Open phase centralized protection for air-cooled m

2022-07-28
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Transformer air-cooled motor open phase centralized protection

years of operation experience has proved that a certain number of fan motors of air-cooled transformers are burned every year. Except for a few mechanical failures, most of them are caused by long-term phase failure operation or phase failure startup of three-phase power supply, and most of them are caused by breaking one phase to create closed-loop insurance of the industrial chain, or by loose insurance and poor contact. This benefit. There was no signal when the situation occurred. Only the motor stopped running during the inspection, but it was too late. The motor had been burned and the loss had been caused. Therefore, the signal can be sent in time when the motor just breaks phase, so as to avoid burning the motor

there are many types of motor phase loss protection, but most of them are single machine or multi machine protection with neutral point outgoing line. The other sequence protection is the most common, simple and reliable, but each motor needs to have another phase outgoing line. However, we usually have no neutral point outgoing line for fan motors. If each fan motor is equipped with a set of protection, it is both wasteful and unnecessary. We can only find a way on the total power circuit of each group of main transformer fans. Therefore, the total current change is very small due to the lack of phase during the operation of fan motors; It is difficult to detect, so we have to find a way from the qualitative aspect. Therefore, negative sequence current detection method is adopted. In order to ensure sufficient sensitivity, a transistor amplification link is used, and a delay of 3 When the air conditioner housing is in the circuit, the switch circuit is used to drive the relay

circuit working principle

1. Square root diagram (Fig. 1):

2. Circuit analysis:

① negative sequence current filter (Fig. 2):

when the motor operates normally, there is only positive sequence current, and the AC output voltage umn=ua+uc=0

when an unbalanced fault (such as phase failure) occurs, in addition to the positive sequence component, there is also a negative sequence current component in the three-phase AC power supply system. The negative sequence current component makes the m N two points have voltage output. Umn=ua+uc o, the negative sequence current generated by phase failure of a single motor is much smaller than the working current of all motors, but this negative sequence component can still be detected

② AC amplifier (Fig. 3):

because the negative sequence voltage detected by the negative sequence filter is very weak and the load generation capacity is poor, the emitter follower and AC amplifier are added. The signal is large enough to be sent to the lower threshold circuit after voltage doubling rectification and filtering

③ Schmidt circuit (Figure 4):

when there is a phase failure, the output bit current voltage of the voltage doubling strip current circuit is high to a certain extent, the Schmidt trigger acts to output a high level, which is sent to the R-C delay circuit. Note: W and WL are used to adjust the setting value

. ④ Time delay switch circuit (Figure 5)

r-c constitutes a delay element for anti-interference. When the voltage at both ends of C is charged until G5 is turned on, G6 is turned on successively, and j acts to send a signal. "Polypropylene" product is a uniform mixture of recycled carbon fiber and thermoplastic polypropylene fiber

d is the discharge diode when the device is reset

experimental conditions of the device after installation

location: Niulanshan No.1 tr main transformer air cooling motor as the object. This main transformer is equipped with seven groups of radiators, two fans in each group, 14 sets in total, and each motor is 0.15kw. The negative sequence output caused by unbalanced current during normal operation shall not be greater than 85mv. During operation, one TV loses phase, and the negative sequence output is not less than 160mv. The action signal voltage is set at 120mV. When starting the motor, the phase loss negative Hall output shall not be less than 250Mv

devices and have been used in artificial satellites, aerospace and energy storage. Adjustable range: 80 ~ 250Mv,

device power supply uses DC 12V, and the internal current resistance is obtained by partial voltage

transmission content

1. The fan shall not be misoperated when it is put into operation. The potential of each point is normal, and the relay is closed

2. Stop the fan, remove the one-phase fuse of a TV, and then turn on the fan power supply. The device operates reliably. After the 6S relay is released, its normally closed contact is closed to send a signal

3. The primary fuse of a fan is taken during the operation of the fan. The device operates reliably. After a 6-second delay, the relay is released, and the normally closed contact of the relay sends a signal (the normally closed contact is used to monitor the power supply of the device)

4. During the normal operation of the fan and the device, the three-phase motor (4.5kw) starts and stops. There is continuous and intermittent welding of the electric welding machine, and the device has no misoperation

the group test is done in a hurry, and the circuit is not deliberated again and again. The best value of each element of the negative sequence filter can still be found. The circuit should also be further improved so that it can automatically cut off the power supply when starting under the relay phase

in addition, this device can also be used for other types of single motor or motor group. It can send signals to protect the motor against asymmetric faults

see Figure 6 for the full picture

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