Abstract
Voltage source converter (VSC) based HVDC systems are one of the most promising technologies for high voltage bulk power transmission. The reliability and stability of a VSC-based HVDC system greatly depends on the design of a proper controller for the inner decoupled d-q current loop. One of the major causes of instability in a properly tuned controller is due to system parameter variation. This paper presents the design of a fixed parameter robust controller for the inner decoupled d-q current loop for a VSC-based HVDC system to deal with the uncertainties due to system parameter variations. The method of multiplicative uncertainty is employed in the robust design to model the variations in the system parameters. The robust control design was realized through a graphical procedure known as the loop-shaping technique. The graphical loop shaping technique is a much simpler and more straightforward method compared to the traditional H∞-based algorithms for robust controller design. The designed robust controller was experimentally verified using a real-time hardware in loop (HIL) system and was tested on a VSC HVDC system. The performance of the designed robust controller is compared to that of a traditional PI controller. It has been observed that a classical PI controller is effective for a given operating point, and its performance deteriorates when the operating point changes or when the system parameters change. The studies conducted using real-time hardware in the loop (HIL) system prove that the designed loop-shaping-based robust controller provides very good performance and stability for a wide range of system parameter variations, such as changes in resistance and the inductance of the VSC HVDC system compared to the PI controller tuned using conventional methods.
Highlights
High Voltage Direct Current (HVDC) transmission is used to transmit bulk power over long distances or to connect two asynchronous AC networks
Modern HVDC systems are based on voltage source converters (VSC) and use fast-acting Insulated Gate Bipolar Transistor (IGBT) switching technology
Important future application fields that are best met by using VSC-based HVDC systems are power transmission to oil or gas platforms from land, power transmission and distribution from offshore wind farms to land, and as an improved and upgraded power supply for megacities [3,4] In these applications, the converters must be able to stabilize an AC-grid of relatively low short circuit power through the fast and independent control of the active and reactive power flow [5]
Summary
High Voltage Direct Current (HVDC) transmission is used to transmit bulk power over long distances or to connect two asynchronous AC networks. Pradhan et al designed a multi-variable PI controller for a VSC-based HVDC transmission link [10] These methods are simple, but the controller operates best at the designed operating point and does not guarantee robust performance. The H∞-based robust control design approach is an appealing technique, as it addresses the problem of model uncertainty This method is mathematically too complex and involves non-linear modeling. The other attempts to design robust controllers for the VSC-based HVDC have been reported in the literature, but most of these approaches have been tested at a system level and not on the device level for tuning the inner decoupled d-q current loop of the VSC converters [16]. A robust controller design for VSC-based HVDC using the loop-shaping technique is presented.
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