Commissioning & Grid Connection
The staged sequence that turns a finished construction site into a running power station - clean it, energise it, fire it, raise steam, then synchronise and prove it. The network operator has the final say on when it connects.
Last updated 2026-08-25
Typical duration
Typically 4-9 months from the first system turnover to commercial operation, with pre-commissioning starting well before construction finishes. The connection window set by the network operator, not the plant, decides the end date.
What is Commissioning & Grid Connection?
Commissioning is not the last few weeks of a power station project. It is a separate discipline with its own team, its own programme and its own way of thinking, and it starts long before construction finishes. Systems are handed across from construction one at a time, and each one goes through the same staged progression: inspection and checking, cleaning and flushing, instrument calibration and loop testing, functional testing without process, testing with process, and finally operation. Across the station as a whole the same shape repeats at a larger scale - pre-commissioning, cold commissioning, first fire and steam raising, synchronisation, then performance and reliability testing. Each stage begins only when the one before it is complete and signed off. The detail of every step, the sequence, the holds and the criteria for moving on all come from the equipment suppliers and the commissioning engineer, and they are specific to the plant being commissioned rather than general knowledge.
The point at which commissioning takes a system is also the point at which the site changes character. A construction site is worked on. A commissioning site is operated. Systems become live and stay live, isolation and permit control take over from method statements, energy that was not there yesterday is there today, and the person who can authorise work on a system is no longer the person who built it but the commissioning engineer who now owns it. That transition is managed formally, with unmistakable boundaries between what is still construction and what is not, because the most dangerous period on a power station project is the long overlap when both are happening at once in the same building. Running through all of it is a preoccupation with cleanliness. Everything left inside the pipework during construction - weld spatter, scale, offcuts, rag, mill debris - will be carried by the first flow of water, oil or steam into something with fine clearances that cost a great deal of money. Cleaning the systems out is the largest single activity in the early stages and the one that most often takes longer than the programme allowed.
The last stage is not in the project's gift. Connecting a generator to the network is controlled by the network operator, who agreed the connection date years earlier and who sets what the plant has to demonstrate before and after it is connected. There is a compliance process to work through, tests to be carried out and witnessed, data to be submitted and accepted, and permission to be granted for each step - the first energisation of the connection, the first synchronisation, and the release to full output. None of it happens because the project feels ready. It happens when the operator says so, in a window the operator can move for reasons that have nothing to do with the station. After synchronisation come the tests that matter commercially: the performance tests that establish what the plant actually produces and what it consumes doing it, and a reliability run that requires the plant to hold that condition continuously. Those results are what the contract, the lenders and the offtaker have been waiting for, and they are the real end of the project.
Compare the methods at a glance
When and why is Commissioning & Grid Connection used?
Commissioning starts far earlier than most people expect and it does not really stop until the plant is handed over. The control building is needed early because the commissioning team works from it. The control system is installed, powered and configured long before the plant it controls is finished. Auxiliary power, instrument air, service water and demineralised water are commissioned ahead of everything else because everything else consumes them. That is why the commissioning team is embedded in the project during construction rather than mobilised at the end - the sequence they need has to be built into the construction programme, and a system-based completion strategy is the only thing that makes it possible. Handing over a floor of a building means nothing to a commissioning engineer. Handing over a complete, tested, documented system means everything. The reason for the staged approach is that each stage protects the one after it. Cleaning protects the machine. Calibration and loop testing protect the controls. Cold commissioning finds the wiring and rotation errors while nothing is hot and nothing is turning fast. First fire and steam raising are approached in steps because that is when the plant first behaves like a plant, and problems found then are found in the presence of the people who can fix them. Skipping a stage never saves time. It moves the discovery of the same problem to a point where the plant is running, the damage is real and the repair is measured in weeks. All of it sits under a fixed date. The connection date was committed to the network operator years in advance, the commercial arrangements are built on it, and commissioning is the last activity before it, which means commissioning absorbs every delay upstream of it. That is precisely the wrong place for a delay to land, because it is the one phase where hurrying causes damage rather than saving days.
Types of Commissioning & Grid Connection
Explore each method in depth - benefits, limitations, plant and quality control on its own page.
Pre-commissioning and system turnover
The unglamorous majority of the work. Systems are accepted from construction, checked against their drawings, cleaned and flushed, instruments calibrated, control loops proved end to end, motors checked for direction and rotating equipment run in. It happens system by system, quietly, over months, and it rarely appears on a progress photograph. Almost everything that goes wrong later is either found here or created here.
Cold commissioning
Testing with the plant filled and running but without fire or steam. Auxiliary systems in service, pumps and fans operating, valves stroking, protection and interlocks proved, alarms and trips tested, and the control system exercised against the plant it is actually wired to rather than against a simulation. The stage where a wiring error is a nuisance instead of a disaster, which is exactly why it is done thoroughly.
Hot commissioning
First fire, steam raising and the staged bringing of the plant up to operating condition, together with the steam-side cleaning the equipment supplier requires before steam is admitted to the turbine. Carried out by the commissioning engineer to the suppliers' sequences, with the plant taken up in steps and held at each one while it is checked. The most technically demanding stage and the one with the least tolerance for improvisation.
Grid compliance, performance and reliability testing
Everything from first synchronisation onwards. The network operator's compliance testing, the contractual performance tests that establish what the plant produces and what it consumes, and a continuous reliability run that proves it can hold that condition. Witnessed, recorded and disputed more often than any other part of the project, because these results decide when the plant is accepted and paid for.
Best suited for
- New build combined cycle and thermal stations working backwards from a fixed connection date
- Repowering and major refurbishment schemes reconnecting existing plant to the network
- Projects committed to system-based completion, where turnover packages can actually be accepted
- Schemes whose commercial acceptance depends on witnessed performance and reliability testing
- Any plant where cleanliness and staged testing protect equipment that cannot easily be repaired in place
Commissioning & Grid Connection: step by step
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Step 1: Build the commissioning sequence into the construction programme
The commissioning team is involved while the plant is still being drawn, because the order systems are needed in is not the order they would naturally be built in. Systems are defined, boundaries are drawn, and a turnover sequence is agreed that puts the station's own services first and works outwards from there. The construction programme is then re-cut around that sequence rather than the other way round. Documentation requirements are set at the same time - what a turnover package has to contain before commissioning will accept it - because a system that arrives without its records is not a system that can be commissioned. The control building, the control system and the auxiliary supplies are all pulled forward, since the commissioning team needs somewhere to work from and something to work with. This step costs nothing and it is the difference between a commissioning phase that runs and one that spends its first two months arguing about what has actually been finished.
- 2
Step 2: Accept systems from construction
Turnover is a formal transaction, not a handshake at a door. Construction presents a system as mechanically complete, with its marked-up drawings, its test records, its vendor documentation and its punch list. The commissioning team walks it down against the drawings, checks that it is genuinely complete and genuinely separable from what is not, and either accepts it with the outstanding items agreed and owned, or refuses it. Once accepted, control of that system passes: it comes under the commissioning permit and isolation regime, and any further construction work on it happens only with the commissioning engineer's authority. Doing this properly is what keeps a site safe through the long overlap where some systems are live and others are still being built. Doing it loosely is how somebody ends up working on a system that somebody else has just energised.
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Step 3: Clean and flush the systems
Everything that will carry water, steam, oil, air or fuel is cleaned out before it is allowed to carry it. The methods vary by system and by what the equipment supplier will accept - flushing, blowing, circulation through temporary filters and screens, and chemical cleaning where the design calls for it - and the procedures, the sequences and the acceptance criteria all come from the equipment supplier and the commissioning engineer rather than from general practice. Temporary strainers, screens, pumps, hoses and connections are installed for the purpose and removed afterwards, and keeping track of what has been fitted and what has been taken out again is a real task in itself. Cleanliness is demonstrated and recorded rather than assumed, because the party accepting the system is the party who will live with whatever was left inside it. This activity routinely takes longer than planned, and it is the one place where hurrying reliably produces damage that appears later, inside a machine, where nobody can reach it.
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Step 4: Calibrate the instruments and prove the control loops
Every instrument on the plant is calibrated and recorded, and every control loop is checked from the field device through the cabling and the marshalling to the control system and back out to whatever it operates. That is thousands of individually trivial checks, and it is where wiring errors, reversed signals, wrong ranges and mislabelled cores are found. The control system is configured, its graphics built, its alarms set up and its interlock and protection logic tested against the plant rather than against a simulator, because the entire point is to prove the connection between the two. Trip and protection systems get particular attention, since they are what will be relied on when something goes wrong and they are also the hardest things to test convincingly. The record produced here becomes the baseline the operator maintains the plant against for the rest of its life, so it is written to be used rather than filed.
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Step 5: First energisation and the electrical systems
Electrical commissioning runs ahead of most of the plant, because everything else needs power. The auxiliary transformers, switchboards, motor control centres, battery and uninterruptible systems and the emergency generation are tested and progressively brought into service, with protection applied to the settings the protection engineer has issued and proved before anything is energised. Energisation is staged, controlled and announced, with the boundaries of what is live made unmistakable to everyone on site, and from that moment the site has permanent live equipment on it and the discipline around it changes for good. The connection out to the network is energised only with the network operator's permission and to their sequence. The generator, its excitation and its protection are tested to the requirements of both the equipment supplier and the operator, and those results form part of the compliance evidence the operator will want to see before any synchronisation is allowed.
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Step 6: Cold commissioning of the plant
With the systems clean, calibrated and powered, the plant is run without fire. Pumps, fans, compressors and the rest of the rotating equipment are checked for direction, run in, and monitored for vibration and temperature against what their suppliers expect. Water systems are filled and circulated and the cycle chemistry is established. Valves are stroked and their positions proved, dampers and actuators exercised, and interlocks, permissives and trips tested as functioning systems rather than as individual devices on a list. Lubrication and control oil systems are commissioned, cleaned and proved, and the turbine protection is tested. The whole purpose of the stage is to find every fault that can be found while the plant is cold, because each of those faults costs many times as much to find after first fire. It is also where the operating team, if they are on site, begin to learn the plant they will spend their careers running.
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Step 7: First fire, steam raising and hot commissioning
First fire is a controlled event with a defined readiness check behind it, and it happens only when the fuel system, the protection systems, the water quality, the controls and the permissions are all in place and confirmed. From there the plant is brought up in stages, held at each one and checked before the next, and the steam side is cleaned to the arrangement the equipment supplier requires before steam is admitted to the turbine. Every part of that sequence - the steps, the holds, the criteria for moving on and the checks at each stage - is issued by the equipment suppliers and directed by the commissioning engineer, and it is specific to the machine rather than transferable from the last job. Expansion, movement, vibration, drainage and support behaviour are watched throughout, because this is the first time the plant has been hot and it is when the consequences of an erection error finally appear. The plant is taken up deliberately slowly. Nothing about this stage is improved by going faster.
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Step 8: Synchronise, test, run reliably and hand over
Synchronisation to the network happens with the network operator's agreement, in a window they set, and it is the first time the plant does the thing it was built to do. What follows is a programme of testing rather than a celebration: the operator's compliance testing to demonstrate the plant behaves the way the connection requires, then the contractual performance tests to establish output, efficiency and consumption under witnessed conditions, then a continuous reliability run to prove the plant can hold that condition without stopping. A failure during a reliability run usually means starting the run again, which is why the small auxiliary systems that were rushed months earlier tend to reappear here at their most expensive. Alongside the testing runs the handover itself - operator training, spares, final documentation, the as-built record, the maintenance regime and the outstanding punch list with an owner and a date against every item. When the tests are accepted and the paperwork is complete, the plant stops being a project and becomes a power station.
Plant & equipment
- Temporary flushing pumps, hoses, screens, strainers and filtration rigs, installed for cleaning and removed afterwards
- Temporary air, water and power supplies to serve systems before their permanent services are available
- Instrument calibration equipment and loop test sets, with certification traceable and in date
- High-voltage test equipment for cable, transformer, switchgear and protection testing
- Vibration, temperature and alignment monitoring equipment for running in rotating plant
- Data acquisition and recording equipment for witnessed performance and compliance testing
- Temporary exhaust silencing and noise control for the stages where plant vents to atmosphere
- Isolation, lock-off and tagging equipment for the permit regime, controlled by the commissioning engineer
Quality control & testing
- Formal system turnover, with documentation completeness a condition of acceptance rather than a follow-up action
- Cleanliness demonstrated and recorded against the acceptance the equipment supplier states, never assumed
- Every instrument calibrated and recorded, with certification traceable to in-date test equipment
- Every control loop proved end to end, from field device through to control system and back out again
- Protection settings issued by the protection engineer, applied and proved before anything is energised
- Interlocks, permissives and trips tested as functioning systems against the plant rather than simulated
- Each stage formally signed off before the next begins, with the readiness check recorded
- Performance and compliance tests witnessed, with instrumentation, conditions and results recorded together
Safety watchpoints
- The transition from construction control to permit and isolation control, and the long overlap where both run
- Live electrical systems on a site where construction is still under way elsewhere
- Stored energy in pressurised and hot systems, with surfaces, drains and vents that are now dangerous
- Fuel introduced to the plant for the first time, with hazardous area and ignition control in force
- Noise during venting and high-flow stages, well above normal construction levels
- Rotating machinery brought into service, with guards, exclusions and trip systems proved beforehand
- Chemicals in use at the water plant and during any cleaning the design requires
- Fatigue during extended shift working across first fire, steam raising and the reliability run
Common defects to hunt
- Systems turned over incomplete or undocumented, so commissioning inherits construction work it cannot control
- Cleaning cut short, leaving debris that damages fine-clearance equipment the first time the plant runs
- Station services commissioned too late, stalling everything that depends on them
- Calibration and loop testing rushed, so instrument and wiring errors surface after first fire instead of before
- Protection and interlocks proved on paper rather than against the plant they are there to protect
- Erection errors in supports and expansion arrangements appearing the first time the plant is heated
- Grid compliance evidence assembled late, delaying permission to synchronise after the plant is ready
- Reliability runs failed and restarted because of small auxiliary systems that were never properly finished
How long does Commissioning & Grid Connection take?
Typical duration: Typically 4-9 months from the first system turnover to commercial operation, with pre-commissioning starting well before construction finishes. The connection window set by the network operator, not the plant, decides the end date..
Related processes
Previous in sequence
04 - Balance of Plant & Stack