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Fault analysis and protection of HVDC system is very important. Protection of DC systems can be done with conventional AC devices such as circuit breakers and fuses or with DC devices, such as IGBT circuit breakers and converter embedded devices. Placing AC circuit breakers on the AC side of the VSC is the most economical way to protect the DC system. They are commonly available and can be replaced in a shorter

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amount of time. However, AC circuit breakers result in the longest interruption time as a result of their mechanical restrictions. As of March 2011, the best interrupting time for an AC circuit breaker is two cycles [26].

Fuses on the AC side are generally not a good solution for the protection of the VSC. The fuse will only operate in the event that the DC protection fails.

DC protective devices such as IGBT circuit breakers, can act faster than their AC counterparts. When a fault occurs on the DC line, the IGBT is able to block the fault current. If the fault occurs on the converter side, the anti-parallel diodes conduct and allow current to flow, illustrated in Fig 2.9.

Fig 2.9 IGBT-CB Fault Blocking Capability [26]

Fast acting DC mechanical switches are used with the solid state DC switches (IGBT- CB), which is used to isolate the line once the fault current has been cleared. It should be noted that the switch cannot break current and may only be opened once the fault has been extinguished. Once the control system senses a fault on the line, an appropriate IGBT-CB will receive a gate signal to block the current. Once the fault current has been extinguished the fast acting DC switches will open, isolating the line. To determine if the fault is temporary or permanent, the DC switches and the IGBT-CB will close [26].

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AC Protection in MTDC: DC protection by using AC circuit breakers on the AC system can be achieved. A “hand shaking” method is proposed [40]. In this method, AC circuit breakers with fast acting DC switches are implemented. The switches are only used to isolate lines and cannot break load or fault current.

DC Protection in MTDC: DC protection utilizes IGBT-CB’s and fast acting DC switches. The IGBT-CB’s can be placed at the terminals of each VSC or at the end of each line. Presently AC protection devices are widely in use for protection. AC side protection appears to be a good solution on two terminal systems, but may cause unnecessary outages in multi terminal systems. Converter embedded devices provide better and more versatile protection than AC side protection; but still cause complete converter shutdown in the event of a permanent fault. DC device protection provides the best form of non-active protection. DC devices operate faster than AC devices under fault conditions and allow for more flexibility in MT-VSC-HVDC systems. Controllers can provide good protection in terms of allowing the system to continue to operate under fault conditions. Ultimately, the best form of protection appears to be a combination of active controllers and DC devices [26].

HVDC circuit breakers to break DC short-circuit currents have only been realized in very limited numbers and maximum ratings are 250 kV, 8kA or 500 kV, 4 kA, which is not more than 1.6 times the rated nominal current. The breaking time is in the order of 35ms, but for CSC based systems, the large inductance limits the rate of rise of fault current, and this time is sufficiently fast.

Most HVDC lines are used for transmission power over long distances, inevitably passing through complex terrain and operating under harsh weather conditions. It is extremely difficult to determine where a fault is occurring on the line. The inability of quickly locating and removing faults on an HVDC transmission line will compromise the stability of the power system. In fault location techniques for HVDC transmission

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lines, travelling wave based methods have fast responses and high accuracy, in which the time it takes for the travelling wave front to propagate from the fault point to the terminals implies the fault distance. However, they also face significant technical problems as follows:

1. The detection of the wavefront is the key to travelling wave fault location. If the wavefront cannot be captured successfully or the wavefront does not exist at all on the occurrence of a fault, the fault location detection will fail.

2. In the method, the time is measured for the wavefront to arrive at the point where the device is installed, and the fault distance is the product of the time and the wave speed. Therefore the accuracy of fault location is dependent, to a great extent, on the wave speed which, in turn, depends on the parameters of the line. 3. Accuracy in fault location depends upon sampling frequency. Since the speed at

which the wave travels over transmission lines is slightly lower than the speed of light, in order to achieve higher accuracy a very high sampling frequency has to be used in the travelling wave fault location methods.

4. The travelling wave fault location is vulnerable to interference signals.

In order to overcome the above difficulties a non-travelling wave fault location principle is proposed in [41]. For the protection of VSC HVDC systems with overhead lines, a superconducting fault current limiter (SCFCL) has been proposed and evaluated under DC faults [42]. The transient characteristic of the transmission system with a cable-to- ground fault is analyzed in detail and the result shows that the designed protective inductors can effectively prevent overcurrent and protect converters [43]. A novel hybrid travelling wave/boundary protection scheme for bipolar HVDC lines has been proposed and tested in real time using a Field Programmable Gate Array (FPGA) board [44]. Two main factors that affect the performance of the protection are: fault resistance and fault location. The relationship between the two factors and the sensitivity of

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transient harmonic current protection is discussed [45-46]. Papers [47-50] discussed about the development of a protection device for HVDC system. From all the reviews so far the following points can be identified:

 In HVDC applications, control and protection system plays an essential role

 Presently travelling-wave based protection and voltage derivative protection are usually used in HVDC protection

 Fault analysis is a vital part of any protection system

 Researchers proposed fault analysis specifically fault detection and classification

 But no researchers have been proposed so far a complete detection and classification of different types of faults in HVDC system. This research work is going to address this issue for the better protection of the HVDC system

 To identify and classify different faults in the HVDC system this research work is used three different signal processing techniques. These are wavelet transform, artificial neural network and fuzzy logic based approach. So the review of these techniques is discussed in the next section.