Case study: Roof-mounted PV system

A giant solar photovoltaic (PV) array in Las Vegas offers a resort a sustainable power option.

By Robert R. Jones Jr., PE, LEED AP; and Leslie Fernandez, PE, LEED AP, JBA Cons September 26, 2016

Mandalay Bay Resort and Casino in Las Vegas has the biggest roof-mount solar photovoltaic (PV) array (as of July 2016) in the U.S., with a 6.4-MW dc-rated system on a single roof (see Figure 5). It is also the second-largest roof-mount solar system in the world. The parent company that owns Mandalay Bay and the resort operational management group are committed to being a leader in sustainability and stewardship of the environment. The property is currently expanding the existing array, which will increase the system rating to 8.3-MW dc, making it the largest rooftop solar array in the world. After completion, the roof-mounted PV system will generate energy to power about 20% to 25% of the annual electricity consumption of the resort property.

The PV array produces enough energy to power more than 1,300 homes. The project is estimated to displace approximately 6,300 metric tons of carbon dioxide emissions per year, the emission equivalent of taking 1,326 automobiles off the road.

Mandalay Bay and New Jersey-based NRG Energy installed the roof-mount system in two phases. The initial Phase I construction 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

Phase I includes the roof installation of 21,324 PV modules and associated string inverters, dc combiner panels, and 480 V 3-phase combiner panels. In the basement, there are four ac recombiner switchboards installed to feed power to four step-up transformers. The transformers step up the voltage to match the public electric utility’s (NV Energy) distribution voltage of 12,470 V, 3-phase. There are three 1-MVA transformers and one 1.5-MVA transformer. The 12.47-kV medium-voltage collection switchgear, located outside of the existing convention center, delivers PV-generated power to the new interconnect 12.47-kV switchgear.

Phase I overview:

  • The dc rating: 6.4 MW
  • The ac rating: 4.87 MW
  • PV module quantity: 21,324
  • PV module manufacturers: JA Solar and Hanwha QCells
  • Inverter manufacturer: SMA America.

Phase II:

The PV solar array installation was completed in phases due to an under-construction expansion of the property convention center. The expanded roof space was not available until the convention center expansion was completed in August 2015. After the expansion was complete, the Phase II solar PV work commenced and is projected was completed in July 2016. Phase II will solidify what already is one of the largest roof-mount solar arrays in the world by expanding the PV array on the new 8-acre roof. Upon completion, the added portion of the array will generate approximately 1.49 MW, with an approximate annual production of 3.4 million kWh. Covering more than 28 acres on a single roof, the combined Phase I and Phase II array will generate enough electricity to power an equivalent of more than 1,300 homes.

Phase II includes the roof installation of 4,532 PV modules. However, the modules were not wired in a conventional string topology. Instead, it uses a Ten-K Solar-designed system with module cells wired in serial and parallel connections with dc-to-dc converters/optimizers and redundant microinverters. The advantage of this system is its increased system reliability, reliance, and safety. 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 PV combiners and six Eaton ac combiner 480 V, 3-phase, four-wire panels. In turn, each Eaton combiner panel supplies power to a 2,500-amp, 480 V, 3-phase, four-wire, ac recombiner panel that supplies power to the step-up 1.5-MVA wye-wye transformer. Additionally, the existing Phase I 12.47-kV medium-voltage collection switchgear had to be reconfigured to accept the new PV system along with the associated control and monitoring system.

Phase II overview:

  • The dc rating: 1.86 MW
  • The ac rating: 1.5 MW
  • PV module quantity: 4,532
  • PV module manufacturer: Ten-K (410 W)
  • Inverter manufacturer: Leadsolar 700S micro inverter in a redundant inverter bus configuration

The total combined dc capacity is 8.3 MW. The total combined ac capacity is 6.37 MW.

System design challenges—utility integration

A challenging aspect of the design of this project is that there is no power purchase agreement in place with the local utility. Contractually, all solar-generated power must be delivered to and consumed by the property. The utility point of interconnect for the PV system is a 15-kV feeder from the serving substation. The utility required a reverse-power relay (device No. 32), overvoltage (device No. 59), undervoltage (device No. 27), and under/over frequency (device No. 81) to sense and prevent any backfeeding of PV-generated power into the utility system. If the PV system feeds power into or distorts the utility distribution systems, then the property medium-voltage switchgear must trip offline.

There are multiple safeguards to ensure that the PV system will not backfeed into the utility system. The solution includes a Kirk Key system located at both the designated solar plant switchgear and the main utility interconnection point to the facility, which ensures that no one can open the main utility switch without opening the PV system switches first. This prevents the second 15-kV service feeder from paralleling with the property PV system at any time. In addition, the relay and communication system measures the facility load and the solar power output. If the PV system output reaches 92% of the total load, a curtailment feature in the relay system forces an automatic reduction in power output. Any issues that occur will trigger alarms, to which the utility, installing contractor, and property operations have the ability to remotely limit plant output of the Phase I SMA inverters. The SMA inverter software also features a curtailment function that a system manager can use remotely if the power generated is reaching power-usage levels.

As of May 2016, the property consumes all of the PV-generated power and there has not been any curtailment of PV power. Based on the resort’s historical load profile and expected output of the PV arrays, it should not be necessary to curtail any PV power during the life of the system.

System design challenges—existing gear modification

The existing PV interconnect 12.47-kV switchgear had to be modified to tap the new Phase II circuit. Modifications included:

  • The existing switch was rebuilt to accommodate the Phase II cable entry.
  • Providing circuit overcurrent protection for the Phase 1 circuit. The non-fused mechanism had to be replaced with a fused unit.
  • Finally, the existing Kirk Key system had to be modified to comply with utility requirements.

All modifications required extensive planning and was staged so as not to interrupt the Phase I solar production. It was determined that major components of this work would occur during the night, under the cover of darkness and under one nighttime shift.

System design challenges—control modification

Upgrading a solar PV monitoring and control system in an existing large PV plant can be daunting. The control software had to be updated and tested. New 12.47-kV switch-position monitors and remote tripping were required, along with new meters, remote monitoring of the PV plant, weather monitoring equipment, new input/output cabinets, and so on. All of these points had to be thoroughly tested and validated. As a final point, utility-monitoring points and remote-tripping requirements also had to be integrated and witness-tested by the utility.

Robert R. Jones Jr. is the associate director of electrical for the Las Vegas office at JBA Consulting Engineers. He has experience in multiple market sectors including hospitality, commercial, medical, and government projects.

Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers. He specializes in renewable energy systems and complex medium and high voltage distribution systems.