Simple arc detection circuit
A simple arc detection circuit for a solar inverter comprises three main parts (Figure 4):
1. An analog front end (e.g. SM73307/73308): a current transformer that measures current on the panel strings, and acts as a bandpass filter and adds gain to the signal.
2. A fast high-resolution analog-to-digital converter (ADC; e.g. SM73201) then samples the signal and converts the voltage into a digital signal. Sufficient resolution and speed is required in order to detect arc events. UL 1699B stipulates that after an arc event a switch should be opened and the system shut down within 2 seconds. Sufficient resolution and speed can be provided by a16-bit ADC sampling at 250 Ksamples.
3. A DSP (e.g. Piccolo F2803x microcontroller) then processes the incoming digital signal from the ADC. Real-time capability is essential as complex algorithms are needed to process the digital signal in the frequency domain. Fast and efficient signal processing can be achieved using a 32-bit architecture. To establish whether there has been an arc event, changes in amplitude (peaking) of high frequency noise on the DC bus are detected by multiple Fast Fourier transform (FFT)-based filters
Figure 4: A simple arc detection circuit
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If an arc event occurs, front end relays are opened by analog outputs driven by a Pulse-Width Modulator (PWM) resulting in the PV panel strings being disconnected and the inverter shutting down. At the same time, the annunciator is triggered giving a visual or audible warning that an arc event has been detected and that the system has been shut down. A communications interface is provided by the digital outputs and enables communication with a remote controller, as well as remote self-testing.
Testing arc detection capabilities
Equipment is required that can generate an arc under controlled laboratory conditions in order to test arc detection reliability when developing an arc detection system. An arc generator will in most cases need to be constructed as they are not routinely available. An arc generation circuit requires a DC power supply that simulates the solar panel string, and a knife switch that generates an arc to enable the developer to test the arc detection system (Figure 5). To optimize the arc detection system’s ability to detect an arc, the detection algorithm will need to be adjusted. To ensure accuracy, environmental factors (e.g. temperature and humidity) must also be taken into account.
Figure 5: The basic architecture of an arc generation circuit
As dangerous voltages need to be generated (although the current can be low) to produce the test arc, safe guards must be in place to prevent the user being injured and the application under development being damaged. Measures include using a ballasting resistor capable of withstanding >200 W, and a long-handled knife switch surrounded by polycarbonate.