One Terrace, Two Uses: 85 kWp Elevated Solar with DG Synchronisation at Steadfast Limited

The problem: one terrace, two uses

Most rooftop solar projects start with an empty terrace. At Steadfast, the fifth-floor terrace was the only open area with a clear view of the sky, and it was also needed as a covered godown. A conventional rooftop plant would have taken the terrace for solar alone.

So the brief had several parts at once:

• Keep a full-height, usable godown on the terrace
• Carry the solar array on a structure strong enough for the module and wind loads
• Keep the roof watertight over stored material for the life of the plant
• Survive the stronger wind that acts on a structure five floors up
• Give the modules a proper slope
• Leave safe access for installation and for maintenance over 25 years
• Connect 85 kWp into a building served by 1 MVA of grid supply and 1 MVA of DG backup, without reverse power ever reaching the generator

The solution: one structure, two jobs

We engineered a raised steel shed on the terrace. Steel columns rise about 15 ft at the front and 18 ft at the rear, and carry a sheet roof. The solar panels are fixed on that roof, and the space underneath is the godown.

The 3 ft difference between front and rear columns does three things at once: it gives the modules their slope towards the sun, it lets rainwater and dust run off the roof, and it leaves full working height inside the godown.

What this bought the client:

• Two uses from one terrace: covered storage below and a power plant above, on the same footprint
• Unused sky turned into an asset: the roof is designed to produce about 1.36 lakh units a year
• A cooler godown: the panels shade the sheet roof, cutting the direct sun it absorbs during the day

Engineering and building a structure five floors up

A roofed structure 15 to 18 ft tall on top of a five-storey building behaves very differently from a low rooftop frame. Wind speed rises with height, building edges accelerate it, tall columns act as levers, and a roof covered in panels presents a large surface to the wind. The design had to deal with:

• Wind uplift on the roof and panels, and lateral wind pressure on the whole frame
• Higher exposure near the building edges
• Bending and sway in the tall columns, and vibration over time
• Stability in both directions, along and across the array
• A clear load path from the panels, through the roof framing and columns, down into the building structure
• Corrosion resistance for a 25-year life
• Safe access for inspection and maintenance

Diagonal and lateral bracing was built into the frame to stiffen it and spread wind forces across the whole structure. The weight of the array and the wind acting on it travel through the roof framing and down the columns into the building.

Everything had to go up by hand and hoist: modules, long structural sections, mounting rails, DC and AC cable, cable trays, protection equipment and earthing material. We sequenced the work in controlled stages, erecting and bracing the structure before any module went up, so that nothing was left unsecured at height. Tools and members were controlled throughout to prevent falling objects, and the building stayed in normal use below.

Running solar alongside a 1 MVA DG set

A grid-tied solar inverter takes its voltage and frequency reference from the grid. When the grid fails, anti-islanding protection switches the inverter off within moments. On a site with no DG, that is the end of the story.

At Steadfast, switching off was not good enough. The solar plant is synchronised with the 1 MVA generator, so the two share the same electrical bus during an outage, and the solar plant must never push power back into the generator. The plant works in four modes:

• Grid available: the inverters synchronise with the utility supply. Solar feeds the running load, the grid supplies the balance, and the DG stays off.
• Grid fails: anti-islanding protection disconnects the inverters. The DG starts and builds a stable bus. Once voltage and frequency are within limits, the inverters synchronise with the DG and solar resumes.
• Running on DG: solar output is held within the live building load, so the DG always carries a safe minimum share. If the load drops too low, solar output is curtailed or disconnected. Generator stability takes priority over maximum solar output.
• Grid restored: incoming supply is checked, changeover takes place, the DG runs its cool-down and stops, and the inverters reconnect to the grid after their programmed delay.

What this means for the client: during a power cut the solar plant does not sit idle. It carries part of the building's load, so the DG supplies less power for every hour it runs.

Why reverse power matters, in one example. Suppose the building draws 150 kW while running on DG, and the solar plant is producing 70 kW. The DG supplies the remaining 80 kW. Now a large load switches off and the building drops to 50 kW. If solar kept producing 70 kW, 20 kW would try to flow backwards into the generator. The result is reverse-power trips, DG hunting, voltage and frequency swings, and a generator running at unhealthy low load. The control arrangement prevents this by tying solar output to the live building load and to the generator's minimum safe loading.

Sizing the plant

The 85 kWp size was set by the shadow-free roof area of the raised shed, its geometry, the building's daytime consumption, the capacity of its LT electrical system and interconnection point, and the need to stay safely below the building's load at all times so the DG never sees reverse power.

At about 8.5% of the 1 MVA sanctioned load, the plant is small relative to the building. That is deliberate. The building absorbs every unit of solar during working hours, so none of it has to be exported at a lower rate, and there is ample load headroom for DG operation.

Design basis: generation and carbon

The figures below are the plant's design values, not metered results.

• Design specific yield: about 1,600 kWh per kWp per year
• Year-1 generation: about 1,36,000 kWh (1.36 lakh units)
• Average generation: about 11,333 kWh a month, or 373 kWh a day
• Generation over 25 years: about 32 lakh kWh, allowing 0.5% module degradation a year
• CO2 avoided: about 95 tonnes in Year 1, and about 2,240 tonnes over 25 years

Carbon figures use an indicative grid emission factor of 0.70 kg CO2 per kWh. Actual generation varies with season, fog, monsoon cloud and soiling. Because every unit is consumed on site, each kWh offsets electricity the building would otherwise buy at its full commercial tariff, which is where captive rooftop solar earns its best return.

Maintaining an elevated structure

A tall frame five floors up needs more than panel cleaning. On top of routine electrical checks, the structure itself is inspected for:

• Loose fasteners and deformed bracing members
• Corrosion at welded and bolted joints
• Movement at the support points and wind-induced vibration
• Sagging or chafed cables along the elevated route
• The sheet roof around every panel fixing point, and any water ingress into the godown
• Bird nesting, debris and blocked drainage paths
• The safety of the access route itself

Because the plant shares a bus with the DG, the solar and DG interlocks and reverse-power protection are also tested periodically, not assumed.

What this project shows

• A terrace does not have to choose between storage and solar. One raised structure can give covered space below and a full power plant above.
• Height is an engineering problem, not a blocker. Wind, load path and bracing were designed for from the start.
• Grid-tied solar can live alongside a large DG set when the control and protection logic is engineered for it, rather than switched off and forgotten.
• Sizing to the load pays. An 85 kWp plant on a 1 MVA building means every unit is used on site.

This combination, structural work at height plus DG integration, is common in hospitals, healthcare facilities and multi-storey corporate buildings. Planning solar on a roof that is already in use? Our engineering team will assess your roof and your electrical system before recommending anything. Email wecare@mgetenergy.com.

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