In an ungrounded double wye capacitor bank, unbalance protection measures the current or voltage between the two neutrals. According to IEEE C37.99-2000 (cl. 8.2.6.2), this differential measurement avoids the effect of system voltage unbalance and is not affected by balanced third harmonics. In exchange, the CT or VT must be rated for system voltage, and faults can cancel between the two wyes.
Short answer
The ungrounded double wye splits the bank into two equal wyes whose neutrals are not connected to ground. The unbalance protection measures the current or voltage between those two neutrals: with the bank healthy, it is almost zero; when one wye loses a unit, a difference appears. According to IEEE C37.99-2000 (clause 8.2.6.2), this method avoids the effect of system voltage unbalance and is not affected by third harmonics when they are balanced. Care must be taken with the insulation of the CT or VT, which is rated for system voltage, and with faults that cancel between the two wyes.
How it works
With the two wyes equal, any external effect that acts on both equally, such as a variation of the system voltage, also appears at both neutrals and cancels in the measurement between them. Only a difference between one wye and the other produces a signal. If one wye loses a unit or an element, the neutral of that wye shifts relative to the other, and a current (or a voltage) proportional to that difference flows (or appears) between the two neutrals.
The standard states that, for this reason, an ungrounded bank can be split into two equal halves and thus inherently compensate for system unbalance, since the neutral current or voltage is measured differentially. The effect of manufacturing tolerance remains, and it is estimated with the equations of Annex C. It is covered in the guide on inherent unbalance.
The schemes for the other connections are neutral voltage in an ungrounded wye, neutral current in a grounded wye and voltage differential. The connections are compared in bank connection: grounded wye, ungrounded wye or delta.
The methods of figure 13
Clause 8.2.6.2 presents four methods. Three are for ungrounded banks and one for grounded banks; the latter is mentioned only to complete the picture.
| Scheme | Bank | What it measures | Standard’s remark |
|---|---|---|---|
| (a) | Ungrounded | Current between neutrals with a CT and overcurrent relay, or a shunt and voltage relay | CT rated for system voltage |
| (b) | Ungrounded, isolated neutrals | Voltage between neutrals with a VT and overvoltage relay | VT rated for system voltage |
| (c) | Grounded | Neutrals grounded through separate CTs with secondaries crossed to an overcurrent relay | Covered in separate guides |
| (d) | Ungrounded, neutrals tied together | VT between the common neutral and ground | The relay must have a harmonic filter |
Schemes (a) and (b) avoid the effect of system unbalance, and neither is affected by third-harmonic current or voltage when the bank is balanced.
What to know about each element
Neutral current. It is half that of a grounded bank of the same rating. Although small, the CT ratio and the relay range can be chosen for the desired sensitivity, because in the ungrounded scheme they are not subject to the switching currents or single-phase load currents that do affect the CT of a grounded neutral.
System voltage. The CT or VT between neutrals must have system-voltage insulation, because the neutral is not grounded and can rise during switching. If the neutral displacement is measured with a VT, the transformation ratio is high, and the voltage signal reaching the relay can be very small. That is a disadvantage compared with current.
Neutral displacement. The voltage displacement can be determined in the same way as for a simple wye bank of the same rating as one section of the double wye. The calculation details (tables 2 and 3 of the standard) are left to the study of each bank.
Harmonics. In scheme (d), with the neutrals tied together, the relay must have a harmonic filter so that it does not respond to components at other frequencies.
What it sees and what it does not
The ungrounded double wye detects well the loss of units in a single wye, and does not depend on system unbalance. It has two limits of its own:
- Ambiguous indication. The standard points to the double wye as the classic example: fuse operations or shorted elements in one wye can cancel the signal generated in the other, and the bank is left unbalanced without the relay seeing it. It is covered in the guide on ambiguous indications.
- Rack-to-rack faults. In banks with groups connected phase over phase, the racks must be connected to alternating sides of each group so that a flashover between them produces current between neutrals (clause 8.2.2 and figure 10).
To reduce the first limit, the alarm must be as sensitive as possible, for a single fuse or a single element, and the trip must allow for faults that cancel.
The readings that can mislead the relay are in ambiguous indications and faults that unbalance does not see, and the signal of the healthy bank is in inherent unbalance and measurement error.
When this configuration is chosen
The standard gives a practical reason: splitting a wye bank into a double wye doubles the sensitivity of the protection and removes the effect of system voltage unbalance (clause 8.2.3). It is the usual choice when the bank is large enough to split and you do not want to compensate for the system with additional VTs.
If, instead, the bank is an ungrounded wye with a single neutral, the measurement is the neutral voltage. If the bank is grounded, the analysis changes: neutral current or voltage differential is used.
The function that uses the signal is unbalance protection (ANSI 60).
What not to do
- Do not choose a CT rated below the system voltage for the isolated neutral.
- Do not assume that a current near zero means a healthy bank (ambiguity).
- Do not connect the racks to only one side in phase-over-phase banks.
- Do not forget the harmonic filter when the neutrals are tied together.
When to contact us
If your bank is a double wye and you are not sure which faults your relay covers or which CT ratio suits it, at Tiza Group we review it and document the scheme. We do remote relay configuration and quote the work according to its scope. Write to us.
Related guides
- IEEE C37.99: capacitor bank protection guide
- Neutral voltage unbalance in an ungrounded wye
- Neutral current unbalance in a grounded wye
- Voltage differential unbalance in a grounded wye
- Ambiguous unbalance: faults the protection does not detect
References
- IEEE C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 8.2.2, 8.2.3, 8.2.6.2 and Annex C (table C.1), figures 7, 10 and 13.
Frequently asked questions
Why is protection between neutrals not affected by system unbalance?
Because it compares two halves of the same bank. A system voltage unbalance acts on both wyes equally and produces no difference between their neutrals. IEEE C37.99-2000 (cl. 8.2.6.2) states that both ungrounded schemes avoid that effect. The effect of the manufacturing tolerance of the capacitors remains.
How much current flows through the CT between neutrals compared with a single bank?
The standard states that the neutral current is half that of a grounded bank of the same rating. Even so, the CT ratio and the relay range can be chosen for the desired sensitivity, because in the ungrounded scheme they are not exposed to switching currents or single-phase load currents.
Which fault can go unnoticed in a double wye?
A fault that cancels between the two wyes, for example the same number of units out in the same phase on both sides. The current between neutrals is almost zero, just as with a healthy bank. That is why a sensitive alarm, for a single fuse or element, is recommended.