Research and development of multi-directional intelligence of Nickel Hydride Batteries energy storage system

1 Nickel Hydride Batteries energy storage system can store "valley electricity" at night or surplus electricity on weekdays. It can not only cope with emergencies such as power grid outages or large-scale power outages, but also is a "dispatching expert" for urban power grid peak shaving and valley filling. At the same time, Nickel Hydride Batteries energy storage system can also be used in wind and solar storage system, which will make the grid-connected power generation of renewable energy such as solar energy and wind energy more stable. Nickel Hydride Batteries energy storage system has two important components, namely system Nickel Hydride Batteries and grid-connected access system. Both are indispensable and inseparable. Traditional unidirectional PWM rectifier or unidirectional PWM inverter can no longer meet the requirements of grid-connected access of energy storage system, so a new type of grid-connected access system is proposed, namely bidirectional power converter (PCS) topology, which can not only meet the Nickel Hydride Batteries charging and discharging, but also supply power to key loads when the power grid is out of power, playing the role of EPS, also known as island operation. Three operation control modes of PCS were studied, and a 12kW Nickel Hydride Batteries energy storage system bidirectional PCS was developed and successfully used in vanadium Nickel Hydride Batteries energy storage system. 2 Main circuit topology The main circuit topology of the bidirectional power converter PCS developed here is shown in Figure 1. The core of the main circuit is the PWM1 three-phase IGBT full-bridge converter, which can perform AC/DC conversion and DC/AC conversion. The DC filter unit (C5, L1, C4) can filter the output when the PCS is rectifying, effectively reducing the ripple current and ripple voltage of the input Nickel Hydride Batteries. At the same time, when the PCS is inverting the output, it reduces the impact of the switching frequency pulse current on the Nickel Hydride Batteries, playing a dual filtering role. The isolation transformer T1 not only plays the role of isolation and voltage matching, but also plays the role of AC filter. When designing the transformer, a certain leakage inductance can be designed into it, so that it and the filter capacitors C1~C3 form an AC filter unit, thereby effectively reducing the size of the PCS and making the system simpler. The main function of the AC contactor KM1 and KM2 is to isolate the critical load from the power grid when the PCS is operating in an island mode, so as to prevent the sudden recovery of the power grid voltage from causing an impact on the system. 3 Control method Nickel Hydride Batteries charging requires constant DC current or DC voltage, while when the Nickel Hydride Batteries is discharged, DC current constant current discharge is generally required. Therefore, in the design of PCS charge/discharge control system, dual-loop control is generally adopted, namely, the DC voltage (or current) outer loop and the AC current inner loop, as shown in Figure 2. The purpose of the DC voltage (or current) outer loop is to maintain the DC voltage (or current) constant. In order to achieve the steady-state difference of the DC voltage, the outer loop adopts a PI regulator. For the inner loop, its main function is to control the current according to the current command output by the DC voltage (or current) outer loop, which can realize the unit power factor sine wave current control. The specific method is to make a difference between the command current value output by the outer loop regulator and the actual detected grid current to obtain the deviation value, and then obtain the grid-side output voltage command signal through the current inner loop controller and the grid voltage phase-locked loop signal, where id is the active current of the three-phase current ia, ib, ic output by the PWM1 converter after Park transformation. When the grid is powered off, PCS first disconnects KM1 in 1 to isolate the AC output from the grid, and then starts island operation. Its control block diagram is shown in Figure 3. The control goal of island operation is to keep the output AC voltage stable under different output load conditions. Therefore, the island operation control also adopts a dual-loop structure. The outer loop is the output AC voltage effective value loop, which aims to maintain the stability of the output AC voltage. The inner loop is the output AC voltage instantaneous value loop, which aims to ensure that the output AC voltage has good dynamic performance. The specific method is to perform PI integral control on the difference between the given AC voltage effective value of the outer loop and the feedback AC voltage effective value, which can effectively reduce the static error of the output. The outer loop PI output is multiplied with the sinusoidal signal as the given instantaneous value of the inner loop output voltage, and the difference is compared with the AC voltage instantaneous value feedback, and then amplified by the ratio as the input of the PWM generator. In Figure 3, Uip is the bias component, which can prevent the output current from being biased and causing the output transformer (preliminary diagnosis method for transformer faults) to saturate. Its value is the average value of the output AC current and then converted into the bias voltage. 4 Control circuit PCS control circuit block diagram is shown in Figure 4. The control circuit adopts a dual DSP+FPGA structure. The DSP is TMS320C2812 with a main frequency of 150MHz and a PWM event manager. The control circuit is divided into the control side and the logic side. The control side is managed by DSP1, which mainly implements the control algorithm and pulse output, and the key protection signal input; the logic side is managed by DSP2, which mainly completes A/D acquisition, external communication, and input and output. DSP1 and DSP2 exchange information through dual-port RAM. At the same time, external A/D, PWM output, input and output, etc. can be flexibly allocated through FPGA, that is, A/D acquisition can be collected by DSP1 or DSP2, and FPGA only needs to be configured according to system needs. The above control circuit can meet the complex control needs of PCS. For example, the charge/discharge characteristic curve of PCS for the Nickel Hydride Batteries can be stored in the logic side DSP2, and the setting of charge/discharge time can also be completed by DSP2. It only needs to tell the control side in real time when to operate under what parameters and what working conditions. DSP1 calculates the control parameters of each operating condition, generates PWM output pulses, and completes various control functions of PCS. 5 Experimental results and conclusions A 12kW vanadium Nickel Hydride Batteries energy storage system PCS was developed, and its main technical parameters are as follows: rated capacity 12kW; rated AC voltage of the grid 380V; rated current of each phase of the grid 20A; rated voltage of the Nickel Hydride Batteries 160V; Nickel Hydride Batteries voltage range 100-200V; maximum charging current and maximum discharge current of the Nickel Hydride Batteries are both 100A; switching frequency 10kHz. The experimental waveform is shown in Figure 5. As can be seen from Figure 5a, the ripple current is less than 0.5% during constant current charging; as can be seen from Figure 5b, the ripple current is less than 0.5% during constant current discharge; Figure 5c shows that when the grid is powered off, the PCS operates as an island. When the load is suddenly added, the inverter outputs a sinusoidal voltage without overshoot, and the waveform is smooth, which can fully meet the power supply requirements of the critical load; Figure 5d shows the output current THD curve of the PCS when it is connected to the grid and discharged at a constant current. At rated power, the output current THD is only 2%, which meets the requirements of relevant standards. A 120kW PCS for vanadium Nickel Hydride Batteries energy storage system was developed using a dual DSP controller structure and bidirectional PWM conversion topology. The PCS is powerful and has multiple functions including Nickel Hydride Batteries charging, Nickel Hydride Batteries discharging, island operation, pre-charging, alarm, protection, communication, touch screen display, and meter display. The experimental results show that the PCS has flexible operation mode and small charge and discharge ripple current. Only one PCS can completely replace the traditional inverter, rectifier, and EPS to form an energy storage charging and discharging system, which has great promotion value.

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