Setting safety standard for arc detection in solar industry
Unlike conventional electrical applications, most solar photovoltaic (PV) systems cannot contain arcs. Although alternative topologies using micro-inverter technologies are being developed with DC voltages below 80 V and a direct AC voltage output, the vast majority of solar PV systems available today utilize high voltage series DC circuits as they provide a superior cost per watt.
These systems use a central or string topology where many PV panels are connected in series to a central DC to AC inverter (Figure 1), which in a typical residential solar panel system carries 200-600 volts. These high voltages pose potentially serious safety issues including electric shock and fire. If there is faulty wiring or connectors, arcing can occur over high voltage DC lines.
This can lead to the installation being electrified shocking anyone who touches it, or cause a fire that can extensively damage the solar equipment and property. Consequently, arc detection is required between the inverter and the string of panels.
With the rapid expansion of solar technology both for small scale residential/commercial installations and large scale power generation using "solar farms," there has been a need to develop safety measures to address the potential dangers of high voltage DC arcing. This has led the solar industry in the USA to develop the UL 1699B photovoltaic arc-fault circuit protection standard to increase personal safety and protect equipment.
Key requirements of UL 1699B
UL 1699B stipulates the requirement for solar technology companies to include arc detection in high-voltage PV systems with a DC bus voltage of ≥80 V and <1000 V. The full development of UL 1699B is expected to be completed by the end of 2012. A similar standard in Europe is then expected to be implemented. Meeting this standard will undoubtedly pose a challenge for designers of solar inverters, converters, charge controllers, and standalone DC arc-fault interrupters.
Key standard requirements for arc detection systems include:
• An annunciator (i.e. siren or flashing light) to signal when arcing has shut down the system. A wired or wireless communication link may be necessary to enable remote detection e.g. to notify a utility company that an arc event has triggered the shutdown of the system
• A self-test circuit (initiated directly via a switch, or remotely) that is capable of simulating an arc event and shutting down the system if the test fails. The requirement for automated self-testing to verify that the arc detection unit is operating correctly is under consideration in order to ensure testing is carried out routinely
• A manual reset following tripping of the system. This presents a problem for developers who must balance the potential consequences of not detecting an arc (e.g. electrocution, and fires and destruction of equipment) and the cost of false arc detection and the unnecessary shutting down of the system (loss of power and expensive technician visits), particularly if located remotely.
How can an arc be detected?
This is not an insignificant problem and arc detection requires complex algorithms and the simultaneous evaluation of multiple filters to ensure false detections are prevented. In order to stop catastrophic arc events (fire and electrocution) the circuit must be broken immediately after the arc, which requires that the algorithms are executed as rapidly as possible.
Arcing events cause spectral noise in the nominal power signature of a string inverter over specific frequency bands (40-100 KHz), compared with when there is no arcing in the system (Figure 2). The spectral noise in the nominal power is determined from the digital signal which is converted from the DC voltage. A simple way of arc detection is to establish a baseline nominal power value for the system and then measure when the spectral noise level suddenly exceeds it. The major drawback of this approach, however, is that during power up arcs cannot be detected before the baseline has been established. As this is a requirement of UL 1699B, a detection algorithm should be included that performs without a baseline nominal power measurement.
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Furthermore, the AC inverters that tend to be used in string inverter topology produce a pattern of noise that is very similar to the pattern generated during an arc event (Figure 3). It is, therefore, difficult to differentiate between normal operating conditions and an arc event.
Figure 3: AC inverters generate similar noise levels during normal operation and when there is an arc event
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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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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
Meeting the UL 1699B standard
As the vast majority of existing solar installations run DC lines over 80 V, to meet UL 1699B when it is introduced they will require retrofitting with a fairly simple standalone arc detection unit (arc detection subsystem and circuit breaker) placed between the inverter and the string of panels. In contrast, in new installations arc detection and string inverter functionality can be integrated on the same processor cost-effectively. Texas Instruments’ (TI) high-performance F2803x and F2806x microcontrollers are well suited to high voltage solar applications:
• A high level of integration with full-featured peripheral sets minimizes arc detection and breakaway circuitry external components
• TI’s C2000 microcontroller platform offers the optimal balance of performance, peripherals, and memory
• Enables maximum system efficiency by providing sufficient capacity to perform additional tasks
Maximizing cost-effectiveness
A single arc detection unit using a high-performance processor can simultaneously analyze many strings. This enables the component count to be minimized, which lowers the cost of the system and reduces potential failure points (Figure 6).
Systems can be made further cost effective by combining arc detection and other solar application processing tasks on a single multiple/dual core processor. This reduces system complexity and provides addition processing power that enables greater efficiency/reliability. TI’s C2000 Piccolo microcontroller platform, for example, enables Maximum Power Point Tracking (MPPT) by providing a second core (the Control Law Accelerator [CLA]) that operates independently of the arc detection DSP core as it has access to the C2000 Piccolo microcontroller ADC and PWM peripherals. Having MPPT (which maximizes PV panel efficiency) and arc detection on a single processor adds considerable value to solar applications. TI’s SM73201-Arc-Eval Photo-voltaic arc detection system
Figure 7 shows TI’s SM73201-Arc-Eval Photo-voltaic arc detection system containing a C2000 Piccolo microcontroller. The SM73201-Arc-Eval Photo-voltaic arc detection system provides a number of important features that allow developers to create reliable arc detection systems capable of accurately identifying arcs without producing unwanted false positive detections (Table 1).
Table 1: Features of TI’s SM73201-Arc-Eval Photo-voltaic arc detection system
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New micro-inverter topology: no need for arc detection
As mentioned earlier, an alternative micro-inverter topology is becoming available that is aimed at residential installations (Figure 8). Each PV panel has its own micro-inverter and as the DC voltage is less than 80 V for each micro-inverter it falls outside the scope of UL 1699B as it can operate safely without arc detection. The key benefits of micro-inverter-based over string-based topologies are greater flexibility, ease of installation, and scalability.
Micro-inverters, however, have cost disadvantages i.e. they are costly to install and the cost per watt is inferior to string-based topologies. Consequently, until the cost of micro-inverters falls significantly, the majority of solar installations (99 percent) will continue to be developed using the string approach. Furthermore, the additional cost of complying with UL 1699B and the need to include an arc detection unit is minimal, compared with the overall system cost. As string inverters will continue be used in solar installations for some considerable time to come, developers with need to take in to consideration the requirements of UL 1699B when designing solar equipment.
Wider application of arc detection
Arc detection can be employed in a range of other areas where there is a safety risk associated with high DC voltages e.g. hybrid and electric cars. These vehicles utilize a 400-500 V battery pack and a significant amount of high-voltage DC. In an electric car, the high-voltage DC busses between the primary batteries and inverter stages are a potential cause of disastrous car fires (Figure 9). By including an arc detection system it is possible to monitor the high-voltage DC busses and ensure the safety of these increasingly popular vehicles.
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Many industries recognize that there is a need to optimize the safety of high voltage applications. The solar industry with the development of UL 1699B is leading the way with the introduction of a requirement for arc detection. As routine arc detection becomes an established principle in solar installations, other industries will no doubt follow suit.
About the author:
Brett Novak is marketing manager for C2000 microcontrollers and solar end equipment at Texas Instruments.
See related links:
Circuit protection design for photovoltaic power systems
Enhancing the efficiency of photovoltaic systems
Balancing safety and cost-effectiveness in solar-power inverter installations
Energy harvesting module suits thermoelectric, vibrational, photovoltaic sources
Major shakeout expected in PV supply chain
Europe accounts for over 70% of installed PV capacity in 2011
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