Harmonic filter for multipulse converter systems

Abstract

An improved scheme to filter harmonics and damp resonance for multipulse converter systems using a single characteristic passive filter branch without a plurality of non-characteristic harmonic filters is disclosed. The filter includes a first capacitor, a first inductor and a frequency-dependent resistor block connected in series. The frequency-dependent resistor block includes a second inductor connected in parallel with a circuit which is assembled by a second capacitor in series with a resistor.

Claims

1. A filter for a multipulse-converter system that has characteristic harmonic currents, and causes non-characteristic harmonic resonance at harmonic frequencies including a third harmonic frequency, fifth harmonic frequency and a seventh harmonic frequency, the filter comprising: a first series capacitor; a first series inductor; a frequency-dependent impedance block providing high equivalent resistance for damping non-characteristic harmonic resonance, the frequency dependent impedance block including a second capacitor in series with a resistor, and a second inductor in parallel with the second capacitor and the resistor, the frequency-dependent impedance block having a parallel resonance frequency; the first series capacitor, first series inductor and the frequency-dependent impedance block being jointly tuned so that the filter has a tuning frequency to filter the characteristic harmonic currents, the filter having an equivalent impedance; and wherein the second capacitor has a capacitance and the second inductor has an inductance, the capacitance and inductance being selected so that the parallel resonance frequency is greater than the third harmonic frequency and less than the seventh harmonic frequency; and wherein the equivalent impedance of the filter approaches proportionality with frequency for frequencies greater than the tuning frequency; each of the first series capacitor, first series inductor, frequency-dependent impedance block being suitable for use in multipulse converter systems.

2. The filter of claim 1 in which the characteristic harmonic currents comprises the 11.sup.th harmonic or higher.

3. The filter of claim 2 in which the non-characteristic harmonic comprises the 3.sup.rd, 5.sup.th and 7.sup.th harmonics.

4. The filter of claim 1 in which the non-characteristic harmonic comprises the 3.sup.rd, 5.sup.th and 7.sup.th harmonics.

5. The filter of claim 1 in which the tuning frequency of the filter is selected to be equal to a lowest characteristic harmonic frequency.

6. The filter of claim 1 in which the first series capacitor, first series inductor and the frequency-dependent impedance block are also jointly tuned to minimize harmonic amplification in a frequency range.

7. The filter of claim 1 in which each of the first series capacitor, first series inductor and frequency-dependent impedance block are suitable for use in 12, 18 or 24 pulse converter systems.

8. An apparatus comprising: a multipulse converter system; and a filter connected to the multipulse converter system that has characteristic harmonic currents, and causes non-characteristic harmonic resonance at harmonic frequencies including a third harmonic frequency, fifth harmonic frequency and a seventh harmonic frequency, the filter having, a first series capacitor; a first series inductor and a frequency-dependent impedance block providing high equivalent resistance for damping non-characteristic harmonic resonance, the frequency dependent impedance block including a second capacitor in series with a resistor, and a second inductor in parallel with the second capacitor and the resistor, the frequency-dependent impedance block having a parallel resonance frequency; the first series capacitor, first series inductor and the frequency-dependent impedance block being jointly tuned so that the filter has a tuning frequency to filter the characteristic harmonic currents, the filter having an equivalent impedance; wherein the second capacitor has a capacitance and the second inductor has an inductance, the capacitance and inductance being selected so that the parallel resonance frequency is greater than the third harmonic frequency and less than the seventh harmonic frequency; wherein the equivalent impedance of the filter approaches proportionality with frequency for frequencies greater than the tuning frequency; and each of the first series capacitor, first series inductor, frequency-dependent impedance block being suitable for use in multipulse converter systems.

9. The apparatus of claim 8 in which the multipulse converter system comprises a high voltage direct current transmission system.

10. The apparatus of claim 8 in which the multipulse converter system comprises a variable frequency drive.

11. The apparatus of claim 8 in which the multipulse converter system comprises a 12, 18 or 24 pulse converter system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of a filter will now be explained in further detail with reference to the accompanying figures, by way of example, wherein:

(2) FIG. 1 is an example of existing filtering scheme for a 12-pulse system known.

(3) FIG. 2 is an example of harmonic distortions in a 12-pulse system for different filter configurations.

(4) FIG. 3 discloses an embodiment of the disclosed filter.

(5) FIG. 4 shows the frequency response diagram of the disclosed configuration, including filter impedance Z, filter equivalent resistance R.sub.eq and filter equivalent reactance X.sub.eq.

(6) FIG. 5 shows the frequency response diagram of the combined system and filter impedance.

(7) FIG. 6 shows an alternative filter location which is at a transformer tertiary winding.

(8) FIG. 7 shows an alternative filter location in an industrial plant with several multipulse converters.

DETAILED DESCRIPTION

(9) FIG. 1 shows the filtering scheme commonly used for filtering harmonic currents generated by a 12-pulse converter system 140. The filter package 120 is connected at the interconnection point 130 between the supply system 110 and the 12-pulse converter 140, which is composed by a plurality of filter branches 121, 122 and 123. The filter branch 123 is composed by a main capacitor together with a shunt circuit including an inductor parallel with a second capacitor and a resistor. It serves as the characteristic filter to sink the characteristic harmonic currents, such as the 11.sup.th, 13.sup.th 23.sup.rd and 25.sup.th harmonics for a 12-pulse system. This branch is tuned to the 11.sup.th harmonic. In some literatures, there are also other configuration variations for the filter branch 123 (IEEE Standard 1531-2003, IEEE Guide for Application and Specification of Harmonic Filters, November 2003, pp. 57-57). The filter branches 121 and 122, composing of a series connected capacitor and inductor circuit, serve as the non-characteristic filters to suppress potential resonance at non-characteristic harmonic frequencies, namely, the 5.sup.th and 7.sup.th harmonic frequencies for a 12-pulse system.

(10) Now tuning to FIG. 2 which illustrates an example of harmonic distortions in a 12-pulse converter system with different filtering options. The amount of each harmonic current is presented as a percentage value with respect to the fundamental frequency current. Case Without Filters shows that the non-characteristic harmonics (5.sup.th and 7.sup.th harmonics) of the 12-pulse system are able to meet the harmonic limits even without any filter branch. But the 11.sup.th, 13.sup.th, 23.sup.rd and 25.sup.th characteristic harmonics exceed the limits. Therefore, the filtering objective should be the reduction of the characteristic harmonics. However, if only the characteristic filter branch 123 is applied, the low-frequency non-characteristic harmonics are significantly amplified and will exceed the limits due to the abovementioned resonance phenomenon. In accordance to this condition, non-characteristic filter branch 121 and 122 must be inserted to reduce the non-characteristic harmonics.

(11) Referring to FIG. 3 the single filter branch 320 is applied at the interconnection point 330 between the supply system 310 and the multipulse converter system 340 to replace the conventional filter package 120 illustrated in FIG. 1. The filter branch 320 is composed of the first capacitor C.sub.1 and inductor L.sub.1 in series with a frequency-dependent resistor block 321. The frequency-dependent resistor block 321 is a shunt circuit which includes a second capacitor C.sub.2 and damping resistor R together connecting in parallel with a second inductor L.sub.2.

(12) The frequency-dependent resistor block 321 can yield a different total resistance at different frequencies. If capacitor C.sub.2 is selected to resonate with L.sub.2 at, say, a frequency close or equal to the 6.sup.th harmonic, the resistor block will exhibit a large resistance component at the 5.sup.th harmonic and 7.sup.th harmonic. Since a resistor damps resonance, the large resistance produced by the resistor block 321 creates a significant damping effect at the 5.sup.th and 7.sup.th harmonics. Consequently, the resonance at the 5.sup.th and 7.sup.th non-characteristic harmonics caused by the filter elements C.sub.1 and L.sub.1 is suppressed. Since the resonance has been suppressed, filter branches such as 121 and 122 shown in FIG. 1 are no longer needed for mitigating the non-characteristic harmonics. The above phenomenon can be seen from FIG. 4. This figure shows the frequency response of the entire filter branch 320, i.e. its equivalent impedance 410, resistance 420, reactance 430 as a function of frequency (expressed as harmonic number). A high resistance around the 5.sup.th and 7.sup.th harmonic can be seen. The double-tuned filters described in Reference (E. W. Kimbark, Direct Current Transmission, Wiley-interscience, 1971, pp. 363-364.) and U.S. Pat. No. 3,038,134 are not able to produce the damping effect for non-characteristic harmonics, as its goal is to filter two characteristic harmonics using one filter branch.

(13) A design method embodies the above principles for the disclosed filter is explained below.

(14) Referring back to FIG. 3, the impedance of the disclosed filter 320 can be expressed as

(15) $\begin{array}{cc}Z\left(\right)=\frac{jL\left(j{C}_{2}R+1\right)}{\left(1-{}^2{C}_{2}R\right)+j{C}_{2}R}-j\frac{1}{C_1}+j{L}_1=R_{\mathrm{eq}}\left(\right)+j{X}_{\mathrm{eq}}\left(\right)& \left(1\right)\end{array}$
where

(16) $\begin{array}{cc}{R}_{\mathrm{eq}}\left(\right)=\frac{{}^4{\mathrm{RL}}_2^2{C}_2^2}{{\left({\mathrm{RC}}_2\right)}^2+{\left({}^2{L}_2{C}_2-1\right)}^2}& \left(2\right)\end{array}$

(17) $\begin{array}{cc}{X}_{\mathrm{eq}}\left(\right)=\frac{L_2\left(1-{}^2{L}_2{C}_2\right)+{}^3{R}^2{L}_2{C}_2^2}{{\left({\mathrm{RC}}_2\right)}^2+{\left(1-{}^2{L}_2{C}_2\right)}^2}-\frac{1}{C_1}+L_1.& \left(3\right)\end{array}$

(18) Design Condition 1: Reactive Power Support.

(19) The filter reactive power output should be determined based on the system reactive power requirement Q.sub.C. This can be used to determine the first series capacitor C.sub.1 as shown below

(20) $\begin{array}{cc}{C}_1=\frac{Q_c}{{}_0{V}_0^2}& \left(4\right)\end{array}$
where V.sub.0 is the system nominal voltage and .sub.0 is the system fundamental frequency.

(21) Design Condition 2: Filter Tuning Frequency.

(22) The filter tuning frequency .sub.t should be equal to the lowest characteristic harmonic frequency. For example, it is the 11th harmonic for a 12-pulse system or the 17th harmonic for a 18-pulse system. The filter tuning frequency Wt is defined here as the frequency where the reactive component of the filter impedance equals to zero, i.e.
X.sub.eq(.sub.t)=0.(5)

(23) Design Condition 3: Harmonic Reduction Capability.

(24) The filter should be able to filter the characteristic harmonic it is tuned to. The degree of harmonic reduction is defined as a user input parameter %, i.e.
I.sub.s(.sub.t)=(1 %)I.sub.L(.sub.t)(6)
where I.sub.L(.sub.t) is the harmonic current generated by the multipulse converter 340, and I.sub.s(.sub.t) is the harmonic current flowing into the system 310 (referring to FIG. 3).

(25) Design Condition 4: Parallel Resonance Frequency of the Frequency-Dependent Resistor Block.

(26) The parallel resonance of the frequency-dependent resistor block 321 is intended to boost the disclosed filter resistance for the non-characteristic harmonics. Therefore, the resonance frequency .sub.p can be selected to be between the two lowest order non-characteristic harmonics (such as between the 5.sup.th and the 7.sup.th harmonics, i.e. the 6.sup.th harmonic). The parallel resonance frequency .sub.p determines the C.sub.2 and L.sub.2 relationship as

(27) $\begin{array}{cc}{C}_2=\frac{1}{{}_p^2{L}_2}.& \left(7\right)\end{array}$

(28) Design Condition 5: Damping Performance of the Non-Characteristic Harmonic Resonance.

(29) The filter must limit the amplification of non-characteristic harmonics. This is achieved by minimizing the worst case harmonic amplification factor (HAR.sub.worst) as illustrated in (8). This factor presents the worst case voltage amplification ratio at a specific harmonic frequency after filter connection.

(30) $\begin{array}{cc}\min \left\{{\mathrm{HAR}}_{\mathrm{worst}}\left(\right)=\sqrt{1+\frac{X_{\mathrm{eq}}^2\left(\right)}{R_{\mathrm{eq}}^2\left(\right)}}\left({}_{5\mathrm{th}}{}_t\right)\right\}.& \left(8\right)\end{array}$

(31) The disclosed filter 320 is composed of five components (first series capacitor C.sub.1, first series inductor L.sub.1, second parallel capacitor C.sub.2, second parallel inductor L.sub.2 and damping resistor R). By solving the above five equations, parameters of all filter components can be determined.

(32) Using an example industry case, Table 1 shows that the disclosed filter can achieve a performance comparable to that of the filter package 120 of FIG. 2. Accordingly, the use of non-characteristic filter branches may be eliminated.

(33) TABLE-US-00001 TABLE 1 HARMONIC CURRENT FLOWING INTO SYSTEM Harmonic currents as a percentage of the fundamental frequency current (h harmonic order) Option 5 7 11 13 17 19 23 25 TDD Without Filter 2.6 1.6 4.5 2.9 0.2 0.1 0.9 0.8 6.3 With Filter Branch 123 4.3 8.2 1.4 1.1 0.1 0.0 0.4 0.3 9.5 With Filter Branch 121, 1.9 0.6 1.3 1.0 0.1 0.0 0.4 0.3 2.7 122 and 123 With Filter Branch 320 3.2 2.0 1.4 0.8 0.1 0.0 0.4 0.3 4.1 IEEE Harmonic 4.0 4.0 2.0 2.0 1.5 1.5 0.6 0.6 5.0 Limitations

(34) Now referring to impedance curve 510 in FIG. 5 which shows the frequency response of the combined system and filter package 120, as seen by the multipulse converter system. The response shows that the impedances are low at the frequencies to which the filter branches 121, 122, 123 are tuned to. However, the impedance can be quite high between two harmonic frequencies. This implies that the filter package 120 causes a resonance at interharmonics frequencies, although it has avoided resonance at harmonic frequencies. Such an interharmonics resonance can amplify interharmonic components, leading to voltage flicker and other power quality issues.

(35) Now referring to impedance curve 520 in FIG. 5 which shows the frequency response of the combined system and the disclosed filter 320, as seen by the multipulse converter system. This figure reveals that the combined impedance is consistently low at various frequencies. As a result, interharmonics resonance is also eliminated. Accordingly, there may be provided a resonance free impedance profile below the filter tuning frequency to suppress the interharmonic amplification.

(36) Although the example case given is for a 12-pulse system, embodiments of the disclosed filter may also be used for other multipulse converter systems such as the 18-pulse or 24-pulse systems.

(37) The location of the disclosed filter is not restricted to that shown in FIG. 3. For example, if the transformer connecting power system and the multipulse system has tertiary winding, the tertiary winding can be a candidate location to connect the disclosed filter (as shown in FIG. 6). Using tertiary winding to connect filter package 120 is disclosed in US patent 2010/0118568 A1. An industrial plant may contains several multipulse converter systems, for this case, the disclosed filter 320 can be located at the point of common coupling of the industrial plant as shown in FIG. 7.

(38) The filter is primarily intended for use in three-phase power systems. It can also be used for single-phase power systems.

(39) Embodiments of the filtering scheme may eliminate the use of non-characteristic harmonic filters for multipulse converter systems. This is achieved through the use of a novel characteristic filter that can provide a high resistance below its tuning frequency. This resistance, in turn, damps the aforementioned resonance. As a result, a single filter branch is sufficient for multipulse convert systems.

(40) Immaterial changes may be made to the disclosed filter without departing from what is claimed.