Online Resistance Monitoring

INTRODUCTION

PROPER grounding is the first line of defense against lightning or other system contingency to ensure the safety of operators and power apparatus. A poor grounding system not only results in unnecessary transient damages, but also causes data and equipment loss, plant shutdown, as well as increases fire and personnel risk. As a result, Utility companies are actively seeking techniques that can effectively and reliably evaluate the grounding grid conditions to ensure personnel safety and prevent equipments damage.

The performance of grounding grid is affected by various factors such as unqualified jointing while building, electromotive force of grounding current, soil erosion and theft of grounding rods. Thus, monitoring and diagnosing the conditions of grounding grid has been an active research field for many years. However, almost all techniques implemented or proposed for grounding monitoring are offline types where special instruments are installed for grounding condition check on a regular or as-needed basis. These existing methods can generally be categorized into two types: measurement of grounding impedance and detection of grounding integrity.

Fall-of-Potential (FOP) method is the basic scheme for grounding impedance measurement and it has been implemented for many years. Its key point is to correctly locate the potential probe, which is quite time-consuming. A lot of variations have been proposed to improve this scheme, such as by using variable frequency source or implementing multiple electrodes. The methods taking account of current split in transmission and distribution grounding system are further developed in for accurately measuring the impedance of in-service substations. However, the potential probes are still indispensable in these FOP-based schemes. Several enhanced grounding grid computer models are developed recently with considering soil layer depth in or based on electro- magnetic field methods. But, the accuracy of these models relies on the soil resistivity measurement. Once the soil condition is changed, potential electrode needs to be relocated and it obviously increases the labor.

Monitoring the integrity of grounding grid is another way to evaluate the performance of grounding grid. However, the computation of this method depends on many uncertain factors such as soil conductivity, humidity and climate. A device based on measuring magnetic induction intensity is designed to diagnose the grounding grid corrosion in. It requires the current injection between all possible grounding leads on the ground surface to increase accuracy, which is not practical in a large scale substation.

All of the aforementioned methods are offline-based, which at best give one-shot measurement results. If another set of results is needed, the measurement system must be redeployed. The offline-based methods have significant disadvantages. Firstly, the results are largely dependent on the soil condition at the time of measurement. Secondly, sudden changes of the grounding grid such as those caused by theft cannot be identified timely. To solve these problems, the methods that can monitor the grounding condition on a continuous, i.e., online, basis is highly desired.

We have developed an online resistance monitoring system which can measures the grid resistance as per the desired interval (5 minutes to 30 minutes) and communicate to the user in login screen in numeric and graphic form and also through SMS to 5 subscribers and also allow upper threshold and threshold value setting for resistance using SMS & SERVER.