IEEE C37.99-2000 recommends grounding the neutral of a bank only in effectively grounded systems, and from 121 kV asks for a grounded bank and system for the breaker. When there are several banks in the same substation, their neutrals are joined and connected to the ground grid at a single point (single-point ground) or through a peninsula conductor, so that back-to-back energization currents between banks do not flow through the grid.
Short answer
A wye-connected capacitor bank can have the neutral grounded or isolated. IEEE C37.99-2000 (cl. 6.2) recommends grounding it only in effectively grounded systems, and suggests a grounded bank and system from 121 kV. When there are two or more wye banks at the same location, their neutrals are connected directly to each other with a single ground connection (cl. 6.1): at a single point or as a peninsula. The goal is that high-frequency currents between banks do not pass through the substation ground grid.
Neutral grounded or isolated (cl. 6.2)
When to ground it. In general, only in effectively grounded systems. In a system that is not, a ground fault with the bank neutral grounded can cause high transient overvoltages, in the system and in the bank, from fault arc restrikes.
Advantage of a grounded neutral. It reduces the recovery voltage on the first pole to clear when the bank is disconnected. That voltage is the sum of the trapped charge in the capacitors and the swing of the system voltage:
| Bank | Recovery voltage on the first pole |
|---|---|
| Grounded | About 2 times the normal peak voltage |
| Ungrounded | Up to 3 times the phase-to-ground peak voltage on the first peak |
Since the recovery voltage determines the breaker’s capability to switch capacitive currents, from the breaker’s point of view it is better to ground the neutral. IEEE C37.04-1979 and ANSI C37.06-1997 recommend a grounded bank and system at voltages of 121 kV and above. Many banks at higher voltages are installed ungrounded, but then the breaker manufacturer must state whether it is suitable for that application. For the equipment see capacitive current switching.
Tertiaries. A bank connected to a transformer tertiary, for example the delta tertiary of an autotransformer, is an isolated source. Grounding the bank neutral makes that side grounded through the capacitance. Overvoltages can appear in line-to-ground faults for certain X0/X1 ratios. If the neutral is grounded in a system that is not effectively grounded, the guide calls for a complete analysis of surge arresters, bank configuration and switching equipment.
The concept of the system neutral is in isolated neutral versus grounded neutral.
How the unbalance signal changes with the neutral connection is in bank connection: grounded wye, ungrounded wye or delta.
Several banks at the same location (cl. 6.1)
If there are two or more grounded-wye banks at the same location, their neutrals are joined directly with a single ground connection. The design of the grid and the connection of the neutrals to it is of paramount importance: inadequate grounding can damage the neutral current and voltage transformers or the control cables.
When a bank is energized with a nearby bank already energized, or when the arc restrikes in the breaker, a very high, oscillatory inrush current appears between the banks. The guide places it in the order of 50 to 100 times the normal peak current of the bank. That current can damage the breaker’s contacts or mechanism. A reactor of 0.5% to 1% of the capacitive reactance of the banks, placed in series, greatly reduces the rate of rise and the peak value.
Single-point ground
The neutrals of all banks of the same voltage are joined with insulated cable or an insulated bus, and connected to the grid at a single point. That way, the high-frequency currents that flow between banks on a nearby energization do not pass through the grid. What is not eliminated are the high frequencies that return to the system through the grid during a nearby ground fault.
During switching, tens of kilovolts appear between the ends of the neutral bus and the ground point. That voltage acts on the primary-to-secondary insulation of the neutral current or voltage transformers, and increases the chance of failure. The guide says to use a two-bushing voltage transformer, with the primary between the bank neutral and the substation ground grid.
Peninsula
One or several conductors of the grid are run under the rack of each phase of each group. They are connected to the main grid at one point on the edge of the capacitor area. All the bank neutrals are connected only to that insulated peninsula conductor.
With this scheme, the potential of the bank neutral (and of its current and voltage transformers) rises during switching, but transients are reduced in the rest of the substation. Since all the neutral equipment tends to rise to the same potential, the high voltage between ends of the previous case does not appear. If it is coordinated with the shielding and grounding of the control cables, common-mode voltages in the control building stay at safe levels.
The two schemes can be used together. The guide considers it desirable that all banks of the same voltage in a substation use the same scheme.
What problems it avoids (cl. 7.2.3)
Every time a large grounded-wye bank is energized, momentary charging currents flow through a phase and the neutral that can approach the ground trip level. If another bank is already energized in parallel, they are of the order of thousands of amperes. They can cause:
- false relay operations and relay failures,
- current transformer failures,
- energized fences,
- ground grid problems.
That is why the guide says the neutral connection between several banks must not be made through the ground grid, but with a single-point or peninsula ground.
The transients that energization induces in CTs, VTs and control cables are explained in switching transients in CTs, VTs and control circuits; energization itself, in bank switching.
What to check in an existing bank
- Whether the neutrals of several banks share a single ground point or a peninsula.
- The withstand voltage between primary and secondary of the neutral current or voltage transformer, against the switching voltages.
- Coordination with the control cables: shielding and grounding points.
- Whether the bank neutral is consistent with the system grounding type.
The ground grid itself, with its resistance and its step and touch voltages, is covered in ground grid and in system and neutral grounding. To estimate the resistance of rods and grids you can use our grounding resistance tool.
What we do
The way the neutrals are connected, the insulation of the neutral measuring transformers and the coordination with the ground grid are defined for each substation. At Tiza Group we do it within the bank’s engineering design and studies and quote the study according to the scope.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Capacitor bank connections: grounded, ungrounded wye, delta
- Bank protection: threat, device and notes
- Bank switching: breaker, inrush and surge arresters
- Ground grid
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 5.1, 6.1 (Figure 4), 6.2 and 7.2.3.
- IEEE Std C37.04-1979 and ANSI C37.06-1997, cited by the guide for bank and system grounding from 121 kV.
Frequently asked questions
Can the neutral of a bank be grounded in an isolated system?
The guide advises against it: in a ground fault on an ungrounded system, a grounded bank neutral can produce high transient overvoltages from fault arc restrikes. A grounded bank neutral is recommended only in effectively grounded systems.
Why isn't the neutral of several banks connected to the grid at different points?
Because when a bank is energized with another already connected, high-frequency currents flow between them. If the path is the ground grid, there are false relay operations, current transformer failures and energized fences. That is why a single point or a peninsula is used.
Which is better, single-point ground or peninsula ground?
The guide says both have been used successfully. The single point keeps the currents between banks out of the grid, but it subjects the neutral transformers to tens of kilovolts. The peninsula makes all the neutral equipment rise to the same potential.