Optimization Research On Distributed Suppressing Harmonic Sources And Placements Of APFs In Coal Mine Power Grid

In order to improve the efficiency and automation of coalmine manufacturing equipment,the power electronic devices,such as rectifier and inverter,are widely used in coal mine.There is no doubt that these equipment improves the efficiency of coalmine production,but it inevitably brings harmonic pollution to the power quality due to their nonlinear characteristic impedance.In the coal mine underground power grid,the harm of harmonics mainly includes: The power equipment powered by motor,such as driving coal mining machine and entry-driving machine,will vibrate;The underground power supply and communication system will be disturbed;The underground power cable generate additional loss and increase insulated heating of cables;The local parallel resonance or series resonance will be caused by harmonic.Hamonic brings a potential danger for coal mine safety production.Serious harmonic pollution maybe result in a production accident and casualties.At present,one of the basic passive ideas to suppress harmonic is to install harmonic compensation devices,which can be classified into passive technology and active technology.The LC filters are usually adopted in passive technology,whose disadvantages include: only fixed-frequency harmonic compensating,the bad compensation result and compensation characteristics that are easily affected by the impedance or state of power grid.Therefore,the application of LC filters is less in the explosive environment of underground coal mine.Active technology,such as active power filter,is to detect the harmonic component from the compensation object through the detection circuit and extracts a compensating current that has equal amplitude and opposite phase compared to harmonic component.A PWM inverter is employed to inject the compensating current so as to the fundamental component is only left in compensation object.Active power filter are good in filtering performance,small in physical size,and flexible in application resulting in a widely attraction and research.In this paper,the power supply in coal mine underground is investigated and analyzed.The measurement and analysis of harmonic pollution in representative coal mine underground power grid is also carried out.The results show that the harmonic sources mainly exists in the inverters applied in belt conveyor,scraper conveyor,endless rope winch,shearer and other equipment.The harmonic distortion of underground power grid branch is very serious.Most coal mining enterprises adopted the method of installation of filter equipment in the grounding transformer substations to reduce harmonic pollution.However,this method only controls the harmonic impact of underground power grid on the grounding transformer substations.The harmonic pollution in the underground power grid is still unreduced.In order to solve the problem of harmonic pollution in coal mine power grid,active power filter is employed in the paper.The operating principle,harmonic extract algorithm,control algorithm is descripted.The environment of coal mine underground power grid,which has its own particularity,is different from the general industry environment.Therefore,it is very important to deeply understand the characteristics of coal mine underground power grid when active power filter is adopted in underground power grid for harmonic pollution controlling.The equipment employed in the coal mining underground includes dewatering pump,air machine,belt conveyor and coal mining machine and so on.Those equipment usually has a high rated voltage and power,and always work on the start-stop state.The range of rated voltages needed by those equipment are from 660 V to 3300 V that results in multi voltage classes in underground power grid.In order to adapt to the multi voltage classes,active power filter can be designed in the form of standardized module which effectively reduces the repeated explosion test caused by the different voltage classes and capacities.Series-parallel system consists of modular meet the demand of different capacity.The module can be designed to support cold swaps,which means the fault module can be replaced quickly and conveniently.The complexity of the maintenance can be reduced greatly.The nominal capacity of active power filter need to be small because of the limiting factor such as narrow space,explosion and thermal design and frequency moving.Small nominal capacity increase the difficulty to control harmonic pollution in underground mine.The size and nominal capacity can be reduced by decreased the compensating current decided by the compensating purpose.When active power filter is only adopted to suppress the harmonic component,the size and nominal capacity is the smallest.With the consideration of distributed harmonic sources in coal mine power grid,active power filters can be installed to achieve distributed harmonic suppress.Therefore,the placement environment of an active power filter and corresponding harmonic detecting algorithm is concerned first.In order to solve the problem between harmonic suppressing and unit power factor in distortion supply,a harmonic detecting algorithm based on active power filter capacity constraint is established in this paper.Considering of the limited nominal capacity of active power filter,an individual harmonic compensating current constraint is proposed.To overcome the issue of large computation time required to solve the optimization model using iteration technology,a non-iterative method is presented in this paper.This method decouple the optimization problem into two sub-problems: primary problem and secondary problem,and solve them separately.In the primary problem,with the only consideration of power equality constraint and current total harmonic distortion,conductance factors are derived.In the secondary problem,taking the constraints of individual harmonic rate and individual harmonic compensating current into account,the conductance factors derived in the primary problem are modified.The proposed approach is proven to be correct and effective in four different test cases by comparing other methods.The results show that the non-iteration optimized detecting method maximize the power factor satisfying harmonic distortion constraints and avoid the iteration process which hugely decrease the computer time computation.With the proposed approach,active power filter has a reliable and faster response for the dynamic load compensation.Thinking and dealing with problems from the economic viewpoint,the cost of harmonic suppress is mainly related to the capacity of active power filter.In practice,the cost increases proportional with the capacity of active power filter.Therefore,with the consideration the special underground environment,it is necessary to optimize the placements of active power filters to minimize the project cost meeting the harmonic distortion standard.Assuming that the PWM circuit of active power filter is ideal and does not generate harmonic and time delay,the mathematical model of active power filter in power grid can be replaced by the instruction current approximately.Therefore,the decision variables based on the instantaneous reactive power theory is proposed,which can hugely decrease the number of decision variables.In the special underground environment,many factors directly or indirectly put forward certain requirements to the size and the placement of active power filter.Taking this into consideration,a size constraint and a placement constraint are present in this paper.Base on the presented size constraint,placement constraint,voltage total harmonic distortion constraint and individual voltage harmonic rate constraint,cooperating with the objection function formulated from project cost,an optimization model of the placements of active power filters in underground power grid is established.Then,the optimization model is solved by the method based on standard particle swarm optimization algorithm.Finally,the placements of the active power filters are optimized to minimize the project cost satisfying the requirement of harmonic pollution standard.In the face of the increasing trend of the coal mine power grid,a method based on co-evolutionary particle swarm optimization is proposed to optimize the placements of active power filters when the method based on standard particle swarm optimization algorithm failed to deal with the large-scale power grid.To verify the validity of the proposed method,the simulation validation of the non-iterative detection optimized algorithm based on active power filter capacity constraint is carried out.Meanwhile,the power grid model based on an actual coal mine underground power grid in our country is established.The proposed optimization method for the placements of active power filters are verified by simulation.Simulation results show that the proposed methods is correct and effective.