| With the continuous development of society and economy,the exploitation of fossil energy is increasing day by day,which has led to many global challenges such as energy shortage,environmental pollution and climate change.In recent years,many governments have successively issued policies to encourage the development and use of renewable energy power generation to gradually replace fossil energy.The main goal of energy revolution in China will be to gradually replace fossil energy with renewable energy,so that clean energy such as renewable energy will account for a larger share of primary energy production and consumption and a new generation energy system with clean,low-carbon,safe and efficient features will be built.In order to achieve the goal of energy transition,renewable energy power generation is integrated into the grid on a large scale in a centralized or distributed form.On the power source side,large-scale wind farms,photovoltaic power stations,energy storage power stations,etc.are centrally connected to the grid,which improves the proportion of renewable energy.On the transmission side,the gradual application of high voltage direct current(HVDC)and flexible AC transmission systems(FACTS)devices have improved the transmission capacity of transmission lines.On the distribution network side,a large number of DC-driven rail transit,electric vehicles,data centers and other power electronic equipment are connected to the grid.At the same time,distributed wind power,photovoltaic and energy storage devices are widely connected to the distribution network.These increase the flexibility of the distribution network but also increase the complexity of the system.With a large number of power electronic devices of different types and voltage levels connected to the grid,the trend of power electronics in the power system has gradually emerged,bringing many challenges to system operation safety,system analysis and control,simulation modeling calculations,etc.,including: 1)The reduced system inertia causes frequency fluctuations and frequency stability problems.2)Power quality problems caused by the nonlinear process of power electronic equipment.3)Interaction of multiple power electronic devices induces broadband oscillation problems.To address these challenges,the flexible and efficient converter control method,power quality improvement method and the wideband-frequency oscillation analysis method for a multi-converter system have been extensively studied.(1)Control method of interfacing convertersThe control of power converters is the key for robust integration of DG units in power distribution systems.Previous control methods can be divided into two categories: the current control method(CCM)and voltage control method(VCM).CCM is widely used in power converter of DG unit,due to the merit of fast and accurate current tracking.However,it should be mentioned that conventional CCM can hardly be used in islanded power systems if there is no voltage controlled electric sources to provide voltage support.On the other hand,the recent development of microgrid and active power distribution system concepts further requires a DG unit to keep operating when the grid mains are not available.To fulfill this requirement,the voltage control method for DG unit is also proposed in order to provide proper voltage support to standalone power distribution systems.Compared to a CCM-based DG unit,it is well understood that VCM-based DG units can operate in both grid-tied and islanding.In addition,multiple parallel DG units can operate like a set of distributed synchronous generators where the power sharing of load demand is automatically realized by the real power-frequency droop and reactive power-voltage magnitude droop controllers.However,it is important to note that a drawback of a VCM-based DG unit is the slow power control dynamics during grid-tied operation,as the method is developed based a steady-state power flow between a controlled line frequency voltage source and the main grid.Another trend of DG interfacing converter topology is to use multiple low voltage converter modules connected in parallel.This can be more flexible to expand power level and improve the system reliability.To realize these objectives,relevant research is mainly focused on circulating current suppression,DC-link ripple reduction,modulation schemes,and interleaving angle optimization.However,except for the slight difference in the modulation stage,the power control method of each parallel module remains the same.As a result,the output characteristic of the DG unit is either a current source or a voltage source,depending on how the control scheme is implemented.Thus,the parallel-connection of multiple converter modules does not bring any changes of the output response comparing to a single converter based DG unit.It is necessary to note that control schemes for modules in a multiple-converter based DG unit are not necessarily to be the same,in order to have better control of the DG unit in a microgrid with dual operation modes.This provides a new idea for the dual-converter coordinated control method in single DG unit.(2)Power quality improvement methodThe nonlinear process of power converter switching devices will bring a series of power quality problems such as harmonics,three-phase unbalance,voltage fluctuations and flicker to the grid.There are growing demands of using power conditioning circuits in power systems.Comparing to bulky passive filters that are highly sensitive to circuit parameters variations,the active power conditioning equipment including active power filter(APF),dynamic voltage restorer(DVR),static var compensator(SVC),static var generator(SVG),and unified power quality conditioner(UPQC)is preferred due the fast dynamic response and the good immunity to system parameter changes.However,the investment and maintenance costs of active filter devices are high.On the other hand,the high penetration of distributed generation(DG)unit with power electronics interfacing converter offers the possibility of power distribution system harmonic current compensation using multi-functional DG interfacing converter.Previous research mainly focused on the control of a single DG shunt interfacing converter as an APF,as their power electronics circuits have similar topology.To realize an enhanced active filtering objective,the conventional current control methods for grid-tied DG interfacing converter shall be modified.Nevertheless,it is important to emphasis that even when the local load harmonic current is properly compensated using various controllers as mentioned above,high quality supply voltage to local load cannot be guaranteed at the same time.This problem is particularly serious when the DG interfacing converter is interconnected to a weak microgrid with nontrivial upstream grid voltage distortions.To overcome this limitation,a DVR with series harmonic voltage compensation capability can be installed in the power distribution system.Unfortunately,the functionality of a DVR can hardly be implemented in a shunt DG interfacing converter.Using an additional series power conditioning equipment to ensure very low steady-state harmonic supply voltage to local loads is definitely feasible.However,it is associated with more expenses which might not be accepted for cost-effective power distribution systems.How to achieve the two control objectives of harmonic current compensation and harmonic voltage compensation in a cost-effective manner is an urgent problem to be solved.(3)Wideband-frequency oscillation caused by multiple-converter systemsNowadays,an increasing number of DG units have been integrated into the power grid,which causes serious stability and resonance issues.In order to conduct resonance analysis,establishing an accurate model is essential for resonance analysis.The resonance analysis of multiple converter parallel systems has been extensively studied in the previous literature.The analysis method can be generally divided into state-space method based on time-domain and transfer function method based on frequencydomain.The state-space method establishes the model in time-domain,and reveals the oscillation mode of the system through eigenvalue analysis,which has a solid theoretical basis.However,as the number of parallel converters increases,the order of the system increases rapidly,making the calculation hard to realize.The transfer function method establishes the transfer function matrix model of the system to investigate the resonance characteristics with the variety of control parameters,grid strength and number of parallel converters.However,this method is only effective for a white-box system,which all system and control parameters should be known.To overcome this drawback,the impedance-based analysis method focuses on the external behavior of converters and has been widely applied with the advantages of measurable characteristics,intuitive physical meaning,and strong applicability.Generally,there are two types of impedance models: dq impedance model or sequence impedance model.The dq impedance model is usually a 2×2 transfer function matrix,while the sequence impedance model is composed of positive-sequence impedance and negative-sequence impedance.When the influence of frequency coupling characteristics is not considered,the two terms are decoupled from each other,which is more convenient for impedance aggregation modeling and stability analysis for multiple inverter systems.The accurate sequence impedance model including all control loops for single converter is established in detail by harmonic linearization method.Then the impact of grid strength,PLL parameters and current loop parameters on the single converter stability is analyzed based on Nyquist criterion.However,the accurate aggregate sequence impedance model for a multi-converter system is still absent in the previous literature.On the other hand,voltage feedforward control has been widely applied in the grid-tied converters due to the merits of good startup performance,rapid current response dynamics and grid disturbance rejection.However,it brings an additional positive feedback of the grid current and tends to reduce the system stability margin under weak grid condition.In order to overcome this drawback,some enhanced methods are proposed to automatically adjust the parameters of PCC voltage feedforward based on an adaptive rule.However,these methods are focused on improving the performance of voltage feedforward itself and the impact of power operation condition on the system stability is not considered.The influence of power operating conditions on the stability of a multi-converter system is rarely involved,and the coupling effect with the voltage feedforward types has not been reported in the previous literature.In addition,the above literatures for resonance analysis are all based on a relatively simple parallel structure,where some identical inverters are connected to a single AC bus.The scenarios of multiple buses,introduced by different feeder lines or group transformers are not taken into consideration.In high-capacity centralized power generation system,such as photovoltaic(PV)station or energy storage system,multiple DGs are usually divided into several low-capacity groups.In each group,a set number of DG units in parallel are connected to the secondary of the matching transformer.This topology also introduces more complex power quality problems,due to the interaction between inverters,transformers,and feeders.To address these challenges,the coordinated control strategy of two parallel converters in single DG,stability analysis and resonance mechanism of multi-DG parallel systems are mainly discussed in this thesis.(1)Enhanced power control strategies of dual converters in single DG unit.A novel control method is proposed for a DG unit with two parallel interfacing converters.One interfacing converter is controlled by VCM and the other is controlled by CCM.With this modification,the proposed DG unit can have the following features: 1)Enhanced power control dynamics in grid-tied mode;2)Improved load sharing performance in islanding operation mode;3)Seamless mode transfer at any moment.Comprehensive verification results are provided to validate the performance.(2)Enhanced power quality strategies of dual converters in single DG unit.To realize simultaneous mitigation of the grid current and the supply voltage harmonics,the voltage control algorithm and current control algorithm in the dual-converter system are modified,which enables the system to have the power quality control capability.The local load supply voltage quality is enhanced by the first interfacing converter through harmonic voltage control.The harmonic current produced by the interactions between the local nonlinear load and the first converter is then compensated by the second converter.With cooperative operation of two converters,the proposed comprehensive power quality control objective can be realized.(3)Stability analysis of a parallel-converter single-AC-bus system.An aggregated sequence impedance model is established for parallel-converter-based grid-tied generation systems.It is found that for radial connection of the same type of parallel converters,the power allocation among converters has no impact to the system stability.Further analysis has indicated that the total output power of the system has strong coupling with the feedforward voltage loop,regarding system harmonic stability.In detail,increasing the output reactive power can always help to stabilize the system when no voltage feedforward loop or voltage feedforward with a low pass filter(LPF)is adopted by each converter.However,when the direct PCC voltage feedforward control is applied to converters,reactive power injection within a certain range can improve system stability.(4)Stability analysis of converter dominated multi-AC-bus systems.An equivalent circuit model of the proposed system is established.It reveals that there are different resonance types in such a system,and the determining factors of these resonance types are analyzed.For parallel resonance problems,under normal circumstances,the harmonic source tends to cause resonance with its neighboring converters,but it has little effect on remote converters with long electrical distances.However,under special circumstances,the harmonic source has almost no effect on neighboring converters,but causes resonance with the remote converters.In this situation,the resonance path passes through the transformer and multiple AC buses.This phenomenon poses a new challenge to the location of the harmonic source and the resonance suppression strategy.This part of the research is helpful to the optimization design and resonance damping of large-capacity photovoltaic power plants and energy storage systems. |
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