| The figures published by the National Bureau of Statistics represented that in 2014 China's primary energy consumption structure,coal accounted for 66.0%,17.1% for oil,natural gas accounted for 5.7%,hydropower,nuclear power and wind power accounted for a total of 11.2%,while the proportion of natural gas in the energy structure is far below the world average of 23.8%.During the “Twelve Five” period,China's natural gas gap have reached 1600 billion cubic meters,and the external dependence as high as 45%.Although the natural gas gap can be resolved through the liquefied petroleum gas and import of natural gas,but still many problems existed in cost and safety.The composition of coalbed methane(CBM),which is a mixture of methane-based gas formed in the process of coalification and stored in the coal seam,is similar to that of natural gas.The geological resource of CBM with depth less than 2000 m is about 38 trillion cubic meters,which shows China's CBM resources are abundant.Compared with other unconventional gases such as shale gas,natural gas hydrate and so on,CBM is one of the most realistic gas sources in the near future and has great development potential.In summary,the development of CBM industry will be constructive to alleviate current shortage in the supply of natural gas in China,improve the energy production-consumption structure and work safety conditions of coal mine enterprises as well as reduce the environmental pressure caused by excessive consumption of fossil fuels.China's CBM production had exceeded 10 billion cubic meters by the end of 2011,which means CBM industry has entered the initial stage of rapid development.However,with the continuous expansion of the scale of CBM industry,the high cost of electricity caused by high energy consumption in the process of CBM extraction has become the primary problem that restricts the sustainable development of CBM industry.The unreasonable use of electricity in the process of CBM extraction is mainly in: 1)As an important part of CBM drainage system,CBM-well pumping unit has a large amplitude of active power variation in one cycle,while the reactive power demand is almost unchanged,which results in power factor of AC side varying drastically and generally low,serious harmonic pollution and energy waste,and with the increase of load penetration AC networks may be impacted or even destroyed;2)Due to the cyclical and dynamical load of CBM-well pumping units affected by suspension load,the issues of strong voltage fluctuation at DC power supply side and high energy consumption caused by the CBM-well pumping motor operating on motoring and regenerating modes periodically often occur;3)Because of several objective factors such as the distribution of CBM resources,there are many problems such as large supply radius,long supply lines,poor environmental conditions in line corridors,high difficulty in erecting lines and high one-time investment costs existing in the conventional AC power supply for CBM ground extraction system,especially a large number of eddy current loss,reactive circulation and operating costs resulted from long distribution lines;4)The conventional AC power supply mode is adopted in the power supply for CBM ground extraction system.Three-stage power supply mode,namely 35 kV substation/10 kV high voltage transmission/380 V low voltage distribution,requires a large number of transformers.Considering the actual situation of quantity and capacity of CBM-well pumping units,the distribution transformers selected are usually large in capacity and operate in non-economic operating areas,which greatly contribute to the active and reactive losses.In order to response to the high electricity costs of CBM extraction,avoid “clean energy for clean energy”,realize energy-saving and efficient production of CBM drainage system and improve the economic efficiency,there is a pressing need to change the conventional AC power network form for CBM ground extraction system.For this reason,my research team proposed a novel DC microgrid power supply for CBM ground extraction system,making full use of distributed energy resource(DER)such as photovoltaics and batteries supply power to loads,which can reduce the dependence of CBM-well pumping units on AC network and system efficiency and energy consumption will be significantly improved due to absence of reactive power and harmonics.The project is sponsored by the NSFC(51577187),the Fundamental research funds for the Central Universities(2010YJ03),and the Natural Science and Technology Major Project(2011ZX05063).Based on the centralized distribution of CBM-well pumping units in the block,the DC microgrid power supply for CBM ground extraction system is an autonomous power grid,making full use of photovoltaics and batteries and connecting with AC network through bidirectional PWM rectifier.In contrast with AC microgrids,DC micrgrids have drawn growing concerns for the advantages of no need to track the frequency and phase of voltage,high controllability and reliability,and more suitable access to DERs and loads.Theoretically,DC microgrids have higher conversion efficiency and less line losses owing to integrating various DERs and loads with only one-stage converter as well as irrespective of eddy current loss and reactive power in the process of DC transmission.CBM-well pumping unit,which is a primary load of the DC microgrid power supply for CBM ground extraction system,consists of motor,four-bar linkage,pumping rod,and underground pump.The motor as the power source to drive CBM-well pumping unit,in addition to the existence of motoring state,also has regenerating status generated by CBM-well pumping unit driving motor over-speed running caused by potential energy load dropping in the downstroke due to the cyclical and dynamical load of CBM-well pumping units and the unbalanced weight.The DC bus voltage is increased due to the motor feeding to the DC bus in the regenerating state,while in the motoring status the DC bus voltage decreases due to the DC bus supplying the motor,resulting in DC bus voltage fluctuations.The more severe DC bus voltage fluctuations,the higher the reactive power required for the DC capacitor,thus when the active power is certain the energy force index(product of efficiency and power factor)will be lower.Moreover,since the motor operates in no-load and regenerating states in a considerable period of time under alternating load,the motor is in a low energy force index running with only 30% efficiency.Meanwhile,due to the difficulty of starting load,the installed motor rated power is significantly greater than its actual operating power,resulting in a serious motor “big horse car” problem.In conclusion,the cyclical and dynamical load of CBM-well pumping units is the main cause of the DC bus voltage severe fluctuations and the high energy consumption of the system.Therefore,an energy-saving control strategy based on optimal speed curve of CBM-well pumping motor is proposed in this paper.Firstly,the four-bar linkage mechanism of CBM-well pumping unit is analyzed,the mathematic model of the suspension point load is established,and the equivalent load torque expression of CBM-well pumping motor is derived.Secondly,based on vector control technology,the field-oriented control mathematical model of CBM-well pumping motor is established,and the mapping relationship between the bus voltage of DC power supply and the acceleration of CBM-well pumping motor is investigated.Further the absolute function of DC bus voltage fluctuation difference and the maximum voltage fluctuation are constructed,respectively.Finally,by seeking the extremum of the absolute value function and analyzing the monotonicity of the maximum voltage fluctuation function,the optimal acceleration that meets the minimum DC bus voltage fluctuation is found,and thereby the optimal speed curve of CBM-well pumping motor is obtained.The disordered operation between multiple CBM-well pumping units in the DC microgrid power supply for CBM ground extraction system will further aggravate the DC bus voltage fluctuations.In order to make multiple CBM-well pumping units in the network coordinated operation,suppress DC bus voltage fluctuations to maximum extent from the load side,and reduce system loss,the coordinated operation of two CBM-well pumping units under DC microgrid power supply is studied,and a coordinated control strategy based on complementary operation of CBM-well pumping units is proposed in this paper.Firstly,the operation characteristics of CBM-well pumping unit and its equivalent load are analyzed in this paper.Then,through further analysis of the relationship between the DC bus voltage fluctuation and the electromagnetic torque and acceleration of CBM-well pumping motor,the optimal acceleration that meets the minimum DC bus voltage fluctuation is found,and thus the optimal speed curve of coordinated operation of CBM-well pumping motors is obtained.Combining with the analysis and discussion of DC microgrid networking mode at home and abroad,it can be seen that the reasonable selection of DC microgrid structure is crucial for the whole system in practical application.To satisfy that the system structure is simple,easy to control and the performance of the system under multi-control objectives meets the practical application,the DC microgrid power supply for CBM ground extraction system based on hierarchical structure is proposed in this paper.The first level is the source level where combined sources of PV and battery are utilized to establish the source bus.The PV works in maximum power point tracking(MPPT)mode and the battery works in voltage output mode to build the bus voltage.The second level is the transfer and distribution level where the bidirectional Buck/Boost converter is utilized to achieve the bidirectional energy transmission,stable DC bus voltage and dynamic load power sharing.The input of the converter is the source bus with stable voltage,and the output is the DC bus.The third level is the load level including CBM-well pumping units driven by the inverter?motor system.For the DC microgrid power supply for CBM ground extraction system,when the cyclical and dynamical load of CBM-well pumping units with negative impedance characteristics and DERs are integrated into a DC microgrid via converters,oscillation may easily occur,causing the bus voltage stability problem and bringing the hidden trouble to the stable operation of the DC microgrid.For this reason,the stability of the DC microgrid power supply for CBM ground extraction system is investigated in this paper.A global small-signal model is derived based on the output impedance Zo(s)of the first and second levels and the input impedance Zin(s)in series with the controlled source Ym? of the third level.According to the instability issues induced by the dynamical and cyclical load of CBM-well pumping units with negative impedance characteristics,a stability control strategy based on virtual impedance is analyzed and discussed in the second level.Simultaneously,droop control is employed to achieve dynamic load power sharing.The impedance criteria is selected to evaluate the overall system stability with and without this stability control strategy by solving the dominant poles of polynomial 1/(1(10)Zo(s)/Zin(s)).Additionally,theories proposed above are validated through the experimental platform of the DC microgrid power supply for CBM ground extraction system based on Matlab/Simulink,field test,and advanced hardware-in-the-loop simulation technology.The results show that the energy-saving control strategy based on optimal speed curve of CBM-well pumping motor proposed can effectively suppress DC bus voltage fluctuations and reduce the energy consumption.The proposed stability control strategy of the DC microgrid power supply for CBM ground extraction system is correct and feasible.The simulation results and experimental results lay a good foundation for the follow-up research. |
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