Transformer DC bias online monitoring systemEquivalent to the direct current component forced into each phase winding of the transformer through the neutral wire. Similarly, expressions for the peak value of magnetic current, effective value of excitation current, fundamental frequency component, and various high-order harmonic components can also be derived. When analyzing, it is assumed that the transformer is operating at the rated voltage U, and the peak value of the rated no-load excitation current is used as the base value for calculating the current. Table 1 shows the correlation characteristics between the neutral line DC current and the excitation current of the transformer.

System functions

1. Real time continuous monitoring of AC and DC currents flowing through the neutral point of the transformer;

2. Monitor and record parameters of transformer status, timely and effectively track changes in transformer status, effectively assess the condition of monitored equipment, and provide early warning for latent faults.

3. The data measurement speed is once per second, and the sampling interval can be set and changed through the system platform.

4. The system has multiple output interfaces and interfaces that can be connected to other monitoring systems according to a unified communication protocol; The system should also have multiple alarm output interfaces, which can be used to trigger alarms through other monitoring systems or connected to conventional alarm devices.

5. The device structure of the system is simple, easy to use and maintain, and has good stability and reliability in the electromagnetic interference environment on site;

6. The system software has a good human-machine interface, simple operation, and is easy to use.

7. The system provides self checking function and uploads the self checking results (status information) according to requirements; Being able to correctly record dynamic data and establish event identifiers correctly in case of system anomalies.

8. The service life of the system should not be less than 10 years.

9. Capable of automatic acquisition of equipment status parameters, signal conditioning, analog-to-digital conversion, data preprocessing, and transmission to upper level units;

10. Capable of receiving control commands such as parameter configuration, data summoning, timing, and forced restart from upper level units;

11. Without affecting the safe operation of on-site equipment, the grounding down lead should ensure reliable grounding and meet the corresponding current carrying capacity.

Main technical indicators

Working environment

Working voltage: AC 220V ± 10%, 50Hz

Power consumption:<5W at rated voltage

Working environment temperature

Indoor system: -25 ℃~+55 ℃

Outdoor system: -40 ℃~+70 ℃

Working environment humidity: ≤ 95%

Monitoring parameters

Measurement values: DC (average) current, AC/DC effective value

Current measurement range: AC/DC current, peak value 0~± 200A

Measurement accuracy: 1%

Resolution: 0.05%

Data measurement speed: 1 time/second

Data storage category: time, current

Data storage capacity: 90 days

Other technical indicators

System capacity: 80 local modules

CAN network structure, network speed:<500Kbps

RS485 structure, network speed: ≤ 2400bps

The system has a lifespan of over 10 years.

The average working time (MTBF) of the system: indoor: greater than 5 × 104 hours. Outdoor: More than 3 x 104 hours.