Low-voltage flexible direct-current power system and dynamic adjustment method thereof

Abstract

The invention discloses a low-voltage flexible direct-current power system and a dynamic adjustment method thereof, and the system comprises a power distribution network which is located at the foremost end of the system, is connected to the input end of a series transformer, and provides electric energy for an alternating-current bus, a direct-current bus, an alternating-current load and a direct-current load; the series transformer, the converter AC / DC, the converter DC / AC, the alternating current bus and the direct current bus, a power supply path starts from a power distribution network and is divided into two paths through the series transformer, the first branch is connected to an alternating current load through the alternating current bus, and the second branch is connected to a direct current load and an energy storage battery through the converter AC / DC and the direct current bus in sequence. The problems that an existing LVFDC system is still poor in complex fluctuation response and control precision, and system stability and scheduling flexibility cannot be guaranteed are solved, by introducing intelligent PID control and a self-adaptive adjustment mechanism, load changes can be responded in real time, system voltage and current output can be optimized, and the system stability and scheduling flexibility can be guaranteed. And the efficiency, the stability and the intelligent level of the system are obviously improved.

Description

Technical Field

[0001] This invention relates to a method for regulating low-voltage flexible DC power, specifically to a low-voltage flexible DC power system and its dynamic regulation method. Background Technology

[0002] With the advancement of global energy transition and sustainable development goals, low-voltage flexible DC power systems are gradually becoming an important component of modern power networks. Especially in applications such as electric vehicle charging, distributed energy integration, and smart grids, low-voltage DC power systems demonstrate significant advantages in improving energy utilization, optimizing power dispatch, and reducing energy losses due to their high efficiency, stability, and flexibility.

[0003] While low-voltage direct current (LDDC) power systems perform well in many applications, traditional DC power systems generally have some limitations. Traditional low-voltage DC power systems typically output a fixed voltage, lacking the ability to respond quickly to load fluctuations and sudden load changes. Under significant load variations, they are prone to overload or energy waste, affecting system efficiency and stability. Furthermore, their energy dispatch flexibility is insufficient, especially when renewable energy sources fluctuate. Existing inverter technology and voltage / current control methods struggle to accurately respond to instantaneous load changes, easily leading to load mismatch and voltage fluctuations, which in turn affect the normal operation of equipment.

[0004] The application of modern control theory provides an effective way to solve the above problems. In particular, by adopting advanced control algorithms, more precise and flexible voltage regulation and more intelligent energy dispatch can be achieved in power systems. For example, existing literature 1 (Chinese invention patent application with publication number CN112234597B) discloses a low-voltage DC coordinated control method, system and device. The coordinated control side determines the dispatch strategy and issues dispatch instructions based on the current optimal operating mode of the low-voltage DC system. The low-voltage DC system side uses a master-slave control method or a supplementary master-slave control method to allocate power, which effectively realizes the optimized management and coordinated control of the low-voltage DC system, and helps to effectively handle faults and improve operational efficiency.

[0005] The application of modern control theory provides an effective way to solve the above problems. In particular, by adopting advanced control algorithms, more precise and flexible voltage regulation and more intelligent energy dispatch can be achieved in power systems. For example, existing literature 1 (Chinese invention patent application with publication number CN112234597B) discloses a low-voltage DC coordinated control method, system and device. The coordinated control side determines the dispatch strategy and issues dispatch instructions based on the current optimal operating mode of the low-voltage DC system. The low-voltage DC system side uses a master-slave control method or a supplementary master-slave control method to allocate power, which effectively realizes the optimized management and coordinated control of the low-voltage DC system, and can achieve effective fault handling and improved operating efficiency. However, the response and control accuracy of this LVFDC system to complex fluctuations are still not good, and it cannot guarantee system stability and flexible dispatch. Summary of the Invention

[0006] The purpose of this invention is to provide a low-voltage flexible DC power system and its dynamic regulation method, which solves the problem that the response and control accuracy of existing LVFDC systems to complex fluctuations are still not good, and the system cannot guarantee stability and flexible scheduling. By introducing intelligent PID control and adaptive regulation mechanism, it can respond to load changes in real time and optimize the system voltage and current output, which significantly improves the system efficiency, stability and intelligence level, and has broad application prospects.

[0007] To achieve the above objectives, the present invention provides a low-voltage flexible DC power system, the system comprising: The distribution network, located at the front end of the system, is connected to the input of the series transformer, providing power to the AC bus, DC bus, AC loads, and DC loads, and stabilizing the system voltage. The series transformer, located between the distribution network and the AC bus, has its first output connected to the AC bus, distributing power from the distribution network to the AC bus. The AC / DC converter, with its input connected to the second output of the series transformer, distributes power from the distribution network to the DC system; its output is connected to the DC bus, converting AC to DC for use by the DC bus. The DC / AC converter, with its input connected to the DC bus and its output connected to the AC bus, converts DC to AC, feeding back to the AC bus. The AC bus, as a centralized distribution node for AC power, is located downstream of the series transformer and upstream of the AC loads. It is interconnected with the DC bus via the DC / AC converter, receiving power from the distribution network or the DC / AC converter, and simultaneously... The system connects to AC loads and supplies power to them. A DC bus connects unidirectionally to distributed generation and DC loads, and bidirectionally to energy storage batteries, concentrating the DC power from distributed generation and directly supplying it to DC loads or storing it in the energy storage batteries, reducing AC-DC conversion losses. It forms a vertical energy channel with the AC bus through the AC / DC converter and the DC / AC converter, maintaining a stable 750V voltage for use by DC equipment, the AC / DC converter, and the DC / AC converter, mitigating the intermittency of distributed generation. The power supply path of this system is as follows: starting from the distribution network, it branches into two paths via the series transformer. The first branch goes through the AC bus to the AC load, and the second branch goes through the AC / DC converter and the DC bus to the DC load and the energy storage battery. The energy storage path of this system is as follows: from the distributed generation or the energy storage battery, it goes through the DC bus, the DC / AC converter, and the AC bus to the AC load.

[0008] Preferably, the distributed generation outputs power to the DC bus in one direction, and the electrical energy can be transmitted in reverse to the AC bus through the DC / AC converter, while simultaneously supplying DC loads or energy storage batteries.

[0009] Preferably, when the energy storage battery is charging, the electrical energy from the distribution network is stored through the AC / DC converter or the electrical energy generated by distributed generation; when the energy storage battery is discharging, its electrical energy is supplied to the DC load or supports the AC load through the DC / AC converter.

[0010] Preferably, the electrical energy of the AC load is supplied from the distribution network through the series transformer to the AC bus, or from the distributed generation or the energy storage battery through the DC bus and DC / AC converter to the AC bus, thereby realizing power mutual assistance between the AC and DC systems.

[0011] Preferably, the electrical energy of the DC load comes from the distributed generation, energy storage battery, or is converted from the distribution network via an AC / DC converter.

[0012] Preferably, the AC bus is a 380V / 50Hz AC bus, the AC / DC converter is a low-voltage flexible DC / DC converter, the DC / AC converter is a low-voltage flexible DC / AC converter, the DC bus voltage is 750V, and the distribution network voltage is 380V.

[0013] This invention provides a dynamic regulation method for a low-voltage flexible DC power system as described above, the method comprising: Step 1: Collect three-phase load current i Labc The collected load current i Labc pass abc coordinates to dqo Coordinate system transformation yields the load current. dqo exist d Components of the axis i Ld ; Step 2: The components i Ld The positive sequence current is obtained through a second-order low-pass filter. d Axial fundamental wave component i Ld + ; Step 3: The DC bus voltage control adopts a typical Type II control system, and the DC bus reference voltage is acquired through a voltage sensor. u dc * and feedback DC voltage u dc DC bus voltage reference voltage u dc * With feedback DC voltage u dc The difference is combined with the DC bus voltage control to calculate the DC bus current. i dc ; Step 4: Power based on the state-of-charge balance of the energy storage battery k p Adjustments to the state of charge (SOC) of energy storage battery 1 and energy storage battery 2, and the input power of the low-voltage flexible DC / AC converter. P pc Powered by the energy storage unit, the rated output power of energy storage battery 1 is P Rate1 The rated output power of energy storage battery 2 isP Rate2 The actual output power of state-of-charge energy storage battery 1 and energy storage battery 2 are respectively P battery1 and P battery2 The sum is P battery1 + P battery2 = P pc ,but k p =1-( P battery1 + P battery2 ) / P L； Step 5: Convert the positive sequence current d Axial fundamental wave component i Ld + After passing through the PI controller, it is converted into current. i ap = k p i Ld + The obtained reference current is converted into the AC / DC converter. d Shaft reference input current i scd * = i ap +i dc ; Step Six: Using a three-phase balanced power grid, the AC / DC converter... q shaft and o The reference currents of the shafts are respectively i scq * and i sc0 * ,and i scq * =0, i sc0 * =0; Step 7: Following Step 6, a DC bus with stable voltage is obtained, whose AC load power... P L AC power grid output power P s and low-voltage flexible DC-DC converter AC / DC input power Ppc Between satisfy P L = P s + P pc = k p P L +(1- k p ) P L Because the DC voltage is constant, i Ld = i s + i pc ， in i s = k p i Ld ，i pc =(1- k p ) i Ld ,along with P L The changes can be adjusted. k p This enables flexible dispatching of active power in low-voltage flexible DC power systems.

[0014] Preferably, in step one, the specific method of the conversion is as follows: (1); In equation (1), i Ld , i Lq and i L0 They are respectively the load current dqo exist d axis q shaft and o The components of the axis; =2πf, where f is the AC frequency. t For time.

[0015] Preferably, in step two, the low-pass filter is: (2); In equation (2), K P This refers to the proportional coefficient in a PID controller. K 1 The integral coefficient in PID control; Udc This is the DC bus voltage; s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; L s Equivalent inductor for low-voltage flexible DC system equivalent circuit; R s This is the equivalent resistor in the equivalent circuit; i Ld Load current dqo exist d The components of the axis; i Ld + Positive sequence current d Axial basis wave component.

[0016] Preferably, the formula for calculating the equivalent inductance of the equivalent circuit of the low-voltage flexible DC system is as follows: (3); In equation (3), L d , L g and L fs These are the leakage inductance of the series transformer, the equivalent inductance of the power grid, and the filter inductance of the filter. n T This refers to the turns ratio of a series transformer.

[0017] Preferably, the formula for calculating the equivalent resistance of the equivalent circuit is: (4); In equation (4), R t and R Lfs These are the resistance of the series transformer and the parasitic resistance of the filter inductance, respectively. n T This refers to the turns ratio of a series transformer.

[0018] Preferably, in step three, the typical Type II control system is: (5); In equation (5), s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; K dc , z and p These are the controller gain, zero frequency, and pole frequency, respectively. The DC bus current The calculation formula is: (6); In equation (6), u dc* This is the DC bus reference voltage, with a value of 750V. u dc For feedback DC voltage; G dc ( s This is for DC bus voltage control; u dc * - u dc This is the difference between the DC bus voltage reference voltage and the feedback DC voltage.

[0019] Preferably, in step four, the rated output power of the energy storage battery 1 is... P Rate1 The rated output power of the energy storage battery 2 is P Rate2 The calculation methods are as follows: (7); (8); In equations (7) and (8), P Rate1 This is the rated output power of energy storage battery 1; P Rate2 Rated output power for energy storage battery 2; , SOC1 ( t ) represents the state of charge (SOC) of energy storage battery 1; SOC2 ( t () represents the state of charge of energy storage battery 2.

[0020] Preferably, in step five, the PI controller is: (9); In equation (9), s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; G sc Pass functions to the PI controller. k i = 0, k p This is for dynamic power adjustment gain.

[0021] The present invention provides a dynamic adjustment device and method for a low-voltage flexible direct current (LVFDC) power system, which solves the problem that existing LVFDC systems still have poor response and control accuracy to complex fluctuations, and cannot guarantee system stability and flexible dispatch. It has the following advantages: 1. This invention effectively improves the operational stability and reliability of a low-voltage flexible DC power system by dynamically adjusting key parameters such as voltage and current. The introduction of intelligent PID control and adaptive adjustment mechanisms, along with real-time monitoring and adjustment mechanisms, enables rapid response to system fluctuations and sudden faults, significantly reducing the risks associated with unstable operation and laying a solid foundation for efficient system operation.

[0022] 2. This invention optimizes power transmission efficiency, performs precise power allocation according to load demand, reduces energy loss, and improves adaptability to various types of loads and new energy access, meeting the needs of diverse application scenarios. Through power allocation control based on energy storage SOC balance, it avoids over-discharge of a single battery pack, balances energy storage battery life and thermal load, further controls system operating costs, and improves overall economic efficiency and environmental friendliness.

[0023] 3. The series transformer of this invention serves as both a voltage adapter and a key node in power distribution. It establishes a hybrid AC / DC architecture, enabling flexible interconnection between the 380V AC bus and the 750V DC bus through low-voltage flexible DC / DC converters and DC / AC converters. This bidirectional energy flow enhances system reliability and supports a high proportion of renewable energy integration. Furthermore, the system exhibits excellent flexibility and scalability, adapting to low-voltage DC power demands of varying scales and scenarios, providing technical support for the efficient integration and promotion of new energy sources. While achieving efficient and reliable system operation, it also contributes to green and low-carbon development, demonstrating broad application prospects and promotional value. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the low-voltage flexible DC power system of the present invention.

[0025] Figure 2 Simulation analysis of the system of this invention Figure 1 .

[0026] Figure 3 Simulation analysis of the system of this invention Figure 2 . Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The technical solution of the present invention will be described in detail below through specific embodiments. It should be noted that the following embodiments may exist independently or in combination with each other, and the same or similar content will not be described again in different embodiments.

[0029] Example 1: A low-voltage flexible DC power system, such as Figure 1 The diagram shows a schematic of the low-voltage flexible DC power system of the present invention. The system includes: a series transformer, a low-voltage flexible DC-DC converter (AC / DC), a low-voltage flexible DC-DC converter (DC / AC), a distribution network, a 750V DC bus and a 380V AC bus (380V / 50Hz); distributed generation, energy storage batteries and AC / DC loads.

[0030] The 380V distribution network is located at the front end of the system, directly connected to the input of the series transformer. It provides power to the 380V AC bus, 750V DC bus, and loads, stabilizing the system voltage level (380V AC). The series transformer is located between the 380V distribution network and the 380V AC bus. It is the first-level interface device for power transmission from the 380V distribution network. Its input is directly connected to the 380V distribution network, and its first output is connected to the 380V AC bus. It adapts to the voltage level between the 380V distribution network and the bus, distributing power from the 380V distribution network to the 380V AC bus and providing electrical isolation to reduce the impact of grid disturbances on the bus. Its second output is connected to a low-voltage flexible DC-DC converter, which distributes power from the 380V distribution network to the DC system. Both the low-voltage flexible DC-DC converter (AC / DC) and the low-voltage flexible DC-DC converter (DC / AC) are located between the series transformer and the 750V DC bus. The low-voltage flexible DC-DC converter (AC / DC) is closer to the series transformer, with its input connected to the second output terminal (380V AC side) of the series transformer. Its output is connected to the 750V DC bus, controlling the energy transfer from the 380V distribution network to the 750V DC bus, converting 380V AC to 750V DC for the use of the 750V DC bus. The low-voltage flexible DC-DC converter (DC / AC) is closer to the DC side, with its input connected to the 750V DC bus and its output connected to the 380V AC bus. It converts the 750V DC from the 750V DC bus to 380V AC, feeding back to the 380V AC bus, and supporting energy storage batteries or distributed generation to supply power to AC loads. The 380V AC bus, serving as a centralized distribution node for AC power, is located downstream of the series transformer and upstream of the load. It is directly connected to the first output terminal of the series transformer and interconnected with the 750V DC bus via a low-voltage flexible DC / AC converter. This interconnection connects to the AC load, supplies power to it, and receives power from the 380V distribution network or the DC / AC converter, maintaining voltage stability in the AC system. Power balance in the AC / DC hybrid system is achieved through the converter and the 750V DC bus. The 750V DC bus, as the core hub on the DC side of the entire AC / DC hybrid system, connects DC equipment on both sides. It connects unidirectionally to distributed generation and DC loads, and bidirectionally to energy storage batteries, centralizing DC power from distributed generation and directly supplying it to DC loads or storing it in the batteries, reducing AC / DC conversion losses. Through low-voltage flexible DC / AC and DC / AC converters, it forms a vertical energy channel with the 380V AC bus above, maintaining a stable 750V voltage for DC equipment and converters, and mitigating the intermittency of distributed generation. When AC power is insufficient, DC power is converted into AC power through a low-voltage flexible DC / AC converter to support AC loads; when DC power is excessive, the power from the distribution network is rectified and stored or utilized through a low-voltage flexible DC / DC converter.

[0031] Distributed generation is connected only to the 750V DC bus. Its output power can be reverse-transmitted to the 380V AC bus via a low-voltage flexible DC / AC converter, converting renewable energy sources such as solar and wind power into DC power to supply DC loads or energy storage batteries. When the 380V distribution network is insufficient, the 750V DC bus supports the system's power demand. Simultaneously, the DC power supply is directly connected to the 750V DC bus, avoiding multiple AC / DC conversion losses. Energy storage batteries are bidirectionally connected to the 750V DC bus. During charging, power can come from the 380V distribution network, via the low-voltage flexible DC / AC converter or distributed generation. During discharging, power can supply DC loads or support AC loads via the low-voltage flexible DC / AC converter, smoothing out the volatility of distributed generation and improving system stability. Charging during off-peak hours and discharging during peak hours reduces electricity costs. Furthermore, it serves as an emergency power source for islanded operation during grid failures. AC loads are powered by a 380V AC bus. Power originates from the 380V distribution network, passing through a series transformer to the 380V AC bus, or from distributed generation or energy storage batteries, passing through a 750V DC bus and a low-voltage flexible DC / AC converter to the 380V AC bus. Relying on a stable AC voltage / frequency, the inverter capability of the low-voltage flexible DC / AC converter enables power exchange between AC and DC systems. DC loads are directly connected to the 750V DC bus, with power directly from distributed generation, energy storage batteries, or the 380V distribution network via a low-voltage flexible DC / DC converter. This is suitable for DC equipment such as data centers and LED lighting, reducing AC / DC conversion losses, directly absorbing DC power from distributed generation, and improving system efficiency.

[0032] 380V power distribution network power supply path: Starting from the 380V power distribution network, two branches are formed via series transformers. The first branch goes to the AC load via the 380V AC bus; the second branch goes to the DC load and energy storage battery via the low-voltage flexible DC / DC converter and the 750V DC bus. Energy storage path: From distributed generation / energy storage battery, through the 750V DC bus, the low-voltage flexible DC / AC converter, and the 380V AC bus to the AC load.

[0033] The dynamic regulation method for the aforementioned low-voltage flexible DC power system specifically includes the following steps: Step 1: Collect three-phase load current i Labc ,in i Labc =[i La i Lb i Lc The collected load current i Labc pass abc coordinates to dqo Coordinate system transformation yields the load current. dqo exist d Components of the axis i Ld 、q Components of the axis i Lq and o Components of the axis i Lo The specific conversion method is as follows: (1); In equation (1), i Ld , i Lq and i Lo They are respectively the load current dqo exist d axis q shaft and o The components of the axis; =2πf, where f is the AC frequency. t For time; Step 2: Convert the above load current dqo exist d Components of the axis i Ld The positive sequence current is obtained through a second-order low-pass filter. d Axial fundamental wave component i Ld + .

[0034] The low-pass filter is: (2); In equation (2), K P This refers to the proportional coefficient in a PID controller. K 1 The integral coefficient in PID control; U dc This is the DC bus voltage; s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; L s Equivalent inductor for low-voltage flexible DC system equivalent circuit; R s This is the equivalent resistor in the equivalent circuit; i Ld Load current dqo exist d The components of the axis; i Ld + Positive sequence current d Axial basis wave component.

[0035] The formula for calculating the equivalent inductance of the equivalent circuit in a low-voltage flexible DC system is as follows: (3); In equation (3), L d , L g and L fs These are the leakage inductance of the series transformer, the equivalent inductance of the power grid, and the filter inductance of the filter. n T This refers to the turns ratio of a series transformer.

[0036] The formula for calculating the equivalent resistance in an equivalent circuit is: (4); In equation (4), R t and R Lfs These are the resistance of the series transformer and the parasitic resistance of the filter inductance, respectively. n T This refers to the turns ratio of a series transformer.

[0037] Step 3: The DC bus voltage control adopts a typical Type II control system, and the DC bus reference voltage is acquired through a voltage sensor. u dc * and feedback DC voltage u dc DC bus voltage reference voltage u dc * With feedback DC voltage u dc The difference is combined with the DC bus voltage control G dc ( s The change in the reference DC current output of the DC bus control is calculated. .

[0038] A typical Type II control system is: (5); In equation (5), s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; K dc , z and p These are the controller gain, zero frequency, and pole frequency, respectively.

[0039] Reference DC current variation of DC bus control output The calculation formula is: ; In equation (6), udc * This is the DC bus reference voltage, with a value of 750V. u dc For feedback DC voltage; G dc ( s This is for DC bus voltage control; u dc * - u dc This is the difference between the DC bus voltage reference voltage and the feedback DC voltage.

[0040] Step 4: Power based on the state of charge (SOC) balance of the energy storage battery k p Dynamically adjust the state of charge (SOC1) of energy storage battery 1. t The state of charge (SOC2) of energy storage battery 2 t Assuming the input power of the low-voltage flexible DC-DC converter / AC converter is... P pc Powered by the energy storage unit, the rated output power of energy storage battery 1 is P Rate1 The rated output power of the energy storage battery 2 is P Rate2 Considering the state of charge, the actual output power of energy storage battery 1 and energy storage battery 2 are respectively P battery1 and P battery2 The sum is P battery1 + P battery2 = P pc Then the corresponding k p =1-( P battery1 + P battery2 ) / P L .

[0041] The calculation method for the rated output power of energy storage battery 1 is as follows: (7); (8); In equations (7) and (8), P Rate1 This is the rated output power of energy storage battery 1; P Rate2 Rated output power for energy storage battery 2; , SOC1 ( t ) represents the state of charge (SOC) of energy storage battery 1; SOC2 ( t () represents the state of charge of energy storage battery 2.

[0042] Step 5: Positive Sequence Current d Axial fundamental wave component i Ld + After passing through the PI controller, it is converted into current. i ap = k p i Ld + (From step four below) k p The calculation method yields k p Finally, a reference current is obtained, which is then converted into AC / DC power by a low-voltage flexible DC-DC converter. d Shaft reference input current ; The PI controller is: (9); In equation (9), s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; G sc Pass functions to the PI controller. k i = 0, k p For dynamic power gain adjustment; Step Six: Employ a three-phase balanced power grid and a low-voltage flexible DC-DC converter. q shaft and o The reference currents of the shafts are respectively i scq * and i sc0 * ,and i scq * =0, i sc0 * =0.

[0043] Step 7: Following Step 6, a DC bus with stable voltage is obtained, whose AC load power... P L AC power grid output power P s and low-voltage flexible DC-DC converter AC / DC input power Ppc Between satisfy P L = P s + P pc = k p P L +(1- k p ) P L Because the DC voltage is constant, i Ld = i s + i pc = k p i Ld +(1- k p ) i Ld (Since the DC bus is constant, the formula is...) P L = P s + P pc = k p P L +(1- k p ) P L Divide both ends by the DC bus voltage V This yields the load current. i Ld = i s + i pc = k p i Ld +(1- k p ) i Ld ， in i s = k p i Ld ， i pc =(1- k p )i Ld ), i Ld For load current, i s For the current of the AC power grid, i pc The output current of the DC / AC converter, as P L Changes can be dynamically adjusted. k p This enables flexible dispatching of active power in low-voltage flexible DC power systems.

[0044] like Figure 2 As shown, the simulation analysis of the system of the present invention Figure 1 ,in i Ld That is, the load current. i s It is the current of the AC power grid. i pc This refers to the output current of the DC / AC converter.

[0045] like Figure 3 As shown, the simulation analysis of the system of the present invention Figure 2 ,in P L Power required by the load, P s For AC grid output power, P pc This refers to the output power of the parallel DC / AC converter (i.e., the AC / DC input power of the low-voltage flexible DC converter).

[0046] Depend on Figure 2 and Figure 3 It can be seen that the times 0.1s-0.2s and 0.2s-0.3s correspond to respectively k p =1 and k p =0. Load during the period of 0.1s to 0.2s P L =1 pu, at this time k p =1, the low-voltage distribution network supplies active power to the load. P pc and i pc All values ​​are 0; load during the period of 0.2s to 0.3s. P L =1 pu, at this time k p=0, the low-voltage distribution network supply power does not supply active power to the load; the active power of the load is all supplied by the DC / AC converter on the DC side. P pc Supply. Therefore, as long as it is properly regulated... k p This enables dynamic, adaptive, and flexible adjustment.

[0047] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A low-voltage flexible DC power system, characterized in that, The system includes: The distribution network, located at the front end of the system, is connected to the input terminal of the series transformer to provide power to the AC bus, DC bus, AC load and DC load, and stabilize the system voltage; A series transformer is located between the distribution network and the AC bus, with its first output terminal connected to the AC bus, to distribute the power from the distribution network to the AC bus. The AC / DC converter has its input terminal connected to the second output terminal of the series transformer, which distributes the power from the distribution network to the DC system; its output terminal is connected to the DC bus, which converts AC power into DC power for use by the DC bus. The DC / AC converter has its input connected to the DC bus and its output connected to the AC bus, converting the DC power from the DC bus into AC power and feeding it back to the AC bus. The AC bus, as a centralized distribution node for AC power, is located downstream of the series transformer and upstream of the AC load. It is interconnected with the DC bus through the DC / AC converter, receiving power from the distribution network or the DC / AC converter. At the same time, it connects to the AC load and supplies power to the AC load. The DC bus connects unidirectionally to distributed generation and DC loads, and bidirectionally to energy storage batteries. It centralizes the DC power generated by distributed generation and directly supplies it to DC loads or stores it in energy storage batteries, reducing AC-DC conversion losses. It forms a vertical energy channel with the AC bus through the AC / DC converter and the DC / AC converter, maintaining a stable voltage of 750V for use by DC equipment, the AC / DC converter, and the DC / AC converter, thus mitigating the intermittency of distributed generation. The power supply path of the system is as follows: starting from the distribution network, it branches into two paths through the series transformer. The first branch goes through the AC bus to the AC load, and the second branch goes through the AC / DC converter and the DC bus to the DC load and the energy storage battery. The energy storage path of the system is as follows: from the distributed generation or the energy storage battery through the DC bus, the DC / AC converter, and the AC bus to the AC load.

2. The low-voltage flexible DC power system according to claim 1, characterized in that, The distributed generation system outputs power to the DC bus in one direction, and can transmit electrical energy in reverse to the AC bus via a DC / AC converter, while simultaneously supplying DC loads or energy storage batteries.

3. The low-voltage flexible DC power system according to claim 1, characterized in that, When the energy storage battery is charging, the electrical energy from the distribution network is stored through the AC / DC converter or distributed generation; when the energy storage battery is discharging, its electrical energy is supplied to the DC load or supports the AC load through the DC / AC converter.

4. The low-voltage flexible DC power system according to claim 1, characterized in that, The electrical energy of the AC load is supplied from the distribution network through the series transformer to the AC bus, or from the distributed generation or the energy storage battery through the DC bus and DC / AC converter to the AC bus, thereby realizing power mutual assistance between the AC and DC systems.

5. The low-voltage flexible DC power system according to claim 1, characterized in that, The electrical energy of the DC load comes from the distributed generation, energy storage batteries, or is converted from the distribution network via an AC / DC converter.

6. The low-voltage flexible DC power system according to claim 1, characterized in that, The AC bus is a 380V / 50Hz AC bus, the AC / DC converter is a low-voltage flexible DC / DC converter, the DC / AC converter is a low-voltage flexible DC / AC converter, the DC bus voltage is 750V, and the distribution network voltage is 380V.

7. A dynamic adjustment method for a low-voltage flexible DC power system as described in any one of claims 1 to 6, characterized in that, The method includes: Step 1: Collect three-phase load current i Labc The collected load current i Labc pass abc coordinates to dqo Coordinate system transformation yields the load current. dqo exist d Components of the axis i Ld ; Step 2: The components i Ld The positive sequence current is obtained through a second-order low-pass filter. d Axial fundamental wave component i Ld + ; Step 3: The DC bus voltage control adopts a typical Type II control system, and the DC bus reference voltage is acquired through a voltage sensor. u dc * and feedback DC voltage u dc DC bus voltage reference voltage u dc * With feedback DC voltage u dc The difference is combined with the DC bus voltage control to calculate the DC bus current. i dc ; Step 4: Power based on the state-of-charge balance of the energy storage battery k p Adjustments to the state of charge (SOC) of energy storage battery 1 and energy storage battery 2, and the input power of the low-voltage flexible DC / AC converter. P pc Powered by the energy storage unit, the rated output power of energy storage battery 1 is P Rate1 The rated output power of energy storage battery 2 is P Rate2 The actual output power of state-of-charge energy storage battery 1 and energy storage battery 2 are respectively P battery1 and P battery2 The sum is P battery1 + P battery2 = P pc ,but k p =1-( P battery1 + P battery2 ) / P L； Step 5: Convert the positive sequence current d Axial fundamental wave component i Ld + After passing through the PI controller, it is converted into current. i ap = k p i Ld + The obtained reference current is converted into the AC / DC converter. d Shaft reference input current i scd * = i ap +i dc ; Step Six: Using a three-phase balanced power grid, the AC / DC converter... q shaft and o The reference currents of the shafts are respectively i scq * and i sc0 * ,and i scq * =0, i sc0 * =0; Step 7: Following Step 6, a DC bus with stable voltage is obtained, whose AC load power... P L AC power grid output power P s and low-voltage flexible DC-DC converter AC / DC input power P pc Between satisfy P L = P s + P pc = k p P L +(1- k p ) P L Because the DC voltage is constant, i Ld = i s + i pc ， in i s = k p i Ld ，i pc =(1- k p ) i Ld ,along with P L The changes can be adjusted. k p This enables flexible dispatching of active power in low-voltage flexible DC power systems.

8. The dynamic adjustment method according to claim 7, characterized in that, In step one, the specific method of the conversion is as follows: (1); In equation (1), i Ld , i Lq and i L0 They are respectively the load current dqo exist d axis q shaft and o The components of the axis; =2πf, where f is the AC frequency. t For time.

9. The dynamic adjustment method according to claim 7, characterized in that, In step two, the low-pass filter is: (2); In equation (2), K P This refers to the proportional coefficient in a PID controller. K 1 The integral coefficient in PID control; U dc This is the DC bus voltage; s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; L s Equivalent inductor for low-voltage flexible DC system equivalent circuit; R s This is the equivalent resistor in the equivalent circuit; i Ld Load current dqo exist d The components of the axis; i Ld + Positive sequence current d Axial basis wave component.

10. The dynamic adjustment method according to claim 9, characterized in that, The formula for calculating the equivalent inductance of the equivalent circuit of the low-voltage flexible DC system is as follows: (3); In equation (3), L d , L g and L fs These are the leakage inductance of the series transformer, the equivalent inductance of the power grid, and the filter inductance of the filter. n T For series transformer turns ratio; The formula for calculating the equivalent resistance of the equivalent circuit is: (4); In equation (4), R t and R Lfs These are the resistance of the series transformer and the parasitic resistance of the filter inductance, respectively. n T For series transformer turns ratio; In step three, the typical Type II control system is: (5); In equation (5), s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; K dc , z and p These are the controller gain, zero frequency, and pole frequency, respectively. The DC bus current The calculation formula is: (6); In equation (6), u dc * This is the DC bus reference voltage, with a value of 750V. u dc For feedback DC voltage; G dc ( s This is for DC bus voltage control; u dc * - u dc This is the difference between the DC bus voltage reference voltage and the feedback DC voltage; In step four, the rated output power of the energy storage battery 1 is P Rate1 The rated output power of the energy storage battery 2 is P Rate2 The calculation methods are as follows: (7); （8）； In equations (7) and (8), P Rate1 This is the rated output power of energy storage battery 1; P Rate2 Rated output power for energy storage battery 2; , SOC1 ( t ) represents the state of charge (SOC) of energy storage battery 1; SOC2 ( t () represents the state of charge of energy storage battery 2; In step five, the PI controller is: (9); In equation (9), s Let be a complex variable in the transfer function, representing the frequency in the Laplace transform domain; G sc Pass functions to the PI controller. k i = 0, k p This is for dynamic power adjustment gain.

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