EMC & Circuit Protection
Detecting hazardous arc faults
Michele Sclocchi highlights the technical challenges design engineers face when looking to detect potential arcs on photovoltaic systems and then automatically disconnecting each PV panel
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hotovoltaic system arc faults lead to system damage and potential fires. The National Electrical Code requires the inclusion of a new PV safety protection system with the possibility to manually disconnect the high voltage bus of the photovoltaic array. The technical challenges of detecting potential arcs on PV systems and automatically disconnecting each PV panel are significant. A standard PV system is defined as a
structure made of a number of parallel strings of series connected PV modules. All the strings are connected to a centralised PV inverter that has the double function of tracking the global maximum power point (MPPT) of the PV array and converting the DC power to AC in the grid. Figure 1 shows a typical residential photovoltaic roof top installation, where two strings of 12 panels are connected in series to obtain a maximum open circuit DC voltage slightly below 600V and a minimum operating MPP (maximum power point) voltage above 250Volts. The DC electricity produced by each string is fed by cables into a combiner box, and a cable from this combiner box feeds the DC electricity to the central inverter. The combiner box is used to facilitate testing of module strings, connect string fuses, surge overvoltage protection devices, and for housing additional electronic devices such as monitoring-communication, arc detection and remote disconnect switch devices.
Systems operating at voltages up to 600VDC with a considerably high number of cables and connectors increase the probability of electrical arcs across deteriorated or loosing connectors, cables and cold solder joins.
An electric arc is particularly difficult to detect and prevent in photovoltaic installations because the system operates with DC voltage arrays, with no zero cross current like AC systems, arcs to grounds leads to currents limited by the short circuit current of the panels and are difficult to detect and prevent with standard fuse or peak current sense protections. Three different types of arcs can be defined: the most common is the series arc where a loosing cable or connector suddenly disconnects while the current is flowing, less common is the parallel arc when there is a failure of the isolation between two cables with different potentials, or the arc to ground system. When an arc fault occurs, it’s possible to detect and disconnect part of the system to prevent potential fires. The PV industry has been reluctant about introducing dedicated safety solutions, however with the introduction of the NEC (National Electric Code) in the US in 2011, an arc detection device and respective counter measures are now required as a new safety requirement. Enforcement will happen within the next 2-3 years, other countries are expected to follow with similar requirements in form of codes or industry self-regulation. According to the NEC requirements the system should detect and interrupt “series” arc-faults in modules, connections, wiring, and other components in a PV System. It requires inverters, charge controllers or other devices in the arcing circuit to be disconnected and disabled, and requires manual resets and reconnects once an arc is detected and fixed. Arcing events in PV arrays are very difficult to detect because an electric arc
does not leave a uniquely identifiable electronic signature. In addition, the power lines of a PV array act as very effective antennas for a wide range of electromagnetic interference in the vicinity of the array; inverters also contribute additional noise that is inducted onto the power lines. A sophisticated signal processing approach is required to reliably detect the entire range of dangerous arcing events without false alarms when the PV array is operating under safe conditions.
The A/D converter must have enough dynamic range to process the high level CW signals while detecting the low level noise signal created by the arc. In order to eliminate the unwanted noise signals typically present in large PV system arrays, the signal is then fed to a DSP for signal processing and arc detection. The Arc detect signal can be output as a digital output or over UART or wireless signal. The entire system is able to detect parallel and serial arcs, to operate in the presence of noise due to switching power
Figure 2: Simplified system diagram of the arc detection reference design
National Semiconductor has developed a patent pending signal processing approach and multi-band dynamic filtering firmware; together, they detect the arc fault condition, and provide an alert to shutdown system power or disconnect part of the PV system arrays. Arcing present in a PV system creates random noise current in the cabling used for the PV string. The current noise of the arc itself has a Gaussian distribution with a spectrum extending to several MHz. Because of the geometry of the cabling in a typical PV system, the noise current density above 200kHz, varies significantly with frequency. We should also take into consideration that inverters used in PV systems generate high level noise in their switching frequencies, usually below 50 kHz in range. For these reasons, noise in the range of
frequencies between 40 kHz and 100 kHz has been used to detect a possible arc. The hardware has been designed to meet typical PV system requirements, such as 1000V maximum string voltage and string current up to 13 Amps DC. Figure 2 shows a system diagram of the
arc detection solution developed by the SolarMagic application team of National Semiconductor. To measure the current of the PV string and isolate the high DC voltage and current, an isolation transformer is used. The isolated signal on the secondary of the transformer is amplified and filtered to be fed to an analogue to digital converter.
electronics (inverters, power optimiser, etc) and special algorithms have been developed to recognise arc signals and to avoid false triggers. The arc detect reference design has
been architected to operate on a single string or to be integrated in a “smart” combiner box where several strings can be monitored and powered by the PV bus voltage. The arc detection signal can be used in various configurations to trigger the shut- down of the affected part of the array: through a PLC or wireless communication that disconnects each panel of the array/string, or alternatively through an electro-mechanical string shut down or simply through the inverter based centralised shut down switch. Figure 3 shows the spectrum in dB of the sensed current after filtering and sampling with arc and no arc condition. The arc is accruing in a PV string with a DC current of 12A.
National Semiconductor |
www.national.com
Michele Sclocchi is an Applications Engineer at National Semiconductor
For additional information regarding the arc detection reference design solution developed by National Semiconductor refer to the SolarMagic SM73201 DC Arc evaluation board and application note AN2154 written by Florent Boico and Chris Oberhauser, June 6, 2011.
Figure 1: Typical photovoltaic system with two strings and a combiner box with arc detection units 26 September 2011 Components in Electronics
Figure 3: Spectrum of digitised current of an arcing system versus not arcing system
www.cieonline.co.uk
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