INVESTIGATIONS ON THE DEVELOPMENT OF NEWTON POWER FLOW MODELS OF HYBRID AC-DC SYSTEMS

Abstract

For the past few decades, the construction of generation facilities and new transmission lines have been delayed in light of rising energy cost, environmental concerns, rights-of-way (RoW) restrictions and other legislative and cost problems. In addition, system stability issues may render long distance AC transmission infeasible. In this respect, High-Voltage DC (HVDC) transmission requires a smaller RoW, simpler and cheaper transmission towers, reduced conductor and insulator costs, reduced losses and is not limited by stability considerations. A HVDC link can augment system reliability by interconnecting two asynchronous AC grids and can integrate offshore wind farms with onshore AC grids. The first commercial application of HVDC transmission took place between the Swedish mainland and the island of Gotland in 1954, using mercury arc valves. Subsequently, the first 320 MW thyristor based HVDC system was commissioned in 1972 between the Canadian provinces of New Brunswick and Quebec. Continuous development in conversion equipment led to reduced size and cost which resulted in more widespread use of HVDC transmission. The thyristor based line commutated converter (LCC) based HVDC (LCC-HVDC) technology now constitutes the bulk of the installed HVDC transmission corridors over the world. With LCC-HVDC, for controlling the active power, both the rectification and inversion processes consume reactive power. This necessitates the use of reactive power sources to match the reactive power demand at both ends. To reduce the effects of harmonic voltages and currents generated by the converters, harmonic filters are used on both the AC and the DC sides. Also, a minimum short circuit level is required to avoid voltage instability. However, despite its limitations, LCC-HVDC possesses xxviii high reliability, good overload capability and lower converter losses. It requires low maintenance and capital cost and is robust to DC fault currents due to its current regulating nature. Subsequently, the development of the Insulated Gate Bipolar Transistor (IGBT) paved the way for the Voltage Sourced Converter (VSC) based HVDC (VSCHVDC) technology, which offered significant advantages over the LCC-HVDC. VSC-HVDC facilitates independent active and reactive power control, along with reduction in filter size [8] - [18]. VSC-HVDC also enables the integration of offshore wind farms with AC grids. Compact, modular designs of the VSCs enable rapid installation, commissioning and relocation. Unlike LCC-HVDC, fixed DC voltage polarity in the VSC-HVDC enables the use of stronger and lighter XLPE cables, suitable for under-sea environment and attractive for offshore transmission. In addition, VSC-HVDC systems can be integrated with AC systems having low short circuit ratios. The first 3-MW, VSC-HVDC link was commissioned at Hellsjon in Sweden in 1997. Subsequently, rapid development in the VSC technology has now resulted in the availability of higher rated (up to 2000 MW) VSC-HVDC links. This has resulted in the installation and commissioning of a large number of VSC-HVDC systems worldwide. Now, in both LCC-HVDC and VSC-HVDC systems, the converter stations can be connected in two ways - back-to-back (BTB) and point-to-point (PTP). Most of the MTDC systems installed worldwide are in PTP configurations, their DC sides being interconnected through DC links or cables. xxix Unlike a two-terminal HVDC interconnection, a multi-terminal HVDC (MTDC) system is more versatile and better capable of utilizing the economic and technical advantages of HVDC technology. Moreover, sources of renewable energy can be easily integrated with a MTDC system, as and when the need arises. For proper MTDC operation, DC voltage control is an essential requirement. In this respect, several control techniques have been envisaged. These include DC slack bus control (also known as DC master-slave control), distributed DC voltage droop control, power synchronization control, hierarchical power control and transient management control. However, among all the DC voltage control techniques, the DC slack bus control and distributed DC voltage droop control have been the more popular and widely employed ones. In DC slack bus control, the voltage of one DC terminal, known as the DC slack bus, is maintained constant by the master converter. The main disadvantage of this control scheme is the DC grid instability following a failure of the master converter. The above problem can be tackled by ensuring that individual converters contribute to the DC voltage regulation scheme by adjusting their active power flow in response to changes in the DC voltage with the operating point, known as DC voltage droop control. For MTDC control, both linear and nonlinear types of DC voltage droop characteristics have been envisaged to ensure proper sharing based on the converter ratings. Voltage-Power (V-P), Voltage-Current (V-I), Voltage Margin (VM), V-P droop with power Dead-Band (DB) and V-P droop with voltage limits are some of the more widely used characteristics. xxx To manage power-flows within the DC grids, DC power-flow control devices have been conceptualized and developed. They include the use of DC transformers, variable resistors, current flow controllers (CFCs), thyristor power flow controllers (TPFCs), DC series voltage sources and Interline DC Power Flow Controllers (IDCPFCs) for power-flow control in meshed DC grids. The IDCPFC is a DC powerflow controller without an external AC or DC source and is used for power-flow management of DC grids, similar to its AC counterpart - the flexible AC transmission systems (FACTS) based Interline Power Flow Controller (IPFC). Now, for proper planning, design and operation of AC power systems integrated with multi-terminal DC grids, the development of suitable power-flow models of both LCC and VSC based hybrid AC-DC systems is a fundamental requirement. Because of the need of suitable power-flow models of both LCC and VSC based hybrid AC-DC systems and the adoption of the Newton-Raphson algorithm as the de-facto standard for industrial power-flow solutions, a lot of attention is being paid towards the development of Newton-Raphson power-flow models of such hybrid AC-DC systems. The development of Newton-Raphson power-flow models of both LCC and VSC based integrated AC–DC systems has resulted in two distinctly different approaches known as the unified and the sequential Newton algorithms, respectively. In the former, the AC and the DC quantities are solved simultaneously, while in the latter, the AC and the DC systems are solved separately in each iteration. Unlike the unified method, the sequential method is easier to implement and poses lesser computational burden due to the smaller size of the Jacobian matrix. Many xxxi comprehensive research works have been carried out for the development of unified and sequential Newton power-flow models of both LCC and VSC based hybrid ACDC systems.