ELCOME Dear friends of protection and control engineering! For the selective and fast measurement of distance protection, the falsification of measured variables due to current transformer saturation is to be limited to defined lines. The following article presents the verification of these limits within the framework of an automatic reclosure retrofit.
IFigure 1: System configuration
A distance relay of the type 7SA6 is used in the present system configuration (Fig. 1). This is integrated into the primary installation by means of a classic iron-enclosed current transfomer of the class TPX. For economic reasons, TPX cores are usually not designed for the saturation-free transmission of the fully displaced short-circuit current for the entire short-circuit duration. Since a too high impedance is measured with a saturated transmitted current and the zone range is thereby shortened, there is the danger of triggering into the next higher zone. Therefore, in order to achieve a sufficiently accurate measuring result in the area of the breakover point, the CT must be designed in such a way that, in the case of remote errors, saturation-free transmission is carried out until the end of the elapsed measuring time.
CT EVALUATION WITHOUT SB (SB = short break)
Modern digital distance relays operate with measurement times between 25 and 50 ms. The 7SA6 used here is indicated with a minimum necessary measuring time of 25 ms (Siemens). The software-based simulation shows that the TPX core can transfer saturation-free for 29 ms. The converter thus fulfills almost the requirement of the distance protection relay. Fig. 2 shows the resulting trend of the short-circuit current without SB for a period of approx. 100 ms. The model was terminated with an operating voltage of 0.9 ohms (50 m at 2.5 qmm copper and 80 ° C).
Fig. 2 Short-circuit current curve without SB over approx. 100 ms
CT EVALUATION WITH SB
A SB cycle Closed-Open-Closed-Open with a dead time of 0.5 s is set for a 3-pole automatic reclosure to be retrofitted. The autonomous time of the circuit breaker used is 60 ms. Since the TPX CT can not demagnetize during the voltage-free break, it must already be assumed that the measurement is experiencing an impermissible saturation in the event of a recurring short-circuit. A re-calculation based on the SB scenario confirms this forecast. The calculation results in a saturation-free transmission time of approximately 7 ms. Figure 3 shows the entire SB curve. The detailed time curve of the current measurement of the second short-circuit event is shown in Fig. 3.
Fig.3 Trend of the short-circuit current over the entire SB curve
The planned three-pole automatic reclosure with short-circuit can not be implemented in this configuration under using the existing current transformer. Saturated short-circuit currents would lead to measurement inaccuracies and possibly faulty protection decisions with longer tripping times.
In order to achieve the required values, the CT would have to supply at least a nominal power of 50 VA and the primary rated current should be doubled from 600 A to 1200 A. Due to the size achieved, the retrofit of a linear core is recommended. A corresponding TPZ core requires only a small oversizing factor, even in case of the unsuccessful reclosure, since the core is completely demagnetized during the voltage-free break. In addition, space savings are achieved compared to a conventional TPX converter with comparable performance.
Since the settings for the distance protection zones, in particular for small impedances and small short-circuit voltages, depend on the accuracy of the current and voltage transformers used, the measuring accuracy resulting from the short-circuit current calculation should be included in the design of the X range of the zones to be determined.
This contribution comes from the new issue of the Austrian network protection magazine. You can read more interesting articles from "Netzschutzmagazine" directly (simply click on the picture below).