At present, there are two main problems in this type of protection configuration:
(1) For motors of 2000 kW or more, differential protection needs to be configured. Therefore, in the case of the inverter dragging the motor, the motor differential protection is withdrawn and the reliability of the protection is affected.
(2) At any time, only one transformer protection device and motor protection device are put into use, which reduces the efficiency of the device.
3 inverter motor differential protection
In the case of using the inverter to drive the motor, the conventional motor differential protection cannot be used because the motor terminal CT is the current frequency of the CT1 at the switchgear in Fig. 1 and the CT3 at the neutral side of the motor, that is, CT3. Not the same. The literature proposes to use magnetic balance differential protection to achieve, but there are several problems in practice:
(1) At present, the electric motor used in the power plant basically cannot provide the neutral side cable extraction required for the magnetic balance differential.
(2) The magnetic balance differential current is below the inverter, non-power frequency current. For microcomputer protection, the fixed value according to the power frequency 50Hz is not applicable to non-power frequency conditions.
Since the current on both sides of the differential protection must be the current at the same frequency. It can be considered to install a set of CT under the inverter and above the motor, namely CT2. This group of CT can be installed in the inverter cabinet, and the two sets of currents of CT2 and CT3 constitute differential protection.
Conventional differential protection is a phasor differential. The principle is to use the Fourier algorithm to calculate the real imaginary part of the inflow and outflow currents according to the sampling point of one cycle, and then calculate the amplitude and phase of the differential and braking current. The criteria are constructed by means of phasor comparison. Since the current is not 50 Hz, the frequency tracking is required to ensure the correctness of the calculation when performing Fourier calculation. Since no voltage is introduced under the inverter, it cannot be realized by the conventional voltage tracking frequency method. Some manufacturers have proposed to use the current tracking frequency, but because of the large error of the current tracking frequency, it is easy to cause the misoperation and rejection of the protection, which is not used in practice.
For the differential value of the sample used in the differential protection, all the channels in the microcomputer protection are sampled as the instantaneous value of the current at the same time: when the protected device has no lateral internal fault, the sum of the sampled current values is zero; when the internal occurs In the event of a fault, the sum of the values of the sampled currents is not zero. The sampled value differential protection is formed by using the sum of the sampled currents according to certain motion criteria.
Compared with the conventional phasor differential protection, the sampled value differential has the characteristics of fast action speed and small calculation amount. It is a breakthrough in the field of microcomputer differential protection and has been applied to the protection of the mother and transformer. The sampling value difference does not involve the Fourier calculation, and the harmonics brought by the inverter will not affect the calculation accuracy. Therefore, for the high-voltage variable frequency motor operating at 25-50 Hz, the differential protection can be realized by the algorithm.