CN104242291A - Series multi-terminal direct-current power transmission system - Google Patents

Abstract

The invention provides a series multi-terminal direct-current power transmission system. The series multi-terminal direct-current power transmission system comprises converter unit groups, an external high-voltage direct-current polar line and a filter. The converter unit groups include a rectifier unit group and an inverter unit group, the rectifier unit group comprises at least two converter units in sequentially serial connection through at least one internal direct-current bus, and the inserter unit group comprises at least two converter units in sequentially serial connection through at least one internal direct-current bus. A first direct-current terminal of the rectifier unit group and a first direct-current terminal of the inverter unit group are electrically coupled with a first terminal and a second terminal of the external high-voltage direct-current polar line respectively; a second direct-current terminal of the rectifier unit group and a second direct-current terminal of the inverter unit group are electrically coupled with a first earthing pole and a second earthing pole respectively. The filter is positioned between the external high-voltage direct-current polar line and a first direct-current terminal of a first converter unit group in the converter unit groups. By adoption of the topology, external faults and internal faults can be distinguished.

Description

Series-connection multi-terminal direct current system

Technical field

The present invention relates to multi-terminal direct current transmission system, more particularly, relate to series-connection multi-terminal direct current system.

Background technology

Along with the development of power electronic technology, direct current network becomes a development trend, comprises series-connection multi-terminal direct current system and multi-terminal direct current transmission system in parallel etc.Wherein series-connection multi-terminal direct current system has the advantage that construction cost is low, transmission capacity is large.

Fig. 1 illustrates existing ambipolar multi-terminal direct current transmission system.As shown in Figure 1, ambipolar multi-terminal direct current transmission system 1 comprises positive pole multi-terminal direct current transmission system 10 and negative pole multi-terminal direct current transmission system 11.Positive pole multi-terminal direct current transmission system 10 comprises two groups of power converter cells groups, and it is respectively positive pole rectifier unit group 100 and positive pole inverter unit group 101.Positive pole rectifier unit group 100 comprises two the positive pole rectifier unit 100b connected successively by DC bus 100a in the first positive polar region; If need higher electric pressure, the number of positive pole rectifier unit 100b also can higher than two, and in the first positive polar region correspondingly connecting positive pole rectifier unit 100b, DC bus 100a also can more than one.Positive pole inverter unit group 101, comprises at least two the positive pole inverter unit 101b connected successively by DC bus 101a in the second positive polar region; If need higher electric pressure, the number of positive pole inverter unit 101b also can higher than two, and in the second positive polar region correspondingly connecting positive pole rectifier unit 100b, DC bus 101a also can more than one.Ambipolar multi-terminal direct current transmission system 1 also comprises high voltage direct current polar curve 102 outside positive polar region, wherein: a DC terminal of positive pole rectifier unit group 100 and a DC terminal of positive pole inverter unit group 101 respectively with first end and the second end electrical couplings of high voltage direct current polar curve 102 outside positive polar region; And another DC terminal of another DC terminal of positive pole rectifier unit group 100 and positive pole inverter unit group 101 respectively with earth electrode electrical couplings.

Usually, DC filter 100c is had in the two ends cross-over connection of the positive pole rectifier unit 100b near positive pole high voltage direct current polar curve 102, DC filter 101c is had, to suppress to be fed to high fdrequency component from positive pole rectifier unit 100b and positive pole inverter unit 101b to positive pole high voltage direct current polar curve 102 in the two ends cross-over connection of the positive pole inverter unit 101b near positive pole high voltage direct current polar curve 102; DC filter 100d is furnished with between the series connection point and earth electrode of the positive pole rectifier unit 100b of two series connection, DC filter 101d is furnished with, to suppress to be fed to high fdrequency component from positive pole rectifier unit 100b and positive pole inverter unit 101b to positive pole high voltage direct current polar curve 102 between the series connection point and earth electrode of the positive pole inverter unit 101b of two series connection.

Negative pole multi-terminal direct current transmission system 11 comprises two groups of power converter cells groups, and it is respectively negative pole rectifier unit group 110 and negative pole inverter unit group 111.Negative pole rectifier unit group 110 comprises two the negative pole rectifier unit 110b connected successively by DC bus 110a in the first negative pole district; If need higher electric pressure, the number of negative pole rectifier unit 110b also can higher than two, and in the first negative pole district correspondingly connecting negative pole rectifier unit 110b, DC bus 110a also can more than one.Negative pole inverter unit group 111, comprises at least two the negative pole inverter unit 111b connected successively by DC bus 111a in the second negative pole district; If need higher electric pressure, the number of negative pole inverter unit 111b also can higher than two, and in the second negative pole district correspondingly connecting negative pole rectifier unit 110b, DC bus 111a also can more than one.Ambipolar multi-terminal direct current transmission system 1 also comprises high voltage direct current polar curve 112 outside negative pole district, wherein: a DC terminal of negative pole rectifier unit group 110 and a DC terminal of negative pole inverter unit group 111 respectively with first end and the second end electrical couplings of high voltage direct current polar curve 112 outside negative pole district; And another DC terminal of another DC terminal of negative pole rectifier unit group 110 and negative pole inverter unit group 111 respectively with earth electrode electrical couplings.

Usually, DC filter 110c is had in the two ends cross-over connection of the negative pole rectifier unit 110b near negative pole high voltage direct current polar curve 112, DC filter 111c is had, to suppress to be fed to high fdrequency component from negative pole rectifier unit 110b and negative pole inverter unit 111b to positive pole high voltage direct current polar curve 112 in the two ends cross-over connection of the negative pole inverter unit 111b near negative pole high voltage direct current polar curve 112.DC filter 110d is furnished with between the series connection point and earth electrode of the negative pole rectifier unit 110b of two series connection, DC filter 111d is furnished with, to suppress to be fed to high fdrequency component from negative pole rectifier unit 101b and negative pole inverter unit 111b to negative pole high voltage direct current polar curve 112 between the series connection point and earth electrode of the negative pole inverter unit 111b of two series connection.

Principles of Relay Protection for HVDC Transmission line mainly contains two kinds: a kind of is protection based on wavefront amplitude, protection act speed is fast, but there is the problem that anti-transition resistance ability is inadequate, if transition resistance exceedes a certain numerical value, can there is tripping in this protection; Another is current differential protection, for avoiding the impact of line distribution capacitance electric current, must adopt longer time delay, being generally not less than 0.5 second.

Be different from traditional point-to-point two-terminal direct current transmission system, series connection MTDC transmission system has new fault signature.Such as, for traditional point-to-point two-terminal direct current transmission system, not having this fault type of DC Line Fault in district, then there is this fault type in multiterminal series direct current electrical network.Traditional route protection for point-to-point two-terminal direct current transmission system can encounter new problem.Specifically, as shown in Figure 1, for two adjacent circuits, such as electric pressure be 400kV the first positive polar region in DC bus 100a and electric pressure be the positive pole high voltage direct current polar curve 102 of 800kV.For the protection of 400kV circuit; when breaking down in 800kV AC line; row ripple can not through alternating current filter (not shown) outside first DC terminal and district of the first power converter cells group 100 between high voltage direct current polar curve 102, and is through DC filter 100c and directly enters 400kV circuit.Like this, this fault occurs, for the protection of 400kv circuit, row ripple during fault has the amplitude of voltage change ratio with close during external area error and Geng Gao, thus can not distinguish with the fault phase on 400kV circuit.

Fig. 2 illustrates the high-pressure side power converter cells of the traditional bipolar multi-terminal direct current transmission system according to Fig. 1.As shown in Figure 2, two DC terminal of the valve group of power converter cells 100b are connected in series two smoothing reactor L10a respectively, L10b, wherein smoothing reactor L10a can be electrically connected high voltage direct current polar curve 102 outside district, and smoothing reactor L10b can be electrically connected DC bus 100a in district; Two DC terminal of the valve group of power converter cells 101b are connected in series two smoothing reactor L10c, L10d respectively, and wherein smoothing reactor L10c can be electrically connected high voltage direct current polar curve 102 outside district, and smoothing reactor L10b can be electrically connected DC bus 101a in district; Two DC terminal of the valve group of power converter cells 110b are connected in series two smoothing reactor L11a, L11b respectively, and wherein smoothing reactor L11a can be electrically connected high voltage direct current polar curve 112 outside district, and smoothing reactor L10b can be electrically connected DC bus 110a in district; Two DC terminal of the valve group of power converter cells 111b are connected in series two smoothing reactor L11c respectively, and L11d wherein smoothing reactor L11c can be electrically connected high voltage direct current polar curve 112 outside district, and smoothing reactor L11b can be electrically connected DC bus 111a in district.

Summary of the invention

For above-mentioned technical problem, the invention provides a kind of series-connection multi-terminal direct current system, comprising: power converter cells group, it is respectively rectifier unit group and inverter unit group; With high voltage direct current polar curve outside district.Rectifier unit group comprises at least two power converter cells of being connected successively by DC bus at least oneth district; Inverter unit group comprises at least two power converter cells of being connected successively by DC bus at least oneth district; First DC terminal of rectifier unit group and the first DC terminal of inverter unit group respectively with first end and the second end electrical couplings of high voltage direct current polar curve outside district; Second DC terminal of rectifier unit group and the second DC terminal of inverter unit group respectively with the first earth electrode and the second earth electrode electrical couplings; Filter, outside first DC terminal of its first power converter cells group in power converter cells group and district between high voltage direct current polar curve.This restrained effectively the protection for DC bus in the first positive polar region, and row ripple during fault has the amplitude of voltage change ratio with close during external area error and Geng Gao, thus external area error can be distinguished mutually with troubles inside the sample space.

Accompanying drawing explanation

Fig. 1 illustrates existing ambipolar multi-terminal direct current transmission system;

Fig. 2 illustrates the high-pressure side power converter cells of the traditional bipolar multi-terminal direct current transmission system according to Fig. 1;

Fig. 3 illustrates ambipolar multi-terminal direct current transmission system according to an embodiment of the invention; With

Fig. 4 illustrates ambipolar multi-terminal direct current transmission system according to another embodiment of the invention.

Embodiment

Fig. 3 illustrates ambipolar multi-terminal direct current transmission system according to an embodiment of the invention.Shown in Fig. 3, ambipolar multi-terminal direct current transmission system 3 is at least one on high-tension side power converter cells 100b with the difference of the ambipolar multi-terminal direct current transmission system 1 shown in Fig. 1, 101b, 110b, 111b (being with hypographous that to represent on high-tension side power converter cells in same group of power converter cells group) improves, filter is arranged between high voltage direct current polar curve outside first DC terminal and described district of the first power converter cells group in power converter cells group, it is for carrying out ac filter the fault component signal from high voltage direct current polar curve outside described district and carry out DC filtering for the output signal from described power converter cells group.Such as, in the first power converter cells group, (the first power converter cells group can be power converter cells group 100,101,110, any one group in 111, or wherein some groups, or series AC filter (such as smoothing reactor) between high voltage direct current polar curve outside the first DC terminal all) and district, and cross-over connection DC filter between being connected in series a little of at least two power converter cells of connecting successively in first DC terminal and the first power converter cells group of the first power converter cells group.Specifically, series AC filter 300a between high voltage direct current polar curve 102 outside first DC terminal and positive polar region of the first power converter cells group 100, and at least two the power converter cells 100b connected successively in first DC terminal and the first power converter cells group 100 of the first power converter cells group 100 are connected in series cross-over connection DC filter 100c between a 300c, series AC filter 301a between high voltage direct current polar curve 102 outside first DC terminal and positive polar region of the first power converter cells group 101, and at least two the power converter cells 101b connected successively in first DC terminal and the first power converter cells group 101 of the first power converter cells group 101 are connected in series cross-over connection DC filter 101c between a 301c, series AC filter 310a between high voltage direct current polar curve 112 outside first DC terminal and negative pole district of the first power converter cells group 110, and at least two the power converter cells 110b connected successively in first DC terminal and the first power converter cells group 110 of the first power converter cells group 110 be connected in series cross-over connection DC filter 110c between a 310c, series AC filter 311a between high voltage direct current polar curve 112 outside first DC terminal and negative pole district of the first power converter cells group 111, and at least two the power converter cells 111b connected successively in first DC terminal and the first power converter cells group 111 of the first power converter cells group 111 be connected in series cross-over connection DC filter 111c between a 311c.By above-mentioned improvement, for power converter cells group 100, radio circuit is formed by DC filter 100c, can suppress to be fed to high fdrequency component from positive pole rectifier unit 100b to positive pole high voltage direct current polar curve 102, when alternating current filter 300a can make to break down on positive pole high voltage direct current polar curve 102 simultaneously, row ripple can not DC filter outside the first DC terminal of the first power converter cells group 100 and district between high voltage direct current polar curve 102, and be through alternating current filter 300a and enter DC bus 100a in the first positive polar region, this restrained effectively the protection for DC bus 100a in the first positive polar region, row ripple during fault has the amplitude of voltage change ratio with close during external area error and Geng Gao, thus external area error can be distinguished mutually with troubles inside the sample space.

Preferably, shown in Fig. 3, ambipolar multi-terminal direct current transmission system 3 is also to change high-pressure side power converter cells 100b, 101b with the difference of the ambipolar multi-terminal direct current transmission system 1 shown in Fig. 1, the smoothing reactor L10a of 110b, 111b, L10c, the inductance value of L11a, L11c.Specifically, the alternating current filter 300a according to Fig. 3,301a, the high-pressure side power converter cells 100b that the inductance value of 310a, 311a is connected with it, 101b, the smoothing reactor L10a of 110b, 111b, L10c, L11a, the inductance value of L11c is identical respectively, and itself and equal the smoothing reactor L10a of the ambipolar multi-terminal direct current transmission system 1 shown in Fig. 1 respectively, L10c, the inductance value of L11a, L11c.Such as, the inductance value of the alternating current filter 300a shown in Fig. 3 and the inductance value sum of smoothing reactor L10a equal the inductance value of the smoothing reactor L10a according to Fig. 1.The row ripple that external fault produces can be stopped like this, make this protection philosophy can distinguish inside and outside fault.And this configuration makes cost compare low, more existing scheme cost increases few.

Fig. 4 illustrates ambipolar multi-terminal direct current transmission system according to another embodiment of the invention.Equally, shown in Fig. 4, ambipolar multi-terminal direct current transmission system 4 is at least one on high-tension side power converter cells 100b with the difference of the ambipolar multi-terminal direct current transmission system 1 shown in Fig. 1, 101b, 110b, 111b (being with hypographous that to represent on high-tension side power converter cells in same group of power converter cells group) improves, filter is arranged between high voltage direct current polar curve outside first DC terminal and described district of the first power converter cells group in power converter cells group, it is for carrying out ac filter the fault component signal from high voltage direct current polar curve outside described district and carry out DC filtering for the output signal from described power converter cells group.Such as, in described first power converter cells group, (the first power converter cells group can be power converter cells group 100,101,110, any one group in 111, or wherein some groups, or all) the first DC terminal and the 3rd earth electrode between arrange DC filter.Specifically, DC filter 400 is arranged between first DC terminal of the first power converter cells group 100 and the 3rd earth electrode, DC filter 401 is arranged between first DC terminal of the first power converter cells group 101 and the 3rd earth electrode, arrange DC filter 410 between first DC terminal of the first power converter cells group 110 and the 3rd earth electrode, between the first DC terminal of the first power converter cells group 111 and the 3rd earth electrode, arrange DC filter 411.By above-mentioned improvement, for power converter cells group 100, radio circuit is formed through the earth by DC filter 400 and DC filter 100d, can suppress to be fed to high fdrequency component from positive pole rectifier unit 100b to positive pole high voltage direct current polar curve 102, when alternating current filter 400 can make to break down on positive pole high voltage direct current polar curve 102 simultaneously, row ripple can not DC filter outside the first DC terminal of the first power converter cells group 100 and district between high voltage direct current polar curve 102, and be through alternating current filter 300a and enter DC bus 100a in the first positive polar region, this restrained effectively the protection for DC bus 100a in the first positive polar region, row ripple during fault has the amplitude of voltage change ratio with close during external area error and Geng Gao, thus external area error can be distinguished mutually with troubles inside the sample space.

Above-mentioned describing from circuit topological structure angle according to embodiments of the invention distinguishes external area error and troubles inside the sample space.From the angle of signal transacting, the ambipolar multi-terminal direct current transmission system 3,4 shown in Fig. 3 and Fig. 4 also comprises detection part 30,31,32, and 33 and processing unit 34.Detection part 30,31,32,33 can be voltage transformer or current transformer for detecting voltage or the current signal of DC bus 110a, 111a in DC bus 100a, 101a and negative pole district in positive polar region.Processing unit 34 can according to detection part 30, and the rate of change of 31,32, the 33 fault component signals detected judges whether described first converter cell group exists troubles inside the sample space.Processing unit 34 and detection part 30,31,32,33 communicate by network 35.

The algorithm distinguishing external area error and troubles inside the sample space is described for the first converter cell group 100 below.

Step 1: calculate fault component electric current and fault component voltage

$\left\{\begin{array}{c}{\mathrm{\Δu}}_{\mathrm{LP}}\left(t\right)=u_{\mathrm{LP}}\left(t\right)-u_{\mathrm{LP}}(t-T)\\ {\mathrm{\Δu}}_{\mathrm{LN}}\left(t\right)=u_{\mathrm{LN}}\left(t\right)-u_{\mathrm{LN}}(t-T)\\ {\mathrm{\Δi}}_{\mathrm{LP}}\left(t\right)=i_{\mathrm{LP}}\left(t\right)-i_{\mathrm{LP}}(t-T)\\ {\mathrm{\Δi}}_{\mathrm{LN}}\left(t\right)=i_{\mathrm{LN}}\left(t\right)-i_{\mathrm{LN}}(t-T)\end{array}\right.---\left(1\right)$

U _lPand u _lNdC line positive pole and cathode voltage respectively, i _lPand i _lNdC line positive pole and cathodal current respectively.Δ u _lPwith Δ u _lNcalculate gained fault component positive pole and negative pole fault component voltage, Δ i according to above formula _lPwith Δ i _lNcalculate gained fault component positive pole and negative pole fault component electric current according to above formula.Above in formula, T time definite value, e.g.10ms or100ms, sets on demand.

Step 2: the troubles inside the sample space component signal that the troubles inside the sample space component signal detected based on the first detection part 30 and the second detection part 32 detect calculates described first converter cell group troubles inside the sample space component signal decoupling zero value.Such as, carry out pole modular transformation, (becoming modulus from maximum dose), the maximum dose Δ i that wushu (1) calculates _lP(t), Δ u _lP(t), Δ i _lN(t), Δ u _lNt () becomes modulus.

Transformation matrix is as follows.

$\left[\begin{array}{c}{\mathrm{\Δu}}_0\\ {\mathrm{\Δu}}_1\end{array}\right]=\left[\begin{array}{cc}1& 1\\ 1& -1\end{array}\right]\left[\begin{array}{c}{\mathrm{\Δu}}_{\mathrm{LP}}\\ {\mathrm{\Δu}}_{\mathrm{LN}}\end{array}\right]---\left(2\right)$

$\left[\begin{array}{c}{\mathrm{\Δi}}_0\\ {\mathrm{\Δi}}_1\end{array}\right]=\left[\begin{array}{cc}1& 1\\ 1& -1\end{array}\right]\left[\begin{array}{c}{\mathrm{\Δi}}_{\mathrm{LP}}\\ {\mathrm{\Δi}}_{\mathrm{LN}}\end{array}\right]---\left(3\right)$

Δ u ₀with Δ u ₁common-mode voltage and differential mode voltage.Δ i ₀with Δ i ₁common mode current and differential-mode current respectively.Due to the existence of mutual inductance between positive pole high voltage direct current polar curve 102 and negative pole high voltage direct current polar curve 112 in ambipolar multi-terminal direct current transmission system, the voltage and current of a pole is subject to the impact of the voltage and current of another one pole.By pole modular transformation, this impact can be eliminated.

Step 3: calculated direction row ripple

Shown in the capable ripple of forward differential mode and the following formula of the capable ripple of reverse differential mode:

$u_{\mathrm{Fwd}1}=\frac{1}{2}({\mathrm{\Δu}}_1+Z_{c1}\×{\mathrm{\Δi}}_1);u_{\mathrm{Rvs}1}=\frac{1}{2}({\mathrm{\Δu}}_1-Z_{c1}\×{\mathrm{\Δi}}_1)---\left(4\right)$

U _fwd1the capable ripple of forward differential mode, u _rvs1the capable ripple of reverse differential mode.

Common mode direct wave and the following formula of returning wave:

$u_{\mathrm{Fwd}0}=\frac{1}{2}({\mathrm{\Δu}}_0+Z_{c0}\×{\mathrm{\Δi}}_0);u_{\mathrm{Rvs}0}=\frac{1}{2}({\mathrm{\Δu}}_0-Z_{c0}\×{\mathrm{\Δi}}_0)---\left(5\right)$

U _fwd0forward common mode row ripple, u _ivs0it is reverse common mode row ripple

Step 4: based on the rate of change of calculated fault component signal decoupling zero value, judge whether domain of the existence internal fault.Such as:

1 rank are reciprocal for distinguishing inside and outside fault, if be greater than threshold u _set, can internal fault be determined.

Criterion is

$\left|\frac{{\mathrm{du}}_{\mathrm{Rvs}1}}{\mathrm{dt}}\right|>u_{\mathrm{set}}---\left(6\right)$

Similar, also can be used for distinguishing fault.

As embodiment, circuit topological structure of the present invention is described using ambipolar series-connection multi-terminal direct current system above, it comprises the positive pole multi-terminal direct current transmission system 10 and negative pole multi-terminal direct current transmission system 11 that are all monopole type multi-terminal direct current transmission system.Those skilled in the art should understand, and the improvement for positive pole multi-terminal direct current transmission system 10 or negative pole multi-terminal direct current transmission system 11 is equally applicable to monopole type multi-terminal direct current transmission system.From the angle of signal transacting, the algorithm distinguishing external area error and troubles inside the sample space is described below for positive pole multi-terminal direct current transmission system 10.

Step 1: calculate fault component electric current and fault component voltage

$\left\{\begin{array}{c}{\mathrm{\Δu}}_L\left(t\right)=u_L\left(t\right)-u_L(t-T)\\ {\mathrm{\Δi}}_L\left(t\right)=i_L\left(t\right)-i_L(t-T)\end{array}\right.---\left(7\right)$

U _land i _laC line voltage and electric current respectively, Δ u _lwith Δ i _lcalculate gained fault component voltage and current according to above formula.

Above in formula, T time definite value, e.g.10ms or100ms, sets on demand.

Step 2: calculated direction row ripple

Shown in direct wave and the following formula of returning wave:

$u_{\mathrm{Fwd}}=\frac{1}{2}({\mathrm{\Δu}}_L+Z_c\×{\mathrm{\Δi}}_L);u_{\mathrm{Rvs}}=\frac{1}{2}({\mathrm{\Δu}}_L-Z_c\×{\mathrm{\Δi}}_L)---\left(8\right)$

U _fwddirect wave, u _rvsreturning wave, Z _csurge impedance of a line.

Step 4: based on calculated direction row ripple rate of change, judge whether domain of the existence internal fault.Such as:

1 rank are reciprocal for distinguishing inside and outside fault, if be greater than threshold u _set, can internal fault be determined.

Criterion is

$\left|\frac{{\mathrm{du}}_{\mathrm{Rvs}}}{\mathrm{dt}}\right|>u_{\mathrm{set}}---\left(9\right)$

Although illustrate and describe the present invention with reference to some preferred embodiment of the present invention, but it will be appreciated by those skilled in the art that, when not deviating from the spirit and scope of the present invention as defined in appended claims, can make a variety of changes it in the form and details.

Claims (6)

1. a series-connection multi-terminal direct current system, comprising:

Power converter cells group, it is respectively rectifier unit group and inverter unit group; With

High voltage direct current polar curve outside district;

Wherein:

Described rectifier unit group comprises at least two power converter cells of being connected successively by DC bus at least oneth district;

Described inverter unit group comprises at least two power converter cells of being connected successively by DC bus at least oneth district;

First DC terminal of described rectifier unit group and the first DC terminal of described inverter unit group respectively with first end and the second end electrical couplings of high voltage direct current polar curve outside described district;

Second DC terminal of described rectifier unit group and the second DC terminal of described inverter unit group respectively with the first earth electrode and the second earth electrode electrical couplings;

Filter, outside first DC terminal of its first power converter cells group in described power converter cells group and described district between high voltage direct current polar curve, it is for carrying out ac filter the fault component signal from high voltage direct current polar curve outside described district and carry out DC filtering for the output signal from described power converter cells group.

2. series-connection multi-terminal direct current system as claimed in claim 1, wherein:

Described filter comprises:

Alternating current filter outside first DC terminal and described district of described first power converter cells group between high voltage direct current polar curve; With

Be connected across the DC filter between being connected in series a little of at least two power converter cells of connecting successively in the first DC terminal of described first power converter cells group and described first power converter cells group.

3. series-connection multi-terminal direct current system as claimed in claim 1, wherein:

Described filter comprises:

DC filter between first DC terminal and the 3rd earth electrode of described first power converter cells group.

4. as described in series-connection multi-terminal direct current system as described in one of claim, also comprise:

Detection part, it can detect the troubles inside the sample space component signal of described first converter cell group; With

Processing unit, the rate of change of its fault component signal that can detect according to described detection part judges whether described first converter cell group exists troubles inside the sample space.

5. an ambipolar series-connection multi-terminal direct current system, comprises the positive pole series-connection multi-terminal direct current system as described in one of claim 1-3 and the negative pole series-connection multi-terminal direct current system as described in one of claim 1-3, wherein:

First earth electrode of positive pole series-connection multi-terminal direct current system and the very same earth electrode of the first ground connection of negative pole series-connection multi-terminal direct current system;

Second earth electrode of positive pole series-connection multi-terminal direct current system and the very same earth electrode of the second ground connection of negative pole series-connection multi-terminal direct current system; And

First power converter cells of positive pole series-connection multi-terminal direct current system and the first power converter cells of negative pole series-connection multi-terminal direct current system are all rectifier unit or are all inverter unit;

Also comprise:

First detection part, it can detect the troubles inside the sample space component signal of the first converter cell group of described positive pole series-connection multi-terminal direct current system;

Second detection part, it can detect the troubles inside the sample space component signal of the first converter cell group of described negative pole series-connection multi-terminal direct current system;

Processing unit, in first converter cell group of its first converter cell group for described positive pole series-connection multi-terminal direct current system and described negative pole series-connection multi-terminal direct current system one of at least, the troubles inside the sample space component signal that the troubles inside the sample space component signal detected based on the first detection part and the second detection part detect calculates described first converter cell group troubles inside the sample space component signal decoupling zero value, and based on the rate of change of calculated fault component signal decoupling zero value, judge whether domain of the existence internal fault.

6. ambipolar series-connection multi-terminal direct current system as claimed in claim 5, wherein:

Described fault component signal decoupling zero value is differential mode value or common mode value.