Research On Control And Protection Strategy For Stand-Alone Parallel Inverters Supply System

Power network supplied by parallel inverters has been widely applied,such as industial manufacture,locomotive traction,ship power,new energy power generation et al.,due to the modular design of inverter,high redundancy and high power quality.On the one hand,inverters operate in voltage controlled mode in normal operation state,supporting the system voltage and sharing the load power demands.However,influenced by output impedance,conventional droop control cannot achieve accurate power sharing.In the case that there are unbalanced and nonlinear loads in the system,negative-sequence and harmonic powers also cannot be accurately shared.Moreover,the AC bus voltage contains negative-sequence and harmonic components,which deteriorate the power quality.On the other hand,when a short-circuit fault occurs at the load side,the sharply increasing fault current may damage the switch devices in the inverters,reslting in the paralysis of the whole system.To enhance the reliability of the power system,not only should the inverters be capable of riding through the fault,but also the protection devices installed at the lines should isolate the fault with selectivity.To solve these issues,this paper conducts some in-depth researches on control and protection of the parallel inverters system,which include:In the aspect of current sharing control for the parallel inverters,firstly,the characteristic of the inverter output impedance is analyzed.For the resistive output impedance scenarios,two ticipal kinds of paralleling control methods are analyzed.The first one is decentralized logic control,which is based on the maximum active power and reactive power bus.This method could obtain accurate fundamental power sharing,but it needs three communication wires,which reduce the expansibility and relizbility of the system to some extent.The second one is droop control.It does not need any communication wire among among inverters,which enables “plug and play” and hence enhances the system expansibility and relizbility.However,due to the mismatch of the output impedance,the load power cannot be accurately shared among inverters.Finally,by combining the above two control methods,a modified control method is proposed,which is based on the time division multiplexing of a synchronization maximum power bus(SMPB).With the help of the SMPB,an adaptive virtual resistance is introduced,compensating the mismatched output impedance.Therefore,the accurate fundamental power sharing is realized.Moreover,if there are unbalanced and nonlinear loads,the adaptive negativesequence and harmonic virtual resistance is introduced to ensure accurate unbalanced and harominc power sharing with the the time division multiplexing of the SMPB.Simulation and experiments are carried out to validate the effectiveness of the proposed scheme.In the case that the system contains unbalanced and nonlinear loads,the three-phase voltage on the AC bus will be unbalanced and distorted.In response to this issue,firstly,this paper analyzes the reason for arising of the unbalanced and harmonic voltage.Then,a virtual admittance based control strategy is proposed to restrain the unbalanced and haromonic voltage.By introducing a feedforward term of the AC bus to the reference of the current loop in a certain form,an admittance is equivalent to be connected at the AC bus,which reduces the output impedance at the negative-sequence and harmonic frequencies,and naturally reduces negative-sequence and harmonic voltage drop on the output impedance,so the power quality is obviously improved.In this method,the selection of the virtual admittance is of great importance.To ensure the system stability,this paper defines the boundary of the virtual admittance by using the root locus analysis method.Simulation and experimental results are provided to prove the feasiblity of the proposed approach.Once a short-circuit fault occurs at the line or the load side,this paper proposed a current limiting strategy based on the combination of hardware protection and software current control,with the purpose to protect the switch components from damage and ensure the safe and continuous operation of the inverter.In the hardware protection,signal generated by a hysteresis comparator circuit is used to block and release the PWM drivers in real time,limiting the sharp rise of the fault current and protecting the switch component from damage.Meanwhile,once the fault is detected by the software system,inverters switch to the software current control mode to output sinusoidal fault current with magnitude accurately controlled.In addition,a smooth swithching strategy of the inverters between the normal state and the faulty state is proposed to suppress the transient inrush current and shorten the transition time.Simulation and experiments are carried out to verify the feasibility of the methods.To achieve selective protection for the stand-alone inverter supply system,another important issue is how to quickly and selectively clear the fault.Referring to the principle of current protection in traditional power system,this paper firstly studies two kinds of current protection methods for inverter supply system: moulded case circuit breaker based protection and digital relay based protection.In the moulded case circuit breaker based protection,the operation characteristic of molded case circuit breaker is revealed,and the general coordination principles of the upstream and downstream protections are analyzed.In the digital relay based protection,to simulate the fault current droop characteristic in power system,the fault current droop control is synthesized in the current limiting controller,and then the circuit breaker is set using the traditional current protection principle,ensuring the quick and selective clearance of the fault.However,both the two protection methods rely on large fault current,which means that a large current margin should be maintained for the switch devices and the cost of inverter is sharply increased.To address this issue,this paper presents a novel protection scheme based on harmonic components.A certain proportion of the fifth harmonic is injected into the fault current.Meanwhile,the protection devices detect the harmonics with the utilization of digital relays.The harmonics components are used to identy the fault.With the proposed method,the selective protection is achieved without relying on large fault current.Experimental results are provided to verify the feasibility of this protection scheme.