Safe, robust, repeatable and reliable solar energy 

May 28th, 2019, Published in Articles: Energize

Using innovative technology, the latest fixed-string voltage innovation in photovoltaic (PV) solar systems introduces a significant advantage over commonly deployed traditional string inverters. 

Before we look at the advantages offered, allow me to explain what is meant by a “fixed-string voltage”. By using a maximum power point tracking (MPPT) electronic device at each PV module, it is possible to produce a constant string voltage at the inverter, with the MPPT algorithm performed at module level, instead of inverter level only. The technical name for these MPPT electronic devices at each module is module-level power electronics (MLPE) and since we are discussing fixed-string voltage inverters, we can refer to them as power optimisers, which is a more descriptive term.

Each power optimiser will perform MPPT, with a buck or boost function at the module, depending on the voltage required by the inverter. No additional communication lines are required since the DC string wires carry the data as well.

Eco-on recently implemented the first integrated power management solution in South Africa which utilises the fixed-string voltage technology from SolarEdge Technologies. This means the client can now safely use the PV solar system whenever there is an AC power source available at the site, regardless if the source is from the utility supply or from a standby generator supply.

Why this technology? 

Instead of listing the range of benefits offered by the power optimisers and fixed-string voltage inverters, this article will focus on the reasons why owners should choose such a solution, rather than the more commonly deployed traditional string inverters.

Since each module is its own “string”, all available roof space can be utilised, even that one corner receiving a shadow for an hour in the morning, because the reduced exposure to sunlight on these panels will only affect the performance of those specific modules, and not the entire string. Roofs facing different directions can now share a string of modules, and each module will be optimally utilised. In summary, this innovative technology allows for significantly longer string lengths, irrespective of module orientation and partial shading conditions.

Fig. 1: Roof-mounted solar PV installation.

Although PV modules have an inherent temperature susceptibility, the power optimisers attached to the modules reduce the influence such temperature susceptibility will have on the string voltage as experienced by the inverter, or the total string length.

Inverters produce harmonics at the output, which affects the power quality of the attached power network. As in all of life, what you put in is what you get out. In the case of the fixed-string voltage solution, the inverter always sees a fixed voltage, and the DC-AC conversion is optimally designed for this fixed voltage. The result is a better power quality output, reduced stress on the DC-AC electronics, and most importantly, optimal DC-AC efficiency. All of this results in cleaner power from the inverters, while at the same time increasing the inverter reliability.

The effect of module replacement

It is important to consider the effects of a module replacement on the rest of the PV system. The development of PV modules inevitably results in better module efficiency. Ten years ago, the efficiency for reasonably-priced modules was roughly 15%, while today we are pushing towards 17% for similar quality modules. In real values this means a 300 W module then is effectively a 340 W module now. Replacing one 300 W module with a 340 W module ten years later is not too significant.

Let’s consider the instance where a commercial installation with 1000 modules has endured a severe hail storm and 50% of the modules need to be replaced. Bear in mind that in ten years the modules have lost roughly 10% of its capacity, so in reality, what you have is a combination of 270 W and 340 W modules in the installation. In a traditional string inverter setup, all the panels would be treated as 270 W modules, resulting in a loss of 35 kW (500 W x 70) installed capacity. The fixed string-voltage solution protects against this loss because the technology, together with the Eco-on design and application thereof, inherently allows for growth and expansion.

Safety

Ongoing debates about safety in DC strings for PV systems has offered many ideas and opinions on the best approach in dealing with the high string voltages in traditional string inverters. Lethal voltages, however, remain a constant threat when installing and operating these 600 to 1000 V strings. Apart from the lethal voltage, typical short circuit currents during installation and operation can result in a potential energy discharge of 6  to 10 kW. The resulting fire and associated risk can be devastating.

Fig. 2: Installation of inverters in cliet’s premises.

By contrast, the SolarEdge solution allows a maximum voltage of 1 V per power optimiser (at 300mA) if the string is not connected to a SolarEdge inverter, or when the inverter is not connected to an AC source. With a maximum of 30 power optimisers per string, this translates to 30 V at 300 mA, a mere 9 W of power. Ignoring all the other benefits and advantages offered by the fixed-string voltage solution, the improved DC safety feature is the principal reason Eco-on chose to design and implement installations with SolarEdge’s technology.

Monitoring system performance

Monitoring system performance, down to optimiser level, comes standard and, using the available data, can produce a variety of reports delivered with any chosen interval from daily to annual. This means that if a module fails, it can be identified quickly and easily without a maintenance team having to test each module to find it. Closer data analysis allows for pro-active maintenance, by identifying component fatigue before it results in failure, thereby striving for maximum system performance at all times.

An integrated power management solution is also possible. During a power outage, resulting from load-shedding or distribution network failure, when the standby generator is operating, the PV system will safely contribute to the load demand. The system also ensures that the diesel generator remains loaded to at least 30% of its nominal capacity to ensure optimum engine life and a high diesel-to-electric energy conversion efficiency. The integrated power management solution can be applied to various alternative energy sources and is not limited to a diesel generator only.

Contact Dean Marais, Eco-On, Tel 082 889-3026, dean.m@eco-on.co.za

 

 

 

 

 

Related Articles

  • Greater flexibility in electricity generation essential, delegates hear
  • Medupi magic fails Eskom and South Africa – the stuff of nightmares
  • Government urged to expedite IRP
  • Power developments in Africa, October 2019
  • Machine learning solves grinding mill liner monitoring