Research On The Strategy Of The Differential Protection Of The Ultra-high Voltage Voltage-regulating Transformer

The ultra-high voltage voltage-regulating transformer(VRT)has a small capacity and its iron core has a low saturation point,compared with the main transformer.The weakest part of the ultra-high voltage transformer(UHVT)relay protection is the VRT differential protection.The research of this paper is developed from five aspects as advances: the UHVT's main structure and its operating principle,the VRT's special issues of its differential protection,a novel method for the VRT differential protection to identify inrush,an adaptive algorithm for acquiring the VRT's online tap position during on-load voltage regulation,the influence of the bias current on the UHVT's operating conditions and the VRT differential protection.This paper firstly introduces the UHVT's main structure and its operating principle.The protection scheme of the main transformer is inadequate for detecting the VRT's internal fault,so the split-phase differential protection are configured specially for the VRT.1000 k V UHVT model in MATLAB/Simulink is built,according to the electrical structure and parameters under the circumstance of no-load and on-load voltage regulation.The VRT differential protection malfunctioned when the UHVT was energized from the medium-voltage side.The case is reproduced by adjusting the initial closing conditions,of which the reason is also fu rther analyzed.A novel method for the VRT differential protection to identify inrush is proposed based on the performance of the magnetic inductance.Compared with the second-harmonics-based criterion and waveform symmetry principle,the reliability and the sensitivity of these three methods have been put forward while judging the inrush or the internal slight fault current of the VRT.The actual VRT's ratio is unavailable for the protective relay during the tap changing process.In the practical strategy,a fixed base ratio is used to calculat e the VRT differential current and the setting value is raised to avoid current spillage,which also decreases the sensitivity of the VRT differential protection.This paper puts forward to an adaptive algorithm for acquiring the VRT's online ratio in order to eliminate the unbalanced current on-load voltage regulation.The VRT differential current can be adaptively calculated by the online ratio,which makes the protection strategy convert to that under the no-load voltage regulation.The reliability and sensitivity of the proposed algorithm have been verified when the VRT has some internal slight faults.Finally,the flow path and distribution of the bias current in the UHVT under the steady state operation are analyzed.It is pointed out that the direction of the bias current determines the direction of the bias magnetic flux.The bias magnetic flux may lead the core to operating in the linear region and the saturated region by turns.Typical cases that the primary winding of the VRT has a high-impedance-grounded fault are simulated to indicate that the magnitude and the direction of the bias current,and the magnitude of the grounded impedance have impact on the operational characteristics of the VRT differential protection.