A bidirectional DC / DC converter and control method for an active composite electric energy storage system suitable for an electric drive workover rig

Abstract

The present invention discloses a bidirectional DC / DC converter and control method for an active composite energy storage system for an electric-driven workover rig in an oilfield. This converter addresses the complex circuit structure and control difficulties associated with conventional lithium battery-supercapacitor composite energy storage devices that utilize two dual DC / DC converters. The bidirectional DC / DC converter comprises a cascade circuit consisting of a diode acting as a selector switch in the main circuit, a first inductor, a second inductor, and a third inductor, as well as upper and lower bridge arm switches. The unidirectional conduction performance of the diode and the stable control of the switches in the cascade circuit ensure that the lithium battery continuously outputs stable power, while the supercapacitor can be frequently charged and discharged to compensate for the peak power required by the workover rig and absorb the energy generated by the workover rig's motor. The cascade circuit also reduces current ripple and reduces stress on the switching devices during switching. The present invention utilizes a single dual DC / DC converter to simultaneously control both power sources in the workover rig composite energy storage system, achieving a composite power supply charging and discharging mode with the lithium battery as the primary power source and the supercapacitor as the secondary power source. This simplifies the circuit structure, reduces control difficulty and equipment cost, and reduces stress on the switching devices in the circuit, making it an economically viable solution.

Description

Technical Field

[0001] The present invention relates to the technical field of DC / DC converters, and in particular to a low-ripple bidirectional DC converter of an active composite electric energy storage system of an electric-driven workover rig in an oilfield and a control method thereof. Background Art

[0002] Workover rigs are one of the most important operating equipment in oil fields. Electric-driven workover rigs require an energy storage device to compensate for grid power. The composite energy storage device of lithium batteries and supercapacitors combines the performance advantages of lithium batteries and supercapacitors, overcoming the problems of single energy storage devices such as low power density of lithium batteries and low energy density of supercapacitors.

[0003] In order to control the charging and discharging of lithium battery-supercapacitor composite energy storage devices, there are currently four topology circuits: passive composite power supply structure, semi-active supercapacitor end load structure, semi-active battery end load structure and active composite power supply structure. The passive composite power supply structure is simple and easy to use, with low cost, but the power distribution of the composite power supply depends entirely on the internal resistance of the supercapacitor and the battery, which cannot be controlled and has poor adaptability in actual use; although the semi-active supercapacitor end load structure facilitates the supercapacitor to exert its own performance advantages of fast charging and discharging, the voltage change span of the supercapacitor is large, and the load voltage fluctuates greatly, which reduces the working stability of the motor, and requires a large number of single capacitors in series and parallel to make the supercapacitor have a larger total energy, so the cost of this structure will be greatly increased; the semi-active battery end load structure can control the supercapacitor to provide peak power output when the load end requires a large current. At the same time, the bidirectional controller can stabilize the load end output voltage when the supercapacitor voltage changes significantly, and its voltage also does not need to be consistent with the battery end. The battery voltage remains consistent at all times, which is more conducive to the supercapacitor's high-power throughput capacity. The disadvantage is that the supercapacitor passes through a bidirectional controller before supplying power to the load, which affects the response speed of the supercapacitor during high-power charging and discharging. Moreover, the high current of the load is almost entirely provided by the supercapacitor, and the capacity of the bidirectional converter must be increased, thereby increasing the difficulty of manufacturing the bidirectional converter; the active composite power supply structure can control the charging and discharging status of the battery and supercapacitor separately, increasing the system flexibility and making the system control more accurate, so that the supercapacitor and battery can maximize their respective energy storage advantages. However, because of the presence of two DC / DC converters, not only the cost is increased and the control difficulty is increased, but also unnecessary energy loss is caused, resulting in low energy utilization.

[0004] In summary, there are certain technical problems that need to be solved in the existing composite power supply circuit structures. Summary of the Invention

[0005] While an active composite power supply structure for the lithium battery and supercapacitor combined energy storage device of an electric workover rig offers flexible control, it requires two dual DC / DC converters to control the charging and discharging of both the lithium battery and supercapacitor, resulting in a complex circuit structure and increased control complexity. To address this technical issue, the present invention provides a dual DC / DC converter circuit structure that uses a single dual DC / DC converter to control the charging and discharging of both the lithium battery and supercapacitor, simplifying the circuit structure and reducing control complexity.

[0006] The present invention solves its technical problems by employing a dual DC / DC converter designed to control the charging and discharging of two energy storage devices, a lithium battery and a supercapacitor, through the unidirectional conduction performance of the diode and the stable control of the switching transistors in the cascade circuit. This bidirectional DC / DC converter comprises a three-stage cascade circuit consisting of a diode acting as a selector switch in the main circuit, a first inductor, a second inductor, and a third inductor, as well as upper and lower bridge arms. The unidirectional conduction performance of the diode and the control signals of the switching transistors in the three-stage cascade circuit ensure that the lithium battery outputs stable power while the supercapacitor can be frequently charged and discharged to compensate for the peak power required by the workover rig and absorb the energy generated by the reverse power generation of the workover rig's motor. The three-stage cascade circuit reduces current ripple and reduces the stress on the switching devices during the switching process.

[0007] The topology of the circuit includes: lithium battery U b , select the switching diode D, the first inductor L1, the first inductor upper bridge arm switch tube S1, the first inductor lower bridge arm switch tube S2, and the super capacitor U c , second inductor L2, second inductor upper bridge arm switch tube S3, second inductor lower bridge arm switch tube S4, third inductor L3, third inductor upper bridge arm switch tube S5, third inductor lower bridge arm switch tube S6. Lithium battery U bThe positive end is connected to the positive end of the diode D, and the negative end of the lithium battery is connected to the first inductor L1, the second inductor L2, the third inductor L3 and one end of the supercapacitor; the negative end of the diode D is connected to the drain of the upper bridge arm switch tube S1 of the first inductor L1, the negative end of the diode D1, the drain of the upper bridge arm switch tube S3 of the second inductor L2, the negative end of the diode D3, the drain of the upper bridge arm switch tube S5 of the third inductor L3, and the negative end of the diode D5; the other end of the first inductor L1 is connected to the source of the upper bridge arm S1, the positive end of the diode D1, the drain of the lower bridge arm S2, The negative terminal of the diode D2 is connected; the other end of the second inductor L2 is connected to the source of the upper bridge arm S3, the positive terminal of the diode D3, the drain of the lower bridge arm S4, and the negative terminal of the diode D4; the other end of the third inductor L3 is connected to the source of the upper bridge arm S5, the positive terminal of the diode D5, the drain of the lower bridge arm S6, and the negative terminal of the diode D6; one end of the supercapacitor is connected to the source of the lower bridge arm switch S2 of the first inductor L1, the source of the lower bridge arm switch S4 of the second inductor L2, and the source of the lower bridge arm switch S6 of the third inductor L3. The control method of the dual DC / DC converter suitable for the active composite electric energy storage device of the electric drive well workover rig is as follows: based on the operating duty cycle signal of the dual DC / DC converter, there are two operating states in the boost and buck modes, respectively, for a total of four operating states.

[0008] During the workover rig's hoisting phase, the motor demands high power, causing a sudden voltage drop on the busbar. At this point, the energy storage system operates in Boost mode. Due to the unidirectional conduction of diode D and the control signal from the switching transistor, the lithium battery and supercapacitor jointly boost the voltage, while inductors L1, L2, and L3 jointly boost the voltage with the lithium battery, respectively, within the same switching cycles of S2, S4, and S6. If the motor experiences reverse power generation during the workover rig's lowering phase, the energy storage system operates in Buck mode. Similarly, due to the selective conduction of diode D and the control signal from the switching transistor, the busbar charges the supercapacitor and inductors L1, L2, and L3, while inductors L1, L2, and L3 independently charge the supercapacitor, within the same switching cycles of S1, S3, and S5.

[0009] The bidirectional DC / DC converter and control method thereof for an active composite electric energy storage system for an electric-driven workover rig adopt independent PWM control to improve system stability and avoid the shoot-through phenomenon of the upper and lower bridge arm switches that may occur in complementary PWM control. When the switches S1, S3, and S5 are turned off and S2, S4, and S6 are in PWM modulation, the system operates in Boost mode; when the switches S2, S4, and S6 are turned off and S1, S3, and S5 are in PWM modulation, the system operates in Buck mode.

[0010] In the Boost working mode, the switch tubes S2, S4 and S6 are turned on and off, which are two working states. In the Buck working mode, the switch tubes S1, S3 and S5 are turned on and off, which are two working states, for a total of four working states.

[0011] When the converter works in Boost mode, assuming that the currents of inductors L1, L2 and L3 work in a continuous state, the converter switches S2, S4 and S6 use a PWM signal with a duty cycle of d1. In a period T s In Boost mode, there are two working states.

[0012] Working state 1 (t0-t1): During this period, switches S2, S4 and S6 are turned on at the same time, the supercapacitor charges the inductors L1, L2 and L3, and the diode D is turned on by the forward voltage drop. The lithium battery and supercapacitor supply power to the bus side at the same time.

[0013] Working state 2 (t1-t2): During this period, the switches S2, S4 and S6 are turned off at the same time, the diodes D1, D3 and D5 are turned on due to freewheeling, and the supercapacitor and inductors L1, L2 and L3 supply power to the bus side at the same time.

[0014] When the converter works in Boost mode, assuming that the currents of inductors L1, L2 and L3 work in a continuous state, the converter switches S1, S3 and S5 use a PWM signal with a duty cycle of d2. In one cycle T s Buck mode has two working states.

[0015] Working state 1 (t0-t1): Switches S1, S3 and S5 are turned on, and diode D is turned off due to reverse voltage, so the bus side supercapacitor U c Charging, while charging the inductors L1, L2 and L3.

[0016] Working state 2 (t1-t2): Switches S1, S3 and S5 are turned off, diodes D2, D4 and D6 are turned on due to freewheeling, and super inductors L1, L2 and L3 supply power to super capacitor U c powered by.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. Use a single dual DC / DC converter to control the step-up and step-down functions of both lithium battery and supercapacitor power sources, simplifying the circuit structure.

[0019] Second, the diodes in the main circuit that act as selector switches cooperate with the independent PWM control signals of the upper and lower switch tubes of each bridge arm, making the control simple and the system stable.

[0020] 3. The three-stage cascade structure reduces the switching stress of the switch tube during the switching process, avoids the impact on the switch tube during the peak power output of the workover rig, and reduces the current ripple. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0022] Figure 1 Circuit diagram of low-ripple dual DC / DC converter for electric-driven workover rig with lithium battery-supercapacitor composite energy storage

[0023] Figure 2 Equivalent circuit diagram of each working state of the composite energy storage low ripple dual DC / DC converter in Boost mode

[0024] Figure 3 Equivalent circuit diagram of each working state of the composite energy storage low ripple dual DC / DC converter in Buck mode

[0025] Figure 4 The main waveforms of each working state of the composite energy storage low ripple dual DC / DC converter in Boost mode

[0026] Figure 5 The main waveforms of each working state of the low ripple dual DC / DC converter in Buck mode of the composite energy storage DETAILED DESCRIPTION

[0027] The working principle of the present invention "a bidirectional DC / DC converter and its control method for an active composite electric energy storage system for an electric-driven workover rig" is further described in detail below with reference to the accompanying drawings and specific implementations.

[0028] The present invention provides a bidirectional DC / DC converter and control method for an active lithium battery-supercapacitor composite energy storage system suitable for an electric drive workover rig. Figure 1 As shown, the circuit structure includes: lithium battery U b , select the switching diode D, the first inductor L1, the first inductor upper bridge arm switch tube S1, the first inductor lower bridge arm switch tube S2, and the super capacitor U c , second inductor L2, second inductor upper bridge arm switch tube S3, second inductor lower bridge arm switch tube S4, third inductor L3, third inductor upper bridge arm switch tube S5, third inductor lower bridge arm switch tube S6. Lithium battery U bThe positive end is connected to the positive end of the diode D, and the negative end of the lithium battery is connected to the first inductor L1, the second inductor L2, the third inductor L3 and one end of the supercapacitor; the negative end of the diode D is connected to the drain of the upper bridge arm switch tube S1 of the first inductor L1, the negative end of the diode D1, the drain of the upper bridge arm switch tube S3 of the second inductor L2, the negative end of the diode D3, the drain of the upper bridge arm switch tube S5 of the third inductor L3, and the negative end of the diode D5; the other end of the first inductor L1 is connected to the source of the upper bridge arm S1, the positive end of the diode D1, the drain of the lower bridge arm S2, The negative end of the diode D2 is connected; the other end of the second inductor L2 is connected to the source of the upper bridge arm S3, the positive end of the diode D3, the drain of the lower bridge arm S4, and the negative end of the diode D4; the other end of the third inductor L3 is connected to the source of the upper bridge arm S5, the positive end of the diode D5, the drain of the lower bridge arm S6, and the negative end of the diode D6; one end of the supercapacitor is connected to the source of the lower bridge arm switch tube S2 of the first inductor L1, the source of the lower bridge arm switch tube S4 of the second inductor L2, and the source of the lower bridge arm switch tube S6 of the third inductor L3.

[0029] See also Figure 2 and Figure 3 The working status and performance analysis of the above converter in boost and buck modes:

[0030] like Figure 2 As shown in Figure 2, when the system works in Boost mode, assuming that the inductor current works in a continuous state and the PWM signal duty cycle is d1, then in one switching cycle, T s , Boost mode has two states, the equivalent circuit diagram of each working state is as follows Figure 2 As shown in (1) and (2) in .

[0031] 1) Working state 1 (t0-t1): The lower bridge arm switch S2 of the first inductor L1 is turned on, and the supercapacitor starts to charge the inductor L1. The lower bridge arm switch S4 of the second inductor L2 is turned on, and the supercapacitor starts to charge the inductor L2. The lower bridge arm switch S6 of the third inductor L3 is turned on, and the supercapacitor starts to charge the inductor L3. The diode D is turned on due to the forward voltage drop, and the supercapacitor and the battery simultaneously supply power to the bus.

[0032] 2) Working state 2 (t1-t2): Switches S2, S4, and S6 are turned off at the same time, diodes D1, D3, and D5 are turned on due to freewheeling, and the supercapacitor and inductors L1, L2, and L3 supply power to the bus side at the same time.

[0033] like Figure 3 As shown, when the system works in Buck mode, assuming that the inductor current works in a continuous state and the PWM signal duty cycle is d2, then in one switching cycle T s Buck mode has two states, and the equivalent circuit diagram of each working state is as follows Figure 3 As shown in (1) and (2) in .

[0034] 1) Working state 1 (t0-t1): Switches S1, S3, and S5 are turned on, and diode D is turned off due to the reverse voltage drop. Therefore, the bus side charges the supercapacitor and inductors L1, L2, and L3 at the same time.

[0035] 2) Working state 2 (t1-t2): Switches S1, S3, and S5 are turned off at the same time, diodes D2, D4, and D6 are turned on due to freewheeling, and inductors L1, L2, and L3 charge the supercapacitor.

[0036] Figure 4 and Figure 5 Figure 3. Voltage stress on switches S1-S6 in the upper and lower bridge arms of the first, second, and third inductors L1, L2, and L3, as well as current waveforms in the three inductors L1, L2, and L3, of the dual DC / DC converter in both Boost and Buck modes, depending on the PWM signal. The voltages on switches S1-S6 indicate that the triple-cascade structure significantly reduces both the voltage across the switches and the current ripple.

[0037] In summary, the dual DC / DC converter proposed in the present invention can realize the charge and discharge control of two energy storage devices, lithium batteries and supercapacitors, with a single dual DC / DC converter. In order to improve the stability of the system and avoid the possible direct-through phenomenon of the upper and lower bridge arm switches in the complementary PWM control, the dual DC / DC converter adopts independent PWM control. During the charging and discharging process, the lithium battery continuously provides stable power, and provides stable energy output to the bus side when operating after being fully charged. The supercapacitor can be frequently charged and discharged under the action of the control signal, and is responsible for recovering the energy on the bus side (charging) and providing the peak power required by the well repair machine in conjunction with the lithium battery. The three-stage cascade structure greatly reduces the switch stress on the switch tube, and is an efficient and low-cost solution.

[0038] The above descriptions are merely embodiments of the present invention and are not intended to limit the present invention. Any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention shall still fall within the scope of protection of the technical content of the present invention.

Claims

1. A bidirectional DC / DC converter and control method thereof suitable for an active composite electric energy storage system of an electric drive workover rig, characterized in that: The designed bidirectional DC / DC converter uses the unidirectional conduction function of the diode on the main circuit and the control of the switch tube in the three-stage cascade circuit to realize the function of a single bidirectional DC / DC converter to simultaneously control the charging and discharging of two energy storage devices, lithium battery and supercapacitor; the circuit topology includes: lithium battery U b , select the switching diode D, the first inductor L1, the first inductor upper bridge arm switch tube S1, the first inductor lower bridge arm switch tube S2, and the super capacitor U c , second inductor L2, second inductor upper bridge arm switch tube S3, second inductor lower bridge arm switch tube S4, third inductor L3, third inductor upper bridge arm switch tube S5, third inductor lower bridge arm switch tube S6; lithium battery U b The positive end is connected to the positive end of the diode D, and the negative end of the lithium battery is connected to the first inductor L1, the second inductor L2, the third inductor L3 and one end of the supercapacitor; the negative end of the diode D is connected to the drain of the upper bridge arm switch tube S1 of the first inductor L1, the negative end of the diode D1, the drain of the upper bridge arm switch tube S3 of the second inductor L2, the negative end of the diode D3, the drain of the upper bridge arm switch tube S5 of the third inductor L3, and the negative end of the diode D5; the other end of the first inductor L1 is connected to the source of the upper bridge arm S1, the positive end of the diode D1, the drain of the lower bridge arm S2, The negative end of the diode D2 is connected; the other end of the second inductor L2 is connected to the source of the upper bridge arm S3, the positive end of the diode D3, the drain of the lower bridge arm S4, and the negative end of the diode D4; the other end of the third inductor L3 is connected to the source of the upper bridge arm S5, the positive end of the diode D5, the drain of the lower bridge arm S6, and the negative end of the diode D6; one end of the supercapacitor is connected to the source of the lower bridge arm switch S2 of the first inductor L1, the source of the lower bridge arm switch S4 of the second inductor L2, and the source of the lower bridge arm switch S6 of the third inductor L3; The bidirectional DC / DC converter and control method thereof for an active composite electric energy storage system of an electric drive workover rig: based on a duty cycle signal when the bidirectional DC / DC converter is operating, there are two working states in Boost and Buck modes, respectively, for a total of four working states; During the workover rig's hoisting phase, the motor demands high power, causing a sudden voltage drop on the busbar side. At this point, the energy storage system operates in Boost mode. Due to the unidirectional conduction of diode D and the control signals of the switches, voltage boosting is achieved by the lithium battery and supercapacitor, and by inductors L1, L2, L3 and the lithium battery, respectively, within a single switching cycle of switches S2, S4, and S6. If reverse power generation occurs during the workover rig's lowering phase, the energy storage device operates in Buck mode. Similarly, due to the selective conduction of diode D and the control signals of the switches, voltage boosting is achieved by the busbar charging the supercapacitor and inductors L1, L2, and L3, and by inductors L1, L2, and L3 individually charging the supercapacitor, within a single switching cycle of switches S1, S3, and S5.

2. The bidirectional DC / DC converter and control method thereof suitable for an active composite electric energy storage system of an electric drive workover rig according to claim 1, characterized in that: The bidirectional DC / DC converter adopts independent PWM control. When the switches S1, S3 and S5 are turned off and S2, S4 and S6 are in PWM modulation, the system operates in Boost mode; when the switches S2, S4 and S6 are turned off and S1, S3 and S5 are in PWM modulation, the system operates in Buck mode. In Boost mode, the switches S2, S4 and S6 are in two operating states: on and off; in Buck mode, the switches S1, S3 and S5 are in two operating states: on and off, for a total of four operating states.

3. The bidirectional DC / DC converter and control method thereof suitable for an active composite electric energy storage system of an electric drive workover rig according to claim 2, characterized in that: When the system works in Boost mode, if the current of inductors L1, L2 and L3 works in a continuous state, the converter switches S2, S4 and S6 use a PWM signal with a duty cycle of d1. In one cycle T s In Boost mode, there are two working states; Working state 1 is from time t0 to time t1: during this period, switches S2, S4 and S6 are turned on at the same time, the supercapacitor charges inductors L1, L2 and L3, and diode D is turned on by the forward voltage drop, and the lithium battery and supercapacitor simultaneously supply power to the bus side; Working state 2 is from time t1 to time t2: during this period, the switches S2, S4 and S6 are turned off at the same time, the diodes D1, D3 and D5 are turned on due to freewheeling, and the supercapacitor and inductors L1, L2 and L3 supply power to the bus side at the same time.

4. The bidirectional DC / DC converter and control method thereof suitable for an active composite electric energy storage system of an electric drive workover rig according to claim 2, characterized in that: When the system works in Buck mode, if the currents of inductors L1, L2 and L3 work in a continuous state, and the converter switches S1, S3 and S5 use a PWM signal with a duty cycle of d2, then in one cycle T s In Buck mode, there are two working states: Working state 1 is from time t0 to time t1: the switches S1, S3 and S5 are turned on, and the diode D is turned off due to the reverse voltage, so the bus side supercapacitor U c Charging, charging the inductors L1, L2 and L3 at the same time; Working state 2 is from time t1 to time t2: the switches S1, S3 and S5 are turned off, the diodes D2, D4 and D6 are turned on due to the freewheeling current, and the inductors L1, L2 and L3 supply the current to the supercapacitor U c powered by.

5. The bidirectional DC / DC converter and control method thereof suitable for an active composite electric energy storage system of an electric drive workover rig according to claim 1, characterized in that: The bidirectional DC / DC converter and control method simultaneously implement the step-up and step-down control functions of two energy storage devices, a lithium battery and a supercapacitor. The lithium battery continuously provides stable power, while the supercapacitor is frequently charged and discharged under the action of a control signal. This is suitable for the power requirements of electric-driven workover rigs under different working conditions. The three-stage cascade structure reduces the switching stress during the switching process of the switch tube and reduces current ripple.

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