Connecting a solar farm to the MV grid

Which network a PV farm connects to

The voltage level a solar farm connects to depends mainly on its capacity and on what the local network can take:

• micro-installations (up to 50 kW) usually connect to the LV network under simplified rules;
• farms from a few hundred kilowatts to several megawatts or a little more most often connect to the 15 or 20 kV MV network;
• large farms connect to the 110 kV HV network, often through their own HV/MV substation.

The boundaries between these ranges are not fixed — it is the operator who, after analysing the technical conditions of the network at a given location, specifies the connection point and voltage level. This article focuses on the most common case: a PV farm connected to the medium-voltage network.

Step 1: applying for connection conditions

The connection path begins with submitting an application for connection conditions (warunki przyłączenia) to the relevant distribution system operator. The application is usually accompanied by, among other things:

• a document confirming legal title to the property where the farm will be built;
• a document confirming that the location is consistent with the land-use designation — an extract from the local zoning plan or a planning decision, depending on the planning situation;
• a site plan on a map showing the location of the installation;
• information on the planned capacity, type of installation and its technical parameters.

The required advance payment is also made with the application. Each operator publishes its current application form and list of attachments — it is worth checking them before submitting, as missing documents lengthen the procedure.

The operator analyses whether the network at that location can accept the additional capacity. If it can, it issues connection conditions specifying, among other things, the connection point, voltage level, connection capacity, the scope of works required on the operator's and investor's side, and the technical requirements for the installation. If the technical or economic conditions are not met, the operator may refuse to issue conditions — which is why choosing a location with network capacity in mind is one of the key stages of project preparation.

Step 2: the connection agreement

Connection conditions are valid for a set period. Within it, the investor should sign a connection agreement (umowa o przyłączenie) with the operator. The agreement sets out, among other things:

• the scope of works carried out by the operator (e.g. network reinforcement, building the service connection if it is on the DSO's side) and by the investor;
• deadlines for each stage;
• the amount of the connection fee and how it is settled;
• the ownership boundary — the dividing line between the operator's and the investor's assets;
• the conditions for acceptance and start-up of the installation.

The split of works varies. Often the investor builds its own part of the infrastructure — the farm's internal network, its transformer substation or substations and a junction box with metering — while the operator builds or commissions the service connection and any network reinforcement. There are also arrangements in which the investor also builds elements that are then handed over to the operator. The details always follow from the conditions and the agreement.

Step 3: design and permits

Based on the connection conditions, the designer prepares documentation covering both the farm itself and the connection infrastructure. On the electrical side this usually includes:

• the farm's internal LV cable network (DC and AC);
• LV/MV transformer substations at the inverters or inverter groups;
• the internal MV cable network linking the farm's substations;
• a collector substation or MV junction box with switchgear, protection and metering;
• the MV cable connection to the point specified by the operator;
• telecontrol and communication systems with the operator, where required;
• earthing and lightning protection.

The design must be agreed with the operator, and cable routes outside the farm site also with landowners, road authorities and other utility owners. The project obtains the decisions and permits required by building and environmental law — their scope depends on the scale and location of the development.

Technical elements of a PV farm connection

Technically, energy from a solar farm passes through several stages:

Element	Function
PV modules	generate direct current from solar radiation
Inverters	convert direct current into alternating current matching the grid; perform the control functions required by the operator
LV/MV transformer substations	step the voltage up from the inverters' output level to MV
Internal MV cable network	links the farm's transformer substations to the collector substation or junction box
Collector substation / GPO / MV junction box	collects energy from the farm and houses the switchgear, protection and revenue metering
MV cable connection	links the farm to the operator's network at the point specified in the connection conditions

Connected generators must meet the technical requirements set by the operator and the network codes, including those on protection, active and reactive power control, behaviour during network disturbances and the ability to curtail generation remotely. Some of these are handled by the inverters, others by equipment in the collector substation and the control systems. The operator sets out the specific requirements in the connection conditions and its own instructions.

Step 4: building the connection infrastructure

Building the electrical part of a solar farm usually involves:

• earthworks and laying the internal LV and MV cable networks and telecom ducts;
• placing the transformer substations and the collector substation or MV junction box;
• building the MV cable connection, often with directional drilling under roads and watercourses;
• installing cable joints and terminations and connecting the switchgear;
• earthing and equipotential bonding;
• an as-built survey of the completed networks.

This work has to be coordinated with the installation of structures and modules and with the operator, especially if the connection enters an existing substation or requires network outages. A well-planned schedule allows for equipment lead times — transformer substations, switchgear and MV cables — which can be long.

On the farm itself, cable trenches often run between rows of structures, so the sequence of work matters: cables and ducts are best laid before the modules are installed, or in tightly agreed time windows, so as not to block other crews or damage elements already installed. Outside the farm, the connection line runs like any other MV cable line — on an agreed route, with drilled crossings under obstacles and reinstatement of the land.

Step 5: testing, acceptance and start-up

Before the farm is connected, the completed infrastructure is tested: MV cable line tests, earthing and insulation resistance measurements, checks of protection and metering. The full set of test reports, as-built documentation and the as-built survey are handed to the operator.

The operator then carries out a technical acceptance of the connection and the installation and, once requirements are met, the farm is connected and started up. This may include tests of functions required of the generator, such as power control or communication with the operator's systems. In parallel, the investor completes the formalities related to energy trading — depending on the installation's capacity, a licence or an entry in the relevant register kept by the Energy Regulatory Office (URE) may be required — and signs the agreements needed to sell the energy.

What to watch for when planning

Experience of building connection infrastructure shows that many problems can be anticipated at the planning stage. Pay particular attention to:

• The connection route — the MV cable from the farm to the connection point often crosses land owned by others and the road corridor. A lack of consent can delay the project more than the construction itself, so the route is worth analysing and agreeing early.
• Obstacles along the route — roads, drainage ditches, watercourses and woodland usually mean directional drilling. Their number and length have a significant effect on cost and construction time.
• Delivery times — transformer substations, MV switchgear and cables often have long production lead times. Orders are worth planning ahead, linked to the permit schedule.
• The operator's requirements — standards for equipment types, telecontrol, metering and acceptance documentation differ between operators. Taking them into account early in the design and in purchasing avoids changes at acceptance.
• Network outages — connecting into an existing line or operator substation usually requires a planned outage, which has to be agreed well in advance.
• As-built documentation — a complete as-built survey and test reports are a condition of acceptance. They are worth preparing in parallel with construction rather than only at the end.

Every farm is different, and the rules for connecting renewable generators have changed several times in recent years. The outline above should therefore be treated as a general map of the process, with details checked on an ongoing basis with the operator and the project's advisers.

How we support renewable investors

We design and build the electrical infrastructure of solar and wind farms and their connections to the MV grid. In practice this means internal cable networks, transformer substations and MV junction boxes, connection cable lines with directional drilling, accessory installation, testing and preparation of as-built documentation. At the investor's request we can also take on equipment supply.

Our experience of building MV cable lines for operators — including for ENEA Operator — helps us understand DSOs' requirements for infrastructure the operator will take over or that the farm will work with.

Find out more on our solar farms and renewable energy investors pages. Terms such as GPO, connection capacity and connection conditions are explained in our glossary.