Configuration of LCC HVDC systems

HVDC transmission systems may be configured in a number of ways to provide flexi- bility, meet operational requirements, and also to ensure that overall an economically prudent system is achieved. Five main configurations of HVDC transmission systems, available in the literature [19, 23, 18, 24, 25] are discussed below. These configurations are mainly used for LCC HVDC systems but may also be considered for VSC-HVDC systems.

I The Monopolar HVDC Link

In the monopolar link configuration shown in Figure 2.2, the two HVDC con- verter stations are joined by a single conductor line. The earth or sea may be used as the path for the return current. This requires two electrodes capable of carrying the full transmission current. Because of corrosion and magnetic in- terference problems associated with the ground or sea return, a metallic return path may be used. The system can be operated with either a positive or nega- tive polarity. The Caprivi Link Interconnector is an example of a monopolar DC circuit providing an asynchronous link among Namibia, Zambia, and Mozam- bique and also improves power transfer capability in the Southern Africa Power Pool (SAPP) [26].

II The Bipolar HVDC Link

The Bipolar HVDC link configuration, illustrated in Figure 2.3, uses two con- ductors for the positive and negative poles. Each monopolar side can be oper- ated independently if the neutral point at both converter stations are grounded.

2.2. LCC HVDC TRANSMISSION 18

This increases the availability and the power transfer capability of the trans- mission system as one monopolar link can be operated when the other link is temporally out of service. Under normal operation, the current in both links are theoretically equal and cancel out. Therefore, the ground current return path carries no current. This link configuration is the most widely used HVDC link configuration. The Rihand-Delhi ± 500kV (DC) HVDC transmission link uses bipolar link configuration to transmit power from the thermal power station in Uttar Pradesh state to the northern parts of India [14].

A variant of the bipolar HVDC link configuration is the homopolar configuration where both links of the HVDC circuits are operated with the same polarity, often negative. This mode of operation reduces the insulation requirement. However, the earth electrodes carry the total current in the two monopole systems [19, 27]. III Back-to-Back HVDC system

The Back-to-Back HVDC transmission system is the most common configura- tion for connecting adjacent asynchronous AC systems. In this configuration, the two converters are located on the same site, therefore no transmission line or cable is involved. Figure 2.4 is an illustration of a back-to-back HVDC transmis- sion system. The system has been used for the interconnection of asynchronous transmission systems (50Hz/60Hz) in Japan [19].

IV Multiterminal HVDC systems

It is sometimes necessary to connect more than two converter stations to a com- mon DC circuit. This may be achieved with a parallel multiterminal HVDC system, series multiterminal HVDC system or Hybrid multiterminal HVDC system. In multiterminal HVDC transmission, the converter stations are ge- ographically separated and a physical DC circuit is required to interconnect them. In parallel multiterminal HVDC transmission, all the converters involved in the DC transmission system are connected to the same DC voltage as shown in Figure 2.5. When any of the converter stations is connected between any of the monopole links, the multiterminal HVDC transmission is termed series multiterminal HVDC system as illustrated in Figure 2.6.

Multiterminal HVDC using LCC has been considerably investigated [28, 29]. To date, there are only a few commercial installation, the Quebec-New England three terminal HVDC link which uses the parallel multitermianl configuration been one of them. This configuration offers flexibility and low losses compared to the series configuration, however, it requires special switching arrangement to achieve power flow reversal [30]. In the series multiterminal HVDC system Figure 2.6, one converter station controls the current in the circuit allowing the other converters to operate as voltage sources from the DC network which can be connected in series without the need for special switching. This would have been more suitable for multiterminal HVDC applications if not for the high losses, complicated insulation and the interruption of power supply on the occurrence of fault on one of the lines [27]. Technical challenges such as lack of independent control of active and reactive power, possibility of commutation failure, and the need to physically change the DC circuit polarity to achieve power flow reversal have affected the implementation of multiterminal LCC-HVDC systems. Current research trend is focused more on the use of VSC for multiterminal HVDC applications [31, 32, 33]. This is partly due to the many technical ad- vantages of VSC such as ability to independently control active and reactive power, black start capability, no voltage polarity reversal required for power flow reversal and partly due to the low footprint of a VSC station.

2.2. LCC HVDC TRANSMISSION 20 AC SYSTEM 1 AC SYSTEM 2 AC FILTERS AC FILTERS DC FILTERS TRANSFORMERS CONVERTERS DC CIRCUIT

Figure 2.2: A monopolar link HVDC transmission system

AC SYSTEM 1 AC SYSTEM 2 DC FILTERS DC FILTERS AC FILTERS TRANSFORMERS TRANSFORMERS AC FILTERS CONVERTERS CONVERTERS

AC SYSTEM 1 AC SYSTEM 2 AC FILTERS AC FILTERS DC FILTERS TRANSFORMERS CONVERTERS

2.2. LCC HVDC TRANSMISSION 22

AC SYSTEM 1 AC SYSTEM 3 AC SYSTEM 2

Figure 2.5: Parallel multiterminal HVDC system

AC SYSTEM 1 AC SYSTEM 2

AC SYSTEM 3