AC Coupling with Microinverters

As solar adoption accelerates, batteries are becoming a more common addition to photovoltaic installations. According to Bloomberg, the demand for residential energy storage is increasing in the US: more capacity has been installed in Q2 2018 than in all of 2017.

Chilicon

This article will discuss the advantages of using microinverters with AC coupling for residential, single-phase applications. The two leading microinverter manufacturers in the U.S. market include Enphase Energy and Chilicon Power. Both microinverters manufacturers are headquartered in California and have pushed the envelope with AC coupling compatibility. For more information regarding the basics of AC coupling, see HERE for design considerations. If you’re interested in learning more broadly about retrofitting solar PV with battery backup, see HERE. This article covers the basics of adding energy storage with microinverters; moreover, it discusses the methods Enphase and Chilicon use to communicate with battery inverters. Further, battery adoption will likely continue to accelerate as utilities impose more time-of-use rates, cap net metering interconnections, and add demand charges.

 

AC-Coupling with Microinverters

For homeowners interested in maximizing their production on a module-level and or those who would also like an energy storage system to take advantage of a TOU rate and/or backup power solution, AC-coupled storage systems with microinverters are a great option. In this type of system, microinverters are attached to the modules and the branch circuits or strings are combined at a gateway combiner box. They are then fed to a critical loads panel which is also connected to a battery inverter. This battery inverter is responsible for managing the flow of energy to the batteries while also mimicking the frequency of the grid in case of an outage to allow for continued PV production. Hence, when the grid goes down, the battery inverter uses an internal Automatic Transfer Switch (ATS) to isolate the critical loads panel from the grid. You can also configure the system to have an external ATS which feeds the entire main panel in the event of a grid outage. This would eliminate the need for a separate critical loads panel. An external ATS would be required when a customer is looking to power all the circuits in the home; however, it does add extra cost and complexity.

One advantage of using Enphase or Chilicon in an AC coupled system is that microinverters allow for the flow of power to be divided between the backup system and the grid. Thus, the use of microinverters allows for flexibility in the size of storage capacity. For instance, some branch circuits can connect to the sub-panel that the battery inverter feeds while others can exclusively connect to the main panel. Even when some of the microinverters are connected to the battery, the entire array is eligible for net metering credits. However, if the grid goes down, the microinverters that are connected at the main panel will stop producing as they are isolated from the critical loads panel which is being fed by the battery inverter.

Chilicon gateway
The new Chilicon Gateway.

It’s important to keep in mind that the battery inverter needs to be sized for the max AC output from the PV array that is connected to the critical loads panel.  We want to make sure that all the AC output from that array can pass through the battery inverter system. When designing these systems, select the number microinverters that is equal to or less than the battery inverter’s kW capacity and land the rest of microinverter array on the main panel.  If you are providing whole house back-up, you can achieve this same result with AC relays on the microinverter branch circuits to open when the grid is not present, thus reducing the effective off-grid PV array size.

Currently, both Chilicon and Enphase are brand agnostic towards storage inverters, but some work better than others. Installers can use any backup system that fits one’s budget, technical constraints and design preferences. With Chilicon and Enphase, the main feature to keep in mind is where the battery inverter can shift it’s frequency based on the state of charge of the battery bank. This allows the battery inverter to control the PV array output when in off-grid mode. Outback is a popular battery inverter that can’t frequency shift, but it can trigger a relay to knock the PV system offline, thus protecting the battery from overcharging.  In this case, the PV system would shutter on and off based on the battery bank’s state of charge. Put differently, this scenario equates to trying to fill a cup by turning a firehose on and off. However, if a frequency-shifting inverter like Schneider is used, PV would be curtailed to match the loads that are currently on the panel, even if the batteries are at a full state of charge.

 

Power Curtailment Methods

 

Chilicon

Chilicon has two Frequency-Watt modes to control the rate of charge to the battery. The first is to raise the output line frequency to a predetermined grid standard baseline. When the output frequency is higher than the threshold, the microinverters will sense this and halt power production until the output line frequency drops below the threshold. The second option is to set the inverter to steadily increase the frequency and the microinverter array to proportionally decrease production such that an equilibrium can be reached.  

Chilicon offers CT controlled production modes which can create a feedback loop that controls the amount of PV produced and exported to the grid. CTs, or current transducers, are small devices that can measure electrical current by clamping over a line. CT Clamps, located between the utility meter and the main panel, measure the total power consumption to ensure that production is not exceeding the local load. Thus, Chilicon’s Self-supply Mode of Operation allows for the grid-interactive system to produce at a level that matches the local load. This is a great option for states like Hawaii that require systems to operate in self-supply mode. Chilicon’s range of power curtailment is an impressive feature of their equipment. Rather than halting production and limiting the maximization of power production, the CTs curtail overall production potential.

On a side note, Chilicon can follow the voltage of a generator when its connected in parallel. This eliminates the concern of back-feeding it when loads are low and PV is active while a generator is running.  This is a great feature for people that would like to have a generator at their home for back-up. With other inverter options, you would need to add relays to disconnect the PV when the generator is on. This means you can reduce your generator fuel consumption by offsetting generator supported loads with the PV system.

Also, keep an eye out for Chilicon’s exciting new product: a 720w dual MPPT microinverter which will dramatically reduce balance of system costs for a microinverter system.

 

Enphase

Enphase microinverters have the ability to accommodate a wide range of wattages. Most modules, 60-cell or 72-cell, will be compatible with the IQ7 or IQ7+. See the image below for a diagram of the AC Coupled layout:

Enphase power curtailment options include External Relay Control, AC Frequency/Trip, AC Frequency/Watt and Power Export Limiting.

External Relay Control measures voltage and can trigger a full or partial disconnect if need be. Depending on local interconnections, there may be a time delay in reconnection. This operation can lead to a shuttering of the system where the battery system sees all or none of the full power of the PV array depending on the battery’s state of charge.  This is a relatively crude way to AC couple.

AC Frequency / Trip is an interconnection setting that sets an over-frequency trip threshold to halt microinverter production when the frequency reaches the pre-set threshold. This method should be used with older M215 and M250 models which don’t support gradual curtailment.

AC Frequency / Watt can gradually curtail power based on the AC frequency. The microinverters back off the maximum power point voltage thus reducing overall AC output. This feature is available with the IQ lineup of microinverters. Overall, it’s easier on the capacitors within the microinverter and the battery system to ramp the production up and down rather than trip it.  This is the same method that Chilicon uses to curtail power based on the battery inverter’s frequency which is determined by the state of charge of the battery bank.

Similar to the Chilicon’s Self-Supply Mode, Enphase’s Power Export Limiting is for interconnected systems that require an export limit. To limit PV export, the system must be outfitted with consumption metering. It automatically sends power curtailment commands to the microinverters via the powerline communications. Data is pulled from the consumption meter every 500ms and makes adjustments to the microinverter output at 1.5-second intervals. The IQ Envoy adjusts every two to four seconds.

Both of Chilicon and Enphase provide options to maximize the end customers’ return on investment. The right fit depends on the product mix, design preferences, and size of a system. For more information about retrofitting microinverter systems with storage or interest in AC coupled systems using microinverters, contact your CivicSolar Account Manager.