High-voltage dc (HVDC) transmission offers several advantages over alternating current for long-distance power transmission and asynchronous interconnection between two ac systems, including the ability to precisely control the power flow without inadvertent loop flows that can occur in an interconnected ac system. HVDC transmission can be classified into one of three broad categories:
• Back-to-back systems
• Two-terminal, or point-to-point, systems
• Multi-terminal systems
In a back-to-back dc system, shown in Figure 1, both the rectifier and the inverter are located in the same station, usually in the same building. The rectifier and inverter are usually integrated with a reactor, which is generally an air-core design. A back-to-back dc system is used to tie two asynchronous ac systems (systems that are not in synchronism). The two ac systems can be of different operating frequencies, for example, one 50 Hz and the other 60 Hz. Back-to-back dc links are also used to interconnect two ac systems that are of the same frequency but are not operating in synchronism. A dc link offers a practical solution to interconnecting these non-synchronous networks.
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Figure 1 The back-to-back
system of dc transmission.
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Two-terminal dc systems can be either bipolar or mono-polar. The bipolar configuration, shown in Figure 2a, is the commonly used arrangement for systems with overhead lines. In this configuration, there are two conductors, one for each polarity (positive and negative) carrying nearly equal currents.
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Figure 2 Two-terminal dc
transmission systems: (a) bipolar; (b) monopolar ground return; (c) monopolar
metallic
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Only the difference of these currents, which is usually small, flows through the ground return. A mono-polar system has one conductor, either of positive or negative polarity, with current returning through either ground or another metallic return conductor. The mono-polar ground return current configuration, shown in Figure 2b, has been used for undersea cable systems, where current returns through the sea. This configuration can also be used for short-term emergency operation for a two-terminal dc line system in the event of a pole outage. However, concerns about corrosion of underground metallic structures and interference with telephone and other utilities restrict the duration of such operation. The total ampere-hour operation per year is usually the restricting criterion. In a mono-polar metallic return system, shown in Figure 2c, return current flows through a conductor, thus avoiding the problems associated with ground return current. This method is generally used as a contingency mode of operation for a normal bipolar transmission system in the event of a partial converter (one-pole equipment) outage. In the case of outage of a one-pole converter, the conductor of the affected pole will be used as the return current conductor. A metallic return transfer breaker is opened, diverting the return current from the ground path and into the pole conductor. This conductor is grounded at one end and insulated at the other end. This system can transmit half the power of the normal bipolar system capacity, and can be increased if overload capacity is available. However, the percentage of losses will be doubled compared to the normal bipolar operation.
There are two basic configurations in which dc systems can be operated as multi-terminal systems:
• Parallel configuration
• Series configuration
The parallel configuration can be either radial-connected (Figure 3) or mesh-connected (Figure 2b). In a parallel connected multi-terminal dc system, all converters operate at the same nominal dc voltage, similar to ac system interconnections. In this mode of operation, one converter determines the operating voltage, and all other terminals operate in a current-controlling mode.
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| Figure 3 Multiterminal dc transmission systems: (a) parallel-connected radial; (b) parallel connected mesh-type. |
In a series-connected multi-terminal dc system, shown in Figure 4, all converters operate at the same current. One converter sets the current that will be common to all converters in the system. Except for the converter that sets the current, the remaining converters operate in a voltage control mode (constant firing angle or constant extinction angle). The converters operate almost independently without the requirement for high-speed communication between them. The power output of a noncurrent-controlling converter is varied by changing its voltage. At all times, the sum of the voltages across the rectifier stations must be larger than the sum of voltages across the inverter stations. Disadvantages of a series-connected system include the following:
• Reduced efficiency because full line insulation is not used at all times.
• Operation at higher firing angles leads to high converter losses and higher reactive power requirements from the ac system.
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| Figure 4 Series connected multi-terminal dc system. |
