Inverter bus high-frequency harmonic suppression method

Abstract

The invention discloses a high-frequency harmonic suppression method for an inverter bus, and the method comprises the following steps: calculating an AC side ripple in real time based on a modulation mode of comparing a modulation wave with a carrier wave at an AC side; according to the calculation result of the alternating-current side ripple, calculating a first duty ratio of the direct-current side modulation wave, and calculating a second duty ratio required by high-frequency harmonic compensation of an alternating-current side bus at the same time; and if the value of the second duty ratio meets the requirement of the first duty ratio, reversely injecting the alternating-current side carrier wave into the direct-current side modulation wave, so that the direct-current side modulation wave generates phase shift to compensate the high-frequency harmonic wave of the alternating-current side bus. The method has the beneficial effects that bus high-frequency harmonic suppression is carried out by reversely injecting an alternating-current side carrier wave into a direct-current side modulation wave, so that the stability of a system can be effectively improved; and meanwhile, the use number of capacitors in the system can be effectively reduced, and the service life of the capacitors is prolonged.

Description

Technical Field

[0001] The present application relates to the technical field of inverters, and particularly to a method for suppressing high-frequency harmonics of an inverter busbar. Background Art

[0002] In the prior art, the suppression of harmonics in inverters mainly reduces harmonics by increasing the control methods of switching tubes, or optimizes the harmonics by using harmonic suppression algorithms on the AC side of the inverter. However, the methods for harmonic suppression in the prior art mainly have the following disadvantages: the current methods for reducing harmonics are basically aimed at grid-side harmonics and lack the suppression of busbar harmonics; and additional hardware needs to be added to the inverter architecture, resulting in an increase in the cost of the inverter. Summary of the Invention

[0003] One object of the present application is to provide a method for suppressing high-frequency harmonics of an inverter busbar that can solve at least one of the defects in the above background art.

[0004] To achieve at least one of the above objects, the technical solution adopted by the present application is: a method for suppressing high-frequency harmonics of an inverter busbar, including the following steps: based on a modulation method in which a modulation wave is compared with a carrier wave on the AC side, calculate the AC-side ripple in real time; according to the calculation result of the AC-side ripple, calculate the first duty cycle of the DC-side modulation wave, and at the same time calculate the second duty cycle required for compensating the high-frequency harmonics of the AC-side busbar; if the value of the second duty cycle meets the requirement of the first duty cycle, inject the AC-side carrier wave reversely into the DC-side modulation wave, so as to cause a phase shift of the DC-side modulation wave to compensate for the high-frequency harmonics of the AC-side busbar.

[0005] Preferably, the AC-side ripple includes a positive busbar current ripple and a negative busbar current ripple with equal amplitudes and symmetric positive and negative polarities; filter the AC-side ripple to obtain the corresponding high-frequency harmonics; the high-frequency harmonics I out corresponding to a single busbar current ripple is calculated by the formula: I out =S(a)×I(a)+S(b)×I(b)+S(c)×I(c); where S(a), S(b), and S(c) respectively represent the switching states of the three-phase switching tubes of the inverter, and I(a), I(b), and I(c) respectively represent the three-phase current values of the inverter.

[0006] Preferably, the inverter includes a plurality of power generation units, a plurality of DC / DC units, and a DC / AC unit; each power generation unit is connected in parallel with the DC / AC unit through the corresponding DC / DC unit; the formula for calculating the first duty cycle D0 corresponding to the operation of a single DC / DC unit is: V out ×(1 - D0)=V in ; where V out represents the output voltage of the DC / DC unit, Vin Represents the output voltage of the power generation unit.

[0007] Preferably, the minimum value of the conduction required by the AC side harmonics within one switching period T of the DC / AC unit is used as the second duty cycle D1.

[0008] Preferably, based on the carrier reverse injection process in the N-channel DC / DC unit scenario: compare the sum of N first duty cycles D0, N×D0, with the value of the second duty cycle D1; if N×D0≤D1, there is no need to inject the carrier in reverse at this time; if N×D0>D1, calculate the value of N0 that satisfies the inequality N0×D0>D1>(N0 - 1)×D0; according to the calculated value of N0, inject the AC side carrier in reverse into the modulation wave corresponding to the N0-channel DC / DC unit.

[0009] Preferably, for the reverse injection of the AC side carrier, the injection time T a corresponding to each DC / DC unit is calculated as follows: T a = T / N0.

[0010] Preferably, during the reverse injection process of the carrier, the temperature of the IGBT unit of the DC / DC unit is detected; if the temperature of the IGBT unit exceeds the set threshold, increase or decrease the injection time of the carrier for the number of paths N0 of the carrier reverse injection.

[0011] Preferably, when N - N0≥1, if the temperature of the IGBT unit exceeds the set threshold, increase the number of paths N0 of the carrier reverse injection by one path.

[0012] Preferably, when N = N0, if the temperature of the IGBT unit exceeds the set threshold, reduce the injection time T a of the carrier to 60% - 80% of the initial value.

[0013] Preferably, reduce the injection time T a of the carrier to 70% of the initial value.

[0014] Compared with the prior art, the beneficial effects of this application are as follows: This application suppresses the high-frequency harmonics of the bus by injecting the AC side carrier in reverse into the DC side modulation wave, thereby effectively improving the stability of the system; at the same time, it can also effectively reduce the number of capacitors used in the system and increase the service life of the capacitors. Brief Description of the Drawings

[0015] Figure 1 It is a schematic diagram of the working process of this application.

[0016] Figure 2 It is a schematic diagram of the inverter topology structure of this application.

[0017] Figure 3 This is a schematic diagram of the waveform structures of the carrier wave and the modulation wave in this application.

[0018] Figure 4 This is a schematic diagram of the harmonic structure of the positive busbar in this application.

[0019] Figure 5 This is a schematic diagram of the high-frequency harmonic structure of the positive busbar in this application.

[0020] Figure 6 This is a schematic diagram of the high-frequency harmonic structure of the DC / DC unit in the normal state in this application. Detailed implementation manners

[0021] Next, in combination with the detailed implementation manners, the present application will be further described. It should be noted that in the description of this specification, the descriptions with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification.

[0022] In the description of the present application, it should be noted that for the orientation terms, if there are terms such as "center", "transverse", "longitudinal", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicating the orientation and positional relationship are based on the orientation or positional relationship shown in the drawings. This is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and should not be construed as limiting the specific protection scope of the present application.

[0023] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence.

[0024] In this application, unless otherwise clearly specified or limited, terms such as "installed", "connected", "linked", "fixed", etc. shall be understood in a broad sense. For example, it can be a connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0025] In this application, unless otherwise clearly specified or limited, the first feature being "on" or "under" the second feature may include the direct contact between the first and second features, or may include the situation where the first and second features are not in direct contact but in contact through other features between them. Moreover, the first feature being "above", "over" and "on top of" the second feature includes that the first feature is directly above and obliquely above the second feature, or merely indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "beneath" and "underneath" the second feature includes that the first feature is directly below and obliquely below the second feature, or merely indicates that the horizontal height of the first feature is lower than that of the second feature.

[0026] The terms "comprising" and "having" and any variations thereof in the description and claims of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that comprises a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0027] One of the preferred embodiments of this application is, as Figure 1 shown, a method for suppressing high-frequency harmonics of an inverter bus, comprising the following steps: Based on the modulation method of comparing a modulation wave with a carrier wave on the AC side, calculate the AC side ripple in real time. According to the calculation result of the AC side ripple, calculate the first duty cycle D0 of the DC side modulation wave, and at the same time calculate the second duty cycle D1 required for compensating the high-frequency harmonics of the AC side bus. If the value of the second duty cycle D1 meets the requirement of the first duty cycle D0, inject the AC side carrier wave reversely into the DC side modulation wave, thereby causing the DC side modulation wave to generate a phase shift to compensate for the high-frequency harmonics of the AC side bus, so as to achieve the suppression of the high-frequency harmonics of the AC side bus.

[0028] It is understandable that during the operation of an inverter, a large number of thin-film capacitors are required for the bus to suppress high-frequency harmonics. In actual applications, since the capacitor is not an ideal model, the factor restricting the selection of thin-film capacitors is often not its ability to support the bus voltage, but its ability to pass ripple current. If the selected capacitor has insufficient overcurrent capacity, it will cause the capacitor to have an increase in temperature and a reduction in lifespan. When the high-frequency ripple is too large, usually a larger volume is required to select more thin-film capacitors to meet the overcurrent capacity of the thin-film capacitors. However, the installation space of the inverter often cannot meet the installation of large-volume thin-film capacitors, and large-volume thin-film capacitors will also hinder the heat dissipation inside the inverter.

[0029] In this embodiment, by utilizing the characteristic that the inverter's AC corresponds to multiple DCs, first calculate the AC harmonics of the inverter bus thin-film capacitor, that is, the ripple on the AC side, and then use the phase shift of the multiple DC-side modulation waves to compensate for the harmonics on the AC side. This reduces the high-frequency harmonics of the inverter capacitor, extends the capacitor lifespan, and reduces the required capacitance value for the inverter design; at the same time, the suppression of high-frequency harmonics can also improve the stability of the system operation.

[0030] It should be known that there are various types of inverter topologies, such as photovoltaic power generation systems and wind power generation systems, etc. For the convenience of understanding, this application will take a photovoltaic power generation system as an example to briefly describe the specific structure of the inverter topology. As Figure 2 shown, the inverter topology includes N (N>1) PV units as power generation units, N DC / DC units, and one DC / AC unit; each PV unit is connected in parallel to the DC side of the DC / AC unit through the corresponding DC / DC unit, and the AC side of the DC / AC unit is connected to the power grid. Taking Figure 2 the shown inverter topology structure as an example, the bus high-frequency harmonics mainly come from the switching tubes of the DC / DC unit on the DC side and the DC / AC unit on the AC side of the inverter.

[0031] It should also be known that the inverter controls the DC / AC unit on the AC side by the way of the carrier intersecting with the modulation wave. Taking DPWM modulation as an example, the waveform structures of the carrier and the modulation wave are as Figure 3 shown; among them, the low-frequency component is the modulation wave with a frequency of 50Hz, and the high-frequency component is the carrier. The specific intersection method of the carrier and the modulation wave is well-known to those skilled in the art, so it will not be elaborated in detail here.

[0032] In this embodiment, the calculation of the AC-side ripple is mainly to obtain the high-frequency harmonics of the bus; the AC-side ripple includes the positive bus current ripple and the negative bus current ripple. For the convenience of calculation, it can be assumed that the power factor is 1, and at this time, the voltage and current are in the same phase; from the inverter topology, it can be seen that the harmonic amplitudes of the positive bus POS and the negative bus NEG are the same and symmetric in terms of positive and negative. To simplify the process, the bus current harmonic I out will be calculated taking the harmonic of the positive bus POS as an example below.

[0033] The current ripple output by the positive bus POS is mainly composed of three parts: the DC component of the DC / DC-side ripple, the high-frequency component of the DC / DC-side ripple, and the high-frequency component of the DC / AC-side harmonic. For the DC component of the DC / DC-side ripple, it is the effective value of the PV unit output current; for the high-frequency component of the DC / DC-side ripple, it is mainly generated by the on and off of the switching transistor T Figure 2 corresponding to the DC / DC unit in a1 . For the high-frequency component of the DC / AC-side harmonic, it is mainly generated by the alternating on and off of the switching transistors T1 and T5 corresponding to the DC / DA unit in Figure 2 .

[0034] For the modulation method, when the absolute value of the carrier is less than the modulation wave, it corresponds to the conduction of the corresponding switching transistor; when the absolute value of the carrier is lower than the modulation wave, it corresponds to the turn-off of the corresponding switching transistor. The on or off states of the three-phase switches of the DC / AC unit can be represented by S(a), S(b), and S(c). Then the current harmonic of the positive bus POS is as Figure 4 shown, and its period is three times the frequency of the modulation wave. Since the positive bus POS and the negative bus NEG output externally at different times, the current harmonic I out of the positive bus POS is the superposition of the high-frequency component and the low-frequency component.

[0035] It should be noted that after obtaining the bus current harmonic, since the DC component of the DC / DC-side ripple is the effective value of the PV unit output current, the DC component of the DC / DC-side ripple can be directly filtered out; for the low-frequency component of the DC / DC-side ripple, it can be filtered out by the DC-side bus capacitor. The harmonic after filtering out the DC component and the low-frequency component of the DC / DC side is output by the bus capacitor C1 corresponding to the DC / AC unit in Figure 2 . Then the high-frequency harmonic I out corresponding to the positive bus at this time is the harmonic corresponding to the bus capacitor C1, and the specific calculation formula is: I out =S(a)×I(a)+S(b)×I(b)+S(c)×I(c); where I(a), I(b), and I(c) respectively represent the three-phase current values on the AC side of the inverter. At this time, the waveform of the high-frequency harmonic I out corresponding to the positive bus is asFigure 5 as shown

[0036] In this embodiment, the acquisition of the first duty cycle D0 includes the following process: In the actual working state of the photovoltaic inverter, when each DC / DC unit is in use, the operation mode of the switching tube also adopts the form of comparing the carrier wave with the modulation wave. In practical applications, the number of PV modules connected to different DC / DC units is the same, so it can be considered that the voltage corresponding to each DC / DC unit is the same. Therefore, when the DC / DC unit performs MPPT tracking, the bus voltage V out output by the DC / DC unit and the voltage V in output by the PV module satisfy the following relationship: V out ×(1 - D)=V in .

[0037] Among them, D represents the duty cycle and is used to generate the PWM signal for controlling the DC / DC unit. If the value of the DC bus voltage V out is given, then by adjusting the duty cycle D, the operating voltage V in of the PV module can be adjusted in turn to stabilize it at the maximum power point. At this time, according to the law of conservation of energy, the input and output powers are approximately equal. Then, when the duty cycle D is constant, the current relationship between the input and output sides of the DC / DC unit is: I in ×(1 - D)=I out ; among them, I out represents the output-side current of the DC / DC unit, and I in represents the input-side current of the DC / DC unit, that is, the output current of the PV module.

[0038] It should be noted that when the output voltage Vin of the PV module is stable and the grid voltage is stable, the parameters I in , V out and I out are only related to the duty cycle D; then at this time, the duty cycle D is the first duty cycle D0 with a fixed value.

[0039] In this embodiment, for the acquisition of the second duty cycle D1, the minimum value required to turn on the AC-side harmonics within one switching period T of the DC / AC unit is used as the second duty cycle D1.

[0040] It can be understood that after obtaining the corresponding first duty cycle D0 and second duty cycle D1, if it is necessary to compensate the bus high-frequency harmonics, only the second duty cycle D1 needs to be injected reversely into the DC side of the inverter, so as to realize the adjustment of the first duty cycle D0. The specific adjustment process needs to conform to the volt-second product balance principle; that is, as Figure 2 shown, the DC / DC unit includes an inductor and a switching tube T a1; When the inductor meets the usage conditions and will not saturate due to bias, the voltage V applied across the inductor L multiplied by the conduction time T ON is equal to the voltage V across the inductor at the turn-off moment L multiplied by the turn-off time T OFF ; And within one switching period T, the integral of the inductor voltage V L with respect to time is 0. Specifically, it can be expressed by the following formula: V L ×T ON =-V L ×T OFF ; .

[0041] Those skilled in the art should know that when it is necessary to compensate and suppress the high-frequency harmonics of the bus, in the scenario of N DC / DC units, it is necessary to ensure that as many DC / DC units as possible operate near the maximum power point. Then as Figure 1 shown, based on the carrier reverse injection process in the scenario of N DC / DC units is as follows: Compare the sum of N first duty cycles D0, N×D0, with the value of the second duty cycle D1. If N×D0≤D1, there is no need to inject the carrier in reverse at this time; if N×D0>D1, calculate the value of N0 that satisfies the inequality N0×D0>D1>(N0 - 1)×D0. According to the calculated value of N0, inject the AC-side carrier in reverse into the modulation waves corresponding to N0 DC / DC units.

[0042] For the convenience of understanding, the following can be described in detail through specific parameters.

[0043] Assume that the value of N is 7. When the DC / DC unit is stable, calculate the first duty cycle D0 corresponding to each DC / DC unit, and at the same time calculate the second duty cycle D1 required for harmonic compensation on the AC side.

[0044] If D0×N≤D1, it means that the power demand of the inverter is very small at this time, and the demand for the capacitor ripple current capacity is also very small, so no compensation is performed from the DC side to the AC side.

[0045] If D0×N>D1, it means that the power demand of the inverter is very large at this time. Then first judge the number of paths N0 that need to be compensated, and calculate the minimum number of paths that can meet the ripple current; for example, if it is calculated that D0×6>D1>D0×5, then select N0 = 6 paths for compensation, and then inject the second duty cycle D1 in reverse into the six modulation waves for compensation.

[0046] It can be understood that for the reverse injection of the AC-side carrier, the injection time T a corresponding to each DC / DC unit is calculated by the formula: T a=T / N0。

[0047] In this embodiment, during the reverse injection of the carrier, since the value of the first duty cycle D0 on the DC side will increase, that is, the conduction time of the IGBT unit serving as the switching tube in the DC / DC unit will increase, which causes the temperature of the IGBT unit in the DC / DC unit to gradually rise. Therefore, during the reverse injection of the carrier, it is necessary to detect the temperature of the IGBT unit in the DC / DC unit; if the temperature of the IGBT unit exceeds the set threshold, it is necessary to increase the number of paths N0 of the carrier reverse injection or reduce the injection time of the carrier.

[0048] It should be noted that the specific value of the threshold corresponding to the temperature when the IGBT unit works can be selected according to the actual needs of those skilled in the art, so it will not be specifically limited here. When the temperature of the IGBT unit exceeds the set threshold, if the number of paths of the carrier reverse injection is increased, then the duration of the carrier injected per path can be reduced, thereby reducing the temperature of the IGBT unit.

[0049] Specifically, when the temperature of the IGBT unit does not exceed the set threshold, the reverse injection time of the second duty cycle D1 is T a , then the power of all DC / DC units on the DC side can be equivalent to N0×T a ; when the temperature of the IGBT unit exceeds the set threshold, X paths of carrier reverse injection are added; then the power of all DC / DC units on the DC side at this time can be equivalent to (N0+X)×T a ´. Furthermore, when the total output remains unchanged, there is N0×T a =(N0+X)×T a ´, T a ´=[N0 / (N0+X)]×T a ; that is, after increasing the number of paths of the carrier reverse injection, the reverse injection time T a ´ of the carrier will decrease, thereby achieving a reduction in the temperature of the IGBT unit.

[0050] Of course, it is also possible to directly reduce the duration of the carrier injection to achieve a reduction in the temperature of the IGBT unit. Both the IGBT temperature reduction methods of increasing the number of paths N0 of the carrier reverse injection or reducing the injection time of the carrier can meet the requirements of this application. However, considering the overall working stability of the system, the two temperature reduction methods can be specifically selected according to the number of paths of the carrier reverse injection. The specific selection process is as follows: When N-N0≥1, if the temperature of the IGBT unit exceeds the set threshold, add one path to the number of paths N0 of the carrier reverse injection. When N=N0, if the temperature of the IGBT unit exceeds the set threshold, the injection time T of the carrier aReduce to 60% - 80% of the initial value. For the injection time T of the carrier a The specific reduction degree can be selected according to the actual needs of those skilled in the art. Preferably, reduce the injection time T of the carrier a to 70% of the initial value.

[0051] The above describes the basic principle, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited by the above embodiments. What is described in the above embodiments and the specification is only the principle of the present application. Without departing from the spirit and scope of the present application, the present application will have various changes and improvements, and these changes and improvements all fall within the scope of the present application claimed. The scope of protection required by the present application is defined by the appended claims and their equivalents.

Claims

1. A method for suppressing high-frequency harmonics of an inverter bus, characterized in that, It includes the following steps: Based on the modulation method of comparing the modulation wave with the carrier on the AC side, calculate the AC side ripple in real time; According to the calculation result of the AC side ripple, calculate the first duty cycle of the DC side modulation wave, and at the same time calculate the second duty cycle required for high-frequency harmonic compensation of the AC side bus; If the value of the second duty cycle meets the requirements of the first duty cycle, inject the AC side carrier reversely into the DC side modulation wave, so that the DC side modulation wave generates a phase shift to compensate the high-frequency harmonics of the AC side bus.

2. The high-frequency harmonic suppression method for the inverter bus as described in claim 1, characterized in that, The AC side ripple includes a positive bus current ripple and a negative bus current ripple with equal amplitudes and symmetric positive and negative polarities; filter the AC side ripple to obtain the corresponding high-frequency harmonics; High-frequency harmonic I corresponding to the single-bus current ripple out The calculation formula is: I out =S(a)×I(a)+S(b)×I(b)+S(c)×I(c); Among them, S(a), S(b), and S(c) respectively represent the switching states of the three-phase switching tubes of the inverter, and I(a), I(b), and I(c) respectively represent the three-phase current values of the inverter.

3. The method for suppressing high-frequency harmonics of the inverter bus according to claim 1, wherein The inverter includes multiple power generation units, multiple DC / DC units, and a DC / AC unit; Each power generation unit is connected in parallel with the DC / AC unit through the corresponding DC / DC unit; The calculation formula for the first duty cycle D0 corresponding to the operation of a single DC / DC unit is: V out ×(1 - D0)=V in ; Among them, V out represents the output voltage of the DC / DC unit, and V in represents the output voltage of the power generation unit.

4. The method for suppressing high-frequency harmonics of the inverter bus according to claim 3, characterized in that, Take the minimum value of the conduction required by the AC side harmonics within one switching period T of the DC / AC unit as the second duty cycle D1.

5. The method for suppressing high-frequency harmonics of the inverter bus as described in claim 4, characterized in that, The process of carrier reverse injection based on the scenario of N DC / DC units is as follows: Compare the sum N×D0 of N first duty cycles D0 with the value of the second duty cycle D1; If N×D0≤D1, there is no need to inject the carrier reversely at this time; If N×D0>D1, calculate the value of N0 that satisfies the inequality N0×D0>D1>(N0 - 1)×D0; According to the calculated value of N0, inject the AC side carrier reversely into the modulation wave corresponding to the N0 DC / DC units.

6. The method for suppressing high-frequency harmonics of the inverter bus as described in claim 5, wherein, For the reverse injection of the AC-side carrier, the injection time T corresponding to each DC / DC unit a is calculated by the formula: T a = T / N0.

7. The method for suppressing high-frequency harmonics of the inverter bus as claimed in claim 5, wherein During the process of carrier reverse injection, detect the temperature of the IGBT unit of the DC / DC unit; If the temperature of the IGBT unit exceeds the set threshold, increase or decrease the injection time of the carrier for the number of paths N0 of carrier reverse injection.

8. The method for suppressing high-frequency harmonics of the inverter bus according to claim 7, wherein When N - N0≥1, if the temperature of the IGBT unit exceeds the set threshold, increase the number of paths of carrier reverse injection by one.

9. The inverter bus high-frequency harmonic suppression method according to claim 7, characterized in that, When N = N0, if the temperature of the IGBT unit exceeds the set threshold value, the injection time T of the carrier a is reduced to 60% - 80% of the initial value.

10. The method for suppressing high-frequency harmonics of the inverter bus according to claim 9, characterized in that, Reduce the injection time T of the carrier wave a to 70% of the initial value.

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