Retrofitting PV and wind generation systems
View the full story, including all images and figures, in our monthly digital edition
Retrofitting alternative power generation systems such as solar photovoltaics (PV) is much easier than one might anticipate. While these systems typically still have rather long paybacks in the Midwest, where utility power is relatively cheap, other areas such as California are realizing paybacks in 3 to 7 years. It is anticipated that within the next 5 to 10 years, PV electricity will be cost-competitive nationwide with utility power. Other alternative power solutions such as wind, geothermal, hydro, and biomass offer a variety of options for building owners to consider based on finance and regional considerations.
Federal rebates along with state programs now offer building owners an opportunity to further reduce initial costs or, with power purchase agreements, completely eliminate them. Local electric utilities also are offering rebates on alternative power systems because of mandates to reduce cumulative kW hours and cumulative demand load. AIA, ASHRAE, U.S. Green Building Council, Illuminating Engineering Society of North America, and others have issued the 2030 challenge, which calls for all new construction and major renovations to be carbon-neutral by 2030, with a 60% reduction in energy use from the average building in that region by next year. ASHRAE also continues to push consumption requirements’ limits lower, a feat that is becoming increasingly difficult to achieve with traditional products and utility-delivered power. With the rapidly rising cost of utility power, federal and state mandates, local codes, and decreasing cost of alternative power systems due to growing sales, these technologies will be at the forefront of commercial building design nationally in the very near future.
There are two basic ways to connect an alternative power system: grid-tied or off-grid. An off-grid system relies 100% on power it generates. Often this power is stored in battery cells. The advantage of off-grid systems is complete independence from utility costs, immunity from power outages, and reduced carbon footprint since no coal is burned to generate the power. It is important to note that off-grid systems typically are supplemented with a fuel-burning generator to ensure reliable power 24/7, which means the carbon footprint is not completely eliminated. While these systems hold merit in the residential world, the sheer quantity of solar panels, batteries, or other generation devices required to carry the full load of a commercial building still has cost and space ramifications that make it a tough choice.
Grid-tied systems supplement utility power. That is, a grid-tied system will provide all its generated power to the building with any additional power required being drawn from the electric utility. Grid-tied systems can use batteries, but typically do not. This type of system is often the choice for commercial buildings because it reduces energy consumption, still has the reliability of utility service, and is easily expandable. This article focuses on grid-tied PV systems and general design concepts necessary for a clean and functional installation.
Keeping with the flow
The integration of PV and other alternative power systems into existing facilities relies on one basic function typical of ac power distribution. The basic function is the flow of electricity from any power generating device toward the active equipment drawing it. This allows solar arrays, along with any other power generating system, to be installed downstream as if it were distribution in lieu of being in parallel with the utility. This gives designers and building owners greater flexibility in selecting a location for PV arrays, but does create some pitfalls.
Backfeeding building switchgear and the utility service are two major components of implementing PV systems. Circuit protection and voltage control devices used by electric utility companies typically are designed with the assumption that power will only be delivered, not returned. Because a PV system may cause power to flow opposite the utility’s intended design, it is important to consult the local utility prior to installation of a system.
If the load of a building is greater than the generated power of a PV or wind turbine system, all the generated power is used by that building.
However, if the alternative power system is generating more electricity than the load, that generated power will see the utility as load and backfeed power to it. This is often referred to as the “meter spinning backwards” or sold as a way for consumers to get paid by the utility. In general this is OK provided a voltage is present on the utility lines; however, in the event of a utility outage, backfeeding would be very dangerous. Imagine a utility worker going to service a line assuming power is out because the service is down but then discovering that the PV service is still fully operational and thus very dangerous.
Typically this scenario is prevented by a component of the systems inverter. Per NEC 690.61, the inverter must de-energize its output to the connected electrical production network upon loss of voltage, and that system may re-energize only when voltage is returned. This functionality is a must and certainly should be evaluated when retrofitting a PV system to an existing facility. (Figure 1)
Backfeeding through a building power distribution system is often the method of installation for PV.
Certain steps must be taken to ensure it is properly applied. In general, an alternative power system will connect to a power distribution system as a load via standard circuit breaker sized appropriately. That is, if you are adding an 8 kW PV array to an 800 A to 480 V 3-phase facility, and the intent is to focus carrying the 120 V load on the west wing of the building, the PV inverter should feed that distribution section or panelboard with a 30 A circuit breaker. (Figure 2) This is a way of localizing alternative power generation. As with the utility scenario, if the west wing does not draw all the power generated by the system, it will feed back through the building distribution system until all the generated power is used. (Figure 2)
Figure 3 shows problems similar to those a utility experiences during an outage. If the power to the west wing is disconnected, then per NEC 690.61 the PV inverter should see this as an outage and disengage. But, if the power to the west wing is connected and the PV system is generating more power than the panel is drawing, potential concerns at the secondary side of the 480/208 V 3-phase step-down transformer must be evaluated. First, transformers have a typical voltage drop of 3% to 5% from primary to secondary, and they are also designed to compensate for voltage drop from primary to secondary side by factors of 3% to 5%. If power is flowing from secondary to primary, these trends are reversed. Thus you could experience voltage drops of up to 10% as the PV array backfeeds the step-down transformer, causing problems upstream.
Another issue is inrush current. Transformers are designed to limit inrush current from the primary side, but not from the secondary side. Because of this inrush, current will be greater from secondary to primary side. Careful planning and consideration is a must when connecting an alternative power system inverter downstream of a step-down transformer. One recommendation is to provide a circuit breaker or fused disconnect at the secondary side of the transformer in addition to the circuit breaker where the PV array is connected. The secondary side of the transformer is protected and allows for isolation of the transformer if necessary. This would be particularly important for maintenance personnel if the transformer is not in the same room as the electrical gear it is feeding. (Figure 3)
Another option is to install the PV array on the main switchboard or panelboard. This application removes the concerns of backfeeding the building distribution system and evenly distributes the generated power from the PV array throughout the building as necessary. This provides a very clean installation but may be slightly more expensive depending on the location of the main switchgear, the solar array, and the size of the array. (Figure 4)
Emergency power protection
One final issue that must be taken into consideration is incorporating PV and wind systems where an emergency generator exists. The combination of PV and generator is commonplace in an off-grid system as the generator steps up when the PV cannot generate enough power to carry the entire load. Here they are working together, but in a typical grid-tied system they will work against each other if the proper precautions are not taken.
Figure 5 is a typical power distribution system for a commercial building with an emergency generator for all loads considered life safety.
Upon loss of power, the result is similar to those discussed above, where the inverter by NEC 690.61 de-energizes when it sees a loss of voltage. The emergency transfer switch then switches power to the generator and the entire load on the emergency panelboard is carried by the generator. (Figure 5)
But what if your facility not only has loads considered life safety but is 100% backed up by an emergency generator? There are obviously countless ways to fully back up a facility. Here, for simplicity, the focus is on the generator configuration indicated in Figure 6.
During an outage, the transfer switch will switch from utility to generator power. Our PV inverter will shut off during the generator startup but will again see voltage once the generator is up and running. In general, the PV or wind will supplement the generator and reduce load on it, but if there is a chance the PV array is generating more power than what the facility is consuming, it will backfeed the generator. This, of course, is highly undesirable and more than likely will damage the generator.
One solution would be to install a pre-primary switchboard or panelboard ahead of the transfer switch and feed the PV or wind inverter directly into it. This eliminates the concern of backfeeding the generator during operation, as it is now completely isolated downstream from the inverter. (Figure 7)
While there may be other solutions for incorporating PV or wind systems into a 100% generator-backed-up system, they must be carefully coordinated with all equipment providers and existing gear to ensure safety.
Incorporating alternative power systems such as PVs creates a great opportunity for reducing carbon footprint, reducing dependency on fossil fuels and utility loads, and increasing community presence. While these systems are relatively straightforward, certain considerations and decisions must be made to ensure a safe and functional system. It is critical that the commercial design community—including engineers, architects, and contractors—gain awareness of the advantages and dangers as well as proper implementation of these fast-arriving technologies.
|Gray is an electrical engineer and project manager at Advanced Engineering Consultants.|