Disconnector Selection for a Substation

Short answer

The disconnector does not interrupt load or fault current: it provides a visible, safe isolating gap. That is why it is selected by its voltage, its continuous current, its fault current withstand, its mechanical endurance and, only if the design calls for it, by its ability to transfer load between busbars or to switch induced currents on earthing switches.

Short answer

The disconnector is the equipment that provides the visible, safe isolating gap for working on a de-energized circuit. It is not made to interrupt load or fault current; the circuit breaker does that. For this reason it is specified differently: its fault withstand matters more than its switching capacity. It is one of the selections in Primary equipment selection for a substation; the circuit breaker that operates together with the disconnector is selected according to Circuit breaker selection for a substation.

To learn what it is and how it works, see the substation disconnector. Here we explain how to decide its data.

1. What it must and must not switch

IEC 62271-102 (IEEE C37.30) defines the disconnector as a device that opens or closes a circuit when the current it interrupts or establishes is negligible, or when there is no significant change in voltage across its terminals. Examples of negligible current: the capacitive currents of bushings, busbars, connections and very short cable lengths, and those of voltage transformers. For rated voltages up to 420 kV, the standard takes 0.5 A as negligible current; above that, consult the manufacturer.

This explains why the disconnector is operated only with the breaker open. That rule is implemented in the busbar disconnector interlocking and the line disconnector interlocking.

2. Voltage, current and fault

They are fixed in the same way as for the circuit breaker, with the same series of IEC 62271-1:

Rated voltage. Equal to or higher than the highest system voltage.
Continuous current. From the R10 series, equal to or higher than the maximum expected load.
Short-time and peak withstand current. They come from the short-circuit study. The preferred duration is 1 s, and the peak is 2.5 times the short-time current at 50 Hz and 2.6 times at 60 Hz when the time constant is 45 ms.

Here the disconnector cannot be a weak point: it must withstand the same fault as the breaker and the busbars, even though it does not interrupt it.

3. The isolation must be real

IEC 62271-102 requires three things about the isolating distance between open contacts:

The isolating distance between open contacts is usually greater than the distance to earth, because the standard requires a higher voltage test across it.
The open position must be visible, or indicated by a reliable indicator mechanically linked to the moving contacts.
There must be no dangerous leakage currents from one side to the other. This is achieved by leading them to earth or by protecting the insulation from pollution.

If a long creepage distance is needed because of pollution, the distance to earth can exceed the distance between contacts; in that case the standard notes that arresters or spark gaps may be needed to keep the probability of flashover across the open gap low. The effect of pollution on insulators is explained in the guide on the conventional AIS substation.

4. Mechanical class

The M class indicates how many operating cycles it withstands:

Class	Cycles	Use
M0	1,000	Standard disconnector
M1	2,000	With a breaker of the same class
M2	10,000	With a breaker of the same class

The standard provides M1 and M2 for disconnectors that work alongside a breaker of the same class. If the breaker is ordered as M2 because of its switching frequency, also check the class of the associated disconnectors.

5. Earthing switches

The earthing switch grounds the circuit and must be able to close onto a fault if someone makes a mistake. Its electrical endurance class is:

E0: no short-circuit making capability.
E1: withstands two closings at the rated making current.
E2: withstands five; the standard defines it only up to 52 kV and it requires little maintenance.

An E1 or E2 earthing switch withstands a mistaken closing onto a fault, but it does not prevent it: interlocking remains mandatory. See line and busbar earthing switch interlocking.

6. Busbar transfer

In a substation with two busbars, a circuit is sometimes moved from one bus to the other with the disconnector, without opening the breaker. The disconnector can do so without damage only if it has a rating assigned for it. IEC 62271-102 allows assigning it to disconnectors of 52 kV and above. Its normal value is 80% of the rated continuous current, normally not exceeding 1,600 A. Disconnectors below 52 kV can do it, but no standardized values or tests are required.

The need comes from the design of the busbar arrangement: if no operating procedure uses it, it is not requested. See the double busbar configuration.

7. Induced currents

A de-energized, grounded line that runs near an energized one receives induced current and voltage. If the line earthing switch is going to open or close in that condition, it must have the capability for it. IEC 62271-102 assigns it to earthing switches of 52 kV and above, with two classes:

Class A: low coupling or relatively short parallel lines.
Class B: high coupling or relatively long parallel lines.

If the expected induction exceeds the standardized values, the rated values are agreed between manufacturer and user. The data comes from the line study; without it, the choice is blind.

8. Terminal loads

The disconnector terminals receive the weight and mechanical tension of the conductor, flexible or rigid, and wind and ice where they apply. Electromagnetic forces from a short circuit are added to that. The manufacturer must declare the rated static terminal load, and the device must be able to open and close with it applied. Strength also depends on the insulators. The loads of busbars and conductors on the terminals are covered in Busbars and insulators of a substation: what to define.

9. Operation and indication

The disconnector must not leave its position through gravity, wind, vibration or accidental contact, and it must be possible to lock it open and closed.
If motor-operated, it must also have manual operation, and connecting the hand crank must cut the motor power.
The position signal changes only when the contacts have reached their position. See motor operating mechanism of the disconnector.

That signal is the one used by the interlocking, the disconnector replica of the busbar protection and the SCADA. If the auxiliary contact does not reflect the real position, everything else goes wrong.

How we help

The definition of the interlocking of each disconnector and its validation are in our free interlocking table tool. We do the engineering design of your substation and the testing and commissioning of disconnectors, with contact resistance measurement, operating times and verification of the position indication. Design is done remotely, worldwide; field testing in Colombia, or elsewhere on request.

References

IEC 62271-102, Alternating current disconnectors and earthing switches; definitions, clauses 4.103, 4.107, 5.102 to 5.104 and annexes B (busbar transfer) and C (induced currents).
IEC 62271-1, Common specifications for high-voltage switchgear and controlgear; clauses 5.2 to 5.8.

Frequently asked questions

Can a disconnector open under load?

No. IEC 62271-102 defines it for opening or closing circuits when the current is negligible or when there is no significant change in voltage, and sets 0.5 A as the negligible current up to 420 kV. The documented exception is busbar transfer, for which a specific rating can be assigned from 52 kV.

What are the E1 and E2 classes of an earthing switch for?

They indicate whether the earthing switch can be closed onto a fault. E0 has no such capability, E1 withstands two closings at the rated short-circuit making current, and E2, defined up to 52 kV, withstands five and needs less maintenance.

Why may a line earthing switch need induced-current capability?

Because a de-energized line running parallel to an energized one receives current and voltage by coupling. IEC 62271-102 assigns this capability to earthing switches of 52 kV and above, and classifies it as class A, for low coupling or short parallel lines, and class B, for high coupling or long parallel lines.