While zero-export systems are frequently associated with residential applications, the 6.4 MWdc MGM Phase I solar project at the Mandalay Bay Convention Center is required contractually to eliminate the possibility of back-feeding solar generation into NV Energy’s system.
In addition to a relay system at the solar plant switchgear and the main point of interconnection, this project’s zero-export strategy includes a system of three locks and two keys that ensures sequencing, allowing closure of only two of the three switches at one time.
The 6.4 MWdc MGM Phase I solar project at the Mandalay Bay Convention Center offsets a significant portion of the convention center’s electrical load. While the facility uses more power than the solar plant can generate, contractually the Mandalay Bay property must use all the power generated from the PV plant. Consequently, the project team installed multiple safeguards to ensure that solar generation would not back-feed into NV Energy’s system. The project also required that Mandalay Bay would not experience a power outage during system interconnection and commissioning. Project partners NRG Renew, MGM Resorts and Eaton worked together to develop a solution.
To meet the nonexport power requirement, the team installed metering at the point of connection between NV Energy and Mandalay Bay. This interconnection point is located at one end of a 12,470 V main-tie-main system. Normally, both mains are closed and the tie is open. A system of three locks and two keys ensures sequencing, allowing closure of only two of the three switches at one time. The new and existing switches are Eaton metal-enclosed switchgear (15 kV).
The PV system feeds into the switchgear connected to the (previously spare) switch between the tie and right main. Sunora Energy Solutions (an NRG Renew subsidiary) installed metering current transformers, potential transformers and a multifunction power meter to manage the power on the NV Energy side of the right main switch. NV Energy helped with the configurations, and enabled the team to avoid power outages to Mandalay Bay during system interconnection.
In the event of a power outage or to facilitate maintenance, the existing key interlocks allow operators to open one main and close the tie. In this condition, if the solar feeder remains on, the PV plant could export generated power through the unmetered left main. This would result in a contractually unacceptable situation. To eliminate the possibility of any power back-feeding onto the grid, installers added interlocks that require isolation of the solar feed before operators can close the tie switch.
The difficulty was adding the interlocks without impacting the power flow to Mandalay Bay. A creative key interlocking system avoided even a momentary power outage. The team installed a system of two locks and one key on the solar feed and tie switch. The new lock on the tie switch physically blocks access to the existing lock, which requires a key from one of the main switches to enable closure of the tie.
The team also installed a relay system at the solar plant switchgear and the main point of interconnection. This system allows NRG, Mandalay