IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 59, NO. 8, AUGUST 2010 2211 D-Dot Probe for Fast-Front High-Voltage Measurement Ibrahim A. Metwally, Senior Member, IEEE Abstract—This paper presents the operating principle, design, and testing of a coaxial D-dot (the time derivative of electric flux density) probe to measure fast-front high voltages, e.g., the residual voltages of surge arresters (SAs). This probe consists of three identical copper toroids placed around a high-voltage electrode, where all are coaxially assembled in a large earthed cylinder. The probe was first simulated by a finite-element package to optimize the assembly and reduce the electric field inside it. This was confirmed by an alternating current test to ensure a corona-free design. Simultaneous impulse voltage measurements were done using the designed D-dot probe—two commercial mixed resistive–capacitive (RC ) probes and a damped capacitive voltage divider. The linearity of the D-dot probe was checked under unloaded and loaded conditions. Results reveal that the larger the toroid separation and/or the lower the attenuator capac- itance is, the higher the measured voltage from the middle “signal” toroid will be. The residual voltage waveforms for an 11-kV SA, measured by two commercial mixed RC probes and the damped capacitive voltage divider, showed an initial inductive overshoot superimposed on the waveform and a significant decay, even before the current peak instant. On the contrary, the voltage measured by the designed D-dot probe gave a voltage waveform that looked like that of the current and slightly led the latter. For the damped capacitive voltage divider and the two commercial mixed RC probes, neither the peak voltage nor the voltage at peak current gave the correct current–voltage characteristics. This confirms the contradiction of some published SA models in the high-conduction regime because most models were based on measurements done by different and large-impulse capacitive or resistive voltage dividers with improper compensation. Index Terms—D-dot probes, electric field sensors, fast-front voltages, impulse voltage measurement, surge arresters (SAs), voltage dividers. I. I NTRODUCTION F AST-FRONT voltages or currents are normally generated by system faults or disconnector operation within gas- insulated switchgear (GIS), operation of metal–oxide or zinc oxide surge arresters (ZnO SA), punctures of insulators, etc. The fastest one is generated within a GIS, where a steep voltage with a rise time in the range of some nanoseconds can be found Manuscript received December 30, 2008; revised July 27, 2009; accepted August 4, 2009. Date of publication October 13, 2009; date of current version July 14, 2010. This work was supported by Sultan Qaboos University, Muscat, Sultanate of Oman. The Associate Editor coordinating the review process for this paper was Dr. Jerome Blair. The author is with the Department of Electrical and Computer Engineering, College of Engineering, Sultan Qaboos University, Muscat 123, Oman (e-mail: metwally@squ.edu.om; metwally@mans.edu.eg). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2009.2030928 [1]. Special measuring systems are needed for the measurement of all previously mentioned fast-front voltages. SAs protect medium- and high-voltage systems and equip- ment against lightning and switching surges, i.e., external and internal overvoltages, respectively. They exhibit an extremely high resistance during normal operation and a very low resis- tance during transient overvoltages, i.e., dynamic characteristic [2], [3]. The highly nonlinear current–voltage (I –V ) character- istic of ZnO SAs obviates the need for series spark gaps. The electrical characteristics are determined solely by the properties of the ZnO blocks. Several models have been proposed to simulate this frequency-dependent characteristic [2], [4]–[10]. Difficulties arise in the calculation and adjustment of their parameters, where in some cases iterative procedures are required, while in others the necessary data are not reported on manufacturers’ datasheets. The differences among the proposed models arise in the difficulties of the parameter estimation. For the IEEE SA model, an early voltage overshoot occurs at about 37% of the current front time irrespective of the current magnitude [5]. In addition, there is a bad response in the voltage tail for fast-front currents (lightning currents), where the tail of voltage waveform dropped very sharply [6]. Moreover, it was observed that for the case of relatively slow surges, only the conventional model suffices [8], [11]. On the other hand, the conventional model proposed in electromagnetic transient program (EMTP) shows that the peak voltage instantaneously occurs with that of the current because the arrester is modeled by a pure nonlinear resistor only [8], [11]. Other models in [7] and [9] also do not give the actual voltage response of the arrester, particularly for the fast-front currents. None of the published SA models gives the expected nonlinear behavior of the arresters because all models are based on measured data, where the residual voltage is inaccurately measured due to the following factors: 1) circuit inductance; 2) voltage divider inductance and response time; and 3) signal cable problems [4]–[10]. Moreover, some of the previously mentioned models considered linear [4], [5], [7]–[10] and nonlinear [6] inductance in series with the nonlinear resistance of the ZnO blocks. While others represented the ZnO blocks without any inductances at all, e.g., [11]. In this paper, the operating principle, design, and testing of a coaxial D-dot probe to measure fast-front high voltages are presented. The designed probe has proved its advanta- geous performance over two commercial mixed RC probes and a damped capacitive voltage divider, particularly when measuring the residual voltage of SA in the high-conduction regime. Adopting these D-dot probes in fast-front voltage 0018-9456/$26.00 © 2009 IEEE