The step-up substation of a solar plant collects the energy of the field transformer stations through the medium-voltage collector system and raises it to grid voltage with a power transformer. It has collector switchgear, a step-up (GSU) transformer, a high-voltage bay with breaker and instrument transformers, interconnection protection, revenue metering, a power plant controller and SCADA. Before it is energized, the equipment, the protection and the communications are tested, and the plant meets the requirements of the local grid code and of the utility it connects to.
What the step-up substation is
In a photovoltaic solar plant, the inverters deliver energy at low voltage. Each field transformer station, next to a group of inverters, raises it to medium voltage. The medium-voltage circuits, called the collector system, carry that energy to the step-up substation, a transformer substation where a power transformer raises it to the voltage of the utility grid.
Small plants can connect directly at medium voltage, through a connection switchgear panel and without a power transformer of their own. Larger plants connect at high voltage to the subtransmission or transmission system and need a complete substation.
Parts of the substation
| Part | What it does |
|---|---|
| Medium-voltage collector switchgear | Receives each collector circuit with its breaker and relay |
| Step-up (GSU) transformer | Raises medium voltage to grid voltage; usually has an on-load tap changer |
| Collector system grounding | If the medium-voltage winding provides no neutral, a grounding transformer creates one to detect ground faults |
| High-voltage bay | Breaker, disconnect switches, current and voltage transformers, surge arresters |
| Protection and control | Transformer, line and interconnection relays; bay controller |
| Revenue metering | Meters and metering instrument transformers at the point of interconnection |
| Power plant controller and SCADA | Regulate active and reactive power at the point of interconnection and send data to the control center |
| Station service | AC and DC supply for protection, control and communications |
The step-up transformer is the most expensive piece of equipment in the substation and the one that receives the most tests; we explain it in power transformer.
Protection
A solar plant is protected in two layers: the inverter protects its own equipment, and the protection at the point of interconnection protects the utility grid. In the step-up substation you find:
- Collector feeders: phase and ground overcurrent.
- Step-up transformer: transformer differential 87T, backup overcurrent and the transformer’s own mechanical protection.
- Point of interconnection: the interconnection functions (voltage, frequency, zero-sequence overvoltage where applicable and anti-islanding) and, if the plant connects through a line, the line protection the utility requires.
The minimum functions and the accepted anti-islanding methods are set by the grid code and the utility’s interconnection requirements. Where it is viable, direct transfer trip from the utility substation is the preferred method. The details are in solar plant protection and in the generation interconnection relay.
The settings of these functions are approved by the utility, and they change when the grid or the plant changes.
Control and supervision
The power plant controller (PPC) measures at the point of interconnection and dispatches active and reactive power setpoints to the inverters so that the plant behaves as a single unit. Plants connected to the transmission or subtransmission system must meet the technical requirements of the grid code: active power and frequency control, voltage control, a reactive capability curve and ride-through of voltage dips.
Utility requirements
Before construction, the plant obtains its transmission capacity and gets its interconnection approved by the utility. The substation engineering (single-line diagram, protection, metering, communications) is reviewed with the utility, and energization requires everything approved to have been tested.
Commissioning the step-up substation
Commissioning follows a general order:
- 01Primary equipment testsTransformer, breakers, disconnect switches, instrument transformers, cables and grounding grid
- 02Secondary testsInjection into each protection relay, logic, interlocks and trips to the actual breaker
- 03Communications and signalsSignals to SCADA, to the power plant controller and to the control center; time synchronization
- 04Tests with the utilityEnd-to-end teleprotection or transfer trip, and verification of the revenue metering
- 05EnergizationFirst the substation is energized from the grid; then the collector circuits and the inverters
- 06Plant testsVerification of the control requirements before the system operator
The energization order follows the real power flow: the grid energizes the substation and the collector system, and only then do the inverters connect and start exporting.
The tests of the PV array, according to IEC 62446-1, and those of the interconnection protection are in tests to energize a solar plant. The stages common to any substation are described in substation commissioning.
Common problems at energization
- Step-up transformer trips on energization because of inrush current and 87T settings without adequate harmonic restraint.
- Wrong polarity or ratio of current transformers in the differential or in the metering.
- Inverters that disconnect because the voltage at the point of interconnection is out of range.
- Signals that do not arrive, or arrive inverted, at the control center.
How we can help with your plant
Our team performs the testing and commissioning of solar plant step-up substations at medium and high voltage: primary equipment, protection, communications and coordination with the utility, in Colombia, or elsewhere on request. See our solar plant testing and commissioning service or write to us with the scope and the date of your energization.
Related guides
- Solar plant protection
- Tests to energize a solar plant
- Solar power plant controller (PPC)
- Generation interconnection relay
- Power transformer
- BESS commissioning: tests and requirements
References
- IEC 62446-1: Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1: Grid connected systems.
- IEC 60076 (series): Power transformers.
- IEEE 1547: Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces.
- IEEE 2800: Standard for Interconnection and Interoperability of Inverter-Based Resources (IBRs) Interconnecting with Associated Transmission Electric Power Systems.
Frequently asked questions
What is the difference between the inverter transformer and the substation transformer?
The transformer of each field transformer station raises the inverter output voltage to the medium voltage of the collector system. The step-up substation transformer receives all the collector circuits and raises that medium voltage to the utility's grid voltage, which is high voltage in large plants.
What protection does a solar plant substation have?
Collector feeder protection, differential and mechanical protection of the step-up transformer, line or point-of-interconnection protection as the utility requires, and the interconnection protection with voltage, frequency and anti-islanding functions. The minimum functions and the accepted anti-islanding methods are set by the grid code and the utility's interconnection requirements.
What is tested when commissioning the step-up substation?
The primary equipment (transformer, breakers, instrument transformers, cables), the secondary injection of the protection relays and their trips, end-to-end tests with the utility when there is teleprotection or transfer trip, the revenue metering and the signals to the control center. After that come the plant performance tests required by the system operator.