Bank Switching Transients in CTs, VTs and Control Circuits

Short answer

When a capacitor bank is switched, transient currents of thousands of amperes and hundreds to thousands of hertz flow. These currents induce a very high voltage in the current transformer secondary that saturation does not limit, and they stress the voltage transformers, relays and control cables. IEEE C37.99-2000 asks for limiters on the secondaries, a careful choice of bank grounding, and attention to the routing and shielding of the cables.

Short answer

Every time a capacitor bank is switched, very high, high-frequency transient currents flow through the bank buses, its ground connections and the substation grid. These currents reach the bank’s current transformers (CTs) and voltage transformers (VTs) and the control cables, and can damage windings, relays and instruments.

IEEE C37.99-2000 (clauses 10.4 to 10.7) asks that the secondaries be protected with voltage limiters, that the bank grounding be chosen with care, and that the control cable routing be handled with care.

What currents appear

The standard describes four situations when a bank is switched:

Case	Peak	Frequency and dI/dt
Pre-insertion resistors (damped pulse)	1000 to 3000 A	Rise of 1 µs, decay of about 600 µs; about 3000 A/µs
Closing that shorts the resistor	1000 to 2500 A	Oscillatory, 1000 to 20 000 Hz; about 100 A/µs
No pre-insertion	Typical 25 000 A crest	Oscillatory, 1000 to 20 000 Hz
Pre-insertion inductors	Less than 3000 A	Hundreds of hertz; less than 10 A/µs

Similar components are superimposed on the 60 Hz fault current in faults on the bank, on the bus and on nearby lines. Transients of several megahertz also appear from traveling waves on the bank bus.

Pre-insertion of resistors lowers the peak and eliminates the oscillation, but does not reduce the initial dI/dt, which depends only on the voltage across the contacts before closing and on the loop inductance: dI/dt = V / L. That value indicates the potential for interference in nearby control circuits.

With such high current and frequency, potential differences appear in the ground grid, and nearby control cables suffer inductive interference.

The current that originates these transients is explained in switching a capacitor bank; how overcurrent relays handle it, in overcurrent and breaker failure in a bank.

Current transformer

Why voltage is generated in the secondary

The transient current flowing through the CT primary produces a transverse-mode interference voltage in its secondary. It is not induced by stray coupling: it is the normal action of the transformer. At those frequencies, the burden reactance can be more than 100 times that at 60 Hz. A few hundred amperes can produce very high voltages across inductive burdens.

It is sometimes said that CT saturation prevents these voltages. The standard says this is not so: the volt-second product of each half cycle is small compared with what is needed to saturate the core at 60 Hz.

A worked example

A 30 Mvar, 115 kV grounded-wye bank, with a nearby phase-to-ground fault. The bus-to-ground loop inductance is 300 µH. Using the equations of the standard:

Bank capacitance: about 6.0 µF.
Oscillation frequency: about 3750 Hz.
Peak discharge current: about 13 300 A.
With a 600:5 CT (ratio 120), the peak secondary current is about 111 A.
If the CT burden is 1 + j1 Ω at 60 Hz, at 3750 Hz its reactance is about 62 Ω.
The peak secondary voltage works out to the order of 6900 V.

That voltage is too high for the relays, the instruments and the secondary insulation, and explains why protection is required.

What the standard asks

High-current-capacity varistors or spark gaps at the secondary terminals. With a wound primary, a spark gap or arrester on the primary may also be needed.
A varistor with enough energy rating, whose peak current does not take it into the upturn region of its V/I curve.
Lower secondary current, through the CT ratio and burden, and no inductive components in the burden. Solid-state relays help because their burden is low and almost resistive.
Relays with a filter that excludes the kilohertz discharge current, or designed to ignore it.

For the neutral CT, the standard considers an insulation of 0.2 times the system phase-to-ground voltage conservative; CTs with 0.1 have been used without problems. It does not recommend grounding the secondary at the CT, because that would carry the bank’s transient potential to the cable and the relay room. If the breaker poles do not operate at the same time, the neutral CT can receive full phase current, for an indefinite time if one or two poles fail to operate.

The neutral CT of the grounded-wye scheme is the critical case. The fundamentals are in current transformer and in CT saturation.

Voltage transformers

A voltage transformer, capacitive divider or potential device that runs from the bank neutral to ground to detect unbalance must withstand switching voltages of 0.5 to 2.5 times the system phase-to-neutral voltage (clause 10.4). At high voltages, higher-ratio transformers are needed, and that can leave neutral voltage unbalance methods insensitive.

Clause 10.6.2 deals with potential differences in the bank area:

Peninsula grounding, with good shielding and cable routing, protects the voltage sources better than single-point grounding. Its higher cost may be justified in large extra-high-voltage banks.
Single-point grounding: the neutral transient potential is around 0.25 of that of the bank bus. A transformer measuring bus-to-neutral voltage must be a two-bushing type so its case can be tied to the grid. If it were a single-bushing type with its case tied to the bank neutral, the transient would appear between the case and the secondary, and the secondary insulation could fail.
Natural resonance. Magnetic high- and extra-high-voltage transformers usually resonate between 500 and 2000 Hz. If the switching coincides with that frequency, the transient is amplified and can affect the relays. A resistor in series or in parallel with the secondary damps the oscillation.

Relays

Switching generates the megahertz transients of any high-voltage switching operation and the kilohertz ones specific to capacitors. Relay protection must be effective against both:

Routing, shielding, grounding and insulation of the cables reduce common-mode interference, in both megahertz and kilohertz.
Passive input filters work in megahertz, but not in kilohertz: they would be very large and would load the CTs and VTs.
In kilohertz the interference is stronger in transverse mode. The standard says to filter it another way, for example with active filters in the relay’s analog processing.
All ac inputs are protected with varistors or spark gaps.

Control cables

Clause 10.7 asks for:

Removing from the bank area any ducts, trenches or buried cables that are not part of the bank control or protection.
Routing the neutral CT and VT cables at right angles to the common neutral, with single-point grounding, or parallel to the link to the grid, if peninsula grounding is used.
Shielding the cables and using a radial configuration: complete circuits within the same cable, with no inductive loops.
Keeping them as close as possible to the grid conductors (at least 4/0 copper). With peninsula grounding it is mandatory.
Grounding the shield at several points, and spare conductors at least at one end.

The outer shield is normally grounded at both ends.

The connection of the cables and the neutral to the grid is explained in bank grounding, neutral and ground grid. Neutral VTs are also covered in inductive voltage transformer.

Common mistakes

Grounding the neutral CT secondary at the CT. It carries the bank transient into the relay room.
A varistor that is too small. It stops limiting or shorts out and leaves the unbalance protection out of service.
Cables of other circuits in the bank area. They receive the interference of every switching operation.

What we do at Tiza Group

In testing and commissioning we review the CTs, VTs, control cables and grounding of your bank before energization, and check how the secondaries behave during switching. We do this in Colombia, or elsewhere on request. Contact us and we will quote the review.

References

IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 10.4 to 10.7, Figures 46 to 49.
IEEE Std C37.90.1-1989, on relay withstand to surge transients, cited by the standard.

Frequently asked questions

Why does CT saturation not limit the secondary voltage during bank switching?

Because the volt-second product of each half cycle of the high-frequency oscillation is small compared with what is needed to saturate the core at 60 Hz. The voltage is produced by the normal action of the transformer, especially with inductive burdens.

Which bank grounding best protects the voltage transformers?

IEEE C37.99-2000 states that peninsula grounding, together with good shielding, grounding and routing of the control cables, protects the ac voltage sources better than single-point grounding. The higher cost may be justified in large extra-high-voltage banks.

Do the passive filters on a relay's inputs help against these transients?

No. Passive surge filters on the inputs are very effective in the megahertz range, but not against the kilohertz transients of capacitor switching. The standard says to achieve the filtering another way, for example with active filters in the relay's analog processing, and to protect the inputs with varistors or spark gaps.