Control Strategy Of Interfacing Converters Based On Enhanced Series/parallel Structure For DC/AC Hybrid Microgrid

With the rapid development of distributed power technology,the penetration of renewable energy such as PV and wind turbine,increases significantly in power system,which makes the conventional power system,especially the distributed and supply system,have changed thoroughly.The intermittent and uncertain features of distributed sources become great challenges to the conventional power system on several aspects including the plan and operation,the control and protection as well as the power quality regulation.The microgrid formed by the coordinated operation of multiple distributed power sources is an important solution to face these challenges of high penetration of distributed renewable energy generation.As a key hub between the microgrid and the external power grid,by proper controller design,microgrid interfacing converters can make the local grid possess a series of characteristics such as strong renewable energy compatibility,high power supply reliability,good power quality and flexible operation control.With varies of power supply and grid demand,microgrid interfacing converter topology can be divided into different types including series and parallel topology.Firstly,for series interfacing converters,they are usually applied for high-voltage grid-connected scenarios.Among these applications,cascaded H-bridge converter(CHB)has been widely used due to their advantages of simple control and convenient extension.The cascade H-bridge converter can realize the flexible interconnection of multiple low-voltage dc microgrids,and realize the coordinated control of each DC microgrid through the centralized controller.When each power module is connected to LC filter to form an independent converter unit,decentralized controller can also be used to realize autonomous control and peer control.Following this trend,CHB converters or series converters can be directly integrated into medium voltage distribution systems such as 6 k V and 10 k V grids.However,it is necessary to point out that,previous studies on grid-connected control of three-phase cascade converters mainly focus on power control,including inter-phase power sharing with zero-voltage injection.Using cascade H-bridge converter to realize the function as an active power regulating device to solve comprehensive voltage and current harmonic problems has not been discussed.Cascaded H-bridge converter,on the other hand,due to its modular structure of low voltage power electronic module,and multilevel output voltage waveform,has been widely applied in power factor correction in medium voltage power distribution system,such as the cascaded H-bridge converter based static synchronous compensator(CHB-STATCOM).As a large number of H-bridge modules are connected in series in CHB-STATCOM,the fault problem of cascade H-bridge system is more serious than that of conventional low-voltage two-level converters with fewer power electronic devices.Among CHB-STATCOM devices including DC capacitor,IGBT diodes and drivers,IGBT fault study is the most prevalent.IGBT failure can be further divided into two types as short-circuit fault and open-circuit fault.For short-circuit fault,inrush current can be immediately trigger the over-current protection of the whole system and whilst open circuit of IGBT failure is more common compared with IGBT short-circuit problem.Before trip of the system,open circuit fault often causes converter output current and dc bus voltage distort,dropped or boosting,which provides an opportunity to apply open-circuit fault diagnosis and tolerance to achieve fault ride-through.Around the fault-tolerant operation problems,there are some pioneering research reports.However,few studies are about IGBT fault fast detection,while the extant methods usually need high-bandwidth high-voltage sensors to capture the detailed step waveforms of output PWM voltage,which is difficult to popularize and implement in low-cost applicationsFor parallel microgrid interfacing converters,especially two level interlinking converters,mainly serve for low voltage AC/DC hybrid microgrid applications.Its modular interlinking configuration makes it easy to realize power capacity expansion.In ac/dc hybrid microgrid,interlinking interfacing converter plays the role of key hub,which connects with distributed generation units,DC microgrid interface and AC subgrid interface.In previous studies,the closed-loop current control has been widely used in the grid-connected operation because of its fast response speed and good antiinterference ability to power grid voltage disturbance and power grid impedance change which is mainly reflected in three aspects: Firstly,using the fundamental positive sequence virtual impedance can avoid the resistance feeder instability problems caused by the impedance in island operation.Secondly,harmonic virtual impedance and fundamental negative sequence virtual impedance can realize the generalized power sharing including harmonic power and unbalanced power.Finally,by optimizing the system harmonic impedance network,virtual impedance can be used to improve the PCC voltage and line current between the microgrid and main grid.However,the conventional virtual impedance coefficient is fixed,so the system under different operating conditions might not maintain its performance.In addition,the realization of the virtual impedance only involves a single converter,while using virtual impedance converter to implement the flexible coordination between multi converters is not documented.In order to solve the above mentioned problems,this paper focuses on the typical structure of parallel-type and series-type microgrid interfacing converters,respectively.The topological structure,control strategy,fault-tolerant operation of cascaded Hbridge microgrid interfacing converters and the virtual impedance control of interlinking interfacing converters are investigated in this paper.Firstly,this paper proposes an improved CHB converter harmonic voltage and current synchronization compensation method.The lower unit of the converter system is equipped with an LC output filter for improving the supply voltage quality of the local load connected to the low voltage AC port.The upper unit is connected to the high voltage grid via an LCL filter.The converter adopts a simple double closed-loop control of the upper and lower units,and can simultaneously provide active power supply voltage harmonic suppression and grid current harmonic compensation without extracting harmonic components.Firstly,through wide bandwidth current regulation,the upper unit suppresses harmonic currents and pushes all local load harmonics and unbalanced currents into the lower unit.Therefore,the method can effectively improve the grid current power quality.At the same time,the voltage supply to the lower unit can ensure the supply voltage of lower unit.In addition,the method can ensure the power quality and power sharing of the system under various adverse conditions such as significant changes of cable impedance,changes of output power in power module,and time-varying loads.It should be noted that when the upper unit grid current and the lower unit supply voltage track the corresponding reference signals,the three-phase imbalanced grid voltage and the local load demand may cause the three-phase DC bus voltage imbalanced,resulting in the three-phase grid current and the supply voltage unable to maintain balanced and sinusoidal.In order to solve this conflict,this paper applies the zero-sequence voltage injection method to the upper and lower units.Through zerosequence voltage control,each unit’s output power can be flexibly adjusted without affecting the line supply voltage and grid current.For the control parameter selection problem,in this system,the tracking of the grid-connected current in the upper unit can be regarded as the interference to the regulation of lower supply voltage.Similarly,the lower power supply voltage regulation can be considered as the interference to the upper cell current tracking.From this point of view,this paper analyzes the interference problem between the upper and lower units in detail,and proves that there is a weak interaction between the upper and lower units,which does not affect the selection of parameters.Therefore,the system can design dual closed-loop voltage and current control parameters according to the typical parameter selection principle.Simulation and experimental results verified the effectiveness of the system control.Firstly,a high-power converter with rated power of 300 KVA is simulated on PSCAD/EMTDC.The system sets the lower unit and the upper unit to three-phase400 V low-voltage bus and 690 V high-voltage bus,respectively.Among them,each DC bus is connected to a 40 k VA constant DC power source,a DC energy storage unit and a DC load.The DC energy storage unit is responsible for the regulation of the DC bus voltage.In order to prove the effectiveness of the method under unfavorable conditions,the three-phase grid voltage is set to have a large number of harmonics and is highly unbalanced,while the three-phase local load is set to a nonlinear harmonic load.The simulation consists of three stages.In the first stage,the system did not use zero sequence voltage injection.In the second stage,the upper three groups and the lower three groups of H-bridge units are dispatched to supply the same power via zerosequence voltage injection.In the third stage,the power reference of the upper and lower units is calculated to minimize the power exchange of the phase-A energy storage unit.Under this condition,the output power of upper unit is completely provided by the constant power DC power supply on the DC bus.The simulation proves that the system can maintain high grid-connected current and power supply voltage quality in all three stages.Secondly,this paper uses Mitsubishi IGBT to construct the H-bridge modules for experiments.Experiments verify the performance of the system under local nonlinear loads and main grid disturbances.Under the above conditions,the three-phase supply voltage maintains good quality,with small distortion and unbalance components.At the same time,local load chop in the grid have no significant effect on the supply voltage.Secondly,for the most common single IGBT open-circuit fault in STATCOM based on CHB converter,in this paper,an online fault diagnosis and corresponding fault-tolerant operation algorithm are proposed.By mapping the fault information to the output current DC component and the DC voltage trend,the IGBT open circuit fault can be quickly and accurately determined before the system overcurrent/overvoltage protection trigger.The faulty H-bridge module then seamlessly transfers to the halfbridge operation mode,continuing to provide a lower output AC voltage.Firstly,this paper gives a brief introduction to the cascaded H-bridge STATCOM controller,which is mainly composed of three parts: First of all,DC voltage control and current control are the main control loops of the system.The DC voltage control is mainly responsible for controlling the DC average voltage regulation of one phase module.According to the reactive power demand,the current control loop regulates the three-phase output current.In addition,a DC voltage secondary balancing control loop is used to ensure the DC bus voltage of the inner-phase power module balanced.Subsequently,this paper further analyzes the relationship between output voltage of module and the output current in the fundamental wave.With the reactive power supply,the steady-state output current is different from the steady-state output voltage of the normal module lagged about 90 degrees.When the IGBT open-circuit fault occurs,harmonics and DC components are present in the low-frequency components of the output voltage,which causes DC bias and distortions of output current,and the corresponding DC bus voltage will rise.Based on the voltage and current changes corresponding to IGBT faults at different cases,in this paper,a fault diagnosis and fault-tolerant operation method is proposed.The method can be divided into four stages.In the stage-1,the three-phase output current DC component is extracted.By detecting the polarity of the DC component current,the phase in which the faulty IGBT occurring is located.Subsequently,in stage-2,the fault module in the phase can be determined by identifying the transient trend of DC bus voltage.Finally,in the stage-3,depending on the polarity of the output current DC component of faulty module,the switch pair in which the open fault occurs can be further specified.Finally,in the stage-4,the fault-tolerant operation of cascaded H-bridge converter based STATCOM is realized by changing the faulty module from the full-bridge operation mode to the half-bridge operation mode and adjusting the modulation reference signals.Thirdly,for the control problems of parallel interfacing converters,this paper applies the coupled virtual impedance coordinated control strategy to the AC/DC hybrid microgrid interlinking interfacing converter.The system consists of two three-phase interlinking converters,and one of converters is operating in voltage control mode with adaptive series virtual impedance droop control while the other is operating in current control mode.However,current-based interfacing converters typically have faster dynamic power response characteristics than voltage-based interfacing converters when the reference power changes instantaneously.In order to achieve good power control of the interconnect unit,the current tracking converter can compensate the transient power control error of the voltage controlled converter by correcting the current control error.Taking advantage of the difference between active power and reactive power control of the two converters,the virtual impedance of voltage mode converter in grid-tied mode is dynamically adjusted to achieve adaptive dynamic power response under different conditions.Therefore,the problem of slow reactive power dynamic response of the droop control system is effectively solved.For conventional inter-linking converters with both grid-tied and islanded operation capacity,the CCM converter is usually used in grid-tied mode and the VCM converter is adopted in islanded mode.For a seamless transfer from grid-tied operation to islanded operation,a direct transfer from CCM to VCM can easily cause inrush current and output LCL/LC filter resonance.This may jeopardize the local load supply voltage quality during transition.To overcome this limitation,the line current of the interlinking unit usually decreases to zero in grid-tied mode before transferring from CCM to VCM.However,the smooth line current attenuation to zero certainly needs a few cycles tuning.This preparation may not be allowed in some cases like the unintentional grid interruptions.In the proposed scheme,the seamless transfer of operation mode can be conveniently realized by the coordination of two converters.At the same time,a small signal model is established to analyze the stability and transient response of proposed interlinking interfacing converter control method.Finally,the effectiveness of the scheme under different conditions is verified by simulation.The simulation results show that the proposed method has a high performance on local load supply voltage,grid-connected current,power sharing and seamless transfer between grid-tied mode and islanded mode.