What are the requirements for commissioning and decommissioning of dry type transformers?

Proper commissioning and operational procedures are critical to ensuring the safety, reliability, and long-term performance of dry type transformers. Below are standardized guidelines tailored to industry best practices, designed to mitigate risks, prevent equipment damage, and optimize operational efficiency.

Contents hide

1 Commissioning & Overhaul Impulse Testing Requirements

2 Cooling System Operation Sequence

3 Neutral Point & Arc Suppression Coil Operation (110kV and Above)

4 Power On/Off Operation Procedures

5 Standby Transformer Activation & Protection Configuration

6 Prohibition of Long-Term Parallel Operation for Station Transformers

Commissioning & Overhaul Impulse Testing Requirements

Dry type transformers newly deployed in power systems must undergo rigorous impulse testing to verify insulation integrity before full operation. Specifically, newly commissioned units require five impulse tests conducted at the rated voltage—these tests simulate transient overvoltage events (such as lightning strikes or switching surges) to ensure the insulation system can withstand extreme electrical stress. For transformers that have undergone maintenance or repair, a simplified but equally critical test protocol applies: three impulse tests at the rated voltage are mandatory to confirm that insulation performance remains uncompromised after component disassembly, replacement, or servicing. These tests are foundational to identifying hidden insulation defects that could lead to catastrophic failures during operation.

Cooling System Operation Sequence

The cooling system plays a vital role in regulating transformer temperature and preventing overheating, so strict adherence to the correct operation sequence is essential. When energizing a dry type transformer:

Activate the cooling system first: Start the cooler and allow it to run continuously for approximately 15 minutes. This pre-operation phase ensures the transformer’s internal components gradually reach a stable temperature, minimizing thermal stress caused by sudden temperature changes. The cooling system should only be shut down once the oil temperature (or winding temperature, for dry type units) stabilizes and no longer rises.
Shutdown sequence: Reverse the process by de-energizing the transformer first. Once the unit is fully disconnected from the power supply, the cooling system can be stopped—this prevents residual heat buildup in the windings or core, which could degrade insulation over time.

Neutral Point & Arc Suppression Coil Operation (110kV and Above)

In 110kV and higher voltage systems with direct neutral point grounding, the handling of the neutral point and associated arc suppression coils requires precise procedural control to avoid operational hazards:

Commissioning/Decommissioning Protocol: Prior to energizing or de-energizing the transformer, the neutral point must be reliably grounded. This grounding provides a low-impedance path for fault currents, reducing the risk of dangerous overvoltage during switching operations. After the transformer is stabilized in its operational or shutdown state, the neutral point grounding can be disconnected in accordance with system design requirements and safety regulations.
Arc Suppression Coil Guidelines: Arc suppression coils connected to the transformer’s neutral point must follow a strict “withdraw first, energize later” sequence. Before commissioning the transformer, the arc suppression coil should be disconnected from the neutral point to prevent interference with the grounding system during startup. Additionally, never connect the neutral points of two transformers to a single arc suppression coil’s neutral bus simultaneously—this configuration can cause unbalanced currents, coil overheating, and potential damage to both the transformers and the coil.

Power On/Off Operation Procedures

The sequence of switching operations during power on and power off directly impacts the stability of the transformer and the broader power system. Adhere to the following protocols:

Power Off Sequence:
1. First, disconnect the load-side switches to reduce the current load on the transformer gradually.
2. If the transformer is supplied by multiple power sources, shut down switches in the order from low-voltage to high-voltage to avoid voltage transients.
3. Finally, disconnect the transformer-side switch, followed by the bus-side switch—this isolates the transformer from the main power grid safely.
Power On Sequence: Reverse the power off steps to ensure a smooth startup:
1. Close the bus-side switch first to connect the transformer to the grid.
2. Activate the transformer-side switch.
3. Gradually close load-side switches (and high-voltage to low-voltage switches for multi-source systems) to restore power to connected loads.

Standby Transformer Activation & Protection Configuration

When activating a standby dry type transformer, proper verification and protection adjustments are essential to avoid operational disruptions:

Load Confirmation: After energizing the standby unit, verify via equipment position indicators and metering data that the load has been successfully transferred from the running transformer. Only once this confirmation is complete should the original running transformer be de-energized.
Special Protection Considerations: For transformers with diagonal or 3/2 wiring configurations, note that even after de-energization, the unit’s heavy gas protection and differential protection systems may still trigger tripping of the closing-side switches. This is due to residual magnetic flux or transient currents in the wiring. To address this, adjust the gas protection settings to the “signal only” mode or temporarily withdraw the gas protection function, following on-site safety regulations and operational guidelines.

Prohibition of Long-Term Parallel Operation for Station Transformers

Station dry type transformers are not designed for prolonged parallel operation, as this can lead to circulating currents, increased energy loss, and premature equipment wear. To mitigate these risks:

For station transformers with low-voltage side connections, use low-voltage knife switches to isolate and eliminate circulating currents between parallel units.
For no-load station transformers (units not connected to active loads), high-voltage knife switches should be used to disconnect the units from the parallel configuration.

Long-term parallel operation is discouraged even if transformers are of the same capacity and specification, as minor differences in impedance or voltage ratio can lead to sustained circulating currents that degrade insulation and reduce efficiency.