Design of short-circuit current detection and identification instrument for power system

When an abnormal connection occurs between phases or between phases and the ground in the power system during operation, a short circuit often occurs, and the loop current may exceed the rated current of the loop by many times, thus causing damage to the power system. To ensure the safety of the power system, the short-circuit fault part should be removed as soon as possible when this happens. To this end, measures to detect, identify and limit the short-circuit current can be adopted. Existing identification methods are mainly divided into two categories: software and hardware. Software implementation requires a series of sampling, conversion and filtering steps, which will directly affect the identification time of the short-circuit current signal. Hardware can be implemented using digital circuits and analog circuits respectively. Due to the lack of a sufficiently fast A/D converter and a sufficiently long filtering window, the digital circuit also has the problem of slow detection response speed, but it is easy to control; while the analog circuit does not have the problem of A/D conversion, so it can achieve rapid detection and identification of short-circuit current signals. Therefore, in practical applications, digital control and analog identification methods can generally be selected.

1. Detection principle and method

When analyzing the basic characteristics of short-circuit current in a power system, we can first assume that the system voltage is:

At the same time, assume that the resistance and inductance of the short-circuit loop are R and L respectively, and a short circuit occurs at t=0, and α is the phase angle at the moment of the short circuit.

Then the approximate expression of the short-circuit current id is:

The approximate expression of short-circuit current steepness did/dt is:

(3) Where Im is the amplitude of the periodic component of the short-circuit current:

φ is the power factor angle:

T is the time constant:

Typically, the power system has T = 0.05s (equivalent to φ = 86.36°).

Thus, when the short-circuit phase angle α=86.36°≌90°, the short-circuit current id does not contain a non-periodic component; and when α=3.64°≌0°, the non-periodic component in the short-circuit current id is the largest.

FIG1 shows the waveform of the short-circuit current (per unit value).

Figure 1 Typical short-circuit current waveform

As shown in Figure 1, the short-circuit current is generally composed of a periodic component and a non-periodic component. The phase angle at the moment of short circuit determines the size of the non-periodic component, thereby affecting the maximum peak value of the short-circuit current; but the steepness of the short-circuit current is mainly determined by the periodic component and is basically not affected by the non-periodic component. According to the above characteristics of the short-circuit current, the short-circuit current detection and identification instrument sets the current instantaneous value i criterion, so that as long as the current amplitude of any two phases of the three-phase current is judged, and when it is greater than 10 times the rated current, the switch is cut off and the power supply is stopped.

2 Hardware Design

The short-circuit current detection and identification instrument is mainly composed of a sensing module, a control module and a dedicated execution circuit. FIG2 shows a hardware structure block diagram of the short-circuit current detection and identification instrument.

Figure 2 System block diagram of short-circuit current detection and identification instrument

2.1 Sensor Module

The sensing module in this system mainly completes the sampling of short-circuit current signals and the judgment of current amplitude. This module is the basis for subsequent control processing. The primary current transformer and the secondary current transformer can convert the large current signal on the power line into a voltage signal suitable for the op amp. The voltage signal is in a certain proportional relationship with the large current signal. The A/D sampling circuit samples the converted voltage signal through the AD736 chip and converts it into a digital signal, which is then compared with the reference voltage signal provided by the digital potentiometer X9C103. The compared signal (low level) is sent to the microcontroller for interrupt processing.

2.2 Control Module

The core device of the control module, STC89C58RD+, has super strong resistance to power supply jitter. In addition, it also has a watchdog circuit inside, which also has strong resistance to electromagnetic interference.

After the signal (low level) after front-end comparison triggers the microcontroller interrupt, it can be controlled by a timer internally and the interrupt pin is scanned regularly. Once an interrupt occurs, the microcontroller outputs a control signal (high level) to control the dedicated execution circuit.

2.3 Power Module

Figure 3 is a block diagram of the power module. Since there is usually strong electromagnetic interference in the working environment of the short-circuit current detection and identification instrument, the AC filter can be used to prevent interference signals from entering the power channel and interfering with the microcontroller and other functional circuits. The power module can provide ±12V and 5V voltages, of which ±12V powers the sensing module and 5V powers the control module.

Figure 3 Power module schematic

3 Software Design

The software part of this system mainly controls the timer inside the microcontroller to scan the interrupt pin and then output the next level control signal. The microcontroller can display the fault information in real time through the interrupt response program. Since it is connected to an external LCD display, it can convert the voltage signal into current information through the internal algorithm and display it in real time. In addition, the microcontroller can also adjust the reference voltage of the comparator within the range of 8 to 10 times the rated current by controlling the digital potentiometer X9C103, so as to meet different needs in different working environments.

4 Conclusion

In this design, the sensing module is used to quickly detect and identify short-circuit currents; the embedded single-chip microcomputer interrupt system can quickly generate action signals to control subsequent circuits and cut off the switches on the power lines to avoid greater losses. The single-chip microcomputer can also provide a good human-computer interaction interface and can save fault information for analysis. In actual operation, this system works very stably and reliably.