Substation switching: opening, closing and safety

What switching is

Switching is the set of operations used to run an electrical network safely. In a substation, switching means changing the state of breakers, disconnect switches and grounding switches to:

• take equipment out of service for maintenance or testing and return it afterwards;
• transfer circuits between buses or between sources;
• energize a line, a transformer or a new expansion;
• restore service after a trip.

Each switching operation is planned on the up-to-date single-line diagram, coordinated with whoever operates the grid, and carried out in an order that never forces a device to do something it was not designed for.

What each device can do

IEC 62271-102 defines the disconnector as the device that opens or closes a circuit when the current is negligible or when there is no significant change in the voltage across its terminals; IEEE C37.30.1 covers the same equipment in North America. Hence the central rule: the breaker opens first and closes last.

General order to de-energize

To energize, the order is reversed (the first time it is done in a new installation has its own checks, described in first energization of a new substation): temporary grounds are removed and grounding switches opened, the disconnect switches are closed with the breaker open, and finally the breaker is closed. If both sides are live, the closing is supervised by the synchronism check 25; if one side is dead, by the dead-line and live-bus mode.

That is the general principle. Each installation has its own switching procedures, approved by the utility or the substation owner, and in substations of the interconnected system switching is coordinated with their control center. When the substation configuration comes into play, the order adapts to it: a double-bus arrangement, for example, allows a circuit to be transferred between buses without interrupting it.

Golden rules

Opening the breaker and the disconnect switches does not make equipment safe. De-energized work is the work done after following the golden rules of electrical safety:

1. Visible isolation.
2. Lock to prevent re-energization.
3. Verification of absence of voltage.
4. Grounding and short-circuiting.
5. Covering adjacent live parts.
6. Marking out the work area.

In Europe, EN 50110-1 groups them into the well-known five rules: disconnect, secure against reconnection, verify absence of voltage, ground and short-circuit, and protect against adjacent live parts. In North America, NFPA 70E and OSHA 29 CFR 1910.269 set the lockout/tagout steps for an electrically safe work condition, and IEEE 1048 covers temporary protective grounding.

What interlocks prevent

Substation interlocking consists of conditions that block a command when the operation would be dangerous. The three basic ones:

• Do not move a disconnect switch with its bay breaker closed (except the parallel transfer cases the logic allows).
• Do not close a grounding switch onto a section that may be live.
• Do not close a disconnect switch onto a grounded section.

They are implemented mechanically, electrically, with keys or as logic in the bay controller (in IEC 61850, the CILO logical node), and they are resolved at equipment, bay or station level; see bay and station interlocking. You can build the condition matrix of a bay with our free interlocking table tool.

Where each operation can be commanded from (control center, substation HMI, panel or equipment) is defined by the control levels and the local/remote selector. Repetitive operations, such as moving a bay to the other bus, can be automated with automatic switching sequences.

Switching at medium voltage

At medium voltage, most switching is done in metal-enclosed switchgear: the breaker sits on a withdrawable truck or has a three-position switch (closed, open, grounded), and the mechanical interlocks of the switchgear prevent racking out the truck with the breaker closed or opening the door without grounding.

Common mistakes

• Operating from an outdated drawing that does not show a tie, a generator or a backup source.
• Trusting position indication alone without checking at the equipment.
• Releasing interlocks to “save time”: every release must be authorized and logged; see interlock bypass.
• Energizing with forgotten temporary grounds, one of the classic causes of a fault on closing.
• Closing a transformer breaker without considering inrush current, which can operate poorly set protection.

We can help with your switching logic

Interlocks, control levels and automatic sequences live in the bay controllers and relays. Our team configures and modifies them remotely, worldwide: see remote relay configuration. On site, we test the complete logic, in Colombia, or elsewhere on request. Contact us with your substation configuration.

Related guides

• Substation interlocking
• Bay and station interlocking
• Control levels and the local/remote selector
• Automatic switching sequences
• Interlock bypass
• Electrical substation: what it is, parts and how it works

References

• IEC 62271-102: AC disconnectors and earthing switches.
• IEEE C37.30.1: requirements for high-voltage air switches.
• IEC 61936-1: power installations exceeding 1 kV AC.
• EN 50110-1: operation of electrical installations, general requirements (five rules).
• NFPA 70E, Standard for Electrical Safety in the Workplace.
• IEEE 1048, IEEE Guide for Protective Grounding of Power Lines.
• IEC 61850-7-4: logical nodes CILO, CSWI, XCBR and XSWI.

Frequently asked questions