Capacitor Bank Connections: Grounded, Ungrounded Wye, Delta

Short answer

IEEE C37.99-2000 describes five common capacitor bank connections: delta, grounded wye, ungrounded wye, ungrounded double wye and grounded double wye, plus the H configuration. Almost all substation banks are wye-connected. The grounded wye gives a path for lightning, triplen harmonics and zero-sequence current. The ungrounded wye and the delta block them. The connection changes which unbalance protection is possible.

Short answer

IEEE C37.99-2000 (cl. 5) describes five common connections: delta, grounded wye, ungrounded wye, ungrounded double wye and grounded double wye. It adds the H configuration for large banks. Practically all substation banks are wye-connected; delta is used in distribution. The best connection depends on making use of the available unit voltages, the fuse and the protection. What changes between them is where zero-sequence and third-harmonic current can flow, and which unbalance signal can be measured.

Quick comparison

Connection	Zero-sequence current	Third harmonic	Use noted by the guide
Grounded wye	Flows	Flows	Practically all substation banks are wye-connected
Ungrounded wye	Does not flow	Does not flow	Neutral insulated for full line voltage
Double wye (grounded or ungrounded)	As in the single wye: flows if the neutral is grounded	As in the single wye: flows if the neutral is grounded	Large banks, to limit parallel energy
Delta	Does not enter	Does not enter	Distribution voltages, with a single series group

Grounded wye (cl. 5.1)

It is the most common connection in substations. Its effects, according to the guide:

Lightning. It gives a low-impedance path to ground for surge currents. Some banks operate without surge arresters, relying on the capacitors absorbing the surge.
Triplen harmonics. It also gives a path to ground for third-harmonic currents, which can cause interference in communication circuits that run parallel to the lines.
Open phase. An open phase produces zero-sequence current, which can operate ground relays. See zero-sequence current.
Switching. With the neutral grounded, recovery voltages during switching are lower. The effect of the inrush current on the relays and the grounding method must be considered (cl. 6.2).

One unit per phase (5.1.1). If an externally fused unit fails and is isolated, the others are not overstressed; unbalance is not needed. The fuse must interrupt the system fault current. With internal fuses or unfused designs, unbalance is needed.

Several series units, single wye (5.1.2). Common above 34.5 kV. The several groups limit the fault current, so current-limiting fuses are normally not needed, unless the parallel power exceeds 4,650 kvar or the manufacturer’s level.

Grounded double wye (5.1.3). When the bank is so large that the parallel energy of a group exceeds what the units or the fuses withstand, it is split into two wyes. The two neutrals are joined directly with a single ground connection. It allows a simple and fast unbalance relay, because system unbalance affects both wyes equally while a failed unit shows in the neutral current. With fuses, time coordination is needed; without fuses the delay can be short.

This connection supports the scheme in unbalance by neutral current in grounded wye; neutral and ground grid grounding is covered in bank grounding: neutral and ground grid.

Ungrounded wye (cl. 5.2)

It allows no zero-sequence or third-harmonic currents, nor large discharge currents during system ground faults. Phase-to-phase faults can produce large discharge currents.

The neutral must be insulated for full line voltage, because it is momentarily at phase potential when the bank is switched or when a unit fails in a bank with a single group.

One unit between phase and neutral (5.2.1). With single-bushing units on an insulated rack. If two-bushing units with the case grounded are used, a fault to the case is a system line-to-ground fault, and the high currents can rupture the case. The current through a failed unit is limited to three times the normal phase current, so current-limiting fuses are normally not required. Care is needed when replacing fuses: failed units in different phases can form a phase-to-phase fault.
Several series units (5.2.2). There is no lightning or third-harmonic path. The whole bank, including the neutral, is insulated for line voltage. If the unbalance scheme is sensitive to system voltage unbalance, the delay must give time for line protections to clear ground faults, or it is accepted that the bank trips with them.
Double wye (5.2.3). Useful when the bank exceeds 4,650 kvar per group with expulsion fuses. The neutrals can be joined through a current or voltage transformer, insulated from ground for the full line-to-ground voltage.

The ungrounded double wye is measured with the current or voltage between neutrals, as in unbalance in double wye.

Delta (cl. 5.3)

It is generally used only at distribution voltage, with a single series group of line-voltage units. It requires two-bushing units or single-bushing units on insulated racks. It is frequent at 2,400 V because units for wye connection are not easily available there.

With a single group, isolating a unit does not raise the voltage on the remaining ones; unbalance is not needed to protect them, but it serves to detect units that are out.
No zero-sequence or third-harmonic current enters.
With one group per leg, the unit fuses must interrupt the phase-to-phase fault current. Current-limiting fuses may be required.
With internal fuses, unbalance is required, because there is no visible indication of a blown fuse, and normally two or more series groups in each unit.
Static var compensators (SVC) use delta for the thyristor-switched capacitors (TSC), for control simplicity.

H configuration (cl. 5.4)

Some large banks use an H in each phase, with a current transformer between the two branches. If all capacitors are healthy, no current flows through it. When a fuse operates, some current flows. It is very sensitive and is used in banks with many parallel units.

The measurement scheme for this configuration is in unbalance in the H-bridge.

How to choose

The connection is chosen according to the bank type (external fuses, internal fuses, fuseless or unfused), the system voltage, the size and the protection that is possible. Neutral grounding has its own criteria. The capacitor bank unbalance protection shows what is measured in each connection, and capacitor bank protection gives the general overview.

What we do

Choosing the connection means deciding the grounding, the unbalance protection and the fuse type at the same time. At Tiza Group we do it within the bank’s engineering design and studies and quote the study according to the scope.

References

IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 5 (Figure 3), 5.1 to 5.4, 6.2 and 4.2.

Frequently asked questions

Why are almost all substation banks wye-connected?

The guide notes it: practically all substation banks are wye-connected. Delta is used only at distribution voltages, with a single series group of line-voltage units.

Can a grounded-wye bank cause ground trips?

Yes, in two cases the guide mentions: if a phase is left open, zero-sequence current flows and can operate ground relays, and at energization the inrush current can affect the relays. Triplen harmonic current also flows through the grounded neutral.

Does a delta bank need unbalance protection?

With a single series group, isolating one unit does not overstress the remaining ones, so unbalance is not needed to protect them. It serves to know that units are out. With internally fused units it is necessary.