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Kr Padiyar Hvdc Power Transmission Systems Pdf 95

Studies Shows transmitting DC is more efficient than AC supply. As losses in HVDC are less than HVAC. But as we mostly generate AC supply hence we need converter stations to convert AC in to DC for efficient transmission. Mostly studies have been done on Transmission line faults or AC faults but Converter station faults or DC faults also cause the stressing of equipments due to overvoltage or current. As in AC system, the faults in DC system are caused by external sources such as lighting, pollution or internally due to failure of converter valves. Electrical disturbance in the power systemcan cause more torsional stressing on the turbine- generatorshafts of the system than in the case of a three-phase fault atthe generator terminals [1], [2]. Asturbine-generator shaft torsional systemscan interact with other power system stabilizers; static-varcompensators, high-voltage direct current (HVDC) systems,high-speed governor controls, and variable speed drive converters[3][5]. In most of the reported studies, attention hasbeen given to the interaction between HVDC systems and theturbine-generator shafts [6], [7]. Fewer studies have investigatedthe impact of HVDC faults on turbine- generator shafttorsional torques. In all these investigations, only dc line faultshave been considered and no attempt has been made to considerthe converter station faults [8]. This paper addresses the study of HVDC converter station faults such as fire-through, misfire,flashover, and a short circuit across the inverter and rectifier side.

kr padiyar hvdc power transmission systems pdf 95

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Fig. 1 shows the system under study, which consists of a six- pulse ac/dc converter station connected to a synchronous machine at its terminals. In the system under investigation, a short transmission line is assumed to connect the converter station to an infinite bus bar. Also, a local ac load (purely resistive load) is connected to the ac bus of the converter station. A capacitor bank is connected to the converter ac bus bar to provide reactive power support to the system.

Fig 1 shows a complete HVDC system required for generation and transmission of AC supply. Its consist of Synchronous Machine, Mechanical System, Converter station, Transmission Network and also showing SMES unit required to improve power quality in case of converter station fault.

High voltage direct current (HVDC) transmission is an economical option for transmitting a large amount of power over long distances. Initially, HVDC was developed using thyristor-based current source converters (CSC). With the development of semiconductor devices, a voltage source converter (VSC)-based HVDC system was introduced, and has been widely applied to integrate large-scale renewables and network interconnection. However, the VSC-based HVDC system is vulnerable to DC faults and its protection becomes ever more important with the fast growth in number of installations. In this paper, detailed characteristics of DC faults in the VSC-HVDC system are presented. The DC fault current has a large peak and steady values within a few milliseconds and thus high-speed fault detection and isolation methods are required in an HVDC grid. Therefore, development of the protection scheme for a multi-terminal VSC-based HVDC system is challenging. Various methods have been developed and this paper presents a comprehensive review of the different techniques for DC fault detection, location and isolation in both CSC and VSC-based HVDC transmission systems in two-terminal and multi-terminal network configurations.

High voltage alternating current (HVAC) is widely used for short to medium distance power transmission but may not be applicable for long distance power transmission because of the high charging current of cable capacitance, high losses, absence of asynchronous operation, difficulty in control of power flow, the need for reactive power compensation and having issues of skin and Ferranti effects. Because of these drawbacks in HVAC transmission, application of high voltage direct current (HVDC) has increased significantly [1,2,3] and HVDC transmission has become an economical choice for the transfer of high power over longer distances.

In the early stage, the current source converter (CSC) based HVDC system was used for the transmission of power. CSC-based HVDC systems use thyristors, and can apply to very high power rating with low losses (typically around 0.7%). However, thyristors can only be turned-on with no turn-off capability which means they cannot be controlled to interrupt a fault current. In addition, a CSC-based HVDC system requires large filters which increase the capital cost and is vulnerable to AC side faults which can lead to commutation failure [4,5,6]. Given these issues, HVDC systems using voltage source converters (VSC) have been developed.

In [38] and [39], a handshaking method is proposed to identify the DC fault in MT VSC-HVDC systems. AC circuit breakers (CB) with DC switches are used. These are cheaper than DC circuit breakers. If the fault occurs in a DC line, the AC circuit breaker opens the line from the AC side and the DC switch will isolate the faulty line. This protection scheme can be applied to a point-to-point HVDC transmission system. However, for a multi-terminal HVDC network, all the converters have to be shut down because of the action of the AC CB which will interrupt the power flow in the entire network. The transient phases such as capacitor discharge and diode freewheeling stages occur very quickly within a few milliseconds, and could damage the semiconductor devices and other components if due care is not taken.

In the past, Fourier transform (FT) was used to extract the spectral components of a signal, but it provides the frequency components in the signal with no time information. Thus, it is not suitable for non-stationary signals which are very important for the protection of an HVDC transmission line. Short time Fourier transform (STFT) was later used to extract the spectral contents of the signal which provides the existing frequency bands and corresponding time intervals, i.e., the resolution is fixed. However, it does not provide the information of the existing frequency at the time instant. Therefore, the wavelet transform (WT) is used to extract the transient signals due to faults and other disturbances. WT is a powerful tool in the signal processing methods for tracking the fault transients in non-stationary signals [62,63,64,65,66]. The continuous wavelet transform (CWT) of a signal f(t) is given by

In [135,136,137], an alternate arm converter (AAC) which is the combination of FB submodules (SM) and the director switches is applied for DC fault protection. The director switches consist of HV series-connected IGBTs. The AAC can block the DC fault current with fewer switches than the FB-MMC. However, the director switches require large numbers of IGBTs, while the flexibility of the AC voltage is restricted and a DC filter is necessary to remove the 6th harmonic in the DC current. In [138,139,140], a hybrid converter which combines the two-level VSC and cascaded FB SM is presented. It can block the DC fault current and provide AC fault ride through (FRT) capability. However, the active switches in the two-level VSC have HV stress, and the synchronization of the two power stages (i.e., the two-level VSC as the main power stage and cascaded FB SM as the low power stage) is challenging. In [141, 142], the hybrid cascaded MMC (HC MMC) which is the combination of half bridge and cascaded full bridge cells is applied to block the DC fault current in the HVDC transmission system. Here, the HB cells are used in the main power stage. This will generate the voltage with low switching frequency and losses. The cascaded FB cells have DC fault current blocking capability and also reduce the harmonics generated by the main power stage. In HC MMC topology, the capacitor voltage balancing and synchronization of the two power stages are achieved, and the FB cells can eliminate the inrush current coming from the AC side during the deblocking action of converter after the fault event.

References [145] and [146] present converter faults such as misfire, fire-through, flashover and DC link capacitor failure. A fire-through is the conduction of the switch before the scheduled instant of time, and its occurrence in the VSC can interrupt power transmission in the HVDC system. A misfire is the failure of a switch to conduct on the scheduled conduction period, while flashover occurring in the non-conducting switch can cause a short circuit and overcurrent in the converter. Short circuit or open circuit faults can occur in the DC link capacitor which can affect the performance of HVDC systems.


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