تغییرات ولتاژ توالی منفی در سمت ولتاژ پایین ترانسفورماتورهای توزیع
Abstract
1- Introduction
2- The nature of the Vdip method for an earth fault location
3- The Vdip method principle
4- Verification of the Vdip method
5- Conclusion
References
Abstract
The paper presents a new approach for earth fault localization which is based on evaluation of phase to ground voltages recorded by voltage monitors installed at low-voltage side of distribution transformer station deployed under smart metering projects. The introduced method “Vdip” determines the probability of asymmetrical fault location based on comparison of the changes in negative sequence current recorded at the feeding medium voltage substation and changes in negative sequence voltages recorded at the LV sides of distribution transformers. The paper describes the nature of the method and presents its principle using simulation of a MV resonant earthed distribution network where short-term connection of an auxiliary resistor is used. The method was also evaluated in real resonant earthed 22 kV network, where totally 15 earth faults with fault resistance up to 1.2 kΩ were carried out. As the result of this field test, the location error of the Vdip method is presented.
Introduction
Despite decades of experience with operation of resonant earthed medium voltage (MV) distribution networks, fast and accurate location of an earth fault (EF) in such networks is still very challenging. This is due to a high complexity of distribution networks caused by its purpose of distribution of electrical energy to all customers and relatively low earth fault currents due to reactive component compensation. At the present time, the identification of faulty feeder at supply substation is relatively reliable; however localization of the earth fault on affected feeder is not suitably solved. One of the most widespread and most effective methods for EF localization is still trial switching, i.e. successive process of disconnection and reclosing of individual feeder sections until the fault is found. This multiple switching operation has negative impact to the continuity of power supply and can cause subsequent EF or short circuit due to switching over-voltages and elevated voltages on the two healthy phases. Another technical solution for an EF localization utilizes fault indicator units. Those devices can indicate the EF direction based on monitoring of voltage and current specific parameters [12]. The size of indicated faulty area depends on the number of installed indicators and their arrangement within the DS, which requires a large number of installed monitors to achieve satisfactory results. Another disadvantage is the necessity of other supplementary means like trial switching for the final localization of the fault by an operator in the selected part of the network in the case of a small number of fault indicators and complicated fault location in the case of malfunction of one of installed indicators. The next group of the means intended for EF location in a DS is based on calculation of a fault loop impedance/reactance on faulty feeder [3–6]. Resulting reactance calculated by a locator (IED - Intelligent Electronic Device) is used for estimation of distance to the fault. However the calculated electrical distance can correspond to more than one possible faulty point. Therefore complementary method has to be applied, as it was introduced in [7] or base on smart meters measurement in [8]. Similar problem is characteristic also for travelling wave based methods [9–12], where precise time synchronization has to be employed.