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Testing the timing characteristic of the BE1-87T

Posted: 13 May 2002     Print Version  Bookmark and Share



Published for the Basler Electric Power Systems Group #PC-87T01 7 August 1998 APPLICATION i Notes Testing theTiming Characteristic of the BE1-87T Scope The operating and timing characteristics of the 87T relay are displayed in per unit values in the instruc- tion manual, 9 1713 00 990. This normalizing method greatly simplifies the presentation of the characteristics, but tends to be confusing when users need to calculate actual currents for testing the relay. The following additional information and examples will clarify the relationship among the test current, the multiple of tap, and the multiple of pickup used in the timing character- istic of Figure 1-3. Timing Test - Single Input Principle This is the simplest and most accurate method to verify the trip timing of the relay. A test current is applied between any phase and corresponding com- mon. The closing time of the trip contact is expected to be near the values shown in Figure 1-3 which defines the operating time as a function of multiple of pickup. This Multitude of Pickup is not be be confused with the Multiple of Tap. For the single phase test, the operate and restraint currents are equal : the test current as a multiple of tap is It=k.Tap, where k is the multiple of tap. Also : Iop = k.Tap since the operate and restraint currents are equal in this case. The Minimum Pickup (MPU) defining the boundary between the operate and restrain region in Figure 1-2, for test currents greater than the intersection of the slope line with the Iop=.35 line, is : MPU=(slope)x(multiple of tap) xTap. The multiple of pickup (m) is therefore: m=Iop/ MPU=k.Tap/s.k.Tap=1/s. In other words, the multiple of pickup of the test current for single phase testing is always 1/s, regardless of its multiple of tap value. Test current selection Let's say that it is desired to measure the response time at 2X Multiple of Tap for a slope setting of s=0.3 (30%). The MPU is (s)x(multiple of tap)xTap for (multiple of tap)>0.35/(slope). The multiple of pickup is 1/.3=3.3. For a tap of 2A, It=2x2=4A; MPU=0.3x2x2=1.2A; m=4/1.2=3.3. Expected time The timing characteristic shown in Instruction Manual Figure 1-3 represents the total response time as a function of multiple of pickup for a typical relay. For the given example, the expected trip time, correspond- ing to m=3.3, is about 32ms. Timing Test - Two Inputs Principle A restraint current is applied between the same phase and its corresponding common on two input circuits. The currents are adjusted for a given multiple of tap on each circuit, at the proper phase to yield a balanced condition, i.e. the operate current is zero. One of the currents is then reduced to a value that yields an operate current equal to the desired multiple of pickup for which the response time is desired. Test current selection Let's say that the following taps have been set: Circuit 1: Tap=2A Circuit 2: Tap=11.7A Slope=30% Assume that the test is to be performed at a restraint value of 2xTap. The initial (balanced) current is thus 4A and 23.4A for each circuit. Note that the current in one circuit exceeds the continuous rating of 20A; the maximum ON time must take this fact into account. For best results, the operate current is obtained by lowering the current in one circuit. This assures the maximum restraint current remains equal to the desired multiple. The Minimum Pickup (MPU) is derived from the percent restraint characteristic (Figure 1-2). At 2xTap and a slope of 30%, the MPU is equal to the (maximum restraint) x(slope), in multiples of tap. To obtain an operate current of a multiple of pickup (m) at the restraint current of k.Tap, the test current in one of the circuits must be reduced to It=k.Tap.(1-m.s). See Table1. When the current in circuit 2 is reduced from the balance value of 23.4A, pickup occurs below 23.4- (23.4x.3)=16.36A. To obtain an operate current of 2xMPU for the timing test, the test current must be PC-87T01 (8/98) Definitions k=multiple of tap restraint (Fig.1-2) =Irest(max)/tap m=multiple of pickup (Fig.1-3) =Iop/MPU s=slope (p.u.) Tap: relay setting SINGLE SOURCE TWO SOURCES Test current: increase (step) for k>0.35/s : It=k.Tap from 0 to It m=1/s Test current: Reduce (step) It = k.Tap (1-m.s) from balance (k.Tap) to It Table 1 Note: For two source testing using the current reduction method, the maximum multiple of pickup is reached when It=0. In this case m=1/s, as in the single input test. Figure 1-2, simplified. Figure 1-3, simplified. reduced from 23.4 to 23.4-2x(23.4x.3)=23.4x(1- 2x.3)=9.36A. Expected time The operate time corresponding to this multiple of pickup of 2 is found in Figure 1-3 : 52ms. Variations of about 25% are to be expected, due to component tolerances, but particularly due to the slope of the characteristic at multiples of pickup below 3. Conclusion Figures 1-2 and 1-3 have been simplified (below) to clarify the testing concepts. The test currents can be calculated for other tap and slope combinations from the following summary, Table 1. ROUTE 143, BOX 269, HIGHLAND, ILLINOIS U.S.A. 62249 PHONE 618-654-2341 FAX 618-654-2351 P.A.E. Les Pins, 67319 Wasselonne Cedex FRANCE PHONE (33-3-88) 87-1010 FAX (33-3-88) 87-0808, [email protected]

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