IEDs and the Fault Locator function
The IEDs is a general term but mainly are focusing to describe a protection relay which can include some functions in order to detect and trip when a fault is appearing. Besides these devices indicate to high system under a communication channel the distance to the fault and other information to establish the correct location.
The function of the IED is to trip and isolate the fault from the outgoing of the substation. Normally this is the position of the IED, controlling the circuit breakers of the feeders from HV/MV substations collaborating in the information collection to help in the fault location.
These digital devices are multifunction and have protection functions to fulfil other ANSI and IEC normative such as voltage and frequency protection, therefore there are modules with several channels to collect digital inputs in order to implement automatisms.
Another important point of these devices is the communication system, it is possible to integrate them through several communication media and with different protocols. In the electrical market due to this reason this kind of devices have started to be knows as Advanced Electronic Devices (AED).
As mentioned before the AEDs have the possibility to work with several functions. One of these functions is the Fault Locator (FL) which can indicate the distance to the fault from the position of the AED. Although this function is normally used in a high system, then it is necessary to analyse where is the best position to use this protection.
The FL function is described through ANSI 21FL, from the normative ANSI C37.2. This function is defined too through IEC in the normative 60617 as FLOC . The function ANSI 21FL is present normally in some protection devices for the electrical network.
The IEDs use the voltage and current measures in real time and compare with the impedance of the line in order to identify the section of the fault. Through the algorithms of this protection, it is possible to calculate the fault distance to the fault.
Several manufacturers have implemented the function 21FL in their AEDs. The cost of this function in an IED for HV/MV substation outgoing has not a big impact, although in other kind of devices such as FPI or telecontrol system for MV/LV substation the cost difference will be significative.
As the current and voltage are complex values, the obtained result after applying the equation the result will be a real value near to the fault distance fault and another little complex component. In the cases when this result is not correct, the cause is mainly due to not have defined correctly the line impedance.
Although this implementation seems easy, the different faults have different impedances and besides there are compensations and the fault can be added an additional impedance depending on the kind of fault or the earth contact.
The figure 1 shows different AED devices to include in a cabinet in order to install it above or near to the feeder circuit breaker cubicle. In the figure also is showed a set of substation outgoings with their protections. It highlights that these devices are connected to Current Transformers (CT) and Voltage Transformers (VT) which are inside the cubicles with a very reductive transformation constant.
At the end it is an electronic device which works between 0 and 5 V. Also, in the picture there is the picture of internal cabinet with a wired AED to the circuit breaker and other loose AEDs.
In order to define the 21FL function accurately it will be necessary to establish an analysis between two important points. The network analysis and environmental the information influence directly to the calculus of this function.
|Title:||Automatic fault location in electrical distribution networks with distributed generation – Angel Silos-Sánchez, Doctoral Thesis at Universitat Politècnica de Catalunya|
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