power output while the other converter maintains stable voltage, avoiding the need for complex voltage compensation equipment
Solution Approach 2:
The anti-parallel converter acts as an intermediary that handles reactive power compensation independently, allowing the main HVDC transmission system to operate with stable voltage levels without requiring complex voltage regulation equipment
3Ease of operation
If transformer secondary voltage is adjusted to compensate for voltage changes, then reactive power control is possible, but transient response is slow and complex equipment is required
Solution Approach 1:
The reactive power compensation function is separated into a dedicated converter that can respond instantly through firing angle control, providing fast transient response without relying on slow mechanical transformer tap changers
Solution Approach 2:
The mechanical transformer tap changer system is replaced with electronic firing angle control in the anti-parallel converter, enabling instantaneous reactive power response without mechanical movement delays
Data Source
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AI summary
The invention relates to a converter station (1) having two line-commutated converters (4, 5) for energy transmission via a bipolar high voltage direct current transmission line (30), and to a method for operating the converter station (1). The two converters (4, 5) are electrically connected in an anti-parallel circuit to the same pole (21, 23) of the high-voltage direct current transmission line (30). One of the converters (4, 5) is operated as a rectifier in an AC network (27) and the other converter (4, 5) is operated as an inverter in the AC network (27). A station reactive power (Q1) exchanged by the converter station (1) with the AC network (27) is controlled by active power specifications for converter active powers (P11, P12) which are exchanged between the converters (4, 5) and the AC network (27).