Dual Channel Model Fitting Techniques

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thyristor valves. The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.

• Forced Commutated Converters.

This type of converters introduces a spectrum of advantages, e.g. feed of passive networks (without generation), independent control of active and reactive power, power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off (Ekici, Yildirim and Poyraz, 2009). They are known as VSC (Voltage Source Converters). Two types of semiconductors are normally used in the voltage source converters:

the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of them have been in frequent use in industrial applications since early eighties.

The VSC commutates with high frequency (not with the net frequency). The operation of the converter is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase 3 angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneously (Anshika, 2016). Thus, PWM offers the possibility to control both active and reactive power independently. This makes the PWM Voltage Source Converter a close to ideal component in the transmission network. From a transmission network viewpoint, it acts as a motor or generator without mass that can control active and reactive power almost instantaneously.

- 52 - 2.10.1 Monopolar DC Link

As the name suggests, monopolar link has only one conductor and return path is provided by permanent earth or sea. The line usually operates with negative polarity with respect to ground so as to reduce corona loss and radio interference.

The earth electrodes are designed for continuous rated current operation and for any overload capacity required in the specific case. The sea or ground return is permanent and of continuous rating. The ground return path has a low resistance and, therefore, low power loss in comparison with a metallic line conductor of economical size and equal length provided the ground electrodes are of proper design (Anshika, 2016). Monopolar line is more economical than a bipolar line because the ground return saves the cost of the one metallic conductor and losses in it.

Monopolar HVDC links were used only for low power rating and mainly for cable transmission.

In some cases, the monopolar lines installed earlier are converted into bipolar systems by adding additional substation pole and transmission pole.

Monopolar HVDC line has only the rating equal to half of corresponding bipolar line rating and is; therefore, not economically competitive with EHV ac schemes for submarine cables longer than 25 km and of power rating of about 250 MW. For such cable transmission high voltage ac scheme is not technically feasible due to large charging currents with ac cables beyond thermal limit. Bipolar cable is not justified for ratings up to about 500 MW. Recent HVDC cable schemes are bipolar.

- 53 - Figure 2.5: Monopolar HVDC Link (Anshika, 2016).

2.10.2 Bipolar DC Link

This is most widely used dc link for overhead long distance HVDC transmission systems and also for back-to-back HVDC system. This link has two conductors— one operating with positive polarity and the other with negative polarity with respect to the earthed tower structure.

There are two converters of equal voltage rating and connected in series at each end of the dc line. The neutral points, i.e., the junction between converters may be grounded at one end or at both the ends. If it is grounded at both ends each pole can operate independently.

The rated voltage of a bipolar link is expressed as ± 500 V. Power rating of one pole is about half of bipolar power rating. The earth carries only a small out-of-balance current during the normal operation. When the currents in the two conductors are equal, the ground current is zero.

During fault or trouble on one of the lines, the other line along with ground return can supply half of the rated load (Ekici, Yildirim and Poyraz, 2009). Thus continuity of supply is maintained.

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After taking corrective measures, the system is switched over to normal bipolar operation. Thus the reliability of a bipolar line is equal to that of a double circuit 3-phase line although it has only two conductors instead of 6 for 3-phase line.

Example of bipolar HVDC link is Ranchi-Delhi single bipolar overhead line of length 810 km, capacity 1,500 MW and operating at ± 500 kV for transmission of bulk power. This line is designed to operate in bipolar mode under normal conditions, both the poles sharing the load equally with negligible current (less than 10A) in the ground return path.

Figure 2.6: Bipolar HVDC Link (Anshika, 2016).

In case of fault on one of the poles, the system automatically switches over to monopolar ground return mode supplying 50% of the rated load. Thereafter the system may be changed over to bipolar metallic return mode when the other conductor is used as return conductor.

- 55 - 2.10.3 Homopolar DC Link

A homopolar link has two or more conductors having the same polarity, usually negative, and always operates with ground as the return conductor. In case of a fault on any one of the conductors, the converter equipment can be reconnected so that the healthy conductor can supply power. Such a scheme is very complicated and is preferred to a bipolar link provided continuous ground return does not pose additional problems.

Figure 2.7: Homopolar HVDC Link (Anshika, 2016).