System Design Considerations for DC-coupled Solar + Storage Systems

Issues to Think About When Desiging a Large Scale Solar + Storage System

Today, most solar + storage systems are coupled on the AC side.  DC coupling offers many performance and financial benefits to AC coupling, but system design needs to be carefully considered when attempting to harness those. 

If you are designing a DC-coupled solar + storage system where you are using DC-DC optimizers to connect between the PV and DC bus connected to the batteries and inverter, you are going to want to make sure the DC bus is protected from any harmful possible faults in the PV array such as arc faults, ground faults or lightning strikes (among others).  If the DC-DC optimizer does not have such protections built in, you are going to want to make sure to source additional balance of system (BoS) hardware that provides these protections.

At Alencon, we solve this issue within our DC-DC optimizer form solar + storage, the SPOT-ES, by providing a variety of protections in our device including cable arc and ground leak detection in addition to protection from lightning strikes as well as over voltage, current and temperate situations.  Alencon’s DC-DC Optimizer for Energy Storage, the SPOT-ES, is able to achieve these protections in a cost effective way as a result of the galvanic isolation built into the product by attaching an isolation transformer to each, individual PV string input. This unique feature provides all the benefits of DC coupling in a protected way while simplifying system design by assuring all the protections you need between the PV array and the DC bus are included in a single device.

 “The ability to isolate a troubled solar array while the rest of the solar and obviously the whole solar plus storage system continues to operate is a huge benefit. Galvanic isolation of solar from the rest of the system simplifies the fundamental protection approach on a system level. Since our system is battery centric, we always have a battery on the DC bus, meaning that the fault issues are much more severe when high battery currents are taken into account,” states Aleksey Toporkov, the President of ARDA Power, a pioneer in DC-microgrids.

Galvanic isolation is a unique feature of Alencon’s SPOT and it is not available from other utility scale DC-DC optimizer on the market today.

The benefits of the electromagnetic isolation provided by the SPOT-ES can be summarized as follows:

1. Robustness:
The SPOT-ES galvanically isolates the PV strings from one another and from the DC bus. Thus, any faults that occur in the PV array such as lightning strikes, ground leaks or arc faults will be confined only to one string and will not affect the inverter, battery or other PV strings. 

2. Fail-safe:
In the unlikely event of an electronic component failure (short of open), the SPOT-ES safely disconnects the failed input channel from the rest of the system.

3. Versatility
The SPOT-ES’s galvanic isolation of each input channel allows complete independence of the grounding scheme of input channels and the output to the DC bus. Each PV string can have negative, positive or floating grounding while the output can be independently grounded in the middle, negative or positive terminals.

4. Wide control range
The SPOT-ES can be configured to map any input voltage regulation range to any output voltage range. For example a 1500 V PV string can be connected into a 600 volt storage DC bus. On the other hand, lower voltage PV can be connected to a higher voltage DC bus.

5. Interconnect Simplicity
The SPOT-ES is very simple to connect into a DC-coupled Sola r + Storage system. The +/- PV cables plug directly into the input connectors and the DC bus cables plug directly into the SPOTs output connectors. With the SPOT-ES, there is no need to split the string in halves or use any other so-called “stretch” techniques.

6. Failure Pinpointing
In the case of a PV string fault such as a ground leak of arc fault, the SPOT-ES will indicate exactly which PV string is in a fault state as well as the severity of the fault.