| symbol | name | Performance parameter |
| T | transformer | 1000kVA, Uk=4%, IN2=1445A |
| M1 | Motor 1 | 100kW, IN=182.4A, Ks=6.5 |
| M2 | Motor 2 | 180kW, IN=329A, Ks=5.8 |
Analysis
1Select motor protection circuit breaker Q3
Since IN=182.4A, a circuit breaker with a rated current of 200A can be selected. Since Ik3 = 19.12 kA, a circuit breaker with a rated on-off capability of 20 kA should be selected. The grid voltage is 380V, so the rated voltage of the circuit breaker is 380V. Therefore, a circuit breaker with a short-circuit breaking capacity of 20kA or 50kA can be selected.
The setting value of the circuit breaker is:
Long delay action current setting value is 200A, instantaneous setting current Iin=12IN=12×182.4=2189A
When a single short circuit occurs at the end of the line, I Ìk4=4.9kA
I Ìk4/Iin=4900/2189=2.238>1.25
Therefore, the circuit breaker Q3 can play a protective role.
Considering that the motor M1 is lightly loaded, when the 6 times long delay action current setting value is reached, the return time can be taken as 5s.
2Select distribution circuit breaker Q2
Because Icl = 600A, a circuit breaker with a rated current of 630A can be used. Since Ik2=29.7kA, a circuit breaker with a short-circuit short-circuit breaking capacity of 30kA should be used.
Looking for a selection manual, the 630A circuit breaker has no such large delay and short-circuit capability. Therefore, the DW15-1000 circuit breaker can only be used across the stage. Its short-circuit short-circuit breaking capacity is 30kA, and the 630A overcurrent release is used.
Circuit breaker Q2 is the second stage of the power distribution system, so the short delay time is 0.2s.
Short delay action current setting value
Isd ≥1.1 (Icl+1.35ksIN)
=1.1 (600 + 1.35 × 6.5 × 182.4)
=2421A
It can be set at 2800A, which is about 4.5 times that of the trip unit.
The return time of the 3 times long delay action current setting value is 8s.
Iin=1.1×19.12=21.03kA. Here, the short-circuit current at the Q3 incoming line is also 19.12kVA, which can be set at 22kA.
3Select circuit breaker Q1
Because Q1 is the main protection switch of the transformer, the secondary side rated current of the transformer is IN2=1445A, so the DW15-1600 type circuit breaker is selected.
However, due to its delay on-off capability of 30KA, it can not meet the requirements of Ik1=31.5kA. It is necessary to select DW15-2500 circuit breaker across the stage, its delay on-off capability is 40kA, and its instantaneous setting current is set at 1.1×29.7. =32.67kA, because this value is less than the DW15-2500 circuit breaker delay short-circuit breaking ability, so the instantaneous release can not work normally.
However, this solution is not economical. On the basis of cost savings, the DW15-1600 circuit breaker can still be selected, and the instantaneous operating current can be set at 30kA. When the short-circuit current is greater than 30kA, let the circuit breaker be disconnected instantaneously.
The short circuit delay time can be 0.2+0.2=0.4s
The short delay action current setting value is:
Isd ≥ 1.1 (1445 + 1.35 × 5.8 × 329) = 4423 (A)
Can be set at 4500A.
Long-delay action setting current setting value can be set at 1600A, and the return time of 3 times long delay action current setting value is 15s.
4Select circuit breaker Q4
The circuit breaker is characterized by direct installation close to the transformer and a large short-circuit current of 31.5 kA. Motor M2 is an infrequent start of the air compressor. It is more economical to use a circuit breaker that can be directly started and short-circuit protected instead of a fuse + contactor, or a general circuit breaker + contactor.
The DWX15-400 current limiting circuit breaker can be used satisfactorily. Its short-circuit breaking capacity is 50kA. It has an electromagnetic operating mechanism and can be operated in the same way as a contactor.
The long-delay action current setting value of the trip unit can be adjusted by 0.85 times the rated current.
Ie 400×0.85=340A
The instantaneous action current setting value is 12 times the rated current of the circuit breaker, that is, 12×400=4800A. The air compressor is a light-load starter, and the return time can be 5s when the 6-time long delay action current value can be selected.
BIOTEPT Farm-duty AC motors, also known as agricultural fan motors, have a high starting torque that allows them to start under large loads. They provide reliable service in dirty and rough environments and are commonly used for crop drying, running poultry equipment, manure pumps, augers, and conveyors. Motors have enclosures that help protect them from moisture, but they are not airtight or waterproof.
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Ningbo Biote Mechanical Electrical Co.,Ltd , https://www.biotept.com