| The development trend of electric power system is regional network and the stability of power system. Micro grid can coordinate distributed power source in the form of the higher concentration, reduce the burden of power grid control and fully play the advantages of distributed power source. Micro grid is a controllable unit on the combination of tens of kilowatts rated power of micro sources, loads, energy storage equipments and control devices. And it can enhance the stability of the system and the safety of power supply. So the development of micro power grid is one of development trend of the power system. In the micro grid, power electronic devices play a key role in energy conversion. DC converter is one of them. When the micro grid operates in the grid-connecting mode, DC converter which has the function of bidirectional power flow can charge up the battery. Once the isolated net running, the converter can send stored electrical energy to the load and meet emergency power supply. Under the above background, this paper designed a set of bidirectional DC converter suitable for micro grid between the DC bus and the energy storage system.Analyzing operation mode, control method of micro power grid, introduce micro power source and control method of energy storage, elaborate stability control of micro power grid, optimal control of power quality and economic operation. Bi-directional DC converter is the key equipment of connecting the DC bus in the micro grid and the energy storage battery. Therefore researching circuit topology and working principle of bidirectional DC converter has very important significance.This article applies small signal analysis method to establish the small signal model of bidirectional DC transform under the model of Buck and the Boost. Based on the small signal model, design the voltage type closed loop control system according to control principle; come to open-loop transfer function of the system. In Matlab use Bode diagram and Nyquist criterion to get the conclusion of system closed-loop stability.Put forward the variable frequency of the bidirectional DC converter using a pulse width modulation PWM control method. Adopted the continuous variable slope integrator delta modulation method, by changing the cycle of pulse sequence to frequency modulation and pulse width or duty cycle to pressure regulating, the output pulse width can change quickly following the output voltage feedback and get the corresponding pulse waveform.Design each sub circuit of the control system, such as voltage and current sampling circuit, voltage regulator circuit, pulse modulation circuit, drive circuit and control signal transmission circuit. Voltage and current sampling circuit uses the simplest method of series parallel resistor to detect voltage and current; Voltage regulation circuit uses PI control links; the pulse modulation circuit consists of an error amplifier, a hysteresis comparator and integrator. Not only it can easily change the output voltage or output voltage stability, can also choose the appropriate working frequency according to load condition; driver circuit uses MOSFET driver circuit of the optical coupler isolation; control signal transmission circuit consists of a four channel analog switch as the core, controls the transmission power flow.In Multisim, set up a set of 1 kW bidirectional DC converter, complete the selection and parameter selection of the main circuit components and control systems components. The simulation results verify the feasibility of the method to control the bidirectional converter. Not only it has a good dynamic characteristic and static characteristic, also has a working frequency variable, fast response characteristics, etc. |
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