Guodian Nanzi PSM692U microcomputer integrated protection
1 Principle description
1.1 Current quick break protection
The current of asynchronous motor is very large during starting, usually up to 5~8 times of rated current (Ie), and the starting time can be up to tens of seconds. The device is set with two quick break settings. During the starting process, the "mid start quick break setting value" is used. The value is set to avoid the starting current of the motor. After the motor is started, the "quick break setting value after starting" is automatically used. The value is based on the self starting current of the motor and the larger feedback current of the motor when the external outlet is short circuited, whichever is greater.
a、 The starting time tst is set to avoid a longer starting time, tst>tst.max.
b、 The setting value Iop. h during startup is set by avoiding the starting current Ist of the motor, namely:
When t ≤ tst, Iop. h=krel × Ist, in order to avoid the influence of aperiodic components, krel is taken as 1.5, Ist is (6~8) Ie.
c、 The setting value Iop. l during operation is set to avoid the self starting current and the large feedback short circuit of the motor in case of short circuit at the outlet of the zone. The size of the self starting current is related to the delay of automatic switching of the standby power supply and other factors. When the fast switching of the auxiliary power supply is successful, the motor almost has no self starting process, because the speed has not been significantly reduced, only when the residual voltage switching or synchronous capture switching, The motor speed has significantly decreased, and the self starting current will be larger. According to the traditional method, the self starting current Iast=5Ie, Iop. l=krel × Iast × Ie=1.3 × 5 × Ie=6.5Ie.
The three-phase short circuit at the outlet of the zone considers the inherent delay of protection (40~60) ms, and the feedback current Ifb=6Ie.
Iop.l =krel×Ifb=1.3×6Ie=7.8Ie。
d、 The short time delay of quick break protection is used to cooperate with F-C circuit.
1.2 Definite time overcurrent protection
When the three-phase current IA, IB and IC of the motor are greater than the setting value of overcurrent protection, it will pass the time delay outlet.
The overcurrent setting can be set according to the starting current, generally (1.2~2) Ie. The time delay is set to avoid the starting time of the motor.
2.3 Two section definite time negative sequence overcurrent protection/inverse time negative sequence protection
When three-phase voltage imbalance, phase failure, reverse phase and inter turn short circuit occur in the motor, negative sequence current will be generated.
The positive sequence current is I1 and the negative sequence current is I2. In order to prevent external fault or imbalance of external power supply system, the feedback negative sequence current of motor may cause false operation of negative sequence overcurrent protection.
According to the different ratio of I2/I1 when asymmetric short circuit occurs inside and outside the area, the negative sequence overcurrent protection can be blocked when the following conditions are met: I2 ≥ 1.2I1, where I1 is positive sequence current and I2 is negative sequence current.
Guodian Nanzi PSM692U microcomputer integrated protection
1.4 Overheating protection
Motor overload, too long starting time, locked rotor, etc. will produce large positive sequence current; In case of phase failure, asymmetric short circuit and asymmetric input voltage, large positive sequence and negative sequence currents will be generated at the same time. According to the heating characteristics caused by the positive sequence and negative sequence currents of the motor stator, overheating protection can be provided for the above faults.
Wherein: τ - overheat time constant.
I ∞ - large current value that allows the motor to operate for a long time, generally set as 1.1
Ip - load current before overload, if it is in cold state before overload, Ip=0
Select one of the above two curves for calculation. When the heat accumulation value reaches τ, the device trips.
1.5 Locked rotor protection
The rotor may be locked due to mechanical failure, excessive load, low voltage and other reasons. The motor with locked rotor under full voltage has large current and is particularly easy to burn out.
The device calculates the positive sequence current according to the collected current of each phase. When the positive sequence current is greater than the fixed value of locked rotor current, the protection will trip after delay.
The locked rotor protection does not quit operation when starting, so the locked rotor protection delay is greater than the motor start time.
2.6 Single phase grounding protection
The measurement range of zero sequence overcurrent is 0.050~30A (secondary value), which is used for indirect grounding system.
When 3I0 is greater than the setting value of zero sequence overcurrent, the protection will trip after delay.
2.7 Low voltage protection and PT disconnection locking
When the motor power supply voltage decreases for a short time or recovers after a short interruption, the secondary motor needs to be cut off to ensure the self starting of the important motor.
When the three line voltages Uab, Ubc and Uca of the input device are lower than the low voltage fixed value at the same time, the low voltage protection acts and acts on the outlet through delay. To prevent maloperation of protection due to PT disconnection, PT disconnection locking is set. In case of PT disconnection, the device will send alarm signal and block low-voltage protection.
The setting of low voltage protection setting is set to avoid the * voltage when the group motor starts automatically.
When the bus is not powered on, the low-voltage protection will act. To avoid this situation, the device is set with low-voltage opening conditions, which must be met before the low-voltage protection can be put into operation. This condition can be set by the user for input (use) or exit (not use).
Low voltage opening condition: one of the three line voltages is greater than 80V, and the delay is 100ms. Once this condition is established, the low-voltage protection is effective. When the low-voltage protection action trips, the device will automatically invalidate the low-voltage opening condition after a 10s delay, and the low-voltage protection will return at the same time. The device uses two methods to identify PT disconnection.
