Research On Grounding Protection Of Flexible DC Distribution Network

With the development of high-power semiconductor,flexible DC power distribution technology is gradually being promoted and applied.Flexible DC power distribution system is usually connected to the AC power system through a converter.The reference potential point can be set on the DC side or AC side of the converter,which are different grounding methods.In some multi-terminal flexible DC power distribution systems with special grounding modes,the fault current is low when grounded fault occurs.If the faulty line can be removed by some simple protection devices before any device blocks,the system’s power supply reliability will be greatly improved.In DC capacitor neutral point grounding method,the equivalent common-mode circuit models of the components in flexible DC distribution power system are analyzed and an equivalent common-mode circuit model of flexible multi-terminal DC distribution power system is built.Subsequently,the calculation formulas of fundamental frequency common-mode fault current and voltage after converter AC side grounded fault occurs are obtained.The influence of various resonances that may exist in a DC system on faulty terminal and non-faulty terminals are analyzed respectively when converter AC side grounded fault occurs.In converter AC side neutral point grounding method,the generation and magnitude of the third harmonic common-mode voltage on converter DC side is analyzed.Taking radial flexible five-terminal DC distribution power system for example,the common-mode networks in normal state and fault-added state are built to calculate the common-mode voltage and current variation in the system after DC side grounded fault occurs.Subsequently the characteristics of common-mode voltage and current variation in the system when the DC side grounded fault occurs are analyzed.To analyze the influence of hall current sensor on current transmission,a mathematical model of closed-loop hall current sensor with nonlinear transmission part is established.The influence of the saturation section in the closed-loop hall current sensor on the current transmission and the variation characteristics when current distortion happens are analyzed.Finally,this paper proposes a multi-terminal flexible DC power distribution system protection strategy based on the common-mode signals of DC terminals for converter AC side grounded fault and a multi-terminal radial flexible DC power distribution system fault line selection protection strategy based on the DC common-mode current and voltage variation and third harmonic common-mode current variation for DC side grounded fault.All these are verified on PSCAD simulation platform.