Grounded vs ungrounded capacitor banks 1

[wfowfo]

We are under the gun to conform with the new NERC power Factor requirements of 97%. This will involve the installation a many new cap banks.
My problem is that we use the Cannon Technologies AMR system and have blocking units in the neutral of each bank to prevent the signal from being shunted to ground. The blocking units are expensive.
Is there any disadvantage to not grounding the cap bank and letting it float so that I can forego the blocking unit?

 

[jghrist]

See

http://www.gilbertelectrical.com/library/technical.discussions/application.floating.wye.htm

Also, if there are single phase fuses or switches on the source side of an ungrounded wye bank, they can get phase-to-phase voltage across the open contacts and must be rated appropriately.

 

[ePOWEReng]

Here is a good article by S&C -

http://www.sandc.com/webzine/101804_1.asp

By the way, I am curious, what is the NERC PF requirement of 97%, could you refer me to the standard or explain it in a little more detail?

 

[supermacc]

IEEE Std C37.99-2000:

The application of large shunt capacitor banks with switched parallel banks in high-voltage transmission systems involves a number of considerations, one of which is grounding. It is generally recommended that the neutral of capacitor banks be grounded only to systems that are effectively grounded. In the event of a phase-to-ground fault, a grounded capacitor bank neutral in an otherwise ungrounded system may lead to high transient overvoltages in the system and capacitor bank as a result of restriking of the arcing fault to ground.

One of the main advantages associated with neutral grounding concerns the severity of the recovery voltage
across the first pole of the switch to clear, interrupting the charging current of the capacitor bank. The recovery
voltage across the first pole to open consists of trapped charges on the capacitors and the variation in the 60 Hz voltage of the system. Due to system parameters and capacitor bank size, the recovery voltage can be approximately two times normal peak voltage when the bank is grounded. On an ungrounded bank, the magnitude of the first peak of the recovery voltage can be as high as three times the peak system line-to-ground voltage when the bank is switched.

Because recovery voltage is a critical factor in determining the capability of a switching device to switch
capacitive reactive power, it may be desirable (in terms of switch performance) to ground the neutral of shunt capacitor banks. IEEE Std C37.04-1979 and ANSI C37.06-1997 recommend that both the shunt capacitor bank and the system be grounded at voltage levels of 121 kV and above. Many capacitor banks of higher voltage are installed ungrounded, but the circuit breaker manufacturer should be consulted for the application of a breaker if these conditions are not met.

and

Ungrounded wye banks do not permit zero sequence currents, third harmonic currents, or large capacitor
discharge currents during system ground faults. (Phase-to-phase faults may still occur and will result in large
discharge currents.) The neutral, however, should be insulated for full line voltage because it is momentarily at phase potential when the bank is switched or when one capacitor unit fails in a bank configured with a single
group of units.

 

[magoo2]

The NERC requirement is designed to permit satisfactory interchange of power between U.S. utilities and the 97% PF applies to their transmission systems.

On the negative side, we use single-bushing capacitor cans on the line and our safety work practices requires us to ground the cans. With a single bushing arrangement, we can simply ground the rack. If we lift the neutral to ground connection, the cans will be at some elevated potential.

An alternative is to use 2-bushing cans. This way the can can be grounded and it doesn't tie it to the neutral. Maybe you're already using 2 bushing cans for your system, but for us that would be a big obstacle to overcome.

The problem isn't really related to Cannon Technology, but rather to the use of power line carrier as the communication means. I believe Cannon's equipment will work with other communications choices.

 

[wfowfo]

I should have been clearer in my question.
As magoo2 states, the 97% PF is aimed at the transmission lines. In the ERCOT Deregulated market, this responsibility has been dumped onto the distribution utilities. The cap banks I was referring to are on 12.5Kv and 24.9 Kv grounded wye distribution systems and are typically two bushing units.

As far as Cannons' Technology, it is simply a case of their 12.5 Khz signal being shunted to ground due to the low impedance of the caps at that frequency (which is the reason for the blocking units).

Not being an engineer (and having a ton of cap banks to install) we were hoping to save some money on the installation by foregoing the blocking units and using the ungrounded banks.

Does supermaccs' expanation of a higher recovery voltage still apply?

 

[magoo2]

The recovery voltage issue applies to switched banks only. As was stated, a 2 per unit recovery voltage is typical for a grounded wye bank and I remember 2.5 applying for ungrounded wye banks.

With 2 bushing units, you should have no difficulty installing the traps (filters) between the bank neutral and the system neutral or simply ungrounding the bank - which is a lot cheaper.

 

[ePOWEReng]

Switching can still be an issue on distribution feeder capacitor banks, due to reclosing at the feeder breaker.

 

[apowerengr]

I have many customers using Cannon's power-line carrier system and a mix of blocking units and floated-neutral cap banks. Two bushing units are required as stated earlier, otherwise if you provide sufficient training to the crews there should not be any significant issues. Switches will need to be rated for ungrounded wye operation which should be easy at 12kV but there's a smaller market at 25kV.