Outback Radian AC Coupling Note

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Application Note

How To AC Couple Grid Tied Inverters

with OutBack Frequency Shifting Inverters

This application note will explain how to AC couple a Grid Tied Inverter (GTI) to an OutBack inverter.

When there is a grid outage, this method employs a frequency shifting technique to prevent the GTI from

overcharging the battery bank during times when the GTI is putting out more power than can be

consumed by the loads. The excess power from the GTI can harm the battery bank in the form of an

unregulated charge exceeding safe charging limits of the batteries. When safe charging limits are

exceeded, the OutBack inverter will shift the frequency upwards from 60 Hz to as much as 64.5 Hz to

make the GTI incrementally reduce its output (Freq/Watt compliant inverters) or disconnect itself from the

AC coupled circuit (any IEEE 1547-compliant inverter).

Introduction

The frequency shifting technique to safeguard overcharging of the battery bank not only curtails the GTI

when the active battery charging set point is exceeded, but also when the AC Charger limit is surpassed.

Additionally, the step rate of the frequency shift is user adjustable from 0.02 to 5.00 seconds to allow

better optimization of system performance. While overall AC coupling performance is highly dependent

on the ratio of GTI current to load current, OutBack’s implementation of voltage and current regulation

with adjustable slew rate will result in the highest performing AC coupled system, especially in the case

of an Freq/Watt compliant GTI that responds to closed loop control.

OutBack’s Frequency Shift AC Coupling is only employed in the Radian GS(A) class inverter at this time.

It requires a firmware upgrade on both the Radian inverter and MATE3s system display and controller.

The firmware is only available by download on the OutBack Power website. See the release notes for

the latest update on regulatory compliance and other product information affected by this update.

Theory of Operation – Live Grid

Figure 1 shows the current path for a normal Grid Tied Inverter from the PV solar panels through the

inverter, to the main service panel and on out to the grid. In a normal GTI application, power produced

from the PV array is consumed by loads connected to the main service panel with excess power going

out to the grid. However, with grid loss the GTI has no way to synchronize itself to the grid – a

requirement for operation – so it shuts down and is unable to use any potential energy from the PV array.

Figure 1 Normal GTI Power Flow

Summary of content (12 pages)

PAGE 1
Application Note How To AC Couple Grid Tied Inverters with OutBack Frequency Shifting Inverters This application note will explain how to AC couple a Grid Tied Inverter (GTI) to an OutBack inverter. When there is a grid outage, this method employs a frequency shifting technique to prevent the GTI from overcharging the battery bank during times when the GTI is putting out more power than can be consumed by the loads.
PAGE 2
Application Note By connecting (coupling) the output of the GTI to the output of a storage based inverter (SBI), the SBI can act like a grid source that the GTI can synchronize to and process power from the PV array to a backup load panel. The backup load panel is required so loads can be powered from either or both inverters without backfeeding the main service panel during a grid outage (see next section on Grid Outage operation).
PAGE 3
Application Note In addition to exporting GTI current out to the main service panel, a separate parallel current path to charge the battery bank from solar can exist if either the Refloat or Rebulk charging targets are reached. However, with a live grid and no PV available, the backup load panel and battery charging (if needed) will be powered from the grid as shown in Figure 3.
PAGE 4
Application Note Theory of Operation – With Grid Outage During a grid outage when the sun is shining, the Radian inverter becomes an AC source to which the GTI can synchronize. This allows the PV power to flow to the backup panel’s connected loads, as well as charge the batteries if the GTI is generating more power than can be absorbed by the loads. Figure 4 shows all possible current flows, the paths of which can change depending on several different factors.
PAGE 5
Application Note Example: There is a grid outage and the Radian becomes the new AC source for the GTI which then delivers 3400 watts to the load. The load then drops to 1,000 watts, meaning the other 2400 watts (10 Aac at 240V) will come back through the Radian inverter to the battery bank. If the 10 Aac GTI charging current is less than the AC Charging limit, then it will continue delivering charge current to the battery bank.
PAGE 6
Application Note Once PV production is gone for the day, then the Radian inverter will power the loads in the backup mode until sun returns the next day, or the battery bank reaches low battery cutout (LBCO). If the battery bank reaches LBCO, then the Radian inverter can no longer power the backed up loads or be a voltage source for the GTI to start again and recharge the batteries.
PAGE 7
Application Note 6kW PV Array 6kW Grid Tie Inverter String Combiner 120V/240V GRID FEED 120V/240V 200 Amp Main Service M 200A Disconnect 120V/240V Critical Load Panel GS8048 Inverter 200A Disconnect GSLC Load Center 50A 120V/240V GTI Disconnect 30A DPST AC Out BUS Grid 50A DPST AC In Bus 50A DPST AC In BUS Gen 50A DPST DC Disconnect 175A SPST (2x) G DC Neg Generator 12V Figure 6 ©2019 12V 12V 12V Single line diagram of GTI inverter connection to Radian inverter OutBack Power, Arlington,
PAGE 8
Application Note Procedure Programming the MATE3s and Radian Inverter 1. Download the MATE3s and Radian AC Coupling firmware from the OutBack Power website. 2. Copy the firmware files to the SD card and install from the MATE3s Main Menu. 3. Enter the Installer password. 4. Select Settings from the MATE3s Main Menu. 5. Select Inverter menu and scroll down to the AC Coupling settings. 6. Change AC Coupling from N to Y, and change the Freq Shift Response Time (0.02 to 5.0 seconds) if desired.
PAGE 9
Application Note Precautions Using a Generator When AC Coupling Most residential generators, and even some commercial grade generators, are not designed to synchronize with another AC source. They may experience catastrophic failure if the other AC source is a “stiffer” source and can overpower the weaker AC coupled generator.
PAGE 10
Application Note For the automatic method using the lockout relay, the generator L1 and L2 phases can be connected to the GEN IN bus bars, with the generator ground and neutral connected to their respective bus bars. The L1 and L2 connections from the AC Gen In bus bars are then connected to the relay coil terminals.
PAGE 11
Application Note If the automated method of disabling the GTI during generator operation cannot be approved by the local authorities having jurisdiction, then there is a manual method where the generator is connected to the GEN IN AC input terminals in the GSLC load center. The Generator input mode should be selected for the Gen AC Input Mode and Limits under the Inverter settings. Figure 8 shows a single line diagram with the generator connected.
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Application Note About OutBack Power OutBack Power is a leader in advanced energy conversion technology. OutBack products include true sine wave inverter/chargers, maximum power point tracking charge controllers, and system communication components, as well as circuit breakers, batteries, accessories, and assembled systems. Contact Information Address: Corporate Headquarters 17825 – 59th Avenue N.E. Suite B Arlington, WA 98223 USA Email: Support@outbackpower.com Website: http://www.outbackpower.