Texas Instruments AN-2154, Application Report

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Vref
Vdig
Vana
PV
String -
Controller
Vana
Vref
Vana
Vref
Array -
Vana
Vin
ADC
Filtering
Vdig
UART
& I/O
Regulation
Gain & Filtering
1:100
16 Bit/250 KSPS
Impedance
Match &
Clamping
FFT(DSP)
Test Signal
Generation
Arcing
Non-arcing
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System Implementation
The 50 to 100 KHz spectrum is used by services such as maritime radio navigation, and standard time
services. The large loop area of a standard PV cabling system may intercept these low frequency signals
and create line current many times that of the noise signals caused by arcing. In addition, the switching
noise from the inverter or other electronics present on the PV systems may also create noise sources at
this frequency. An effective way to eliminate these signals is by digitally processing these signals after A/D
conversion. However, in order to eliminate unwanted signals digitally, the signal chain including the A/D
converter must have the dynamic range to process the high level CW signals while detecting the low level
noise signal created by the arc. This drives the selection of a 16-bit ADC which has a dynamic range of
96dB. This enables the signal path to handle CW signals 93 dB greater than the noise that needs to be
measured for arc detection purposes.
Figure 2 shows the spectrum (10kHz-130kHz) in dB of the sensed
current (after filtering and sampling) with arc and no arc condition. The arc is occurring in a PV string with
a DC current of 12A.
Figure 2. Spectrum of Digitized Current on SM73201
12 System Implementation
The arc detection board is comprised of a transformer, where the string current flows through the primary
and that reacts to AC noise present on the string line. The signal appearing on the secondary is amplified
and filtered to be fed to an A/D converter. The signal is then fed to a microcontroller for signal processing
and arc detection. The board also includes a self test circuit which produces a signal that mimics an arcing
event.
Figure 3. System Diagram
13 Installation
The board is designed to fit in a single string configuration as shown in Figure 4 . While the RD-195 can be
installed at any point in the string, recommended connection is on the negative side of the string for safety
consideration. Arrays containing multiple PV strings can be serviced by connecting one arc detection
board per string.
7 SNOA564F – June 2011 – Revised December 2012 AN-2154 RD-195 DC Arc Detection Evaluation Board
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