An electrical facility is an installation or device, the operating current of which is high enough to become dangerous for an operator or to cause some damage in the surrounding equipment in case of an unwanted earthing. This is usually true for all high and medium voltage facilities used in power generation, transmission and distribution. Whenever maintenance or repair work is to be done on the electrical facility, the facility is disconnected from its power supply and a safety ground connector is installed manually or automatically.
To forget the safety ground can have serious consequences in mainly two different situations. First, in case that the safety ground is not properly installed on the facility and that a worker approaches. Even though the facility is no longer energized itself, it may have picked up induced voltages from energized equipment in its neighbourhood. In the worst case, this situation can be fatal for the worker. The second situation, where a safety ground should not be forgotten, is before the reconnection of the facility to its power supply. If the safety ground connector was forgotten to be removed after finishing the maintenance, an unwanted earthing of the facility occurs. This may cause severe destructions.
The electrical facility may furthermore be temporarily disconnected from the power supply for other reasons than because of maintenance. One such reason is if it is subject to a self clearing fault during operation, like for instance being subject to a short-circuit fault caused by the electrical facility being struck by lightning. In such circumstances the electrical facility is normally disconnected for a certain time period and then it may be reconnected. Such practice is commonly known as reclosing and aims at increasing the continuity of power supply to the electrical facility. However, before it is reconnected any faults that are still present should be removed. Moreover in order to remove a permanent fault an estimation of the location may be needed.
The current invention presents a solution where such safety measures can be provided in a secure way so that an appropriate reaction can be initiated.
A forgotten safety ground or such a short-circuit fault has especially severe consequences in case of a three-phase electrical facility, such as a transformer or a capacitor bank in a substation of a power network or a three-phase power transmission or distribution line. A three-phase fault generally has a very severe impact on the electrical facility and its surrounding equipment. Three-phase faults spread over a large area of the related power network as a sudden depression of voltage or voltage sag, which might trip sensitive devices. The efforts needed to reinstate the devices and the power network, are of serious concern.
Today more stringent constraints are being established by regulatory bodies to limit both the number and the duration of interruptions in the power supply to end customers. These increasing demands on power transmission and distribution companies leave them with little time and few disconnections for scheduled maintenance of lines and substations. Transmission and distribution companies nowadays rely on several external contractors for the execution of the annual maintenance plan which is usually done in one or two annual disconnections of the line or substation. The coordination of maintenance work is difficult, particularly when several crews from different contractors work simultaneously in different points of a line or substation. For safety reasons, each point of work must be grounded at any incoming point. The restoration of the power supply needs to be completed once the maintenance work is done. It is both during the grounding as well as the power supply restoration process where coordination and communication must be perfect to avoid errors that endanger the maintenance personnel, increase the duration of the interruption or cause severe hardware problems. Nowadays, the application and the removal of the safety grounds are checked manually which require considerable man power. And statistics show that in most accidents on power facilities leading to personal injuries or three-phase faults of a line or substation, human errors are involved.
As an alternative solution to the manual checking of the safety grounds, the WO 00/38289 proposes a system for warning regarding the presence of manually attached ground contacts on a high voltage conductor. The system comprises an additional and naked conductor close to the high voltage conductor, which is clamped resiliently to the high voltage conductor when the safety ground is attached. A warning device senses if the additional conductor is in contact with the high voltage conductor and transmits a warning signal to an operational central unit. The central unit makes it then impossible to connect a high voltage to ground to the high voltage conductor. The hardware needed for this warning system is considerable; in particular the additional conductor makes the installation costly.
It is therefore an object of the present invention to provide a system and a method with which the impedance of an electrical facility being disconnected from its high or medium voltage power supply can be determined automatically and at comparatively low costs. This may in turn be used to determine if there is a safety ground present or if a short-circuit fault is still in existence whenever the electrical facility is not energised.