the direction from the right to the left, and exits the lens on the opposite side. When thecomplex impedance exits the lens on the side opposite to its entrance, the 1st pole-sliphas already occurred and more pole-slips can be expected if the generator is notdisconnected. Figure 111 shows two pole-slips. Figures like Figure 111 and Figure113 are always possible to draw by means of the analog output data from the pole-slipfunction, and are of great help with eventual investigations of the performance of theout-of-step function.7.6.7.3 Maximum slip frequencyA pole-slip may be detected if it has a slip frequency lower than a maximum valuefsMax. The specific value of fsMax depends on the setting (parameter) StartAngle(which determines the width of the lens characteristic). A parameter in this calculationroutine is the value of the minimum traverse time, traverseTimeMin. The minimumtraverse time is the minimum time that the travel of the complex impedance Z(R, X)through the lens, from one side to the other, must last in order to recognize that a pole-slip has occurred. The value of the internal constant traverseTimeMin is a function ofthe set StartAngle.For values of StartAngle <= 110°, traverseTimeMin = 50 ms. Forvalues StartAngle > 110°, traverseTimeMin = 40 ms. The expression which relates themaximum slip frequency fsMax and the traverseTimeMin is as follows:fsMax Hz traverseTimeMin msStartAngle[ ] ≅ [ ] ⋅ °[ ]°[1000 1 000 180. - ]]IECEQUATION2319 V1 EN (Equation 48)The maximum slip frequency fsMax for traverseTimeMin = 50 ms is:StartAngle = 90° →fsMax = 20 × 0.500 = 10.000 HzStartAngle = 100° →fsMax = 20 × 0.444 = 8.888 HzStartAngle = 110° →fsMax = 20 × 0.388 = 7.777 Hz (default 110°)The maximum slip frequency fsMax for traverseTimeMin = 40 ms is:StartAngle = 120° →fsMax = 25 × 0.333 = 8.333 HzStartAngle = 130° →fsMax = 25 × 0.277 = 6.944 HzThe minimum value of fsMax is 6.994 Hz. When StartAngle = 110 degrees, fsMax =7.777 Hz. This implies, that the default StartAngle = 110 degrees covers 90% of casesas, the typical final slip frequency is between 2 - 5Hz. In practice, however, before theslip frequency, for example 7.777 Hz, is reached, at least three pole-slips haveoccurred. In other words, if we consider a linear increase of frequency from 50 Hz to57.777 Hz, at least three pole-slips will occur (in fact: (57.777 - 50) / 2 = 3.889). Theexact instantaneous slip-frequency expressed in Hz (corresponding to number of poleslips per second) is difficult to calculate. The easiest and most exact method is tomeasure time between two successive pole slips. This means that, the instantaneousslip-frequency is measured only after the second pole-slip, if the protected machine isSection 7 1MRK502052-UEN BImpedance protection268Technical manual
