The first energization of a new substation is the step from cold to hot commissioning. It requires preconditions closed in writing, a sequence that energizes the bus from the grid first and then the transformers and feeders, no-load checks with voltage (phasing, voltages, synchronism) and, under load, verification of directionality, differentials and measurements. Only then is the installation declared in service.
From cold to hot
The first energization of a new substation is the step from cold to hot commissioning: the moment the installation stops being a construction site. Up to then, everything was tested with the power circuits de-energized: primary equipment, instrument transformers, wiring, protection with secondary injection, controls, interlocks and signals. From energization onward, any work requires a clearance, verification of absence of voltage and grounding, and the remaining tests can only be done with voltage and load.
It is the last stage of substation commissioning and the one that demands the most coordination: the testing contractor, the owner, the substation operator and whoever grants the interconnection, usually the utility or the transmission owner, following its own procedures.
Preconditions
Before applying voltage, a list of conditions should be closed in writing, often in a “ready to energize” certificate:
- Cold commissioning finished, with nonconformities closed or formally accepted.
- Approved settings loaded in every relay and checked against the study; no protection disabled.
- Test switches closed and test modes off in relays and IEDs.
- Temporary grounds removed and counted; grounding switches open.
- Stable station service: DC system, batteries and AC supply.
- Communications with the HMI and, where applicable, with the control center, tested.
- Documentation for whoever grants the interconnection: test reports, inspection approvals required by the local code, and settings.
- Approved switching program, with a single person in charge of operation and the area signposted.
The general sequence
The order follows the flow of energy: first the bus is energized from the grid, then what hangs from it, and on-site generation, if any, is synchronized last.
- 01Line or service entranceThe grid reaches the substation with its protection enabled
- 02BusThe bus is energized and voltages and phasing are checked on its VTs
- 03Transformer at no loadEnergization while watching inrush, noise and alarms
- 04Feeders without loadFeeders and the substation's own station service
- 05Load pickupGradual load and checks with current flowing
- 06GenerationIf there is generation, synchronization with the grid already stable
Each step is done with its breaker, never with a disconnect switch under load, and with an observation time before moving to the next.
No-load checks with voltage
- Voltages and phase sequence on each VT set, compared between bus and line and on the transformer secondaries.
- Phasing between sources that may be paralleled, and correct signals to the synchronism check. If the closing logic uses dead bus or dead line, its behavior is confirmed; see dead-line and live-bus closing.
- Transformer energization: the inrush current must be blocked by the differential; what to watch on the transformer is in power transformer commissioning. If it trips, it is not re-energized until the cause is understood.
- Alarms and statuses in the HMI and the control center consistent with what happens in the switchyard.
- Equipment observation: noise, temperature, leaks and, on high-voltage equipment, no visible or audible discharges.
On-load checks
Some checks only make sense with current flowing:
- Directionality: with a known load, the directional and distance functions must see the flow in the expected direction. Magnitudes and angles are recorded in each relay.
- Differentials: the residual differential current of transformers, buses and lines must be small and stable. A reversed polarity that went unnoticed at no load shows up here.
- Measurements: relays, revenue meters, SCADA and the control center must agree in magnitude and sign.
- Thermography of connections and equipment under load.
- Did any protection operate or did an unexpected alarm appear?
- YesStop, make the installation safe and analyze events and oscillography before retrying
- NoDo voltages, phasing and measurements match what was expected?
- YesContinue with the next step of the program
- NoStop and review ratios, polarities and instrument transformer connections
Risks specific to the first energization
The first time an installation receives voltage, problems can appear that no cold test detects: an insufficient clearance that was not visible on the drawings, moisture in a cable termination, a voltage transformer going into ferroresonance with the bus capacitance, or a protection operating on inrush current. That is why people stay out of the energized areas, each step has an observation time and the program plans from the start how to de-energize safely if something goes wrong.
When it is declared in service
The installation is declared in service when it has operated with voltage and with load, the checks above have been recorded, signals reach the control center correctly and whoever grants the interconnection has given its approval. From then on, maintenance compares against the commissioning test reports. If the installation has generation, the tests required by the utility or system operator are added.
The specific switching of a project depends on its configuration, the grid and the instructions of whoever grants the interconnection, and is defined in the energization program.
We support your energization
If a relay trips unexpectedly during energization, we quote the relay trip analysis. In the field, our crew prepares the ready-to-energize certificate, supports the sequence and performs the no-load and on-load checks in medium, high and extra-high voltage substations and in industry, in Colombia, or elsewhere on request; contact us.
Related guides
- Synchronism check 25
- Controlled switching (point-on-wave)
- Substation switching: opening, closing and safety
- Commissioning dossier: test reports and as-built
- Dead-line and live-bus closing: 25 modes
References
- ANSI/NETA ECS-2024, Standard for Electrical Commissioning Specifications for Electrical Power Equipment and Systems.
- IEEE C37.233-2023, IEEE Guide for Power System Protection Testing.
- IEEE C37.91-2021, Guide for Protecting Power Transformers.
Frequently asked questions
What must be ready before energizing a new substation?
Cold tests finished and nonconformities closed or accepted, approved settings loaded in every relay, protection enabled, temporary grounds removed, communications with the control center working, the authorization of whoever grants the interconnection, and an approved switching program with a single person in charge.
Why is the transformer energized at no load first?
To observe the inrush current, the noise, the alarms and the behavior of the protection with no load connected. If something is wrong, the transformer is taken out without affecting customers and without the load masking the problem.
When can a new substation be declared in service?
When it has operated with voltage and with load, phasing, directionality, differential stability and measurements have been verified, signals reach the control center correctly and whoever grants the interconnection has given its approval. The criteria and the certificate are set by the contract and the applicable requirements.