Soft starter short circuit protection - Solutions - Huaqiang Electronic Network

Let's take the Danfoss 220kW soft starter MCD3220 as an example to discuss the short-circuit protection measures of the soft starter. The rated output capability of the MCD3220 is shown in the following table:

When the starting current is multiplied by 3 times, the rated motor full load current is 430A, and the estimated output current capability is: I _e = 430×3.0 = 1290 A Danfoss soft starter MCD3220 main circuit thyristor module is produced by EUPEC T588N16TOF, the relevant technical parameters are as follows:

Key data 1: On-state current RMS 1250A, which is approximately the same as the soft starter's rated output current capability 1290A; Key data 2: Single 10ms maximum peak effective current 8000A, that is, the maximum short circuit by thyristor natural commutation shutdown The current is not allowed to exceed 8000A; key data 3: The I ² t thermal withstand capability at the highest junction temperature (125 ° C) is 320000 A ² S, which is the short-circuit thermal withstand capability of semiconductor devices, the main reference data for short-circuit protection. The magnitude of the short-circuit current is how large the short-circuit current is, determined by the type of short-circuit and the internal resistance of the grid. The short-circuit type has a three-phase live line and a short circuit at the same time. The short-circuit between the two-phase live line and the short-circuit of the single-phase live line are short-circuited, and the short-circuit with the three-phase live line is the most serious. We analyze the short-circuit current in this short circuit.

Figure 2. Short-circuit current when different grid capacity and grid internal resistance. The short-circuit current when the three-phase live line is short-circuited at the same time is Isca: where S is the capacity of the grid (transformer); Z is the internal resistance of the grid (transformer plus cable). Figure 2 shows the value of Isca by taking the internal resistance of the grid as 5% and 2% as examples. When the MCD3220 is used in a potentially poor 500kVA capacity, 5% internal resistance, the short-circuit current also reaches 13222A, which is greater than the single 10ms maximum peak effective current of 8000A that the thyristor module T588N16TOF can withstand. Therefore, relying on the thyristor natural commutation shutdown can not be a reliable short-circuit overcurrent protection. Therefore, the Danfoss soft starter body only provides two overload protections for overload and transient overload. The figure below shows the tuning parameters of the Danfoss soft starter instantaneous overload protection.

The electrical devices commonly used for short circuit protection are circuit breakers and fuses, which are discussed below for their application. Discussion 1. If the circuit breaker is used for short-circuit protection and the circuit breaker is used for short-circuit protection, it should be selected according to the rated current of the motor 430A. If Schneider's products are selected, the NS630 circuit breaker should be selected. This circuit breaker can be equipped with two types of trip units, one of which is an electromagnetic MA (MAE500) type trip unit. It only has short-circuit current protection function. Because the MCD3000 has its own overload protection, this type of circuit breaker can be selected for the fastest action. The time is fixed for 20ms. The NS630 can also be equipped with another motor-specific electronic trip unit, the STR43ME (500A), which has short-circuit and overload protection. The short-circuit protection has a maximum operating time of 10ms. However, since the circuit breaker is a mechanical device and the mechanical device has a large motion lag, Schneider gives a total breaking time of about 60 ms. The total break time has nothing to do with which trip unit to use. In addition, there is also literature to estimate that the breaking of the circuit breaker is delayed by at least one power frequency cycle, that is, 20 mS. If a domestic circuit breaker is selected, taking Shanghai Huatong's products as an example, when the motor-specific SM30 630A or SM40 630A circuit breaker is selected, the fastest short-circuit protection time is fixed for 20ms, and the total breaking time cannot be found, but the total breaking time is not found. Since most of the domestic products are imitation imported products, it can be estimated that the actual total breaking time will not be superior to Schneider's products. The protection action delay of the above conventional circuit breaker is much longer than the natural commutation turn-off time of the SCR thyristor, and thus is not suitable for short-circuit overcurrent protection of the soft starter. Take a look at the advanced function of the Schneider NS circuit breaker, the reflex energy trip function, which is said to produce a strong gas pressure directly acting on a piston mechanism in the event of a short circuit fault, allowing the circuit breaker to be quickly and reliably disconnected. Since the action is very fast, <10ms, it has the function of naturally suppressing the short-circuit current like a fast-acting fuse. Its current limiting characteristic curve is as follows:

Figure 3. Schneider circuit breaker's reflex suppression of short-circuit current function. It can be seen from Figure 3 that the NS630 has a significant limit on the short-circuit current capability when the expected short-circuit current is >30kA or more, but 30kA is too large for the thyristor, and The action delay time also reaches half a power frequency cycle of 10mS, so the reflex function is not suitable for short-circuit overcurrent protection of the soft starter. In fact, Schneider's introduction of this feature is only positioned to improve the protection of power cables or busbars. Discussion 2: Short-circuit protection with a special fast-acting fuse for semiconductor protection (Required by Danfoss Manual) Danfoss recommends the quick-disconnect of 800A for the front stage of the MCD3220 soft starter. The fuse characteristics of the fast fuse are shown in Figure 3. The fuse action time is divided into two sections. The first four is the pre-arc time, which is the time during which the fuse dissolves. The current is the same as the expected short-circuit current. The specific value of the pre-arc time can be found in Figure 5. A fast fuse with rated current of 800A can be found. When the short-circuit current is 13222A, the pre-arc time is <2mS; when the short-circuit current is 99165A, the pre-arc time is < 0.1mS. The second segment is the arc breaking time, which is the time required to generate the arc and extinguish the arc, during which the current is continuously decreasing. Figure 5. Pre-arc time of the fast fuse

Figure 4. Action characteristics of the fast-blow fuse. Figure 5. Pre-arc time of the fast-blow fuse. In general, the total action time of the fast-blow fuse in the event of a fault is related to the thermal effect of the short-circuit current. Therefore, the key parameter of the fast-blow is not the action time. It is the cumulative current calorific value I ² t during the operation, which is consistent with the data index that the thyristor short-circuit protection needs to refer to. As long as the total I ² t value of the fast melt matches the I ² t value of the protected semiconductor device, effective short circuit protection can be achieved. The following table shows the pre-short-circuit protection fast-melting selection table required by Danfoss soft starters:

Danfoss specification requirements MCD3220 170M6012 BUSSMANN semiconductor fuse the front stage of the installation, the required short-circuit protection thyristor required I ² t value 320000A ² S, which is short-circuited thyristor T588N16TOF MCD3220 soft starter need protection using I ² t value. The data of the semiconductor fuse 170M6012 is shown in the following table:

Since the arc-break I ² t value given in the manual of BUSSMANN is the data of the grid voltage of 660V, it is necessary to look up the correction coefficient when calculating the grid voltage of 400V. From the right figure, the correction factor can be found to be about 0.61. Then available: 170M6012 in the 400V grid in the arc cut I ² t. Arc break I ² t = 465000×0.61= 283650 Total I ² t value of 170M6012 = pre-arc I ² t + arc break I ² t = 69500 ​​+ 283650 = 353150 This value and the I ² t value required for thyristor protection is 320000 similar. Conclusion: The fast fuses for semiconductor device protection provide short-circuit protection for soft starters. For the average customer, Danfoss recommended semiconductor fuses must be required for soft starter short circuit protection. The circuit breaker cannot meet the requirements of the soft starter short circuit protection. Soft Starter Model Fast-Fuse Type Fuse Holder Model MCD3007NGT00C-380V/125 (63A)sist101MCD3015NGT00C-380V/125 (125A)MCD3018NGTC1-380V/250 (160A)sist201MCD3022NGTC1-380V/250 (160A)MCD3030NGTC1-380V/250 ( 160A) MCD3037NGTC2-380V/400 (280A) sist401MCD3045NGTC2-380V/400 (280A) MCD3055NGTC2-380V/400 (280A) MCD3075NGTC2-380V/400 (400A) MCD3090NGTC3-380V/630 (500A) sist601MCD3110NGTC3-380V/630 (560A) MCD3132NGTC3-380V/630 (630A) MCD3185RST3-500V/800 (700A) Seatless MCD3220RST3-500V/800 (800A) MCD3300RST3-500V/1200 (1000A) MCD3315RST3-500V/1200 (1200A) MCD3400RST1-1000V/2000 (1600A) MCD3500RST1-1000V/2000 (2000A) MCD3600RST1-1000V/2000 (2000A) MCD3700RST1-1000V/2000 (2000A) MCD3800RST1-1000V/2000 (2000A)