Roof-mounted photovoltaic system
The roof-mounted photovoltaic system in the USA
Andalay Bay Resort and Casino in Las Vegas has the largest roof-mounted solar photovoltaic (PV) array (in July 2016) in the USA, with a system rated 6.4 MW dc on a single roof (see Figure 5 ). It is also the second largest roof-mounted solar system in the world. The parent company Mandalay Bay and the resort’s operational management group are committed to being leaders in sustainability and environmental stewardship. The property is currently expanding the existing matrix, which will increase the system’s rating to 8.3 MW dc, making it the largest solar array on the roof in the world. Upon completion, the roof-mounted photovoltaic system will generate energy to power about 20% to 25% of the resort property’s annual electricity consumption.
The photovoltaic panel produces enough energy to supply more than 1,300 homes. The project is estimated to displace approximately 6,300 metric tons of carbon dioxide emissions per year, equivalent to the emission of taking 1,326 cars off the road.
Mandalay Bay and NRG Energy, based in New Jersey, installed the ceiling mount system in two phases. The initial construction of Phase I was designed by NRG Energy and the Phase I team. JBA consulting engineers, Bombard Renewable Energy and NRG collaborated in the design and construction of the phase II expansion of the system.
Phase I includes the installation on the roof of 21,324 photovoltaic modules and associated chain inverters, DC combining panels and 480 V three-phase combining panels. In the basement, there are four AC recombination distribution boards installed to power four expansion transformers. The transformers increase the voltage to match the distribution voltage of the 12.470 V, three-phase public electricity utility (NV Energy). There are three 1 MVA transformers and one 1.5 MVA transformer. The 12.47 kV medium voltage collection panel, located outside the existing convention center, supplies energy generated by photovoltaic energy to the new 12.47 kV interconnection distribution panel.
Overview of phase I:
- The DC rating: 6.4 MW
- The AC rating: 4.87 MW
- Number of photovoltaic modules: 21,324
- Photovoltaic module manufacturers: JA Solar and Hanwha QCells
- Investor manufacturer: SMA America.
The installation of the photovoltaic solar panel was completed in phases due to an expansion under construction of the property’s convention center. The expanded roof space was not available until the expansion of the convention center was completed in August 2015. Upon completion, Phase II solar photovoltaic work began and is projected in July 2016. Phase II will solidify what is already one of the largest solar arrays for roof mounting in the world, expanding the photovoltaic array on the new 8-acre roof. Upon completion, the added part of the matrix will generate approximately 1.49 MW, with an annual production of approximately 3.4 million kWh. Covering more than 28 acres on a single roof, the combined Phase I and Phase II complex will generate enough electricity to supply an equivalent of more than 1,300 homes.
Phase II includes the installation of 4,532 photovoltaic modules on the roof. However, the modules were not connected in a conventional chain topology. Instead, it uses a system designed by Ten-K Solar with module cells connected in serial and parallel connections with dc converters / optimizers for dc and redundant microinverters. The advantage of this system is the increased reliability, reliability and security of the system. Safety is greatly increased by limiting the DC bus voltage to less than 60 V DC.
Also installed on the roof are 48 MidNite Solar photovoltaic combiners and six Eaton ac 480 V three-phase, four-wire combiner panels. In turn, each Eaton combiner panel supplies power to a 2,500 A, 480 V, three-phase, four-wire AC recombination panel, which supplies power to the intensive 1.5-MVA Wye Wye transformer. In addition, the 12.47 kV Phase I medium voltage collection switch needed to be reconfigured to accept the new photovoltaic system together with the associated monitoring and control system.
Overview of Phase II:
- The dc rating: 1.86 MW
- The ac rating: 1.5 MW
- Number of photovoltaic modules: 4,532
- Manufacturer of the photovoltaic module: Ten-K (410 W)
- Drive manufacturer: solaracks 700S microacks in a redundant drive bus configuration
The total combined DC capacity is 8.3 MW. The total combined capacity of alternating current is 6.37 MW.
System design challenges – utility integration
A challenging aspect of the design of this project is that there is no energy purchase contract with the local utility. Contractually, all energy generated by solar energy must be delivered and consumed by the property. The interconnection utility point for the photovoltaic system is a 15 kV feeder from the service substation. The concessionaire needed a reverse power relay (device no. 32), over voltage (device no. 59), under voltage (no. 27) and under / over voltage (no. 81) to detect and prevent the PV feedback generated in the system concessionaire. If the photovoltaic system supplies or distorts the utility distribution systems, the medium voltage switchgear must disarm.
There are several safeguards to ensure that the PV system does not return to the utility system. The solution includes a Kirk Key system located on the distribution panel of the designated solar plant and at the interconnection point of the main dealership of the facility, which ensures that no one can open the key to the main dealership without first opening the keys to the PV system. This prevents the second 15 kV service feeder from being parallel to the property’s photovoltaic system at any time. In addition, the relay and communication system measures the installation load and the production of solar energy. If the output of the PV system reaches 92% of the total load, a cut-off feature in the relay system forces an automatic reduction in energy production. Any problems that occur will trigger alarms in which the utility, installation contractor and property operations have the ability to remotely limit the plant output of Phase I SMA inverters. The SMA drive software also has a restriction function that a system manager can use remotely if the generated energy reaches energy usage levels.
In May 2016, the property consumes all the energy generated by photovoltaic energy and there was no cut in photovoltaic energy. Based on the resort’s historical load profile and the expected production of the photovoltaic arrays, it should not be necessary to reduce the photovoltaic energy during the life of the system.
Challenges of system design – modification of existing equipment
The existing 12.47 kV photovoltaic switch had to be modified to explore the new Phase II circuit. The modifications included:
- The existing switch has been rebuilt to accommodate the Phase II cable entry.
- Providing circuit overcurrent protection for the Phase 1 circuit. The unfused mechanism had to be replaced with a fused unit.
- Finally, the existing Kirk Key system had to be modified to meet the dealer’s requirements.
All modifications required extensive planning and were carried out in such a way as not to interrupt Phase I solar production. It was determined that the main components of this work would occur during the night, under the cover of darkness and under a night shift.
Challenges of system design – modification of control
Upgrading a solar photovoltaic monitoring and control system at a large existing photovoltaic plant can be daunting. The control software had to be updated and tested. New 12.47 kV switch position monitors and remote triggering were required, as well as new meters, remote monitoring of the photovoltaic plant, climate monitoring equipment, new input / output cabinets and so on. All of these points had to be thoroughly tested and validated. As a final point, the concessionaire’s monitoring points and remote firing requirements also needed to be integrated and tested by the witness by the company.