## 1 Introduction  
The wide existence of the transient voltage in the power grid threatens the insulation system of high-voltage plants, which often causes the grid faults [1]. The accurate measurement of transient voltage is the key for the fault analysis, the optimized insulation design and the standardization of high-voltage testing techniques. Various types of transient voltages can be found in a power grid. The fastest transient voltages (i.e., very fast front overvoltage, VFFO) usually appear at a gas-insulated switchgear (GIS), which is caused by the operation of the GIS disconnectors [2; 3; 4]. A VFFO commonly has high frequency components of approximately 100 MHz and a rising time of several nanoseconds [5]. For the fast-front transient voltages, the rising time is normally in the range of 0.1 \(\upmu\)s to 20 \(\upmu\)s [5]. In terms of a transient voltage caused by normal operation, the rising time is in the range of microseconds. It is noted that these transient voltages are usually superimposed on the power-frequency voltage, which requires a wide bandwidth and fast response speed for the voltage measuring system to be well measured. Moreover, portability and safety are also advised for developing a voltage measuring system. Subject to the requirements of insulation and capacity, traditional electrotechnical voltage measuring techniques usually bring huge sized and heavy weighted equipment, especially for those applied in an extra-high voltage (EHV) or ultra-high voltage (UHV) power grids. It is also noted that their bandwidth and response speed are unfavorably limited. For example, the capacitance-voltage transformer (CVT) which is a commonly used voltage measuring device in the high-voltage AC power grid typically has a weight of hundreds of kilograms (for 500 kV) and a bandwidth lower than 1 kHz [6; 7; 8]. Besides, the bushing tap is applied mainly to measure the transient voltage during a substation commissioning, of which the performance is mainly affected by the manufacturing process and the bushing structure. As a consequence, the upper bandwidth is generally not higher than 5 MHz for a bushing tap technique [9; 10; 11]. In addition, the portability cannot be achieved on these techniques. Thus, subject to the future demand of the power system, it is highly expected to develop a safe and easy-to-apply voltage measuring technique. For a quasi-static problem, since the electric field is considered to be irrotational, the space electric field around a conductor is linear to the potential of the conductor. Based on the measuring technique for the electric field, a non-contacted measuring method for transient voltage could be possible. On the other hand, as the development of optical sensing technology, several electro-optical effects have been used for measuring electric fields. The electro-chromatic effect has a response time of \(\sim\)100 s, and it is suitable only for extremely low-frequency electric field measurement [12; 13]. The Kerr effect is a quadratic electro-optic effect and normally has a small electro-optic coefficient. Thus, the sensors based on the Kerr effect have a low sensitivity [14; 15]. The Pockels effect is known as a linear electro-optic effect, which means the output voltage is linear to the measured electric field. The sensor based on the Pockels effect normally has a fast response speed, small size, and are passive. It is particularly used for the measurement of high-amplitude transient electric field [16]. In this paper, based on the linear correspondence between the transient voltage and the transient electric-field caused, a non-contact transient voltage measuring technique was proposed. An integrated optical electric-field sensor based on the Pockels effect was introduced to implement the field measurement. The measuring system was developed and tested for its bandwidth and response speed. A series of measured lightning transient voltages and typical switching transient voltages were presented.