Abstract
During reclosure of 275 kV cable circuits used for voltage control, excessive overvoltages were observed on the network. Such events cause onerous and costly failures. Transient simulations have shown that the normal voltage on its own cannot generate such excessive switching overvoltages. Initial investigations by the network operator pointed towards trapped charge on the unearthed as the cause of the failures. Measurement of these trapped charge voltages and their slow decay without interfering with the charge has, to the author’s knowledge, not been done before in an operational substation. This work introduces a technique to measure trapped charge at a 275 kV substation using the Electrostatic Field Mill. Since the electric field is a proxy measurement of surface voltage, field mills can also be used to measure voltage. In this paper, an on-site substation measurement setup using an electrostatic field mill has been developed for the non-contact measurement of trapped charge voltage on a 275 kV underground cable circuit following switching operations at a National Grid substation. Results of field measurements within the substation and laboratory experimentation are discussed. It is demonstrated that with adequate calibration, achieved by using the known pre-switching power frequency steady state voltage, the slowly decaying DC voltage caused by the cable trapped charge can be measured using this non-contact technique. The correlation between the instantaneous time constant and the relative humidity is also analysed.
Highlights
When Circuit Breakers (CBs) or isolator switches operate to switch out cables, successive restrikes occur which equalise the potential between the load and de-energised side [1]
The tests carried out produced an extensive dataset of Relative Humidity (RH), Absolute Humidity (AH), temperature, pressure and field mill voltage on a common time base
The association between the instantaneous time constant, which is derived from a moving window exponential fit, and RH, is investigated
Summary
When Circuit Breakers (CBs) or isolator switches operate to switch out cables, successive restrikes occur which equalise the potential between the load and de-energised side [1] This mechanism is responsible for trapped charge accumulation, which without a significant discharge path, may remain on the cable for a period of hours or even days [2]. The network operator required a non-invasive measurement technique to test for the presence of trapped charge. The solution to this challenging constraint was to use a set of three electrostatic. Humidity (RH) and temperature be measured in tandem with the field measurements such that the role of environmental conditions on the rate of decay can be analysed Such measurements of trapped charge on live networks are rarely reported in the literature. These results are further analysed with emphasis on the change in instantaneous time constant (a sliding window exponential fit) versus RH and time
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