CN115513993A - Leakage current suppression double modulation wave modulation method based on non-isolated photovoltaic inverter - Google Patents
Leakage current suppression double modulation wave modulation method based on non-isolated photovoltaic inverter Download PDFInfo
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention discloses a leakage current suppression double modulation wave modulation method based on a non-isolated photovoltaic inverter, which is characterized in that the topological structure of an inverter system is simplified by establishing a non-isolated photovoltaic grid-connected L-shaped filtering three-level inverter topological structure, an L-shaped filtering leakage current model is established, and an approximate expression of common-mode voltage and leakage current is deduced; then analyzing the change of the working state of the inverter, including the change condition of the common-mode voltage, when a CBPWM carrier in-phase stacking method is adopted; according to the analysis result, the voltage is output in a carrier period when different modulation wave sizes are ensuredu AB The change condition of the inverter is kept unchanged, namely, the common-mode voltage is changed on the basis of ensuring that the output voltage of the inverter is not changedv cm Change of (3)U d /4→U d /2→3U d /4→U d /2→3U d (ii)/4; finally, the common-mode voltage change condition is analyzed, and a double modulation wave modulation strategy is adopted to limit the common-mode voltage to be 3U d /4 andU d a change between/2. The invention provides reference for improving the efficiency of the non-isolated photovoltaic inverter.
Description
Technical Field
The invention relates to the technical field of photovoltaic power generation, in particular to a leakage current inhibiting double modulation wave modulation method based on a non-isolated photovoltaic inverter.
Background
In the renewable energy field, photovoltaic power generation has the characteristics of relatively low cost, low noise and convenient installation, becomes an important member of new energy strategies, and is researched more and more widely. Wherein small capacity single-phase non-isolated inverter systems are becoming more and more important. The Neutral Point Clamped (NPC) three-level topology is one of the most mature and applied topologies. However, the leakage current of the system is relatively large, so that the network access current is distorted, extra loss of the system is increased, and the safety of the system is threatened in serious cases. Therefore, the research on the modulation strategy for reducing the amplitude of the common-mode voltage and inhibiting the high-frequency leakage current is of great significance.
Because the non-isolated inverter system does not have a transformer, electrical connection exists between the direct current side photovoltaic panel and the grid side power supply. High-frequency common mode current (namely leakage current) can pass through a resonant circuit formed by a photovoltaic panel to ground parasitic capacitance, a grid-connected filter and the like, so that grid-connected current is distorted, and the operation safety of a system is threatened when the grid-connected current is serious.
The magnitude of the parasitic capacitance value of the photovoltaic panel to ground is related to many factors, such as the material structure of the photovoltaic panel and weather conditions. The capacitance value is generally considered to be 50-150nF/kW, the specific value depending on the weather conditions and the photovoltaic panel structure. Regarding the safety specification of the leakage current, the German standard adopted internationally generally stipulates that the amplitude of the leakage current should not exceed 300mA, and when the leakage current exceeds the 300mA, the off-grid operation must be stopped within 0.3 s.
The method for suppressing leakage current of the single-phase system mostly focuses on improvement and innovation of a topological structure, for example, with the help of an auxiliary loop, the voltage at two ends of a parasitic capacitor is clamped to be zero by directly connecting a zero line of a power grid and a negative pole of a direct-current side, a clamping branch circuit is added to convert a Heric unit to achieve almost constant common-mode voltage, and a secondary capacitive voltage divider connected to a neutral point end of the power grid is added to keep the common-mode voltage constant. But these all add extra branches and devices, and the economic performance is reduced.
The method for inhibiting the leakage current through software has wide research and application in a three-phase system, and is mainly used for reasonably selecting and matching vectors in a traditional Space Vector Pulse Width Modulation (SVPWM) method. The vector with small common-mode voltage can be selected for synthesis by designing an optimized SVPWM control algorithm, the common-mode voltage is controlled to be one sixth of the voltage on the direct current side, and meanwhile, the output voltage waveform has good quality. Or a modified LMSVM (Large-Medium-Small Vector Modulation) method is used, a Small Vector of high common-mode voltage is used, but the change of the common-mode voltage in each switching period is limited to U d And 6, the leakage current is well suppressed. The method is characterized in that a topological structure with a virtual ground wire is applied, the improvement is further carried out on the aspect of software, and an LMZVM-based modulation strategy is adopted&The vector mixed modulation of the LMSVM further reduces the leakage current by reducing the change rate of the common-mode voltage, and the selection of the strategy vector pair also realizes the neutral point potential balance on the direct current side.
In summary, the leakage current software suppression strategy mostly focuses on the research on a three-phase system, and the algorithm is complex and difficult to implement, while a single-phase system lacks an optional vector for controlling the common-mode voltage due to the reduction of the number of voltage vectors, and there is a great research space for how to suppress the leakage current of the single-phase system through a modulation algorithm.
Disclosure of Invention
In view of the above problems, the present invention provides a leakage current suppression double modulation wave modulation method based on a non-isolated photovoltaic inverter for a non-isolated three-level inverter system, and a novel double modulation wave is applied to change the switching sequence of a switching tube, so as to achieve a modulation strategy for limiting the common-mode voltage amplitude and the common-mode current amplitude. The technical scheme is as follows:
a leakage current suppression double modulation wave modulation method based on a non-isolated photovoltaic inverter comprises the following steps:
step 1: establishing a topological structure of a non-isolated photovoltaic grid-connected L-type filtering three-level inverter;
and 2, step: simplifying the topological structure of an inverter system, constructing an L-shaped filtering leakage current model, and deducing an approximate expression of common-mode voltage and leakage current on the basis;
and step 3: analyzing the change of the working state of the inverter when a CBPWM carrier in-phase stacking method is adopted, including the change condition of a common-mode voltage;
and 4, step 4: according to the analysis result, the voltage u is output in a carrier period when different modulation wave sizes are ensured AB On the basis of ensuring that the output voltage of the inverter is not changed, the common-mode voltage v is changed cm Is 3U d /4→U d /2→3U d /4→U d /2→3U d /4, wherein U d Is a dc bus voltage;
and 5: analyzing the common-mode voltage variation condition, and limiting the common-mode voltage to be 3U by adopting a double modulation wave modulation strategy d /4 and U d A change between/2.
Further, the changing of the inverter operating state in step 3 includes: in a carrier period, the change sequence of the common-mode voltage is consistent, and the modulation wave only influences the ratio of different levels; when the resonant frequency of the resonant circuit through which the leakage current passes is at the switching frequency f s When nearby, the common mode current generated by the common mode voltage will be large; if the amplitude of the common mode voltage is limited to 3U d /4 and U d The amplitude of the leakage current is suppressed by the variation between/2.
Further, the dual modulation wave modulation strategy is specifically as follows:
in a non-isolated photovoltaic grid-connected L-shaped filtering three-level inverter topological structure,
wherein u is m In order to modulate the wave,
and
respectively an A-phase modulation wave and a B-phase modulation wave; u. of am.p And u am.n Respectively, a phase A upper modulation wave with a positive value and a phase A lower modulation wave with a negative value; u. of bm.p And u bm.n Respectively a positive B-phase upper modulation wave and a negative B-phase lower modulation wave;
the output strategy is summarized as follows:
when the upper carrier is > u am.p Or u am.n When the download waves are larger than the download waves, the phase A outputs the P level;
when the upper carrier is > u am.p And u is am.n Downloading > downloadWhen waves are generated, the phase A outputs N level;
when u is bm.p Either the upper carrier or the lower carrier > u bm.n When the voltage is higher than the reference voltage, the phase B outputs a level P;
when u is bm.p Upper and lower carrier > u bm.n When the voltage is higher than the voltage, the phase B outputs N level;
substituting the modulated wave expressions (1) to (6) into the output strategy to find that the phase A is in the phase u m When the output voltage is less than or equal to-0.5, the P level and the N level are simultaneously output, wherein the N level is output correctly, and the P level is not output, so that a switching tube S in a topological structure of the non-isolated photovoltaic grid-connected L-type filtering three-level inverter is controlled a1 And S a4 Is separated to control the switch tube S a1 Is open-and-close
Instead 1, the P level will not be output;
in the same way, phase B is in u m When the output voltage is more than or equal to 0.5, the P level and the N level are simultaneously output, wherein the N level is output correctly, and the P level is not output, so that the switching tube S is controlled b1 And S b4 Is separated to control the switch tube S b1 Is open-and-close
Is changed into-u m The P level will no longer be output;
control switch tube S x3 (signal and control switch tube S) x1 Control the switch tube S by signal complementation x2 Signal and controlled switching tube S x4 Signal complement, where x = a or b;
switch tube S a1 In control signal generating circuits
When the upper carrier is > u am.p Or u am.n When downloading wave, the control signal outputs high level, the switch tube S a1 Conducting, and outputting P level by phase A; switch tube S a4 In control signal generating circuits
As shown in formula (1);
when the upper carrier is > u am.p And u is am.n When downloading wave, switch tube S a4 Control signal output high level, switch tube S a4 Conducting, outputting N level by phase A, and outputting 0 level by other conditions;
switch tube S b1 In control signal generating circuits
When u is bm.p Upper carrier or lower carrier > u bm.n When the control signal is output to high level, the switch tube S is switched b1 Conducting, and outputting P level by B phase; switch tube S b4 In control signal generating circuits
As shown in formula (4);
when u is bm.p Upper and lower carrier > u bm.n Time, switch tube S b4 Control signal output high level, switch tube S b4 And (4) conducting, outputting N level by the phase B, and outputting 0 level by the other conditions.
The invention has the beneficial effects that: aiming at the inherent high-frequency common-mode current problem of a non-isolated three-level inverter system, the invention reasonably sequences the switching sequence by adjusting the working state of the inverter under the condition of not changing any hardware topological structure and limits the common-mode voltage to be 3U d 4 and U d And 2, the modulation strategy for limiting the common-mode voltage amplitude and the common-mode current amplitude is achieved, and the method provides reference for improving the efficiency of the non-isolated photovoltaic inverter.
Drawings
Fig. 1 is a schematic diagram of a topological structure of a non-isolated photovoltaic grid-connected L-type filtering three-level inverter.
FIG. 2 is a leakage current model equivalent diagram of a non-isolated photovoltaic grid-connected L-type filtering three-level inverter topological structure; (a) a leakage current model; (b) Considering only v AO Simplified circuit diagram of action, where v AO Is the a-phase voltage.
FIG. 3 is a diagram showing the change of the operating state of the inverter when the CBPWM carrier in-phase stacking method is adopted; (a) U is more than 0 m ＜0.5；(b)u m ≥0.5；(c)-0.5＜u m ＜0；(d)u m ≤-0.5。
Fig. 4 shows the change of the system operating state and the common mode voltage in a carrier period under different modulation wave sizes.
Fig. 5 shows the variation of the common mode voltage when the output voltage of the inverter is kept constant.
FIG. 6 is a diagram of A, B phase dual modulation wave size and inverter output operating state in one carrier period; (a) U is more than 0 m ＜0.5；(b)u m ≥0.5；(c)-0.5＜u m ＜0；(d)u m ≤-0.5。
FIG. 7 is a comparison graph showing the simulation of leakage current waveforms for two modulation strategies of single modulation wave and double modulation wave of the present invention; adopting a single wave modulation method; and (b) adopting a new double modulation wave method.
Fig. 8 shows the amplitude of the leakage current according to the dual modulation method of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
The invention provides a double modulation wave modulation method for changing the switching sequence of a switching tube by applying a novel double modulation wave aiming at a non-isolated three-level inverter system, which achieves the effect of limiting the common-mode voltage amplitude and the common-mode current amplitude, and the specific scheme is as follows:
1. l-shaped filter modeling calculation
Because the hardware cost is increased by adopting LCL filtering, the grid connection of the invention adopts L filtering, and the non-isolated topological structure is shown in figure 1.
Wherein R is p 、C p Respectively a parasitic resistance and a capacitance of the photovoltaic panel to the ground. A leakage current model for this topology is given below (fig. 2 (a)), and an approximate expression for the leakage current is obtained.
When the A phase outputs P level, v AO ＝U d (ii) a When phase A outputs 0 level, v AO ＝U d /2；(ii) a When the A phase outputs N level, v AO And =0.B is similar. So v AO 、v BO Is a high frequency rectangular pulse wave, and a leakage current is generated thereby. The frequency of the network side power supply is 50Hz which is far less than the switching frequency (set to 10 kHz), the impedance of the resonant circuit is very large, so the leakage current generated by the network side power supply is ignored, and only two high-frequency pulse voltages are considered. First consider v AO Acting alone, the circuit is simplified as in fig. 2 (b).
i cm1 Is v AO The leakage current generated by the single action can be easily obtained by the same method BO Leakage current generated by single action
Integrating the two formulas according to the superposition principle to obtain an approximate expression of the leakage current:
wherein the common mode voltage
The amplitude high frequency transformation of the current is the cause of the leakage current.
2. Software throttling strategy
As can be seen from the expression of the leakage current in the formula (3), the magnitude thereof depends on the common mode voltage v cm Magnitude and frequency of (c). If the common mode voltage can be maintained at a stable DC value, i.e. v cm And constant, the system does not generate leakage current.
In a three-phase non-isolated three-level NPC (neutral vector pulse width modulation) inversion system, an SVPWM (space vector pulse width modulation) method is generally adopted, and a certain value common-mode voltage is avoided by selecting a proper voltage vector, for example, only a medium vector and a zero vector (000) are selected to ensure that the common-mode voltage is not changed to inhibit leakage current.
In a single-phase non-isolated three-level NPC (neutral point clamped) inversion system, if an SVPWM (space vector pulse width modulation) method is adopted, only a vector can be selected to keep a common-mode voltage unchanged
(see table 1), however, the inverter can only output two levels, and the advantage of improving the harmonic characteristics by outputting three levels is lost.
TABLE 1 different working states and their working voltages
The change of the inverter working state is shown in fig. 3 by using the CBPWM carrier in-phase lamination method.
Table 2 lists the change of the system operating state and the common mode voltage in the next carrier cycle under different modulation wave sizes.
TABLE 2 working conditions and common mode Voltage variations
The common-mode voltage is changed in the same sequence in one carrier period, and the modulation wave only influences the occupation ratio of different levels. The variation of the common mode voltage is shown as the actual waveform of fig. 4, and the variation of the fundamental wave. It can be seen that the fundamental frequency is the switching frequency f s Amplitude of U d /4。
When the leakage current passes through the resonant circuit, the resonant frequency is f s The common mode current generated by this common mode voltage will be large when nearby. If the amplitude of the common mode voltage can be limited to 3U d /4 and U d A variation of/2, the magnitude of the leakage current will be well suppressed.
3. Modulation strategy
Ensuring the output voltage of the inverter to be constant, i.e. the output voltage u is within one carrier period when the modulation waves are different AB The changes of (a) remain unchanged as shown in table 2. On the basis of which the common-mode voltage v is varied cm Is 3U d /4→U d /2→3U d /4→U d /2→3U d /4, this can be achieved by changing the operating state to table 3.
TABLE 3 New operating State Change
The common mode voltage changes according to the actual waveform shown in fig. 5 and the fundamental wave changes. It can be seen that the fundamental frequency is twice the switching frequency 2f s Amplitude of U d /8. When the resonant frequency of the resonant circuit is f s Near, at 2f s The impedance of the nearby loop is increased, and the amplitude of the common-mode voltage is reduced by half at the moment, so that the leakage current is well restrained.
A new dual modulation strategy is introduced to realize the operation state change condition shown in Table 3, limiting the common mode voltage to 3U d /4 and U d A change between/2.
(1)0＜u m If < 0.5, A, B phase double modulation wave size and inverter output operation state in one carrier cycle are shown in fig. 6 (a). The red dotted line is a modulated wave of phase a under the monotone modulation strategy (since the frequency of the modulated wave is far less than the carrier frequency, the modulated wave is regarded as a constant in one carrier period), and the green dotted line is a modulated wave of phase B under the original modulation strategy. The red solid line is the modulation wave of the A phase under the modulation strategy of the dual modulation wave, and the value is positive that the modulation wave u on the A phase am.p The value of which is negative is the modulation wave u in phase A am.n (ii) a And the green solid line is the modulation wave of the B phase under the new modulation strategy.
When the upper carrier is > u am.p Or u am.n When downloading waves, phase AOutputting a P level; when u is bm.p Either the upper carrier or the lower carrier > u bm.n At this time, the B phase outputs the P level.
(2)u m When the amplitude is not less than 0.5, the A, B phase double modulation wave size and the inverter output working state in one carrier cycle are shown in fig. 6 (b). When the upper carrier is > u am.p Or u am.n When downloading waves, the phase A outputs P level; when u is bm.p Upper and lower carrier > u bm.n At this time, the B phase outputs N level.
(3)-0.5＜u m When the amplitude is less than 0, A, B phase double modulation wave size and inverter output operation state in one carrier period are shown in fig. 6 (c). When the upper carrier is > u am.p Or u am.n When downloading waves, the phase A outputs P level; when u is bm.p Either the upper carrier or the lower carrier > u bm.n At this time, the B phase outputs the P level.
(4)u m When the amplitude is less than or equal to-0.5, the A, B phase double modulation wave size and the inverter output working state in one carrier period are shown in fig. 6 (d). When the upper carrier is > u am.p And u is am.n When the waves are downloaded, the A phase outputs N level; when u is bm.p Either the upper carrier or the lower carrier > u bm.n At this time, the B phase outputs the P level.
Summarizing the output strategies in (1) - (4), the following can be summarized:
when the upper carrier is > u am.p Or u am.n When downloading waves, the phase A outputs P level; when the upper carrier is > u am.p And u is am.n When the waves are downloaded, the A phase outputs N level; when u is bm.p Either the upper carrier or the lower carrier > u bm.n When the voltage is higher than the reference voltage, the phase B outputs a level P; when u is bm.p Upper and lower carriers > u bm.n At this time, the phase B outputs an N level.
Substituting modulated wave expressions (4) to (9) into the output strategy to find that phase A is in phase u m Outputting P level and N level at the same time when the output voltage is less than or equal to-0.5, wherein the output voltage of N level is correct, and P level should not be output, so S is controlled a1 And S a4 Will control S a1 Is open-and-close
Instead, 1, so that the P level is not output. In the same way, phase B is in u m Outputting P level and N level at the same time when the voltage is not less than 0.5, wherein N level is output correctly, P level should not be output, so S is controlled b1 And S b4 Is divided into control circuits S b1 Is open-and-close
Is changed into-u m So that the P level is not output. Control S x3 (x = a or b) signal and control S x1 Control S by signal complementation x2 Signal and controlled S x4 The signals are complementary.
S a1 In control signal generating circuits
When the upper carrier is > u am.p Or u am.n When the download waves are larger than the preset value, the control signal is output to high powerFlat, S a1 Conducting, and outputting P level by phase A; s. the a4 In control signal generating circuits
As shown in equation (4), when the upper carrier > u am.p And u is am.n When downloading waves, S a4 Control signal output high level, S a4 And conducting, and outputting N level by the phase A.
The remaining cases output a 0 level.
S b1 In control signal generating circuits
When u is bm.p > Up Carrier or Down Carrier > u bm.n The control signal outputs a high level, S b1 Conducting, and outputting P level by the B phase; s b4 In control signal generating circuits
As shown in formula (7), when u bm.p Upper and lower carrier > u bm.n When S is present b4 Control signal output high level, S b4 And conducting, and outputting N level by the phase B. The remaining cases output a 0 level.
4. Examples of the invention
The invention is explained in more detail below with reference to the figures and the description of the embodiments.
Taking the L-filter non-isolated grid-connected NPC inverter system shown in fig. 1 as an example, a grid-connected NPC inverter model is built on a Simulink simulation platform, and leakage current waveforms of the single modulation wave and the new double modulation wave are simulated, contrastingly analyzed by adopting two modulation strategies, namely the single modulation wave and the new double modulation wave, as shown in fig. 7.
Through fig. 7, it is observed that the new dual-modulation wave method proposed by the present invention can effectively suppress the amplitude of the leakage current compared to the single-modulation wave method, and can limit the amplitude within 300mA, which reaches the german standard rule commonly adopted in the international, as shown in fig. 8.
Claims (3)
1. A leakage current suppression double modulation wave modulation method based on a non-isolated photovoltaic inverter is characterized by comprising the following steps:
step 1: establishing a topological structure of a non-isolated photovoltaic grid-connected L-type filtering three-level inverter;
and 2, step: simplifying the topological structure of an inverter system, constructing an L-shaped filtering leakage current model, and deducing an approximate expression of common-mode voltage and leakage current on the basis;
and step 3: analyzing the change of the working state of the inverter when a CBPWM carrier in-phase stacking method is adopted, including the change condition of a common-mode voltage;
and 4, step 4: according to the analysis result, the voltage u is output in a carrier period when different modulation wave sizes are ensured AB On the basis of ensuring that the output voltage of the inverter is not changed, the common-mode voltage v is changed cm Is 3U d /4→U d /2→3U d /4→U d /2→3U d /4, wherein U d Is a dc bus voltage;
and 5: analyzing the common-mode voltage variation condition, and limiting the common-mode voltage to be 3U by adopting a double modulation wave modulation strategy d /4 and U d A change between/2.
2. The leakage current suppression double modulation wave modulation method based on the non-isolated photovoltaic inverter as claimed in claim 1, wherein the changing of the inverter operating state in the step 3 comprises: in a carrier period, the change sequence of the common-mode voltage is consistent, and the modulation wave only influences the ratio of different levels; when the resonant frequency of the resonant tank through which the leakage current passes is at the switching frequency f s When the voltage is close to the set value, the common mode current generated by the common mode voltage is larger than the set value; if the amplitude of the common mode voltage is limited to 3U d 4 and U d The amplitude of the leakage current is suppressed by the variation between/2.
3. The leakage current suppression double modulation wave modulation method based on the non-isolated photovoltaic inverter according to claim 1, wherein the double modulation wave modulation strategy is specifically as follows:
in a non-isolated photovoltaic grid-connected L-shaped filtering three-level inverter topological structure,
wherein u is m In order to modulate the wave,
and
respectively an A-phase modulation wave and a B-phase modulation wave; u. of am.p And u am.n Respectively, a phase A upper modulation wave with a positive value and a phase A lower modulation wave with a negative value; u. of bm.p And u bm.n Respectively a positive B-phase upper modulation wave and a negative B-phase lower modulation wave;
the output strategy is summarized as follows:
when the carrier wave is up＞u am.p Or u am.n When downloading waves, the phase A outputs P level;
when the upper carrier is > u am.p And u is am.n When the download waves are larger than the download waves, the phase A outputs N level;
when u is bm.p Either the upper carrier or the lower carrier > u bm.n When the phase B outputs the P level;
when u is bm.p Upper and lower carrier > u bm.n When the voltage is higher than the voltage, the phase B outputs N level;
substituting the modulated wave expressions (1) to (6) into the output strategy to find that the phase A is in the phase u m Outputting P level and N level at the same time when the output voltage is less than or equal to-0.5, wherein the output of the N level is correct, and the P level should not be output, so that a switching tube S in a topological structure of the non-isolated photovoltaic grid-connected L-type filtering three-level inverter is controlled a1 And S a4 Is separated to control the switch tube S a1 Is open-and-close
Instead 1, the P level will not be output;
in the same way, phase B is in u m When the output voltage is more than or equal to 0.5, the P level and the N level are simultaneously output, wherein the N level is output correctly, and the P level is not output, so that the switching tube S is controlled b1 And S b4 Is separated to control the switch tube S b1 Is open-and-close
Is changed into-u m The P level will no longer be output;
control switch tube S x3 (signal and control switch tube S) x1 Control the switch tube S by signal complementation x2 Signal and controlled switching tube S x4 Signal complement, where x = a or b;
switch tube S a1 In control signal generating circuits
When the upper carrier is > u am.p Or u am.n When > download wave, controlSignal output high level, switch tube S a1 Conducting, and outputting P level by phase A; switch tube S a4 In control signal generating circuits
As shown in formula (1);
when the upper carrier is > u am.p And u is am.n When downloading wave, switch tube S a4 Control signal output high level, switch tube S a4 Conducting, outputting N level by phase A, and outputting 0 level by other conditions;
switch tube S b1 In control signal generating circuits
When u is bm.p Upper carrier or lower carrier > u bm.n When the control signal is output to high level, the switch tube S b1 Conducting, and outputting P level by the B phase; switch tube S b4 In control signal generating circuits
As shown in formula (4);
when u is bm.p Upper and lower carrier > u bm.n Time, switch tube S b4 Control signal output high level, switch tube S b4 And (4) conducting, outputting N level by the phase B, and outputting 0 level by the other conditions.
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