AC Coupling Grid Tie Inverters With OutBack Battery Based Inverters

Transcription

1 AC Coupling Grid Tie Inverters With OutBack Battery Based Inverters This application note will show how to add battery storage to a grid tied (GT) inverter that is limited to photovoltaic (PV) solar conversion only when the utility grid is active. By adding a battery based (BB) inverter like those from OutBack, the GT inverter stays active without utility power synchronization as the OutBack inverter becomes the new AC source to keep the GT inverter actively converting DC power from the PV array to usable AC power for the site s local electrical loads. Not only does the AC coupled storage inverter provide backup power during a grid outage when the GT inverter would normally be offline, but it can also provide local ancillary services to reduce the site s utility bill. For example, using the Support mode of an OutBack inverter set to an incoming current limit of 20 amps (A), the inverter will draw the amperage above 20A from the stored energy in the batteries when the load demand exceeds 20A,. This has the benefit known as peak shaving whereby the peak amp limit is not exceeded and monthly peak demand charges which in some cases exceeds metered watt hour charges can reduce utility bills significantly. Additionally, the OutBack Grid Use Timers can be used to disconnect the inverter loads from the utility grid in an off grid mode and thereby avoid expensive time of use (TOU) surcharges which can be double or triple the normal cents per kilowatt metered rate. TOU surcharges usually occur during the 4 8 PM peak demand period. By going off grid during this four hour period, day after day, month after month, significant savings can be achieved. Introduction to AC Coupling: As mentioned previously, by far the most common configuration of a PV generation system is an array of PV modules feeding a GT inverter that converts the DC power to AC power, then fed through the building s service panel and on out to the electrical grid (see Figure 1). Figure 1 Typical Grid Tie Inverter Utility Connection The grid acts like a battery and the renewable energy is placed on the grid for collective consumption. The grid asa battery is a great concept, until it s no longer there the grid tied inverter requires the grid to stay powered per the UL1741 requirement for safety reasons. Without grid power to keep the GT inverter operating, the available PV power just sits on the roof unused. And during an outage, a home or business with PV electricity potentially available is in the dark just like everyone else OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 1 of 12

2 Adding energy storage through AC coupling: For the owners of these more common grid tied, grid dependent inverters, there is a way to tie in a battery backup inverter system using a method called AC Coupling. It typically requires adding a load center with circuit breakers and electrical connections for the building s critical loads. This allows a point at which the GT inverter and the battery based (BB) inverter to couple and share their energy to the loads. In a normal mode of operation with grid power available, the energy from the PV array flows through the GT inverter to the critical load panel, with any excess energy flowing through the load panel to the BB inverter, and on out to the grid (see Figure 2). When grid power is lost, the BB inverter activates an internal transfer switch which opens its connection to the grid. This keeps the inverter from trying to power other homes on the grid, as well as keeping energy off the power lines so utility workers don t get shocked. The BB inverter also provides a power source to the GT inverter that keeps it online and inverting the DC power to AC power for the critical loads (see Figure 3). Figure 2 Current path when grid is present. When the sun goes down and the PV power is no longer flowing through the GT inverter to the loads, the power stored in the batteries will now provide power to the critical loads panel until morning (see Figure 4 on the next page). When the sun comes out the next day, the system reverts to the power flow as shown in Figure 3, with any extra energy used for recharging the batteries. If no excess is available then some manual load shedding (divesting of less critical loads to prioritize the most critical, such as lighting and refrigeration) may be Figure 3 Current path when grid power is lost OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 2 of 12

3 necessary through the turning off of the critical load panel connected devices until the batteries are charged. If the critical loads are absolutely essential and load shedding is not an option, then adding a generator to the backup system can satisfy the critical load demand while charging the batteries. Figure 4 Current path from the BB inverter with both PV and Grid loss. One might ask: why not just add the generator and forget about the BB based inverter? That might be a viable option for some, but there are some important things to consider before making that decision. 1. The generator will need to run the entire time that electrical energy is demanded by the building s loads, perhaps hours per day. Aside from the noise factor, many low cost generator motors require frequent maintenance, and are very inefficient at low power output. 2. As an example, higher efficiency can mean the difference between filling up a 5 gallon gas tank once a day versus once a week if the generator is tied into a battery based backup system that is running the generator just a couple of hours a day. 3. In a BB inverter system augmented with generator power, the batteries can greatly extend generator runtime the same fuel will go a lot further since the generator does not need to run 24/7. Typical AC Coupling solutions: Like any solution, the devil is in the details, and AC coupling a GT and a BB inverter together is no different, especially if one wants a one size fits all solution for all applications. That one size fits all package usually includes one or more diversion loads, possibly a blackout relay or some other method to take the GT inverter offline to prevent it from overcharging the batteries on the BB inverter when the GT inverter produces more energy than what is demanded by the load panel. And it requires a lot more upfront design to be sure all devices in the system can handle all possible conditions. Some battery based inverter manufacturers attempt to simplify their AC Coupling implementations by dithering or altering the frequency of their power to the GT inverter outside its operational window of 59.3 to 60.5 hertz. This will effectively shut off the GT inverter should excess PV energy start to overcharge the BB inverter batteries and is a simpler solution than using a blackout relay, thereby saving the cost of the relay. However, in many 2015 OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 3 of 12

4 instances frequency dithering prohibits the use of a generator as the generator frequency is not always stable enough for the GT inverter to synchronize to. Even if it synchronizes, there is a risk of back feeding and damaging the generator in low or no load conditions. In most cases, a manual transfer switch will be required to isolate the generator from the GT inverter if one is to be used. It should also be noted that the one size fits all AC Coupling solution really needs diversion loads to divert any excess energy in systems that either have too much PV power and/or too small of a battery bank which could put dangerous charge levels into the batteries. While it can be argued this energy can be used to heat water or run pumps, that benefit often comes when not necessarily demanded, and if the diversion load can no longer accept the available energy, the GT inverter must still be shut down. In addition to the complexity, diversion load implementations can also be expensive, requiring undesirable invasive installations which can make this design a poor choice for those who want simplicity and cost control in their AC Coupling system. The extra expense of all the associated control and coupling hardware plus adding a more expensive inverter/charger often means skimping on energy storage with lower grade batteries and enclosures, lowering overall system performance and utility. OutBack s AC Coupling solution: another approach is to keep things elegantly simple by outlining some basic sizing and operational guidelines by which a simple BB inverter, a battery bank, and a remote operated relay can be added to an existing GT inverter system to tie in the building s available PV power to the critical loads during grid loss. The rest of this article discusses an advanced electro mechanical solution from OutBack Power with an option for automatic generator control if desired. A single line diagram outlining OutBack s solution can be found at the end of this application note. The OutBack control circuit consists of the two OutBack inverter Auxiliary (AU) ports; one controlling a remote operated circuit breaker (ROCB) acting as a blackout relay for the GT inverter, and the other to control an optional generator. This cleaner, more compact component design allows the safe lock out of the GT inverter when the batteries are full, and also keeps the GT inverter locked out if an optional generator is started and running in the system. Integrated with a more advanced, smarter inverter/charger such as OutBack s Radian series with dual AC inputs and advanced generator features the result is a system that achieves higher performance at a lower effective cost. The detailed load center and control wiring can be found at the end of this application note. Notable features of the OutBack GSLC175 AC 120/240 AC Coupling solution include: UL 1741 end to end when used with an OutBack battery rack, the entire system is certified specifically for this application, ensuring fully compliant operation. Split phase design more easily integrated into standard household wiring without costly, inefficient transformers Simple installation only one additional programming step and four new field wire connections. Universal design works well with other brands and models of GT inverters OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 4 of 12

5 Following are a few guidelines on how to size the system and interact with the two types of inverter systems. Guideline Number One: Match PV Array Amps to Maximum Battery Charge Amps Under normal operating conditions when the grid is present, an OutBack BB inverter/charger will keep the batteries in a float charge. However, when the grid is lost, the inverter is no longer in control of the charging current going into the battery, so any remaining power not needed by the loads will flow back through the bidirectional H bridge circuit of the BB inverter into the batteries in an unregulated charge. The following table shows the maximum PV power per string of battery so the maximum charge rate is not exceeded, as well as the associated available power for a given 24 hour period. This system sizing should keep the system in balance depending on the combination of solar radiation available to the array and how much power is needed based on load demand. The PV power column is the maximum amount of power that can be back fed through the inverter to charge the batteries. The calculation for this guideline presumes some losses on the array and in the GT inverter. Table 2 Matching PV power and load demand with the OutBack Energy Cell 200RE AGM battery at the 12 hour discharge rate and 80% depth of discharge (DOD). Based on following calculations: 2000W array x 75% efficiency = 1500W. Charging amps implied: 1500W 48V 30 amps. Ideally, a lower charging current will provide a larger battery capacity, but the batteries are capable of receiving a maximum charging current as listed in column two below. The recommended PV array sizes in the first column will provide a higher capacity charge than the maximum allowed and will also be in line with the maximum array size in Guideline Two below. PV Array kw max (STC) PV Charging Amps (Based on array size) Number of 200RE Battery Strings 1 String: 53A Max 2 Strings: 106A Max 3 Strings: 159A Max Available Capacity at C/24 rate with 80% DOD 2 kw 30 Amps 6.9 kwh 4 kw 60 Amps 13.9 kwh 6 kw 90 Amps 20.8 kwh 2015 OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 5 of 12

6 Guideline Number Two: The OutBack inverter power rating should be 125% of the GT inverter power rating. This guideline ensures that the GT inverter does not overwhelm the charging circuitry in the OutBack inverter if the load demand goes to zero and all available GT inverter power is flowing to the OutBack inverter. For example, the 8 kw rating of the Radian inverter would dictate a GT inverter no bigger than 6 kw. Guideline Number Three: PV array production needs to match combined load and battery charging demand. This ensures that either the daily load demand or battery charging doesn t exceed the power from the PV array. Alternatively, an optional generator can be added to supplement PV energy shortages on the backup system. A detailed look at OutBack s GSLC175 AC 120/240 AC Coupling solution The following is a list of the required AC Coupling components that come with the OutBack pre wired AC Coupling Radian Load Center. 13 kwh 16 kwh 50A DPST Remote Controlled Circuit Breaker (ROCB; takes three CB spaces in the load center) 12 VDC Outback Relay 48 VDC OutBack Relay DIN Rail Hardware 3 kwh Figure 5 Guideline 3: Achieving system balance. The ROCB comes pre wired as do the two OutBack relays and two AU Ports. The L1 and L2 conductors from the GT inverter or its AC disconnect are landed on the open ends of the dualpole circuit breakers ganged to the ROCB. If a generator is installed, then the two wire start lines are connected to the 48V GT lockout/gen start relay, with the L1 and L2 from the generator going to the Gen Input bus bars, and the ground and neutral lines going to the correct bus bars. All other grid input, inverter output, and DC battery connections are landed like any other Radian load center application. The GT inverter and generator connections are indicated by white symbols in the adjacent photo. In addition to the ROCB and relay devices, a new AC Coupling function has been added to the MATE3 user interface. This function uses temperature ROCB ROCB Control Relay GTI Lockout Relay 2015 OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 6 of 12

7 compensated charging set points, will disconnect the GT using the ROCB when the battery voltage is 0.8 VDC above the battery voltage to protect the batteries, and will reconnect the GT inverter when the battery voltage is 0.4 VDC below the current temperature compensated charging set point. If there is abundant surplus power from the GT inverter, then ROCB cycling may be more frequent during grid outage and when loads and batteries are unable to consume all available PV production. Larger loads that cycle off and on such as refrigerators and space heaters could increase ROCB cycling when in their off state. Manually turning on more loads connected to the critical load panel, or using a diversion load, can decrease the cycling. However, it will not harm the ROCB as the cycle life of the circuit breakers and ROCB motor exceed 10,000 cycles. If the system is properly sized and loads defined as critical typically remain running, frequent cycling should not be a problem and will not damage any of the OutBack devices. One other condition could cause short durations of ROCB cycling when either an Absorb or Float charge ends its charge cycle and goes silent which causes the ROCB to cycle until the battery bank self discharges to less than the Offset (formerly Sell RE) voltage setting. This happens because the ROCB trigger target has changed from either the Absorb or Float settings of 57.6V/54.4V respectively, to the Sell RE target whose default setting is 52V. As the battery self discharges down from 57.6V or 54.4V down to 52V, the ROCB will cycle until it falls below 52.8V. To minimize this condition, set the Sell RE voltage to equal the Float voltage and that should all but eliminate ROCB cycling at the end of a charge cycle. Also, set the inverter Aux Port to a function with settings that cannot be achieved to eliminate possible Aux port control conflict between the inverter and the MATE3. Other smart features of the AC Coupling MATE3 function include a regular 15 second check of the high battery set point in the event the occupants manually close the ROCB when it should be open, and will re open the ROCB to ensure the batteries are not overcharged through user error. Another feature is the MATE3 s checking for an active grid when the ROCB has an open command, so if the grid power returns while the ROCB is open then the MATE3 will re close the ROCB and allow the GT to return to selling to the grid. The GSLC175 AC 120/240 AC Coupling load center comes with a Quick Start Guide which includes detailed diagrams of the control circuit and program settings for the MATE3 system controller. The Quick Start Guide can be found on the OutBack Power website under Products>Inverter/Chargers>Radian Series>GS Load Centers. OutBack also offers an AC Coupling kit, part number GS IOB AC 120/240, that can be used for retrofitting existing Radian or FR inverters (photo of typical ROCB kit installation at right). For FR inverters it s recommended to install the ROCB in three unused circuit breaker spaces of the FLEpower (FP) AC load center along with the DIN rail for mounting the two relays. The Radian inverter has both a 12V auxiliary (AU) port and dry contact relay for the generator start, but the FR inverter only has the 12V AU port. A FLEnet DC (FN DC) is recommended for battery monitoring and charging control for every OutBack system so the dry 2015 OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 7 of 12

8 contact relay in the FN DC can be used for AC coupling applications requiring a generator. The kit comes with installation instructions or they can be downloaded as ROCB Installation Instructions on the OutBack website under Products/Inverter Chargers/Radian Series/GS Load Centers. Overall, this AC Coupling solution from OutBack has considerable advantages: it is easy to size, easy to install, does not require external hardware, and is less expensive than most other solutions. It uses intelligent controls and provides back feed protection if a generator is part of the system. The OutBack solution provides backup power for existing grid tied inverter systems up to 6 kw that won t leave all that PV power stranded on the roof the next time disaster strikes. When implemented using the OutBack Radian series inverter, the result is higherperformance and greater reliability in a more compact, cost effective package, enabling the user to invest in higher quality energy storage and thoroughly optimizing their renewable energy system OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 8 of 12

9 Figure 6 below shows a simplified single line diagram of the AC Coupled system. The drawing shows the generator option which is not required if daily sun is expected or critical loads can be shed until more sun is available. 6kW PV Array String Combiner 6kW Grid Tie Inverter 120V/240V 200 Amp Main Service 200A Disconnect 120V/240V GRID FEED M Gen Start Relay Blackout Relay 50A 120V/240V 120V/240V Critical Load Panel 200A Disconnect GS8048 Inverter GSLC Load Center 50A DPST ROCB AC Out BUS Grid 50A DPST AC In Bus 50A DPST AC In BUS Gen 50A DPST G Generator DC Neg DC Disconnect 175A SPST (2x) 12V 12V 12V 12V Figure 6 Single line diagram of an OutBack Radian inverter AC Coupled to a Grid Tied Inverter OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 9 of 12

10 Figure 7 below is a detailed wiring diagram of the Radian GSLC175 AC 120/240 GSLC load center. Field wiring lands on the terminal bus bars as designated below with. If desired, a two wire generator start can be connected to pins 21 and 24 of the 48V relay. GROUND W16 W8 To GS8048 CB1/2 W2 GROUND BUS To GS8048 W1 W17 B W18 T3 T4 CB3/4 W9 DC BREAKER BUS W7 W3 W10 W11 W14 CB5/6 W4 W23 W24 W5 W12 CB7/8 W6 W13 W25 W26 W15 W21 W19 ROCB W20 12VDC RELAY M 24 NO 22 NC 12Vdc Relay 14 NO 12 NC 48Vdc Relay A2 14 DC- NO 11 COM 12 NC 21 COM A1 DC+ A2 DC- 11 COM W27 48VDC RELAY 24 NO 21 COM 22 NC A1 DC+ D A C B W28 Figure 7 Detail diagram of the Radian GSLC175 AC 120/240 GSLC load center wired for AC Coupling. Field connections designated with OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 10 of 12

11 Figure 8 below is nearly identical to Figure 7 above except for the internal wiring labels showing the destination as landing on the FP FR inverters rather than the Radian GS inverter. Field wiring lands on the terminal bus bars as designated below with. If desired, a two wire generator start can be connected to pins 21 and 24 of the 48V relay. Figure 8 Detail diagram of the FP FR load center wired for AC Coupling. Field connections designated with OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 11 of 12

12 About OutBack Power Technologies OutBack Power Technologies 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. Grid/Hybrid As a leader in off-grid energy systems designed around energy storage, OutBack Power is an innovator in Grid/Hybrid system technology, providing the best of both worlds: grid-tied system savings during normal or daylight operation, and off-grid independence during peak energy times or in the event of a power outage or an emergency. Grid/Hybrid systems have the intelligence, agility and interoperability to operate in multiple energy modes quickly, efficiently, and seamlessly, in order to deliver clean, continuous and reliable power to residential and commercial users while maintaining grid stability. Contact Information Address: Corporate Headquarters th Avenue N.E. Suite B Arlington, WA USA European Office Hansastrasse 8 D Schwabach, Germany Telephone: (Technical Support) (Fax) (Fax) Website: Support@outbackpower.com Disclaimer UNLESS SPECIFICALLY AGREED TO IN WRITING, OUTBACK POWER TECHNOLOGIES: (a) MAKES NO WARRANTY AS TO THE ACCURACY, SUFFICIENCY OR SUITABILITY OF ANY TECHNICAL OR OTHER INFORMATION PROVIDED IN ITS MANUALS OR OTHER DOCUMENTATION. (b) ASSUMES NO RESPONSIBILITY OR LIABILITY FOR LOSS OR DAMAGE, WHETHER DIRECT, INDIRECT, CONSEQUENTIAL OR INCIDENTAL, WHICH MIGHT ARISE OUT OF THE USE OF SUCH INFORMATION. THE USE OF ANY SUCH INFORMATION WILL BE ENTIRELY AT THE USER S RISK. OutBack Power Technologies cannot be responsible for system failure, damages, or injury resulting from improper installation of their products. Information included in this manual is subject to change without notice. Notice of Copyright Application Note 2015 by OutBack Power Technologies. All Rights Reserved. Trademarks OutBack Power, the OutBack Power logo, FLEpower ONE, Grid/Hybrid, and OPTICS RE are trademarks owned and used by OutBack Power Technologies, Inc. The ALPHA logo and the phrase member of the Alpha Group are trademarks owned and used by Alpha Technologies Inc. These trademarks may be registered in the United States and other countries OutBack Power Technologies, Arlington, WA MAM Revision 3/FINAL Page 12 of 12