1/15 BCM application note Travel measurement in circuit breaker monitoring Contents 1 Background... 1 2 Why monitoring the travel curve... 1 2.1 Opening speed... 2 2.2 Closing speed... 5 2.3 Overtravel by closing... 6 2.4 Overtravel an d rebound by opening... 6 3 Where to measure the travel curve... 7 4 Speed measurement... 9 5 Transducer characteristics... 10 5.1 Rotary or linear... 10 5.2 Resistive or Encoder type... 11 5.3 Which Pulse number... 11 5.4 Mechanical stress... 14 6 Conclusion... 15 1 Background A travel curve measurement is usually carried out during the routine testing, after which the travel transducer is disassembled.[1] Getting a permanently installed travel transducer allows recording the position of the contact vs. time for every circuit breaker operation. The importance of travel monitoring an enhance the monitoring quality. To provide an accurate and reliable travel monitoring it is important to correctly select the travel transducer. In the following the impact of travel curve on circuit breaker performance are remembered and some guide lines are given how to specify the travel transducer. 2 Why monitoring the travel curve [2] The making and breaking performance of a circuit breaker are very much dependent by the correct travel curve provided by the operating mechanism. Getting a permanently installed travel transducer allows recording the position of the contact vs. time for every circuit breaker operation.
2/15 Figure 1 Typical travel curve for a CO operation. Main calculated parameters are: - Opening speed - Closing speed - Overtravel by closing operation - Overtravel and Rebound by opening operation A straight forward approach is the fingerprint concept, a travel curve is recorded as reference and an alarm is raised if the curve exceeds given tolerances. Additionally, the following parameters can be evaluated as shown in Figure 1: - Opening speed - Closing speed - Overtravel by closing operation - Overtravel and rebound by opening operation In the following it is explaining what is impact of the parameter deviation on the circuit breaker performance. 2.1 Opening speed The opening contact speed of the circuit breaker is a crucial parameter to guarantee its switching capability and mechanical endurance. The manufacturer specifies minimum and maximum tolerances. A too low value can impair the short circuit current interruption as well as the capacitive current switching.
3/15 Short circuit clearing In Figure 2 it is shown as a slower speed results in a smaller pressure build up in the puffer volume which is directly impacting the blowing efficiency at current zero.. Figure 2 Impact of lower speed on short circuit switching performance. A slower speed results in a smaller mechanical pressure build up in the interrupting chamber. This reduces the blowing pressure at current zero with a concequence reduction of thermal switching capability. Additionally the contact distance reached for the same arcing time is also smaller, which increase the probability of dielectric failure around TRV peak. The quenching medium (SF6) decomposed in plasma status by the arc can turn back into its dielectric status only if there is enough cooling power provider by the interrupter. If the cooling power is not enough the plasma status persists as such keeping the gap between contacts conductive. The current crosses the zero line continuing flowing. This is called Thermal failure typically visible in short line fault cases. If the cooling power is still enough to turn back the plasma into dielectric medium, a slower speed results in a shorter contact distance for the same arcing time. The probability of dielectric breakdown around the peak of transient recovery voltage increases. If this happens, after a short pause the current start flowing again. This is called dielectric failure.
4/15 Capacitive switching While switching off a capacitive current, a 1-cos voltage rises across the contacts. To withstand this voltage, the circuit breaker has to keep the dielectric stress below SF6 dielectric strength increasing the contact distance fast enough. A slower speed makes the capacitive switching more severe. Figure 3 Capacitive switching. In the up left diagram 1-cos recovery voltage in blue is shown starting at current zero is shown together with the travel curve (black) and a slower one (red). IN the diagram below the contact distance is plotted vs time. In the up right diagram is plotted the resulting voltage as function of distance applied across the contact gap for the tow given travel curves. As it is possible see, with slower speed, for the same contact distance the applied voltage is higher. A too high opening speed can cause mechanical overstress and result in higher overvoltage in case of inductive current switching.
5/15 2.2 Closing speed It impacts on making capability of the circuit breaker. In Figure 4 a closing operation is shown with current making as it would happen if the circuit breaker closes against a fault.. The current start flowing BEFORE the contact touch. The time between current start and contact touch is called pre-arcing time. An insufficient speed by closing operation increases the duration of pre-arc resulting in a heavier thermal stress of the arcing contacts. The time interval between contact touch and current commutation to main contacts increases as well exposing the tulip to higher current values and consequent electrodynamic forces. The resulting higher friction between tulip and plug could prevent the breaker from completing the closing operation with a failure of latching, or in the worst-case, damage of arcing contact system and catastrophic fault Figure 4 Current making by closing operation. A lower closing speed will increase the time between breakdown and contact touch as well as from contact touch to commutation. This increases the thermal and mechanical stress of tulip and plug. When the closing speed exceeds the maximum value, depending of the specific design, the consequence can go from higher mechanical stress to irreparable damages of nozzle, plug and tulip with fatal consequences on the next opening operation.
6/15 2.3 Overtravel by closing In many circuit breakers equipped with spring operating mechanism, a minimum over travel is required to ensure latching by closing. The manufacturer provides tolerances on these parameters no to be exceeded: A too little over travel would result in a Close Open operation Figure 5 Travel curve of a circuit breaker with spring operating mechanism. - If the overtravel is below the minimum, the latching is not successful and the contacts open again. A Close operation results in a Close-Open 2.4 Overtravel an d rebound by opening Sometimes these two parameters are also specified by the manufacturer. A rebound by opening means the contacts come together after having reach their fully open position representing a risk of re-strike by clearing. An excessive over travel while opening could come from a too high opening speed or a possible problem with the damper of the operating mechanism. The consequence is a general higher mechanical stress up to a damage of the interrupter due to internal collisions. If the opening speed is correct and the over travel and rebound by opening has the tendency to increase, it is a good indication that the damper in the operating mechanism is less and less efficient.
7/15 3 Where to measure the travel curve The final target should be getting the travel curve of the moving contact. All the travel curve parameters are referred to the actual position of the contacts, which is not accessible for measurement. The travel transducer takes the movement from the most convenient accessible point along the linkage between operating mechanism and interrupter. What is measured is not the actual contact displacement and the relationship between the two values is not linear. In Figure 6 is shown a typical case of use of a rotary travel transducer applied to the shaft of the interrupter bell crank. Figure 6 Example of travel measurement at the bell crank shaft. The relationship between the angle and the displacement is not linear The transducer measures an angle that is in relation with the actual contact displacement with a function defined by the linkage geometry.
8/15 A linear re-scaling of the input signal to the actual contact stroke introduces a linearity error that is higher towards the extreme position of the linkage and for a wider angular movement. An example of comparison between actual contact position and linearly rescaled angular measurement is given. Travel curve - measuremtn comparison - deg: read in angle - lin: linerly rescaled travel - poly: translatino curve applied 150 100 50 Difference ~ 1.5 mm 5.00 4.00 3.00 2.00 1.00 0-50 0.00 0 10 20 30 40 50 60 70 80-1.00-2.00-100 -150 deg poly mm lin mm delta mm Overtravel -3.00-4.00-5.00 Figure 7 Travel curve by closing. Comparison between liner rescaling the rotary measurement and actual internal contact position. A difference of 1.5 mm could cause 20 to 30% error in the evaluation of the over travel The get closer to the actual contact position a translation function from rotary measurement to linear contact displacement can be used (Figure 8) Figure 8 Principle data processing for getting the actual contact displacement
9/15 4 Speed measurement For comparison with routine test reference values, the velocity between two defined points will be calculated for C and O operations. The point definition datum points will be independent for closing and opening operations and configurable according to the following criteria: - Time difference from auxiliary contact commutation (52 a/b) - Percentage of the stroke - Distance between datum points based on travel curve Figure 9 Travel curve speed evaluation For both opening and closing operation the speed is the average between two points defined independently - 2 for closing - 2 for opening The points can be defined as % of the total stroke or time. An example of typical defined interval for speed measurement are: Opening operation 1 st point: at Contact Separation 2 nd point: ~7.5-10ms after contact separation Closing operation 2 nd point: at contact touch 1 st point: ~ 5ms before contact touch
10/15 5 Transducer characteristics 5.1 Rotary or linear Of course the transducer has to be selected according to the type of movement to measure. On the other hand, if there is the possibility to choose where to install it, a rotary one should be selected. Rotary transducers are in general more reliable and due to its compactness, also the connection results to be more protected. Figure 10 Linear transducer Figure 11. Rotary transducer
11/15 5.2 Resistive or Encoder type In contrary to routine test travel measurement resistive transducer are very often used providing a quite accurate analogue voltage signal. For an online permanent monitoring they showed to be not reliable enough. The high number of operation combined with quite variable weather conditions was very often damaging the internal sliding contact. In order to ensure a reliable functionality only contact less encoder based transducer should be selected. The encoder is a digital output transducer sending an impulse every of its total stroke. Additionally, the digital output makes the measured signal less sensitive to noise. Figure 12. Typical output signal of an incremental encoder. There are two impulses 90 deg shifted from each other what allows to determine the rotational direction. An additional pulse every revolution (K0) is also given. The number of pulses per revolution is selectable An example of incremental encoder from the company Baumer is shown in Figure 12. Incremental encoders do not remember the starting position. Counting the pulses and knowing the deg/pulse it is possible to measure the incremented position from the beginning of measurement. Absolute encoders are also available, but they are more expensive and more delicate. The number of pulses per revolution is selectable by purchasing. In the specific case given in Figure 12 a value from 10 up to 10000 pulses per revolution can be selected. 5.3 Which Pulse number The number of pulses is function of the stroke to measure. Typical values for circuit breaker stroke and operating time are given in Table 1
12/15 Table 1 - Typical parameters of a circuit breaker travel curve Parameter From To Comment Typical angular stroke deg 60 150 Bell crack design dependent Stroke mm 100 200 Higher values for higher rated voltage ( 145kV --> 550 kv) Closing speed m/s 4 10 Higher values for higher rated voltage ( 145kV --> 550 kv) Opening speed m/s 4 13 Higher values for higher rated voltage ( 145kV --> 550 kv) Acceleration m/s2 500 2000 Acceleration peak during latching or bouncing could be even higher Resolution To best select the pulse number it is crucial to set the wished measurement resolution. 0.2% it is more than enough. Considering a maximum stroke of 200mm this would lead to 0.4 mm/pulse Pulse number Having set the wanted resolution, the number of pulses for the total stroke is 1 = 500 0.2% These are the pulses we need for covering the total linear stroke. If the total linear stroke of the contacts comes from 60 deg of the rotary linkage shat in the bell crack, we need a transducer having 3000 pulses / revolution 500 360 60 = 3000 If the linkage were 90 deg to cover the same stroke 2000 pulses would be enough Max frequency 500 360 90 A higher number of pulses is not necessarily better. = 2000 A frequency limitation of the encoder itself as well as the input card for pulse counting has to be considered. For evaluating the pulse frequency, we have to know the operating speed of the breaker. Considering a circuit breaker having an opening speed of 10 m/s, for a stroke of 200 mm and 0.2% resolution would lead to 25 khz
13/15 10 0.4 = 25 = 25 Assuming that we want to be able to measure speed up to twice the rated value (to evaluate failures) the maximum frequency the encoder must be able to manage and the input card must be able to count is 2 x 25 = 50 khz. As an example the encoder from Baumer can manage up to 250 khz output frequency. Figure 13- Datasheet of incremental encoder [3]
14/15 5.4 Mechanical stress The transducer is connected to moving linkage of the breaker and as such has to withstand the mechanical stress coming from the movement. Acceleration of the movement In the Table 1 values between 500 and 2000 m/s2 are given as typical. This means acceleration up to 200g, what could look like overestimated. A rough calculation to estimate the average acceleration for reaching the rated speed is given the following. Let's assume Speed V=10 m/s Time to reach the speed: t=10ms = 0.01 s (very often shorter) a= v t = 10 0.01 = 1000 m ~100g s Figure 14 contact travel diagram. Speed and acceleration has been calculated as 1 st and 2 nd derivative.
15/15 The Figure 14 gives an example of an actual travel curve measurement of a 170 kv circuit breaker equipped with a spring operating mechanism. It is possible to see how in the damping and latching region the acceleration value goes far above 1000 m/s2. The actual acceleration seen by the transducer should be calculated considering the actual rotary measured stroke. Nevertheless, it is important not to forget this parameter to ensure a reliable measurement and avoid damaging the transducer. 6 Conclusion Monitoring the travel curve can enhance the level of condition monitoring of a circuit breaker. Although apparently very simple to get, a reliable permanent travel measurement for online monitoring application is not trivial to get. Attention must be paid while selecting the travel transducer also from mechanical stress point of view. Selecting the proper pulse number allows getting an accurate travel curve without hitting the upper frequency limit of the transducer and input card. 7 Bibliography [1] C. Baudart, WJ. Bergman, J. Buerger, J., Corbett, E. Colombo, WJ. Franca, RD. Garzon, A. Hyrczak, CJ. Jones, A. Mercier, P. Migaud, K. Nilman Johansson, G de Radigues, L., Mueller, DF. Peelo, C. Rajotte, J. Rodriguez, and Arias, M. Runde, K. Takahashi, JA. Wiersma, USER GUIDE FOR THE APPLICATION OF MONITORING AND DIAGNOSTIC TECHNIQUES FOR SWITCHING EQUIPMENT FOR RATED VOLTAGES OF 72.5 kv AND ABOVE. Paris: CIGRÉ, 2000. [2] N. Gariboldi and P. Corliss, Modern on-line monitoring for high voltage circuit breakers, GCC Power 2017, p. 10, 2017. [3] OG 60 Incremental encoders Baumer. [Online]. Available: https://www.baumer.com/ch/en/product-overview/rotary-encoders-angle-sensors/heavydutyencoders-incremental/complete-portfolio-of-solid-shaft-designs/og-60/p/29157. [Accessed: 05-Dec-2017].