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Note for High Voltage DC Transmission - HVDC by Shataroopa Mohapatra

  • High Voltage DC Transmission - HVDC
  • Note
  • Biju Patnaik University of Technology BPUT - BPUT
  • Electrical Engineering
  • B.Tech
  • 1 Topics
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Shataroopa Mohapatra
Shataroopa Mohapatra
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HARMONIC SUPPRESSION IN HVDC SYSTEM HVDC converters introduce both AC and DC harmonics which are injected into the AC system and DC line respectively. Harmonic voltage and current produced on either side of HVDC system. Since the commutation reactance is low in relation to the DC smoothing reactance, an HVDC converter acts, from the AC point of view as a source of harmonic current (high internal impedance) and from DC point of view as a source of harmonic voltage (low internal impedance). harmonic voltage generated on the AC side distort the supply nature and give rise to various problems that reduce power quality. Hence, they must be eliminated completely and reduced to a minimum level that will not disturb the power system. There are several problems associated with the injection of harmonics listed below: i. ii. iii. iv. v. Telephone interference Extra power losses and consequent heating in machines and capacitors connected in the system Overvoltage due to resonances Instability of converter controls, primarily with individual phase control scheme of firing pulse generation Interference with ripple control system used in load management The obvious place to eliminate the harmonics is the source itself.in theory characteristic harmonics could be eliminated either by some converter complex configuration9which would be uneconomical),or by the use of a series filter preventing the harmonics from arising(which would upset the correct operation of the converter).therefore accepting that the appearance of harmonics is an inherent property of the static conversion process, it will be necessary to reduce their penetration into the AC and DC system. HARMONIC MODEL & EQUIVALENT CIRCUIT: The process by which harmonic currents are bypassed is explained using the following simplified circuit (fig.1. (a)) and one-line diagram of an HVDC station with filter equipment (fig.1. (b),(c)). Fig.1. (a) circuit for harmonic source with filter The converter acts as a source of harmonic current whose amplitude and frequencies may be constant. These harmonics travel into the AC network and AC filters, and produce voltage drops of respective harmonic frequencies. The harmonic currents of the order h generated are divided into two components Harmonic currents in filter branch (Ihf ) = Shataroopa Mohapatra,GIFT,BBSR Vh Zhf I Z = Z hc+Zha ha hf Page 1

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Harmonic currents in AC network (Iha ) = Vh Zha I Z = Z hc+Zhf ha hf And satisfies the condition Ihc = Ihf + Iha Fig.2. (a).one-line diagram of an HVDC station with filter network Fig.2. (b). An HVDC system with filter network The AC filter requirements change with power transfer through the HVDC system and the requirement depends on the AC network condition and their interaction with HVDC systems. Fig.3.typical harmonic impedance diagram The harmonic filter bank is capacitive at low frequencies and the AC system impedance may be inductive.at some frequencies there may be resonance between the two. The net impedance may be very high under these conditions and is resistive in nature. The harmonic current produces high harmonic voltage drops when it flows through the large impedance. The impedance for different harmonic orders is also plotted in fig. 3 h indicates the order of harmonics. The harmonic current is filtered out using AC filters at the converter buses.it is not possible to filter the entire harmonic current and therefore flow of harmonic current into the AC system results in an unbalance in the AC system. Shataroopa Mohapatra,GIFT,BBSR Page 2

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HARMONIC ELIMINATION: There are two principal means of reducing harmonics: - I. Increasing the pulse number: The relationship between pulse number and harmonic order indicates that the higher the pulse number the higher the frequency of the lowest order harmonic produced.in low voltage high current rectifiers high pulse numbers have sometimes been used ranging from 24 to 108.this means of reducing harmonics is very effective as long as all valves are in service, but it requires complicated transformer connections. The use of increased pulse number has the following disadvantages: i. Increased levels of lower order harmonics when converters are temporarily out of service during maintenance ii. Increased number of transformers both in service and spares iii. Increased complexity of transformer connections the consequent problems of insulation Fig.4.12-pulse converter configuration Moreover, as the harmonic order increases its amplitude decreases and it is normally cheaper to eliminate it substantially by filtering. From the analysis of converters, it is clear that the harmonics entering the AC system can be greatly reduced by increasing the number of phases at which the converter operates. Though theoretically it is possible to increase the pulse number from 6 to 108 the method demands a more expensive transformer. therefore a 12-pulse(Fig.4) normal operation is usually the most suitable arrangement. With HVDC schemes only simple transformer connections are used. This is due to the problem of insulating the transformers so that they withstand the alternating voltages combined with the high direct voltages. A pulse number of 12 can be easily obtained with star-star and star-delta transformer connections in parallel which is generally accepted for HVDC transmission. A 24-pulse can be obtained with four six pulse groups by use of a phase shifting transformer bank in conjunction with Shataroopa Mohapatra,GIFT,BBSR Page 3

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two 12-pulse converters. The required phase shift is 150. The effectiveness of 12 or 24-pulse converters in reducing harmonics is somewhat decreased when one valve group is out of service. Fig.5. voltage vector diagrams of autotransformers for shifting three phase voltages by angle Ф=15 0. (a)ring, (b)zigzag II. Use of filters: Any necessary reduction in harmonic output of the converter beyond that accomplished by increase of pulse number must be done by harmonic filters. From an economical point of view, it is a common practice to use a 12-pulse converter with filters. Filters are almost always needed on the ac side of the converter and sometimes on the dc side also. The ac filters serve two purposes simultaneously supplying reactive power of fundamental frequency in addition to reducing harmonics. Hence the part of cost of the filters chargeable to the need for reducing harmonics is usually near to the cost of the filter inductors, the filter capacitor being required for supply of reactive power. Thus, leading to the concept of minimum filter, which is required for harmonic reduction only in installations where the reactive power required by the converter can be supplied by the AC system without reinforcing the latter. A filter costing more than the minimum filter not only supplies additional reactive power but also generally gives better filtering. A filter is a network that responds more vigorously at some frequencies than others and reduces the amplitude of one or more fixed frequency currents or voltages. They normally consist of one or more L-C turned circuits with a Fairley high Q and can also comprise a high pass filter. However damping circuit is used to reduce either the overshoot or rate of rise of a transient current or voltage with unusually low Q. Design of AC filters: Aims are i) to achieve adequate harmonic reduction, ii) to supply adequate reactive power at fundamental frequency, iii) achieving both at minimum cost. The AC filters in each phase comprises of I. Tuned filters for several lower harmonics II. A damped filter for higher harmonics III. Switchable shunt capacitors The major design objective of AC filter is to reduce the telephone interference which can be measured by any of the following performance indices: Shataroopa Mohapatra,GIFT,BBSR Page 4

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