Curso de Transmissão em Corrente Continua Rio de Janeiro, de Junho, 2007

Transcription

1 Curso de Transmissão em Corrente Continua Rio de Janeiro, de Junho, 2007 DC Harmonic Filters Page 1 of 9

2 1 Function of the DC-Side Harmonic Filters Harmonic voltages which occur on the dc-side of a converter station cause ac currents which are superimposed on the direct current in the transmission line. These alternating currents of higher frequencies can create interferences in neighbouring telephone systems despite limitation by smoothing reactors. Harmonic currents of lower frequency can cause risks for human beings and devices through induced voltages. DC filter circuits, which are connected in parallel to the station poles, are an effective tool for combating these problems. DC Filters will always be used if a telephone line is in close proximity to an HVDC overhead line and adequate shielding is not present on both the telephone line and the HVDC line. DC filters are not required in HVDC cable connections and HVDC back-toback links. It can often be more economical to shield individually exposed sections of telephone lines than to install a filter combination to handle the "worst case". Besides the dc filters there are other equipment in the dc circuit, which has significant contribution for limitation of harmonic currents on the dc line and thus shall be considered in the dc filter design as well: smoothing reactor and neutral bus surge capacitor. The smoothing reactors in the hvdc converter stations have following main duties: to avoid resonance at low order harmonics, especially at the fundamental and the second harmonic frequency to limit the high order harmonics in the dc line under the specified limits to smooth the ripple in the dc current, so that the current will not get discontinued in the whole operating range. to limit the overcurrents in case of a failure in the dc circuit Selection of the smoothing reactor size for a hvdc project shall consider all these different aspects. Together with the dc filters the smoothing reactor makes a considerable contribution to reduce the value of harmonic currents in the dc systems. The harmonic currents along the dc line should be calculated for different operation conditions. However, if an active dc filter is used, the harmonic currents on dc line can be effectively damped by the active filter alone. Therefore, the smoothing reactor size can be selected smaller than conventional scheme if the active part has enough power output to meet the performance criteria. The smoothing reactor can also reduce the fault current and its rise rate through the valves at inverter side in case of some dc faults (by pass or bushing fault). This is primary of importance if a long dc cable is used for the transmission. For an overhead line transmission the current stress in valves is lower than the stress which will occur during valve short circuit. Neutral bus surge capacitor has the function to provide a low ohmic path within converter station for the so called triple harmonic currents caused by the stray capacitance in the converter equipment. This capacitor will significantly reduce the triple harmonics on the hvdc lines and the electrode lines as well. The selection of a proper size of this neutral bus capacitor is also a subject in the dc filter design. 2 Design Criteria for DC Filter Circuits The harmonic currents on the HVDC line create a magnetic alternating field. If a telephone line is in the vicinity of the dc line, harmonic voltages will be induced in the telephone line by the magnetic field. These harmonics in the telephone line and the Page 2 of 9

3 associated acoustic noise can significantly impair the quality of the telephone system. Therefore it is common practice to use the telephone interference caused by the dc line as a measure for the design of the dc filter circuits. The harmonic currents flowing in the hvdc lines can be generally split into components of balance mode and ground mode. While the currents in balance mode do not cause any interference in the remote telephone lines, the ground mode currents are actually responsible for the interference. Therefore the ground mode harmonic currents of all conductors at line tower including earth wires are referred as harmonic disturbing currents. The most often used dc filter performance criteria is the equivalent disturbing current Ieq, which is a psophometric weighted equivalent ground mode (=residual) current of all harmonics up to an order defined by the Specification. The equivalent disturbing current Ieq (x) at any location is calculated as geometric sum of contributions form both converter stations: I eq ( x) = I e ( x) r I e ( x) i Where Ieq(x) is the 800 Hz equivalent disturbing current in ma psophometric weighted at any location along the transmission lines Ie(x)r is the magnitude of equivalent disturbing current due to harmonic voltages at rectifier Ie(x)i is the magnitude of equivalent disturbing current due to harmonic voltages at inverter The equivalent disturbing current at any point due to harmonics from either station is calculated as follows: I ( x) = e Where n = m n= 1 [ I ( n, x) P( n) H ] g f 2 Ig(n,x) is the magnitude of residual rms current at harmonic order n in ma at location x P(n) is the psophometric weighting factor at harmonic order n as per CCITT Hf is the coupling factor which represents the frequency dependent coupling effect to open wire circuits. m is the maximum harmonic order, usually 50, to be considered in the calculation. The interference voltage induced on the telephone line can be characterized by the following equation: m ( ) V in(x) = Z Hµ Cµ Iµ ( x) 1 2 = Z I eq Page 3 of 9

4 where V in(x) = Interference voltage on the telephone line at point x (in mv/km) Z = Mutual coupling impedance between the telephone and HVDC lines at 1000 Hz H µ = Weighting factors which reflect the frequencydependency of the coupling between telephone and HVDC lines C µ = "C message" - weighting factors I µ(x) = Resulting harmonic current of the ordinal number µ in the HVDC line at point x as the vector sum of the currents caused by the two HVDC stations I eq = Psophometric weighted equivalent disturbing current It is apparent from the above equation that the interference voltage is dependent only on the equivalent disturbing current and the impedance between the HVDC and the telephone lines. At the time when the system is being designed, this impedance is often unknown. Therefore, in practice, often only the equivalent disturbing current is specified as a design criteria. There are no uniform standards for the maximum permissible equivalent disturbing current. It is dependant on conditions in the vicinity of the HVDC lines and the philosophy of the telephone and power system companies. The intensity of interference currents is strongly dependant on the operating condition of the HVDC. In monopolar operation, telephone interference is significantly stronger than in bipolar operation due to higher ground mode currents. As an example, the following values can be used as guideline for the maximum permissible equivalent interference current: bipolar operation: I eq 500 ma monopolar operation: I eq 1000 ma In light of the above, it is not possible to precisely calculate in advance the equivalent disturbing current caused by a planned HVDC system in existing or planned telephone lines. For this reason, the permissible induced interference voltage in an assumed telephone line, over a length of 1 km parallel to the HVDC system, at a distance of 1 km is in some cases specified. The ground resistance has to be specified, possibly with different values for different sections of the prospective line. The equivalent disturbing current can then be derived from the interference voltage (in mv/km). 3 Calculation of Equivalent Disturbing Current The task is now to determine the most unfavourable combination of harmonic voltages which can occur at one time in view of the operating modes and conditions of the HVDC system which are to be considered, incorporating the anticipated unsymmetries and tolerances. "Most unfavourable" means that combination which produces the highest equivalent disturbing current. In making this calculation, the dependency of the parameters of overhead lines, smoothing reactors, ground electrodes, etc., on frequency must be taken into consideration. Page 4 of 9

5 The harmonic currents which are emanating from a station undergo a frequencydependent phase shift. Standing waves are formed along the HVDC line. These standing waves are the superposition of the waves which are emanating from the two stations. The resulting harmonic profile is derived from the vector addition of the harmonic currents of the same frequency, emanating from the two sources, and being present at a particular location. As the difference of phase angles between the two harmonic currents sources are normally unknown, an angle of 90 is generally assumed. 4 Three Pulse Model for Harmonic Calculation Figure 1 shows the classic equivalent circuit for the calculation of dc-side harmonics of an HVDC converter with the voltage source U 12p. This generates the characteristic harmonic voltages of ordinal numbers k*12. The voltage spectrum will not be altered if the leakage capacitance C s is added. Figure 1 : DC-side equivalent circuit of the 12-pulse converter A 12-pulse HVDC converter can be decomposed into 4 series-connected 3-pulse commutation groups. The state of the art of dc filter design shall base on the 3 pulse model described above. The first step in the dc filter design is to determine the no-load dc harmonic voltages for these four 3 pulse sources. The simplest way for calculating the 3 pulse voltage magnitudes is using a theoretically derived trigonometric Fourier series expressions, which is based on the symmetric ac voltages: 1 = k U3 P Udio cosα cos( α u) (( 1) ( a cos(ϖt) + b3 k sin(ϖt))) 4 k=1 with a b cos( α(1 + )) + cos(( α + u)(1 + )) cos( a(1 )) + cos(( a + u)(1 )) = sin( a(1 + )) + sin(( a + u)(1 + )) sin( a(1 )) + sin(( a + u)(1 )) = Page 5 of 9

6 In order to consider all unsymmetries, a more precise method is needed to calculate the instantaneous voltage/current values for each valve switching interval. The twelve pulse voltage is indeed a superposition of the voltage of 4 three pulse bridges. The voltage wave form of each three pulse source will be constructed piecewise in the time domain. Then the results will be analyzed by Fourier series. Using this method all harmonic distortion on the ac voltage and all unbalances in the firing angle, transformer reactance etc. can be well represented for each chosen combinations of asymmetries. 5 DC Harmonic Filter Configurations DC filter circuits are usually arranged as parallel filters between the dc busbars and the station neutral bus or ground. Series dc filters (so called blocking filter) have been used only in isolated instances and therefore will not be discussed further in this paper. The configuration of the dc filters very strongly resembles the filters on the ac side of the HVDC station. There are several types of filter designs. Single and double tuned filters with or without the high-pass feature are common in the past. In the new scheme triple tuned dc filters are also used due to various technical and economical advantages. One or several types of dc filters can be utilized in a converter station. Basically the same type of ac harmonic filters can also be used in dc side (refer to Chapter AC Harmonic Filters ). Despite the many similarities, there are several important differences between dc and ac filter circuits. AC filter circuits are essentially responsible for providing reactive power of fundamental frequency. Therefore they are usually designed for higher reactive power than would be necessary for the required filter effect. This design aspect does not apply in the case of dc filters. The high voltage capacitor of the dc filter must withstand a high direct voltage. The uniform distribution of voltage among the many series-connected capacitor units, which can be automatically assumed in the case of ac filters due to capacitive current, does not apply for dc filters. It is necessary to ensure the distribution of voltage by means of parallel resistors. The ac system, to which the ac filter is connected in parallel, has a wide impedance range. Thus a resonance can occur under certain network conditions between filter capacitance and ac network inductance. For this reason a certain damping is necessary even in the case of sharply tuned ac filter circuits. In contrast, the impedance of the dc circuit is largely constant and therefore permits the use of sharply tuned filters. The determination of filter configuration is made on the basis of achieving the smallest equivalent disturbing current caused by the HVDC line and with minimum filter costs. Since the characteristic harmonic currents have the largest amplitudes, the dc filters are usually matched to these harmonics (i.e. ordinal numbers 12, 24, 36,...). The portion of cost of the entire HVDC system as represented by the dc filter circuits is normally not great. The most cost-intensive element in a dc filter is the high voltage capacitor. Therefore an effort is always to be made in the design of dc filter circuits to optimize the cost of high voltage capacitors, i.e., to keep them as small as possible, typically in the range from 0.5 uf to 2 uf. One means to this is the construction of double or multiple tuned filter circuits which have a common high voltage capacitor. Page 6 of 9

7 In connection with the dc filter circuits the capacitor between the station neutral bus and station ground must also be mentioned. It represents a low resistance path for transient overvoltages and also for dc-side harmonics. In particular when the dc filter is connected to ground, the capacitor prevents the harmonic currents from flowing through earth electrodes and electrode lines to the station neutral point, causing telephone interference or inducing unacceptable voltages. The harmonic currents described in the previous section, which flow out through leakage capacitances, pass through this capacitor to the station neutral point, which customarily has a capacitance of several uf. 6 Steady- state Stresses of DC Filter Circuits Steady- state stress of the dc filter capacitors is composed of two components: (1) direct voltage, which causes a breakdown risk on the dielectric but will not result in any heat generation, if the resistors built in for voltage grading are ignored, and (2) the harmonic currents of the tuned frequency. In filter reactors and resistors, these harmonic currents account for the entire steady state stresses. In order to determine the maximum steady state stress of individual filter elements, extensive calculations for various operating conditions are required. In particular, the following parameters and/or operating conditions should be taken into consideration: The specified maximum continuous direct voltage The maximum characteristic harmonic voltages which can occur in the steady-state (function of control angle and system direct current) If high-pass filters are provided, the maximum high frequency non-characteristic harmonic voltages which can occur at the same time If low order filters are provided: unsymmetries of the ac side and leakage capacitances in the station, so that the maximum filter current is achieved Harmonic currents flowing from the opposite station Fluctuations of ac-network frequency Filter detuning as a result of temperature fluctuation, manufacturing tolerance, aging of the elements, loss of capacitor sections, and failure of capacitor cans Operating mode of the HVDC system (bipolar, monopolar, reduced direct voltage, etc.) Outage of a filter unit (if several are present) 7 Transient Stresses of DC Filter Circuits Filter circuit components are subjected to various transient stresses as a result of faults and switching procedures. For this reason, the reactors and the resistors in the filter circuit have to be protected from over voltages by arresters where necessary just as in ac filters. The following two types of faults ordinarily result in the most severe stresses for dc filter components: A flash-over to ground occurring when the voltage at the dc busbar prior to the fault is equal to the arrester switching surge protective level. A line to ground fault occurring in the HVDC line at a certain distance from the converter station. Page 7 of 9

8 8 Design Considerations of Filter Equipment Basically the same aspects apply for the components of a dc filter circuit as were presented for ac filter circuits. The high voltage capacitor represents an exception. Since only harmonic currents produce heating, the electric power is very low: This means that only a few capacitor units have to be connected in parallel. In contrast, the direct voltage, which is usually high, requires the series connection of a large number of capacitors. If the classic tiered arrangement on an insulating table is selected, a super slim tower will be obtained which is very sensitive to wind and earthquake stresses. This requires appropriate anchoring. For this reason it might be sometimes advantageous from a cost standpoint to suspend the high voltage capacitor from a mast or portal by elastically fixed insulators. 9 Active Filter "Active" filters represent a rather new development. In contrast to traditional filters, active filters have a controlled voltage source which feeds certain harmonic currents into the line through a power amplifier. These harmonic currents act to cancel the harmonic currents from the converter. The converter, however, represents a voltage source with respect to dc-side harmonics. The theoretically correct action would therefore be to place a controlled harmonic voltage source in series to the converter and to control it in such a manner that the harmonic voltages are precisely cancelled. A series source might present great difficulties since the transmission direct current has to pass it. The active filters placed in service to date are therefore integrated into the passive filter as an auxiliary device in the form of a harmonic voltage source, which injecting an harmonic currents. The high voltage capacitor in the passive filter is used to decouple the active filter from the high dc voltage. 10 Development Trends While ac filters simultaneously provide reactive power compensation, dc filters have no other function than to provide a low impedance path for dc-side harmonic currents and to filter them out of the HVDC line. In view of the growing demand for more cost effective designs of HVDC stations, the question of how to reduce telephone interference more economically has to be addressed. A more precise and realistic specification with respect to telephone interference could significantly reduce the costs of dc filter circuits. In the past, limit values were often specified for telephone interference which were based on the telephone technology of twenty years ago. The interference effect of harmonic currents occurs primarily in openwire analog telephone systems. During recent years telecommunications technology has developed to the extent that higher harmonic currents could well be permitted on HVDC lines. Modern digital transmission technology and mobile phones are both immune to electromagnetic interference by an HVDC line. The need for dc filter circuits in HVDC systems appears, at least in countries with well developed telecommunications technology, to be becoming smaller. In the recent HVDC projects there are rarely telephone interference problems along the DC lines reported. It is often more economical Page 8 of 9

9 to take measures to shield telephone lines which are still sensitive to interference than to install expensive dc filters. This would entail converting sections of telephone lines which are subjected to interference from HVDC overhead lines to cable. For questions please contact: Mario Nelson Lemes Siemens Ltda Tel: Fax: Page 9 of 9