NEUTRON DC Protection and Combiner Scope for a 120 MW Yemen Solar Plant

1. Project context and site constraints

A 120 MW utility solar plant on Yemen's southern coastal plain is a demanding reference for balance-of-system engineering. High ambient temperature, blowing dust and a 1500 V DC array concentrate every design risk on the protection and switching equipment. NEUTRON was engaged to supply the DC protection chain and the low-voltage switchgear that sit between the PV strings and the plant substation, while the inverter, battery and module scope was delivered by the EPC.

The site receives strong year-round irradiation but also sustained air temperatures above 45 °C and frequent dust loading. These conditions drive three specification decisions: rated current must cover bifacial string peaks, enclosure protection class must resist abrasive dust and salt mist, and every thermal limit must be derated for the climate rather than quoted at 25 °C laboratory conditions.

2. Protection challenge: 1500 V DC strings at scale

With more than 85,000 modules arranged in hundreds of strings, the combiner layer is where fault current is first contained. Each string enters a combiner box protected by a gPV fuse sized to 1.25× the module short-circuit current including rear-side gain. A DC-rated disconnect switch and a coordinated DC surge protective device complete the in-box protection before the aggregated feed leaves for the inverter station.

3. NEUTRON supply scope

The supply scope combined the protection and collection equipment required between the PV strings and the grid interface.

• PV combiner boxes rated to 1500 V DC, collecting 16–24 strings each with individual gPV fuse protection.
• DC miniature circuit breakers for auxiliary and monitoring circuits, DC-rated per IEC 60947-2.
• Low-voltage switchgear assemblies at the substation, built to IEC 61439 for the AC collection side.
• Ring main unit coordinating the plant step-up interface where required by the grid connection.
• DC surge protective devices staged to the 1500 V envelope (IEC 61643-31).

4. Combiner box configuration

NEUTRON configured the combiner boxes around four project numbers: maximum system voltage (1500 V DC), string short-circuit current including bifacial gain, number of input strings, and site protection class. The internal busbar and terminal block were rated against the summed continuous current of all protected strings, and the enclosure ventilation path was validated so the fuse and SPD stayed inside their rated temperature bands under peak site load.

5. Substation switchgear and RMU coordination

At the plant substation the AC collection feeders terminate in NEUTRON low-voltage switchgear. A ring main unit provides the controlled interface to the step-up transformer and grid connection, with selective coordination so a downstream fault isolates locally without dropping the whole array. This selectivity was verified against the projected fault levels before release.

6. Thermal and environmental derating for desert sites

The site conditions drove a coordinated environmental review for the current-carrying parts, enclosure and protective devices.

• Current-carrying parts derated for sustained 45 °C+ ambient and solar gain on the enclosure.
• Enclosure specified to IP65 with a corrosion class suited to coastal dust and salt mist.
• Ventilation or passive cooling sized so protection devices stay within rated temperature.
• DC SPD staging reviewed for the local lightning density and soil conditions.

7. Commissioning and monitoring

During commissioning the combiner boxes were inspected for torque, polarity and insulation resistance before energisation. String-level monitoring at the output lets the operator detect a degraded or faulted string early, reducing unplanned downtime across a plant of this size.

8. Outcome and lessons

The protection scope performed to specification through the first hot-season cycle, with no combiner-level nuisance trips attributable to the protection design. The key lesson for similar Yemen utility projects is to derate for climate up front and to coordinate the combiner, switchgear and RMU as one protection system rather than as separate procurements.