Low Voltage Three-phase Solutions

4/29/2020

As any installer may know, commercial solar projects are not created equally. Residential solar installers often have the need to expand their business portfolio into small commercial projects. Why is this?

The need for high powered PV systems at a low AC voltage has increased significantly in recent years. As a result, knowledge of the varying interconnection voltage types has never been more important. Knowing the suitable inverter for interconnection and compatibility is essential when designing the most performance optimal solution for solar customers.

Three-phase power is a three wire Alternating Current (AC) power circuit. Most US commercial buildings use a three-phase 4 Wire 208Y/120V power arrangement because of its power density and flexibility. Compared to single-phase, a three-phase power arrangement provides 1.732 (the square root of 3) times more power with the same current and provides (7) power circuits. *The reason why this voltage configuration remains the most popular in North America is because of its flexibility in handling different load capacities.

Low power loads (lights, etc.) powered using any of (3) 120V single-phase power circuits
Medium power loads (water heaters, etc.) powered using any of (3) 208V single-phase power circuits
High power loads (HVAC systems, etc.) powered using (1) 208V three-phase power circuit

Three phase inverter delta configuration solar system

Three phase inverter WYE configuration solar system

Three phase inverter high leg delta configuration solar system

How it used to be done

Installers who have made the transition from residential into small commercial projects typically want to use the same equipment that they were accustomed to using with their residential systems. Single-phase inverters like the Fronius Primo have a 208V and 240V setting single-phase. To use single-phase inverters on 208V three-phase service voltage an installer had to build the system using three of these inverters connecting one inverter per phase of the 208V service.

It’s critical that the installer established a balanced output on each leg of the three inverters. For example if they had a 15kW 208V system they could not use two 7.6kW Fronius Primos and one 3.8kW Fronius Primo because it would throw off the voltage output. Instead the system design would consist of three 5kW Fronius Primo inverters balancing the output from each of the three legs on each inverter. Utilities often require expensive additional control equipment to make all three inverters shut off which not only can drive up project costs, it’s also a huge inconvenience! In addition, the extra hardware components become yet another task O&M (operations and maintenance) teams need to take on. **Some utilities won’t allow the extra components at all.

The Fronius Symo Advanced commercial inverter

how it's done today

Fronius believes that there is a better way to serve the small commercial segment. With reliability, optimization and long-term flexibility in mind, the Fronius Symo (which services low voltage commercial three-phase system configurations) was created. The Symo 10kW (208/240V), 12.5kW (208/240V), 15kW (208V) can all be utilized for this low voltage commercial configuration (without the need for a transformer to step/down the voltage supply) providing solar installers with an efficient and cost optimal solution.

* http://www.oempanels.com/what-does-single-and-three-phase-power-mean

** https://www.civicsolar.com/support/installer/articles/208v-three-phase-pv-interconnection

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