Ferromagnetic harmonic elimination method for active matched resistance voltage transformer based on PID adjustment

A harmonic elimination method for ferromagnetic resonance for an active resistance-matching voltage transformer based on PID-adjustment, comprising:

• S1, compiling a resistance matching algorithm:
• analysing voltage amplitude and phase sequence of a voltage transformer in different situations of a power grid system, so as to obtain corresponding resistance matching algorithms, respectively, wherein a specific approach for S1 comprises:
• assuming that an effective value of a port voltage of an open triangle winding that is directly measured from an opening is Ur, wherein phasor thereof is expressed as Ur and phase sequence of three-phase winding of the open triangle is expressed as a*, b* and c*, used to distinguish PT primary side and secondary side voltage phase sequence to facilitate vector analysis;
• wherein,
• a neutral point is 0, wherein Ua*=Ub*=Uc*=100V/3 is an effective value of three-phase voltage of the open triangle under a normal operation, where phasor representation corresponds to {dot over (U)}a*, {dot over (U)}b* and {dot over (U)}c*;
• wherein Uo=Ur/3 is an effective value of the neutral point-to-ground voltage, called zero-sequence voltage, and with phasor represents {dot over (U)}o;
• wherein θ=arc cos Ur/100 is an angle of the zero sequence voltage and the phase voltage of the open triangle corresponding to grounding;
• wherein the further conditions comprise:
• when a single-phase ground fault in a system, under a normal situation, no voltage across the opening of the open triangle, as {dot over (U)}r={dot over (U)}a*+{dot over (U)}b*+{dot over (U)}c*=0V;
• when a single-phase metal in the system is grounded, no voltage on a certain phase of a primary winding of the voltage transformer, resulting in no voltage on a corresponding phase of a corresponding PT secondary winding, wherein a 100V voltage appears on the open triangle, as Ur=100V;
• wherein, when non-metal grounding, it is assumed that among three phases of the open triangle, an effective value of the a* phase voltage is Ua*′ which is the largest, and its apparent power is also the largest, so the a* phase is the easiest overloaded;
• if the a* phase is guaranteed not to be overloaded, the other two phases must not be overloaded;
• S2, designing and building a harmonic elimination control system based on a PID control strategy, wherein a specific approach for S2 comprises:
• first, on a basis of active harmonic elimination, adopting the PID control strategy and further according to engineering characteristics of controlling resistance selecting and adopting a PI controlling such that a proportional control algorithm and an integral control algorithm in a PID controller are used;
• wherein, next, a PID automatically adjusts harmonic elimination resistance to run online for continuous operation throughout an entire duration, and gradually puts the harmonic elimination resistance into use through the proportional control algorithm, wherein, at the same time, the PID controls and eliminates a steady-state error in the proportional control algorithm through the integral control algorithm, so that resonance of the voltage transformer approaches a minimum setting target value or almost none, which means preventing ferromagnetic resonance of the voltage transformer is in an active way;
• wherein, at the same time, data that needs to be detected or collected in real time during operation includes:
• voltage of the open triangle of the voltage transformer, phase voltage of three-phase of the open triangle of the voltage transformer, and loop current of the open triangle;
• wherein, finally, the harmonic elimination control system is designed to track change of the effective value Ur of an open triangle port voltage in real time according to a cycle of 100 ms, so as to realize dynamic adjustment of harmonic elimination matching resistance;
• S3, using the above-mentioned harmonic elimination control system, according to different conditions of voltage, presetting an active resistance-matching strategy, and executing the strategy to realize dynamic adjustment of the harmonic elimination matching resistance, comprising:
• first of all, detecting an open voltage Ur, wherein if Ur is 0V, a three-phase grid system is balanced, and a resistance value of a matching input at this time has no effect on the open triangle, wherein, as considering an actual operation of the grid, it is difficult to avoid a three-phase unbalanced situation, and then an matching resistance value is input according to equation
• R
• x
• ⁢
• 1
• ⁢
• 5
• =
• U
• r
• (
• U
• r
• +
• 1
• ⁢
• 0
• ⁢
• 0
• )
• 3
• ⁢
• K
• ⁢
• S
• ,
• where Rx15 in the equation is a minimum value of a harmonic elimination resistance Rx, and wherein if Ur changes, adjustment is made according to strategies as follows:
• strategy 1:
• wherein when Ur is less than or equal to 15V, first, it is determined if a factor of Ur change is caused by any one of load imbalance, single-phase ground fault, high-order harmonic in ferromagnetic resonance, and frequency division ferromagnetic resonance, and then through the harmonic elimination control system according to the equation of
• R
• x
• ⁢
• 1
• ⁢
• 5
• =
• U
• r
• (
• U
• r
• +
• 1
• ⁢
• 0
• ⁢
• 0
• )
• 3
• ⁢
• K
• ⁢
• S
• ,
• where Rx15 is a minimum value of a harmonic elimination resistance Rx, dynamic calculation is performed until a fault is eliminated or resonance energy disappears;
• strategy 2:
• wherein when Ur is greater than 15V and less than or equal to 100V, first, it is determined if Ur change is caused by ferromagnetic resonance or various faults represented by single-phase non-metallic grounding, and then using the harmonic elimination control system according to equation of
• R
• x
• ≥
• U
• Γ
• 3
• ⁢
• K
• ⁢
• S
• ⁢
• U
• r
• 2
• +
• 1
• ⁢
• 0
• ⁢
• 0
• 2
• -
• 2
• ⁢
• 0
• ⁢
• 0
• ⁢
• U
• r
• ⁢
• cos
• ⁡
• (
• 1
• ⁢
• 2
• ⁢
• 0
• +
• arccos
• ⁢
• U
• r
• 1
• ⁢
• 0
• ⁢
• 0
• )
• ,
• where K is a multiple ratio used for designing excitation characteristics of the voltage transformer according to a phase rated voltage, dynamically calculation is performed for a resistance value of a matching resistor until a fault is eliminated or resonance energy disappears;
• wherein after the resonance energy is released, or after the fault is eliminated, when Ur is less than or equal to 15V, it is handled according to the strategy 1;
• strategy 3:
• wherein when Ur is greater than 100V, first, matching resonant resistor is performed with twice resistance of Rx100 through the resonance elimination control system, and, at the same time, the resonance elimination control system dynamically tracks a value of Ur, wherein with release of resonance energy, the value of Ur eventually is less than 100V, then going to the strategy 2;
• S4, designing an engineering scheme for placing resistors in order to avoid ferromagnetic resonance when a system failure is recovered.