Solar Park Engineering and Design: Resource to Grid

Short answer

The engineering of a solar park runs through a chain of decisions: the site solar resource, the array layout, the strings and DC cabling, the inverters and transformer stations, the medium-voltage collector network, the step-up substation and the connection to the transmission or distribution grid. On top of that chain the protections, the plant control demanded by the local grid code and, if present, the storage are designed. Each stage has its standard and its engineering decision.

What the engineering of a solar park covers

A solar park is a power plant. It converts irradiance into direct current in the PV array and turns it into alternating current in the inverters. It then steps it up in the transformer stations, collects it through a medium-voltage network and delivers it to the grid at a step-up substation. The engineering decides how each step is done so that the plant produces what is expected, is safe and meets what the local grid code and the utility require.

These notes follow the plant stage by stage. For each one they indicate the engineering decision that weighs most and the standard that governs it. The utility process is covered in the grid interconnection requirements of each jurisdiction, which the design adapts to.

Engineering chain of a solar park

Resource and layout. Site irradiance, row spacing, fixed or tracker
DC array. Strings, cabling, protection and grounding according to IEC 62548-1 and the local code
Inverters and stations. Conversion to AC and step-up to medium voltage
Collector network and substation. Medium-voltage circuits, step-up transformer and interconnection bay
Interconnection, protection and control. Grid code, PPC and SCADA

Stage 1: the solar resource and the layout

Everything starts with the site irradiance. First estimates come from public databases and are refined with on-site measurement. With that data the layout is decided: tilt of the tables, row spacing, monofacial or bifacial modules, and fixed structure or single-axis trackers. Near the equator the sun passes high and the optimal tilt of a fixed structure is low.

Row spacing sets the ground coverage ratio (GCR): more modules per hectare at the cost of more mutual shading in the first and last hours of the day. It is an economic decision supported by the production simulation; the criteria are in PV array design, and the measurement of the resource during operation is in weather station and monitoring of a park.

Stage 2: the DC array

The number of modules per string is set with the open-circuit voltage corrected to the minimum site temperature, which cannot exceed the maximum module voltage or that of the inverter. The minimum is set by the maximum power point tracking (MPPT) window at high cell temperature. From there the parallel strings, the need for fuses, the combiner boxes if the inverter is central, the cable cross-section and the voltage drop are defined.

IEC 62548-1 covers the design requirements of the PV array: DC wiring, protective devices, switching and grounding. The installation must also meet the local electrical code. The DC side has a particularity that conditions the whole design: as long as there is light, the array is energized and there is no upstream switch that de-energizes it.

Stage 3: inverters and transformer stations

There are two architectures in parks: high-power central inverters in a station with its transformer, or string inverters distributed in the field that reach a transformer station at low voltage. The choice changes the DC cabling, maintenance and availability. The ratio between the installed DC power and the inverter rated power (DC/AC ratio) defines how much energy is clipped in the hours of maximum irradiance.

The transformer station brings together the medium-voltage transformer, its switchgear or an RMU and the station service. Several manufacturers deliver it as an integrated assembly in a container or skid. On this site we publish datasheets of inverters such as Huawei SUN2000, SMA Sunny Central and Sungrow.

Stage 4: collector network and step-up substation

The collector network joins the stations with the substation through medium-voltage circuits, normally underground, radial or in a ring. Its design sets the cable cross-section, the losses, the ground fault current according to the neutral grounding method and the capacitance the collector network adds to the plant, which weighs on reactive power and on resonances.

The step-up substation holds the medium-voltage busbar with the bays of the collector circuits, the power transformer and the interconnection bay toward the grid. There are the revenue metering, the protections at the point of interconnection and the interface with the utility.

Stage 5: interconnection, protection and plant control

Interconnection. The grid capacity and the connection process follow the procedure of the local utility or system operator, with the studies it requires.

Technical requirements. For solar plants connected to the transmission or distribution grid, the grid code sets active power and frequency control, voltage control, the reactive capability curve and voltage ride-through. The reactive power it requires is measured at the point of interconnection, not at the inverter terminals.

Protection. The interconnection protections follow the utility requirements, and their settings are adapted to the local code. The detail by equipment is in solar plant protection.

Control and monitoring. The plant controller (PPC) coordinates the inverters to meet the setpoints at the point of interconnection, and the SCADA, over the network described in solar plant communications, delivers to the system operator the signals and meteorological variables the regulation requires.

Stage 6: storage, testing and operation

If the project includes batteries, it is decided whether they are coupled on the DC bus of the inverters or on the medium-voltage busbar, and how their management system and the plant control are coordinated.

The engineering ends with the tests: IEC 62446-1 for the PV system, the tests of medium and high voltage equipment, those of the protections and those the utility and the system operator require. The path is in tests to energize a solar plant.

Site particularities

Altitude. Many Andean projects sit above 2,000 m. Thinner air reduces dielectric strength and cooling capacity, so transformers, switchgear and inverters are specified for the site altitude according to their product standard and the manufacturer datasheet. In a park at 2,590 m, we measured the power quality and the loading of its transformers.
Corrosion. Near the sea, salt air calls for choosing coatings and materials by atmospheric corrosivity category, classified according to ISO 9223.
Lightning. There are zones of high flash density. The risk assessment is done with IEC 62305, and from it come the shielding, the grounding and the surge protective devices on the DC and AC sides.
Civil works and permits. Structures and foundations follow the local building code, and the project needs its environmental permits. They are disciplines apart from electrical engineering, but their lead times condition the schedule.

We help with the engineering of your park

Our team does the primary engineering of solar plants and their substations: single-line diagrams, medium-voltage network, step-up substation, grounding and drawings for the utility. We do it remotely, worldwide, with a team that works at medium, high and extra-high voltage. When the plant is built, we do its commissioning, in Colombia or elsewhere on request. Tell us about your project.

References

IEC 62548-1:2023, Photovoltaic (PV) arrays – Part 1: Design requirements.
IEC 62446-1, Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1: Grid connected systems.
ISO 9223, Corrosion of metals and alloys – Corrosivity of atmospheres – Classification, determination and estimation.
IEC 62305, protection against lightning.

Frequently asked questions

Which standards apply to the electrical design of a solar park?

The local electrical code and the utility interconnection requirements for the installation and the connection, and the IEC product and design standards, such as IEC 62548-1 for the PV array, as the technical framework. The design and settings are adapted to the local code and to the requirements of the utility and the client.

In what order is a solar park designed?

First the resource and the layout, because the DC power and the shape of the array come from them. Then the strings, inverters and stations, the collector network and the substation. The protections, plant control and communications are designed on that single-line diagram, and the interconnection studies validate it against the grid. In practice there are several iterations: an interconnection study can force a change in the reactive power compensation or the transformer.

What changes when designing a solar park at a high-altitude, coastal or high-lightning site?

Altitude, which affects the insulation and cooling of the equipment, the corrosivity of coastal air, and the lightning flash density, which weighs on lightning protection and surge protective devices. Each is handled with the product standard of the equipment and the site conditions, not with a generic rule.