New power electronic device to manage surplus solar power–Part II


From pv magazine Spain

pv magazine reported last week on a new power electronics technology that is claimed to be able to combine different storage technologies in a single electronic-based board interface. This week, we will reveal the details of the first pilot projects that were carried out with the new technology, which was developed through the EU-funded project ‘renewable penetration levered by efficient low-voltage distribution grids (RESOLvD).

The power electronics device (PED) has a module structure and a power of 75 kVA. It can integrate a heterogeneous array of different kinds of batteries and provide services related to the improvement of energy quality and the ability to integrate renewable energy in a low-voltage system.

The project coordinator, Luisa Candido, explained that two types of batteries were used–a 30 kWh lithium-ion battery based on lithium iron phosphate (LFP), and a 14 kWh lead-acid package. The hybridization of the batteries is aimed at maximizing the technical and economic performance of the devices and, at the same time, providing various services to the final users. “The main challenge is how to change from a traditional network to a more powerful network,” said researcher Ramón Gallart Fernández. “The pilot has low-voltage switches which work as a railroad switch to ‘move' electricity according to the needs of consumers.”

In the proposed configuration, the distribution system operator (DSO) repeatedly sends the PED a schedule of the operation for the next 24 hours, depending on the services it has to provide. This information is received by the management system of the PED, which is called Intelligent Local Energy Manager (ILEM), and more specifically by one of its modules, the power sharing algorithm (PSA).

The PSA is a non-linear model that solves the optimization problem based on the state, technical performance, and degradation mechanisms of each type of battery and distributes the energy demand among them. “Ultimately, it allows each type of battery to be operated synergistically, towards a minimum overall cost and a maximum battery efficiency,” the researchers explained.

The proposed system is claimed to be able to increase power quality through the reduction of phase imbalances and current harmonics, smoothing of rapid fluctuations in power flows, and the compensation of reactive currents. For these services, little or no active power exchange with the grid is needed from the batteries, which must provide a short-term response and high cyclability.

Other services, such as equilibrium–or temporary displacement of renewable generation based on the technical limitations of the network, electricity prices, and consumer habits–require seamlessly exchanging the energy stored in batteries every hour. For these services, the charge of lithium-ion batteries can be reduced by also taking advantage of the cheaper lead-based ones.

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The RESOLvD technology is being tested in the low voltage grid of a village called L'Esquirol, in the north of Catalunya, in Spain. This network is made up of two three-phase lines, which derive from two different secondary substations–SS-A and SS-B–and, in each of them, there are both supply points and photovoltaic installations. The two three-phase transformers of the secondary substations have their operation coupled in a ring configuration. The medium voltage/low voltage transformers are oversized with respect to the load in the area. This is because, a few years ago, the two substations were intended at powering two energy-intensive factories that now have been closed. 

Specific scenarios and use cases were considered for the L'Esquirol grid. First of all, the two radial lines were connected by a link cable that allows the creation of a ring shape. To remotely control and reconfigure the moving edges of the network, three fuses were replaced by motorized switches.

To analyze the energy exchanged between the batteries and the grid, and to evaluate the performance and efficiency of the technology, smart meters were installed at the beginning of the two feeders and the PED at the common coupling point. The PED and batteries, along with the measurement equipment, are the main hardware of the project. The former were developed and provided by the Universitat Politècnica de Catalunya (UPC) and installed on the SS-B substation. 

The PED and the lead and lithium-ion batteries were installed on the second floor. The remote terminal unit (RTU) is located on the ground floor so that it is also easier for the user to control the entire system. To support and distribute its weight on the ground, a metal support was built under the storage systems.

The batteries and the battery management system are electrically connected to the PED by a cable and the link from the low voltage switchgear and a feeder pass through the ground. The measurement components–phase measurement units (PMUs) and power quality meters (PQM)–were provided by Slovenian technology company Comsensus and were installed on both the low voltage and medium voltage side. These analyze the waveform of the energy as well as the voltage and current measurement inputs. The voltage and current ranges allowed by these devices require the installation of voltage and current transformers, as well as Rowosky sensors. 

All these technologies are used to evaluate the impact of the project on the distribution network, mainly with low voltage feeders of 400 V. A series of tests were carried out in the area, related to efficiency, planning, and quality of service.

To test the efficiency of the technology in the demonstrator, the improvement of the energy profile in the SS-A substation and the reduction of power losses due to a better quality of the waveform will be measured. Measurements both before and after the PED should quantify the improvement in energy management. As for the quality of the service, improvements of the voltage profiles, the prevention of critical events, and the operation in island mode will also be studied. For this, the PED and the batteries installed in the substation will be crucial to smooth voltage variations by exchanging energy with the grid, increasing or decreasing the voltage to compensate for the variation in consumption or generation.

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