Connecting a solar power plant to the MV grid

Which network a solar plant connects to

The connection voltage depends on the plant's capacity and the capabilities of the nearby network:

• small plants of households and small consumers are connected to the 0.4 kV network;
• plants from hundreds of kilowatts to several megawatts are most often connected to 6, 10 or 35 kV networks;
• large plants are connected to 110 kV networks and above — to DSO substations or, for the largest facilities, to the networks of the TSO, Ukrenergo (transmission system operator), usually through their own step-up substation.

The boundaries between these ranges are not rigid: the operator determines the connection point and voltage based on an analysis of the network at the specific location. Below we focus on the most common case — connecting a solar plant to a DSO's medium-voltage network.

Step 1: choosing the site and the connection application

Even before applying, it is worth assessing whether there is a network with spare capacity nearby: the distance to a 10–35 kV line or substation and its loading significantly affect the cost and timing of the connection. Many DSOs (oblenergos) publish information on the available capacity of their supply centres.

The procedure begins with a connection application to the DSO in whose area the site is located. It is usually accompanied by documents confirming the right to use the land plot, a site plan showing the installation's location, and data on the planned capacity and type of installation. The current application form and list of documents are set by the Distribution System Code and by the operator itself, so they are worth checking before submission — an incomplete package prolongs the procedure.

Connecting a power plant is, as a rule, a non-standard connection: the operator develops individual connection conditions that take into account the network layout at that location.

Step 2: connection conditions and the connection agreement

The DSO provides the customer with connection conditions and a draft connection agreement; the connection conditions are an integral annex to the agreement. Among other things, they specify:

• the connection point and voltage level;
• the connection capacity;
• the scope of works performed by the DSO (e.g. reconstruction of a supply centre or line) and the customer's scope;
• requirements for the power export scheme, relay protection, telemechanics, communications and commercial metering;
• requirements for power control and the plant's behaviour during network disturbances.

The fee for a non-standard connection is calculated using a methodology approved by NEURC. Some of the works — in particular building the line part of the connection — may, depending on the terms of the agreement, be done either by the operator or by the customer itself, with the result then transferred under an agreed procedure. The specific split always follows from the connection conditions and the agreement.

Step 3: design and approvals

Based on the connection conditions, the designer prepares documentation covering the plant and the connection infrastructure. On the electrical side it usually includes:

• DC and AC networks to and from the inverters;
• 0.4/10(35) kV transformer substations next to inverters or groups of inverters;
• the internal MV cable network between the plant's substations;
• the plant's switching station or MV switchgear with circuit breakers, relay protection and commercial metering;
• the MV cable or overhead line to the connection point;
• telemechanics and communication channels with the operator's dispatcher;
• earthing and lightning protection.

The connection part of the design is approved by the DSO. Routes outside the plant are approved with the owners and users of land plots, road owners and the owners of other utilities. The documents granting the right to carry out construction works, the need for a design review and other permits depend on the facility's consequence class and the planning situation.

Technical elements of the connection

Element	Function
PV modules	generate direct current
Inverters	convert direct current to alternating current matching the grid parameters; perform the control functions required by the operator
Step-up substations	raise the voltage from the inverter level to 10 or 35 kV
Internal MV network	connects the plant's substations to the switching station
Plant switching station / MV switchgear	collects the energy and contains switching devices, relay protection and commercial metering
Connection line	connects the plant to the DSO's network at the point set in the connection conditions

The plant must meet the operator's requirements for protection, active and reactive power control, behaviour during disturbances and the ability to curtail generation on the dispatcher's command. Some of these functions are performed by the inverters, others by the switching station equipment and the plant control system.

Step 4: construction

• earthworks and laying the internal cable networks and communication ducts;
• installing the plant's step-up substations and switching station;
• building the connection line, often with HDD crossings under roads and water bodies;
• fitting joints, connecting switchgear, earthing;
• certificates of concealed works and the as-built survey of the networks.

This work is coordinated with the installation of structures and modules: trenches between rows of structures are best dug before the panels are mounted or in agreed windows, so as not to hinder other crews. The schedule must allow for the lead times of substations, MV switchgear and cables, which can be long, as well as for the timing of outages on the DSO's network to connect to an existing line or substation.

Step 5: testing, acceptance, contracts

Before energisation, the built infrastructure is tested: cable lines, earthing, relay protection, metering equipment, and communications and telemechanics checks. Test reports, as-built documentation and the survey are handed to the operator. The construction project is accepted for operation in the manner established by law — depending on the consequence class, a declaration of readiness for operation or a certificate is issued.

Once the connection conditions have been fulfilled, the operator connects the installation. In parallel, the investor arranges participation in the electricity market: contracts with the DSO for distribution services, commercial metering registration, energy sale contracts or participation in the relevant support mechanisms, and imbalance settlement. Depending on the plant's capacity, a NEURC licence for electricity generation may be required. The exact list is worth confirming at the time of launch, as market rules change.

What to watch out for when planning

• Connection route — the line from the plant to the connection point often crosses other people's land and runs along roads; missing consents can delay a project more than construction itself.
• Obstacles — roads, canals, rivers and shelterbelts usually mean HDD, which affects cost and timing.
• Lead times — substations, MV switchgear and cables should be ordered in advance, in step with the approvals schedule.
• Operator requirements — standards for equipment, telemechanics, metering and documentation vary between DSOs; they should be reflected in the design and orders from the start.
• Outages — connecting to the existing network requires a planned outage, which is agreed with the DSO in advance.
• Documentation — complete as-built documentation is a condition of acceptance; it is best compiled in parallel with construction.

How we help renewable energy investors

We build the electrical infrastructure of solar and wind power plants and their connections to 6–30 kV networks: internal cable networks, substations and switching stations, connection lines with HDD crossings, accessory installation, testing and as-built documentation. At the investor's request we can also take on equipment procurement.

More on the Solar power plants and For renewable investors pages. Terms such as connection conditions or ownership boundary are explained in the glossary.