Patent Publication Number: US-10326275-B2

Title: Method for controlling zero sequence voltage of voltage source converter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 371 of international application of PCT application serial no. PCT/CN2017/080998, filed on Apr. 19, 2017, which claims the priority benefit of China application no. 201610249955.X, filed on Apr. 20, 2016. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of flexible direct current (DC) transmission technologies of an electric power system, and in particular, to a method and device for controlling a zero sequence voltage of a voltage source converter. 
     DESCRIPTION OF RELATED ART 
     In flexible direct current (DC) transmission, a voltage source converter is used, which can independently and rapidly control active power and reactive power of a system, thus improving system stability, suppressing fluctuation of a system frequency and voltage, and enhancing steady-state performance of a grid-connected alternating current (AC) system. 
     For a converter station that is ungrounded, high resistance, or its valve side is grounded through an arc suppression coil, a faulty station generates a zero sequence voltage if a single-phase ground fault occurs in an AC system, and a non-faulty phase voltage of the AC system is likely to become √{square root over (3)} times the value of the original one. In this case, if a DC voltage is remained unchanged, a generated reference wave probably cannot reach a value that is √{square root over (3)} times that of a rated phase voltage. Consequently, overmodulation occurs, fluctuation of a DC side voltage is caused, and a zero sequence voltage like that in the faulty station may occur in other non-faulty converter stations connected to the DC power grid, thus bringing an adverse effect on secure operation of the flexible DC transmission system. 
     Currently, Song Hong-Seok et al. propose “Dual Current Control Scheme for PWM Converter under Unbalance Input Voltage Conditions” (IEEE Transaction on Industrial Electronics. 1999, 46(5):953-959), where negative sequence voltage feedforward and dual-sequence current inner loop control are separately performed for an asymmetrical fault, but only a negative sequence current can be suppressed. A thesis of Dr. Hairong Chen,  Research on Control and Protection Strategy of VSC - HVDC System in AC System Fault , gives a detailed deduction of positive and negative sequence asymmetrical faults, where by use of a dual-sequence current loop, a reference value of a negative sequence current is set for purposes of: suppression of AC side negative sequence and suppression of DC side frequency doubling, but these two purposes cannot be achieved simultaneously. The foregoing deduction and calculation are performed to control only a negative sequence situation, but do not make an analysis and research on suppression of a zero sequence voltage in a non-faulty station that is caused when a zero sequence voltage generated by a current faulty station is transmitted to a peer station. 
     Therefore, it is necessary to find a method for controlling a zero sequence voltage of a voltage source converter, and a corresponding device. A zero sequence voltage at an AC side is detected, a DC side voltage is increased, and an appropriate reference wave is generated to match an AC voltage, so as to control the zero sequence voltage and suppress fluctuation of a DC voltage, thus ensuring stable and secure operation of a flexible DC transmission system. 
     SUMMARY OF THE INVENTION 
     In view of the defects in the prior art, an objective of the present invention is to provide a method for controlling a zero sequence voltage of a voltage source converter. A zero sequence voltage at an alternating current (AC) side or a direct current (DC) side is detected, a DC side voltage is increased, and an appropriate reference wave is generated to match an AC voltage, so as to control overmodulation of a reference wave at a faulty station to suppress fluctuation of a DC voltage, thus ensuring stable and secure operation of a flexible DC transmission system. 
     To achieve the foregoing objective, the present invention adopts the following technical solutions: 
     A zero sequence voltage at an AC side of a converter is detected. If the zero sequence voltage is greater than a set value V 0ref , a DC side voltage of the converter can be increased to a set value U dc0 , thereby generating an appropriate reference wave at the AC side of the converter to match an AC voltage, and accordingly controlling overmodulation of a reference wave at a faulty station to suppress fluctuation of a DC voltage. There are two ways to increase the DC voltage: increasing a capacitor voltage of a sub-module to a set value U c0 , to achieve the objective of increasing the DC voltage of the converter; and investing more redundant sub-modules, to achieve the objective of increasing the DC voltage of the converter. 
     After use of the foregoing solution, the present invention achieves the following beneficial effects: 
     (1) By the method for controlling a zero sequence voltage of a voltage source converter according to the present invention, overmodulation of a reference wave at a faulty station can be controlled, and fluctuation of a DC side voltage can be reduced, thus minimizing an adverse effect of a fault in a current station on other non-faulty stations, and ensuring stable and secure operation of a flexible DC transmission system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a DC system formed by voltage source converters. 
         FIG. 2  is a schematic diagram showing a control over a zero sequence voltage. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The technical solutions of the present invention are described in detail below with reference to the accompanying drawings and specific embodiments. 
     The present invention provides a method for controlling a zero sequence voltage of a voltage source converter. When an alternating current (AC) system ground fault occurs, a zero sequence voltage at an AC side or a direct current (DC) side of a converter is detected. If the detected zero sequence voltage is greater than a set value V 0ref , a DC side voltage of the converter can be increased to a set value U dc0 , thereby generating an appropriate reference wave at the AC side of the converter to match an AC voltage, and accordingly controlling overmodulation of a reference wave at a faulty station to suppress fluctuation of a DC voltage. 
     According to the method described in claim  1  of the present invention, a control over a zero sequence voltage of the voltage source converter shown in  FIG. 1  is described as follows: 
     (1) The zero sequence voltage is detected. If the zero sequence voltage is greater than a set value V 0ref , it indicates that a fault occurs. A constant DC voltage control station starts to increase a DC voltage to U dc0 , where the DC voltage meets a relational expression: U dc0 &gt;2U m , U m  being a non-faulty phase voltage peak at an AC side of the voltage source converter.  FIG. 2  is a schematic diagram showing a control over the zero sequence voltage. 
     (2) The DC voltage is increased from a rated value U dcN  to U dc0 , which can be implemented by increasing a capacitor voltage of a sub-module from U cN  to U c0 , where 
     
       
         
           
             
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             = 
             
               
                 U 
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                       0 
                     
                   
                   
                     U 
                     dcN 
                   
                 
                 . 
               
             
           
         
       
     
     (3) The DC voltage is increased from a rated value U dcN  to U dc0 , which can also be implemented by increasing the number of invested sub-modules from the original n N  to n 0 , where 
     
       
         
           
             
               n 
               0 
             
             = 
             
               
                 n 
                 N 
               
               × 
               
                 
                   
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                       ⁢ 
                       
                           
                       
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                     U 
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                 . 
               
             
           
         
       
     
     (4) After the constant DC voltage station increases the DC voltage, a non-constant DC voltage station connected to the constant DC voltage station needs to increase a capacitor voltage of a sub-module or increase the number of invested sub-modules, where numeric values obtained after the increase are described in steps (2) and (3). 
     As a further preferred solution in the specific embodiment, the zero sequence voltage can be obtained by dividing a sum of AC three-phase voltages by 3, or by dividing a sum of DC positive and negative voltages by 2. A range of a fault set value V 0ref  of the zero sequence voltage is 0.1 pu&lt;V 0ref &lt;1.0 pu. 
     As a further preferred solution in the specific embodiment, a value to which the DC side voltage is increased meets a relational expression: U dc0 &gt;2U m , where U m  is a non-faulty phase voltage peak. 
     As a further preferred solution in the specific embodiment, in a DC transmission system formed by two or more converters, when a zero sequence voltage is first detected in a particular converter station, this converter station notifies other converter stations through inter-station communication; and a constant DC voltage station increases a DC side voltage, the DC side voltage matches a voltage of a sub-module of a set power station, or the number of invested sub-modules. 
     A set value of the voltage of the sub-module is U c0 , and 
                 U     c   ⁢           ⁢   0       =       U   cN     ×       U     dc   ⁢           ⁢   0         U   dcN           ,         
where U cN  is a voltage of the sub-module before the fault, and U dcN  is a DC voltage before the fault.
 
     The number of the invested sub-modules is n 0 , and 
                 n   0     =       n   N     ×       U     dc   ⁢           ⁢   0         U   dcN           ,         
where n N  is the number of invested sub-modules before the fault, and U dcN  is a DC voltage before the fault.
 
     As a further preferred solution in the specific embodiment, a sub-module structure of the converter includes a half-bridge MMC, a full-bridge MMC, or a structure approximating the full-bridge MMC. 
     The foregoing embodiment merely describes the technical idea of the present invention, but is not intended to limit the protection scope of the present invention. Any modification made based on the technical solutions according to the technical idea provided by the present invention falls within the protection scope of the present invention.