Wind-solar hybrid plant design in India: how shared evacuation changes the layout
A hybrid plant is not a wind farm with some panels added. It is one evacuation line that two resources have to share, and almost every design decision follows from that. This guide walks through how a wind-solar hybrid is laid out in India: what the policy requires, how the solar block is sized against the line, where the pooling substation goes, and the checks that keep both resources exporting without curtailing each other.
Why hybrid plants exist in the first place
Wind and solar in India are complementary across the day and across the year. Solar peaks around midday and drops to zero at night. Wind in the main corridors of Gujarat, Tamil Nadu, Karnataka and Maharashtra is strongest in the monsoon months and often picks up in the evening and at night. Put them on the same connection and the line carries energy for more hours of the day and more months of the year.
That is the whole point of the National Wind-Solar Hybrid Policy that MNRE adopted on 14 May 2018: better use of transmission infrastructure and land, lower variability, better grid stability. The policy sets one defining test. A project counts as a hybrid if the rated power capacity of one resource is at least 25 percent of the rated power capacity of the other. It also allows existing wind or solar plants to be hybridised using spare margin in their existing transmission capacity, and it provides for battery storage to be added to firm the output.
For the engineer, those three clauses translate into three design questions. What ratio of wind to solar fits the resource and the rule? How much of each can the line carry? And where does the storage, if any, connect?
Start with the line, not the land
On a standalone solar plant the design usually starts from the available land and the target capacity. On a hybrid the starting point is the evacuation capacity: the connectivity granted at the pooling point, in MW, and the voltage it is granted at.
Everything else is sized to that number. The sum of wind and solar AC capacity will almost always exceed the connectivity, deliberately, because the two resources rarely peak at the same moment. The design task is to choose how far to oversize each side so that the line is well used without clipping too much energy on the days both resources are strong.
A practical sequence:
- Fix the connectivity in MW and the point of interconnection.
- Fix the wind capacity from the turbine layout and the wind resource. Wind capacity is lumpy; it comes in turbine-sized steps.
- Size the solar block as the variable. It can be tuned in finer increments and it is the cheaper resource to oversize.
- Model the combined hourly output against the line limit across a typical meteorological year and count the curtailed energy.
- Iterate the solar DC-to-AC ratio and the solar AC capacity until the curtailment cost is below the value of the extra energy.
This is why the yield study on a hybrid is a combined study, not two separate reports stapled together. The hourly coincidence of the two resources is the number that drives the money.
The 25 percent rule in practice
The policy test is about rated power capacity, not energy. If the wind farm is 100 MW, the solar block must be at least 25 MW for the plant to qualify as a hybrid, and the reverse holds if solar is the larger resource.
Most projects sit well above the minimum. Wind-dominant hybrids in the western corridor commonly add solar at 40 to 70 percent of the wind capacity, because that is roughly where the midday solar output fills the gap the wind leaves. Solar-dominant hybrids, more common where a large solar park has spare evacuation, add wind to lift night-time and monsoon output.
The design implication is simple: decide the ratio from the resource data and the line, then check it against the 25 percent floor, not the other way round. A plant designed to the floor usually under-uses the line.
Where the pooling substation sits
On a hybrid, the pooling substation is the hinge of the layout. The wind collector system comes in at 33 kV from the turbines, the solar block comes in at 33 kV from the inverter stations, and both are stepped up at the same substation to the grid voltage, typically 132 kV, 220 kV or 400 kV depending on the connectivity.
Three placement rules usually hold:
- Put the substation close to the grid interconnection point. HV line is more expensive per kilometre than 33 kV collector cable, and the HV route needs right of way.
- Put the solar block adjacent to the substation. Solar collector runs are many and short; wind collector runs are few and long. The resource with more cable benefits most from proximity.
- Leave space on the substation plot for a storage block and its power conversion system, even if storage is not in the first phase. Retrofitting a bay later is far more disruptive than reserving the land now.
The single-line diagram for a hybrid substation differs from a solar substation in one important way: the two resources need separate metering and separate protection zones at 33 kV before they merge, because they are often scheduled, accounted and sometimes contracted separately. Energy accounting for RPO purposes follows the proportion of wind and solar rated capacity, so the metering must be able to tell the two apart.
Laying the solar block inside a wind farm
When the solar block sits on the same land as the turbines, three constraints shape it that do not exist on a standalone solar plant.
Shadow flicker and tower shade. A turbine tower and its rotor sweep cast a moving shadow that travels across the ground through the day. Solar rows inside that sweep lose output and, worse, see rapidly changing partial shading that stresses inverters and mismatches strings. Keep the solar block outside the shadow envelope of each turbine, which is longest at low sun angles in winter mornings and evenings.
Crane pads and access. Turbine erection and maintenance need crane hardstands and wide access roads with turning radii for blade transport. Those corridors are permanent; a solar row across a crane path will be removed the first time a gearbox fails. Get the turbine supplier’s crane pad and road drawings before the solar layout starts.
Cable corridors. The wind collector cables run from each turbine to the substation. Route the solar block so its own cable trenches do not cross the wind collector trenches more than necessary. Every crossing is a future excavation risk and an earthing coordination point.
Interconnecting at AC or DC
The policy allows integration at the AC or the DC level. In practice almost all utility-scale hybrids integrate at AC, at 33 kV, because wind turbines and solar inverters have their own power electronics and their own protection, and tying them together at 33 kV keeps each resource’s equipment independent.
DC coupling, where solar strings feed into the DC link of a turbine converter, exists in the literature and in a few small projects. It is rarely worth the complexity at scale. The one place DC coupling earns its keep is storage: a battery on the solar block’s DC bus can soak up clipped solar energy before the inverter, which is the cheapest energy a hybrid plant has.
Protection and earthing coordination
Two generation technologies on one substation means two fault-current behaviours. Modern wind turbines with full converters and solar inverters both limit fault current to a little above rated current, so the 33 kV protection must be set for low fault contributions, and the grid-side protection must still see faults on the collector system. This is routine for an engineer who has done both wind and solar; it is a common gap when a solar team adds wind for the first time.
Earthing is the other coordination point. The wind collector system and the solar block will each have their own earthing design, and both tie into the substation earth grid. The step and touch potentials at the substation must be checked for the combined fault current, and the earthing of the two 33 kV systems must be designed so that a fault on one side does not raise the potential of the other beyond its limits.
Scheduling, forecasting and the control system
A hybrid plant forecasts and schedules as one plant at the point of interconnection, but the plant controller has to manage two resources with different controllability. Wind can be curtailed by pitching blades, solar by inverter setpoints, and both respond on different timescales. If storage is present, it becomes the fastest lever.
The design deliverable here is the plant control philosophy: who curtails first when the line limit is reached, how the two resources share reactive power duty, and how the plant meets the grid code’s ramp-rate and voltage requirements as one unit. Writing this down at design stage avoids the common failure where the wind SCADA and the solar SCADA each believe they own the point of interconnection.
Hybridising an existing plant
The policy specifically allows existing wind or solar plants to be hybridised, with higher transmission capacity than originally sanctioned where margin exists, and without additional connectivity or transmission capacity charges for the hybridisation. That makes an existing wind farm with spare evacuation the most attractive hybrid site in the country.
The engineering sequence for a retrofit:
- Confirm the actual available margin at the pooling substation and on the downstream line, in MW and in hours, from the existing plant’s export data.
- Survey the land inside and around the wind farm for the shadow envelope, crane corridors and cable routes described above.
- Size the solar block to the margin, using the existing plant’s hourly export as the hard constraint.
- Design the new 33 kV bay, metering and protection at the existing substation, and check the earth grid for the added fault contribution.
- Update the plant controller so the combined export never exceeds the connectivity.
Adding storage
The policy allows battery storage in a hybrid to reduce variability, lift energy at the delivery point, and provide firm power for defined periods. Recent central tenders for firm and dispatchable renewable energy and round-the-clock supply have pushed developers toward wind, solar and storage on one connection.
For the layout, the storage decision changes two things: the reserved plot at the substation, and the DC bus design on the solar block if DC-coupled storage is chosen. For the yield, it changes the curtailment calculation entirely, because clipped energy becomes stored energy. The sizing of that storage is a topic on its own; see the post on wind plus storage.
Deliverables for a hybrid design
A complete hybrid design package from Eden’s wind and hybrid engineering team contains:
- Combined resource and yield assessment with hourly coincidence analysis and curtailment estimate, P50 and P90
- Turbine micro-siting and solar block layout on one site plan, with shadow envelope, crane corridors and cable routes shown
- 33 kV collector system design for both resources, with separate metering and protection zones
- Pooling substation single-line diagram, general arrangement and earthing design for the combined fault contribution
- Plant control philosophy covering curtailment order, reactive power sharing and grid-code compliance
- Grid connectivity and CEIG approval drawings
- Balance-of-plant BOQ
Inputs we need to start
- Connectivity granted: MW, voltage, point of interconnection, and the hourly export limit if any
- Wind resource data: met mast or LiDAR campaign, with duration
- Turbine model shortlist, with hub height and rotor diameter
- Site boundary, land ownership and any existing plant drawings
- Solar irradiance data for the site, or permission to use a satellite dataset
- Grid code and tender requirements the plant must meet
- Whether storage is in scope now, later, or not at all
Send those and the first layout iteration comes back with a curtailment number attached, so the ratio discussion starts from data.
Next step: see what is included in our wind energy engineering service, or send the inputs above through the quote form or on WhatsApp at +91 95107 18852.
