Wednesday, 30 September 2020

When the proprietor of a forward-looking business in the village of Žulová in the Czech Republic wanted to install four electric vehicle (EV) charging stations; Energon Advanced Energetics, a specialist in modern energy systems, developed a reliable and environmentally friendly solution based on products from Danfoss Drives.

Žulová is a village in the north east of the Czech Republic, close to the Polish border. It has a population of fewer than 1,500 people and, because of its location in a mountainous area, it has a notoriously weak connection to the power grid. This weak grid connection meant that providing four EV charging stations, two of which would be fast-charge DC types with a 40kW power rating, presented an interesting challenge for the design team at Energon Advanced Energetics.

Drawing on their experience with environmentally friendly power systems, the members of the team developed a hybrid solution with solar panels as a green power source to back up the supply from the grid, battery storage to ensure that adequate power would be available at night and in poor weather and an inverter system to provide DC to AC and AC to DC power conversion. For the inverter and associated items, the Energon engineers chose Danfoss equipment, taking into account Danfoss’ expertise in this field, and the strong reputation of its products for combining reliability, performance and efficiency with value for money.

The EV charging system developed by Energon for its customer in Žulová includes a 54kWp solar panel array and a lithium-ion battery bank with a nominal operating voltage of 736V and a capacity of 108.9kWh, together with a Danfoss NXA active front end inverter. The output from the inverter is fed via a Danfoss LCL filter to a power transformer that provides the supplies for the charging stations.

A power management system (PMS) with a colour-screen human machine interface (HMI) panel is provided to allow the operation of the system to be easily controlled and monitored. In addition, a small auxiliary uninterruptible power supply (UPS) is fitted to support “black start” functionality. This allows the system to be started from a fully powered down condition, even if no power is available from the supply grid. In these circumstances, the NXA inverter provides the magnetizing current needed by the transformer, thereby stabilizing the system and ensuring a smooth start up.

The system has two principal operating modes: on-grid, when power is available from the supply grid, and off-grid, when power is available only from the batteries and the solar panels. Transitions between the two operating modes are smooth, and they are invisible to the loads powered by the system.

When operating in on-grid mode, the system can draw power simultaneously from the supply grid, the batteries and the solar panels. This is effectively a peak-shaving arrangement that minimizes the load on the weak supply grid and also saves money by reducing the need to buy energy from the supply grid operator. Even when working in this mode, it is not possible to feed surplus power from the solar panels back into the supply grid, as this is not permitted by the grid operator. All surplus power is therefore used for charging the batteries and for supplying the end-user’s office equipment.

When working off-grid, the system operates in island mode and is entirely independent of the supply grid, drawing power only from the batteries and the solar panels. The microgrid operating mode provided by the NXA inverter, which supports frequency grouping, is active at all times providing effective local control over frequency and voltage in the absence of external references, and ensuring reliable synchronization when power from the supply grid becomes available again.

The hybrid system provides many benefits for the user. First, running costs are low because except at night and in very dull weather, the charging stations operate predominantly with “free” power from the solar panels. Second, even if the grid supply fails, the charging stations can continue to operate using power from the solar panels and the batteries. Third, as the system makes extensive use of clean solar energy, it is exceptionally environmentally-friendly.

This innovative hybrid power system for EV charging has been in use for some six months, at the time of writing. Its performance fully meets – and in some areas exceeds – the end user’s requirements and expectations. With the aid of Danfoss technology, it has brought reliable electric vehicle charging to a remote and mountainous area of the Czech Republic while minimising the dependence on the local power grid infrastructure.