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This post was last edited by chenjinfeng on 2020-2-7 at 16:47. Motor Protection and Calculation I. Quick-break Protection 1. High value for quick-break protection: Calculated using the high set current Isdg. Calculated based on avoiding the maximum starting current of the motor, that is: Isdg = Krel × Kst × In, where In = Ie / nTA. Here, Krel is the reliability factor, equal to 1.5 ； Kst – Multiple of the motor starting current (between 6 and 8). In – Secondary rated current of the motor. Ie – Primary rated current of the motor. nTA – Turns ratio of the current transformer. 2. Quick-break low value: Calculated based on the maximum feedback current of the motor in the event of a short circuit outside the protection zone. When there is a three-phase short circuit at the outlet of the plant busbar, based on past measurements, the transient value of the motor’s feedback current is 5.8–5.9. Taking into account the inherent operating time of the protection device, which is 0.04–0.06 seconds, as well as the attenuation of the transient value of the feedback current, Kfb=6 is used to determine the lower set value for the operating current; that is: Isdd = Krel × Kfb × In = 7.8In. Here, Krel is the reliability factor, equal to 1.3 ； Kfb —— the maximum feedback current multiplier in the case of an external outlet short circuit; take Kfb=6. 3. Calculation of the action time setting value. Protect the inherent operating time; the setting value for the operating time is as follows: quick-break operating time: tsd = 0 s. II. Single-phase ground zero-sequence overcurrent protection (low-voltage motors) 1. Calculation of the primary operating current. There is single-phase ground protection for motors equipped with a zero-sequence current transformer TA0. When the three-phase currents on the primary side are balanced, the leakage magnetic fluxes generated by these three-phase currents are not identical, which results in a magnetic unbalance current within the zero-sequence current transformer. Based on multiple measurement results under different conditions, the value of the magnetic imbalance current is always less than 0.005Ip (where Ip is the balanced three-phase line current). Therefore, by considering the maximum imbalance current that occurs during motor startup, the operating current for the single-phase ground fault protection in low-voltage motors can be set as: I0dz = (0.05–0.15)Ie. Here, I0dz represents the setting value for the primary operating current of the zero-sequence overcurrent protection for single-phase ground faults ； That is—the rated primary current of the motor. When the motor capacity is large, it can be taken as: I0dz = (0.05–0.075)Ie; when the motor capacity is small, it can be taken as: I0dz = (0.1–0.15)Ie. Since the sensitivity of single-phase ground protection is sufficient, in some cases I0dz can be set a bit higher depending on the specific circumstances. Based on experience, the primary operating current for single-phase ground fault protection in low-voltage motor protection is generally set at I0dz = 10–40 A. 2. Calculation of action time t0dz. Take: t0dz=0s. Low-voltage motor protection III: Negative-sequence overcurrent protection. When the three-phase currents of the motor are asymmetric, a negative-sequence current I2 is generated. Large negative-sequence currents (I2) can occur in situations such as a break in one phase of the motor’s primary circuit (when one phase of the high-voltage fuse blows or one phase of the motor’s windings breaks open), an inter-turn short circuit in one or two phases of the motor’s windings, or an incorrect phase sequence of the motor’s power supply (when the phase sequence before the current transformer TA is reversed). In such cases, the negative-sequence current protection or the unbalanced current (△I) protection – with domestic comprehensive protections being referred to as negative-sequence overcurrent protection and imported comprehensive protections as unbalanced △I protection – will activate after a delay to eliminate the fault. 1. Calculation of negative sequence operating current. When the motor operates in two-phase mode, the negative-sequence overcurrent protection should operate reliably. 2. When two-stage negative sequence overcurrent protection is adopted for domestic comprehensive protection settings, the setting calculations can utilize both Stage I and Stage II negative sequence overcurrent protections. (1) Negative sequence Phase I overcurrent protection. The calculation is carried out by taking into comprehensive consideration the negative-sequence feedback current of the motor during asymmetric short circuits outside the protection zone, the transient secondary negative-sequence current that occurs when the motor starts, as well as ensuring sufficient sensitivity when the motor operates under heavy two-phase loads or in the presence of internal asymmetric short circuits. 1) Operating current: An empirical formula is used, given by I22dz = (0.6–1)In; generally, I22dz = 0.6In is adopted. 2) Operating time. Take: t22dz = (0.5–1)s. (2) Negative sequence phase II overcurrent protection. It is designed with consideration for avoiding the maximum negative sequence current that may occur during normal operation of the motor, ensuring sufficient sensitivity when the motor operates in two-phase mode under light loads, and providing protection against inter-turn short circuits in the motor’s stator windings. 1) Operating current: Using an empirical formula, it is given as I22dz = (0.15–0.3)In; generally, I22dz = 0.15In is used. 2) Operating time. Generally, it is taken as: t22dz = (10–25) s. IV. Prolonged Start and Positive Sequence Overcurrent Protection: The prolonged start protection function is activated when the motor stalls during startup or when the heavy-load startup takes too long; if the motor fails to start within the allowed time frame, this protection mechanism triggers a trip. If the motor starts normally, after the start-up is complete, the long-start protection automatically switches to the normal operating positive-sequence overcurrent protection (overload protection during motor operation). 1. Domestic comprehensive protection with long start protection (1) Calculation of the motor’s rated starting current. The formula is: Iqde = Ksta · In, where Ksta is the multiple of the motor’s rated starting current; generally, Ksta is set at 6–7. (2) Calculation of the motor starting time setting value. Calculated based on the measured normal maximum starting time, namely: tyd = (1.2–1.5) × tst.max, where tst.max represents the motor’s normal maximum starting time. For important motors with a longer start-up time, such as exhaust fans, intake fans, circulation water pumps, electric feed water pumps, etc., set tyd=30s. 2. Domestic comprehensive protection positive-sequence overcurrent protection (1) Calculation of the operating current setting value. The formula is: I1g1 = Krel·In, where Krel is the reliability coefficient, taking a value of 1.3–1.5. (2) Calculation of the action time setting value. Based on calculations to ensure the safe operation of the motor within its permissible overload duration: if the motor is allowed to operate for 420 seconds at 1.5Ie, and its self-starting time is 10 seconds, then a moderate value between these two figures can be chosen; that is, t1g1 = 30–60 seconds. For normal-phase protection, a value of t1g1=30-60s is generally suitable. V. Low-voltage protection for motors: (1) To ensure the automatic start-up of important high-voltage motors, it is necessary to install a low-voltage protection device with a 0.5s delay for motors of types I and II; the setting values for this protection are as follows: Operating voltage: Uqy=(0.6-0.7)Un; Operating time: tqy=0.5s. (2) In cases where the production process does not allow motors to start automatically when power is restored after they have come to a complete stop, such as feedwater motors, primary fans, exhaust fans, dust collectors, etc., a low-voltage protection device with a 9s delay should be installed; these motors will be shut down once this protection activates. The setting values for this protection are as follows: Operating voltage: Uqy=(0.4-0.45)Un; Operating time: tqy=9s. VI. FC circuits for high-voltage motors: In circuits where vacuum contactors are used instead of vacuum circuit breakers for high-voltage motors (such as the 6KV coal mills and ash pumps in our plant), these circuits are generally referred to as FC circuits. The FC circuit features a lockout protection trip output to prevent the vacuum contactor from breaking currents exceeding its allowable short-circuit current. Therefore, for a comprehensive protection system that has the function of tripping via FC circuit lockout protection, the setting calculation for phase current quick-break protection does not require taking into account the coordination between the operating time of the quick-break protection and that of the high-voltage fuses; no additional short delay is needed, and the inherent operating time can be used. Since the vacuum contactor in the FC circuit can only turn on and off the motor’s starting current and load current, it is unable to interrupt short-circuit currents that exceed its allowable breaking current value. For example, a vacuum contactor with a rated voltage of 6.3 KV can only interrupt short-circuit currents below 3800 A (the current that needs to be interrupted by the coal mills and ash pumps in our plant is 3200 A); when the short-circuit current exceeds 3800 A, high-voltage fuses must be used to break the circuit and eliminate the short-circuit current. High-voltage fuses used to interrupt the short-circuit current in voltage-driven motors should be able to reliably withstand normal operation and starting of the motor, possess a certain reliability factor, and also take into account compatibility with the protection action time. VII. Differential Protection with Ratio Braking for High-Voltage Motors: For motors with a capacity of 2000 KW or more, or for those that have phase-leading wires at the neutral point due to insufficient sensitivity of the current quick-break protection, differential protection should be installed as a means of protecting against interphase short circuits. The operating conditions of electric motors are essentially similar to those of engines; the longitudinal differential protection for engines should be able to handle the maximum unbalanced current resulting from external short circuits, while the longitudinal differential protection for electric motors should be able to handle the maximum unbalanced current that occurs during motor startup. 1. Calculation of the minimum operating current Iset (also known as the knee current): It is determined using an empirical formula: Iset = (0.3–0.4)IM.N/nTA = (0.3–0.4)In. Here, Iset represents the minimum operating current in amperes ； IM.N —— Rated primary current of the motor ； In —— Motor secondary rated current ； nTA —— Current transformer turns ratio. 2. Calculation of the ratio braking coefficient (slope) K: According to an empirical formula, K = 0.4–0.5