Solar energy is no longer an emerging technology, but a technology that is undergoing major technological changes and is maturing. We are getting closer to grid parity—where the cost of solar energy is comparable to that of conventional energy generation types, and the composition of conventional energy generation types is improved—as the process of converting direct current in panels to usable alternating current becomes more efficient and Affordable.
But while solar panel prices have dropped significantly in recent years, the next wave of solar development will be driven by new technologies in power converter systems. The rise of advanced and complex multi-level power switching topologies will be based on silicon carbide (SiC) and gallium nitride (GaN) materials, coupled with higher operating voltages (up to 1600 VDC), enabling faster power switching, comparable to traditional systems. than, the performance will be greatly improved. Higher switching frequencies mean that the passive components of the power converter—that is, the induction coils and capacitors—can be significantly reduced in size, resulting in reduced weight and cost. Both are key advantages for the further expansion of the solar market.
As a result, these new power switching topologies are driving a revolution in devices that provide associated control and support. Smaller, faster systems require improvements throughout the power conversion signal chain—faster processing and better device integration. But as modern PV inverters get smaller, these innovations exacerbate the challenge of dealing with the important safety issues inherent in power conversion—that is, as these systems shrink in size, the physical isolation of hazardous voltages becomes more complex .
While the solar panel or solar module is the core and more visible part of the solar system, the more complex part of the entire signal chain is the photovoltaic inverter - the brain of the control system. Photovoltaic inverters need to be carefully designed to protect current measurement and calculation circuits from transients caused by power processing circuits and switching. However, this protection comes at a price: multiple redundant isolation devices increase cost and system complexity. And it's clear that the increasingly complex algorithms required to run these systems through programmable processors require code integrity considerations to ensure the security of the systems themselves.
Also, official security certification is a requirement faced by all developers. There are many regulations regarding safe disconnection (and reconnection) that must be followed. How quickly the system must respond, how to handle power outages and blackouts, quick disconnects and arc detection, all must be addressed—and in many cases, the solution varies from country to country. Since certification increases development time (cost), proven devices and methods are attractive and also need to be flexible enough to accommodate multiple evolving regional safety regulations.
Fortunately, these problems can be solved by using a power inverter platform that can integrate an advanced mixed-signal control processor and is surrounded by complementary isolated current sensing and gate driver technologies.
Redundancy - Single Fail Safe
For safety-critical applications such as AC monitors and isolators for solar PV inverters, safety standards require redundant monitoring elements in addition to monitoring equipment to ensure single-failure safety. In conventional PV inverters, this is accomplished by adding a supervisory processor to the system, which acts as a redundant supervisory element and then controls relay K2.
In traditional PV inverter control hardware, a separate monitoring processor is responsible for redundant safety element K2 and related monitoring. Both processors run some safety software and communicate via standard I/O facilities.
It is easy to see that this adds significantly to the overall cost of the system control hardware, since while the monitoring element actually contains a processor with good performance requirements, additional supporting infrastructure must also be added. On the other hand, the separation of redundant elements of this configuration is obvious, so this is an easy-to-understand security layout when a security agency conducts a qualified audit.
While PV inverter manufacturers seek to improve inverter performance, the global market's need to reduce the total operating cost of solar PV systems has put manufacturers under constant pressure and forced to intensify research to improve inverter topologies and shrink PV Cost of inverter safety critical components. Therefore, in order to reduce the cost as much as possible, the redundant monitoring element of the safety isolator has become a device that should be strictly reviewed.
The desire to simultaneously simplify and enhance inverter operation has driven Analog Devices to develop a series of innovative mixed-signal control processors, the ADSP-CM41x family. At the heart of the ADSP-CM41x design is a breakthrough independent dual-core safety concept that integrates safety redundancy and functionality into a single chip. This unprecedented architecture eliminates the need for external monitoring components (which is the current standard), saving considerable development time and system cost.
The new ADSP-CM41x addresses today's power conversion problems with a set of features specific to the requirements of renewable energy conversion systems, including integrated optimized hardware accelerators designed to increase core processing power. In addition, the device's on-board arc fault detection capability simplifies design and enhances safety through intelligent decision-making for improved reliability and accuracy.
By adding a separate M0 monitoring core to the main M4 control core on a single chip, the design of a single-fault fault tolerant system with redundant monitoring and control signal paths is significantly simplified while reducing overall system cost.
The dual-core design greatly simplifies the design of redundant safety elements by integrating an independent M0 supervisory core. The processors communicate through a dedicated mailbox system, including the transmission of heartbeat signals.
While the M0 and M4 cores are on the same wafer—minimum cost from a safety standpoint—the cores are physically separated through an innovative system fabric design. Interprocessor communication via dual-port RAM mailboxes enables independent checking and verification of redundantly obtained process parameters.
In addition to physical power security, care must be taken to ensure that the algorithms running these systems are properly interpreted; a compromised process can result in a security-compromised operating state. Additionally, it may be advantageous to isolate communications between processors using a mailbox communication system that separates processor core functions. The mailbox system allows arbitrary kernels to isolate read/write data at any time, rather than a direct send-receive handshaking communication method.
For code safety, the M4 core has 1 MB of flash and up to 160 kB of SRAM, while the M0 has 32 kB of SRAM. The M4 and M0 processor L1 SRAM, Flash, and mailbox memories are protected with zero-wait-state SECDED ECC and natively protect the 32-bit memory elements. Writing 8-bit, 16-bit data in place will result in automatic read-modify-write ECC updates in the background, usually with no observable processor stalls. The refresh-assist hardware can periodically handle single-bit errors. Multi-bit error detection can also be used to trigger interrupts and/or faults. Additionally, for error detection, a cyclic redundancy check (CRC) hardware module is used to calculate the CRC of the data block. It is based on a CRC32 engine that calculates the CRC value of a 32-bit data word passed to it. In particular, the CRC unit can be used to verify the flash content, a constant block of data (text or code) in SRAM.
As an example of how to take advantage of a dual core design, let's take a look at how AC grid monitoring works in a PV inverter. AC grid monitoring mainly includes two functions - frequency monitoring and voltage monitoring:
For frequency monitoring, time-based measurements with tight tolerance are required, which can be difficult to achieve when using an RC oscillator as a backup time base. Therefore, the processor uses a single oscillator or crystal (XTAL) as the main system clock (SYSCLK) input and uses other XTALs on M0 to monitor the main clock source drift through the mailbox. Clock failures other than drift on the SYSCLK line are handled directly by the internal oscillator comparator unit (OCU). It uses an external low frequency oscillator (LFO) to detect various conditions, such as clock disappearance and clock frequency overrun, and can generate several events to notify the processor of the violation. When a fault event is detected, the clock bad signal (CLKNG) can be configured to put the chip in a reset state, and it also initializes the GPIO pin safe state mechanism.
The AC voltage monitoring must ensure that the phase voltages are within the required tolerances and can also be used for functional self-testing of the two isolating switch relays. For single-fault withstand voltage monitoring, the processor's analog front end (AFE) consists of two separate ADC blocks, each with its own ADC controller, reference voltage, and multiple power supply paths. Of course, one ADC block is controlled by M4 and the other by M0 for fully redundant voltage measurement and integrity checking of the mailbox system. In addition to this, the on-board DAC can be used to individually apply all components of the AFE signal chain internally before networking the PV inverters.
Connecting the Devices - Photovoltaic Inverter Platform
In addition to mixed-signal processors, there are many other critical components in photovoltaic systems that need to be used together in order to communicate, control, and pass data and current securely.
The design employs a redundant signal path concept, including redundant references, ADCs, and XTALs, as well as an internal oscillator and a mailbox system between the voltage monitoring unit and the processor, allowing other external monitoring components to be completely eliminated from the monitoring system (Figure 3). The graphical LCD provides all relevant status information at a glance, and a complete calibration cycle of the entire system can be performed with the push of a button. The unit is equipped with an extensively validated software package and holds the VDE-AR-N4105 conformity certification issued by the German TÜV-SÜD in March 2016.
Analog Devices' VDE-AR-N-4105 Technology Demonstration Unit features two series-connected power relays that form the AC mains path, as well as redundant monitoring of AC power voltage, PV inverter output, and relay-to-relay voltage in a single-phase system. Four independent high-precision isolated voltage measurement channels.
The solar industry has a bright future, but continuous progress is not enough. The intelligent integration of various technologies at the full platform level will ensure the efficiency and safety of power converter designs. Each device must be specifically designed with the safety, efficiency and cost requirements of the energy market in mind. Providing a complete and robust platform—not just devices—will allow manufacturers of future power converter products to create clean, safe and affordable systems.
safety certificate
Since cost reduction efforts can easily weaken the required level of system security, Analog Devices has collaborated with Cologne-based German employers' liability insurance association BGETEM and implementers on how to integrate the monitoring element as a second processor on the same silicon as the main processor. TÜV-SÜED in Traubing collaborates to address potential safety issues. Another consideration is the minimum requirements that such dual-core processors must meet in order to comply with regulatory standards regarding the safe disconnection of AC power on PV inverters.
As a result, the new ADSP-CM41x processor family from Analog Devices now holds the VDE-AR-N4105 compliance certification issued by the German TÜV SÜD in March 2016 (document D8 16 0395142 002). The series is equipped with a set of features specific to the power conversion requirements of renewable energy conversion systems, including all necessary safety elements to form a fully safety compliant AC circuit breaker.
Additionally, to support safety, the Analog Devices power conversion platform is based on its iCoupler® digital isolator technology with integrated gate drivers and current sensors. ADI's digital isolators utilize low-stress thick-film polyimide insulation to achieve thousands of volts of isolation, and can be integrated with standard silicon ICs in single-channel, multi-channel, and bidirectional configurations.
