Hardware design of DC insulation monitoring module based on Cotex-m3

1. System insulation detection principle

1.1 Balanced bridge-unbalanced bridge detection method

The balanced bridge-unbalanced bridge detection method is realized by simulating the balanced state and the unbalanced state. The principle of insulation resistance to ground detection is shown in Figure 1.

When it is necessary to detect the insulation of the DC system, first control K1 to close, and K2 and K3 to open. The CPU collects the CL+ to ground voltage UL1 and the CL- to ground voltage UN1, and obtains formula (1):

Note: R1: Balance bridge CL+ resistance to ground; R2: Balance bridge CL- resistance to ground; R5: CL+ resistance to ground; R6: CL- resistance to ground.

Then, the CL+ voltage to ground and the CL- voltage to ground are compared. When the CL+ voltage to ground is greater than the CL- voltage to ground, the CPU controls K2 to close, collects the CL+ voltage to ground UL2 and the CL- voltage to ground UN2, and obtains formula (2):

Note: R1: balanced bridge CL+ resistance to ground; R2: balanced bridge CL- resistance to ground; R3: unbalanced bridge CL+ resistance to ground; R4: unbalanced bridge CL- resistance to ground; R5: CL+ resistance to ground; R6: CL- resistance to ground.

If the voltage of CL+ to ground is less than the voltage of CL- to ground, the CPU controls K3 to close, collects the voltage of CL+ to ground UL3 and the voltage of CL- to ground UN3, and obtains formula (3):

Note: R1: balanced bridge CL+ resistance to ground; R2: balanced bridge CL- resistance to ground; R5: CL+ resistance to ground; R6: CL- resistance to ground; R3: unbalanced bridge CL+ resistance to ground; R4: unbalanced bridge CL- resistance to ground.

The CL+ ground resistance R5 and CL- ground resistance R6 can be calculated by formula (1) and formula (2) or formula (3).

1.2 Introduction to overall system functions

The overall block diagram of the DC insulation monitoring module is shown in Figure 2:

After receiving the command from the background monitoring, the system starts to detect the insulation status, collects relevant voltage data by controlling the switching of the balanced bridge and the unbalanced bridge, and sends it to the CPU after processing. The CPU processes the data according to the balanced bridge-unbalanced bridge detection method to obtain the actual DC bus insulation value to the ground, and then sends it to the background through communication. When the insulation value reaches a certain range, an alarm is issued to the background, and the background prompts relevant personnel to handle it.

2. System hardware design

2.1 System Core Circuit

This system uses ST's STM32F107RC processor, and mainly uses the chip's USART, CAN, SPI bus, I2C and GPIO functions. The core circuit structure of the system is shown in Figure 3:

The system communicates with the host computer system through the RS485 interface to transmit the insulation status of the DC system in real time; the EEROM memory expanded by I2C is used to store some system given values; the reset circuit ensures the reliable and stable operation of the system, and the open input and output are used to detect some system states and give timely alarms; the system insulation detection circuit is mainly implemented by external ADC+optical coupler+voltage divider bridge arm resistor.

2.2 Insulation detection circuit

The insulation detection circuit mainly uses the balanced bridge-unbalanced bridge principle for detection. The specific detection circuit structure is shown in Figure 4:

Different voltage states are detected through the balanced bridge-unbalanced bridge circuit, amplified by the operational amplifier circuit, input into the ADC to convert the analog voltage value into a digital voltage value, and then enter the CPU for calculation and processing through optocoupler isolation to obtain the actual ground resistance value and give an alarm.

3. Experimental results

Connect a precision resistor between CL+ and PE or between CL- and PE, and use the hardware detection circuit to detect the value of the connected precision resistor. The results are shown in Table 1:

Through actual testing and verification, the detection accuracy in the range of 8KΩ to 100KΩ can reach 5‰, which can fully meet the system requirements.

4. Conclusion

The hardware circuit design in this article is used to measure the insulation resistance of the DC system to ground. It has high detection accuracy and reliability, has a positive impact on the safe operation of the power system, and provides great convenience for on-site operators.