33kV Capacitors 1

[berrycnsk]

I am a planning engineer for a UK distribution utility.

We have been considering the installation of about 6 capacitor banks on a weak part of our 33kV system to provide a voltage boost of a few percent across a wide area.

Each capacitor bank would be switched in and out of service using an appropriately rated vacuum circuit breaker. However one issue we have identified is that the majority of the existing 33kV circuit breakers have not been designed or tested to interrupt capacitive current.

The scenario is that the local capacitor bank is energised and a fault on a nearby 33kV circuit is cleared by the existing breakers and some of the current is capacitive.

The same principle would apply at lower voltages such as 11kV.

Does anyone have any suggestions other than changing all of the existing circuit breakers which would be uneconomic because of the numbers involved?

 

[DanDel]

Is your concern for the VC/Bs at the capacitors or the VC/Bs at the loads? A C/B at a load doesn't know that the reactive power it's load is using (and that it may have to interrupt) is coming from a capacitor or the utility.

 

[busbar]

It is assumed that the capacitors will be somewhat disbursed on the distribution system. Not sure I’m qualified to judge, but isn’t it most likely that capacitors are not connected in low-load conditions? It may be possible that during usual conditions there is sufficient real current to damp the capacitive/high-frequency components to allow for safe interruption at non-capacitor nodes, where capacitive current is ‘diluted’ by the real component? Admitted during a contingency condition, capacitors may be disconnected during a low-load situation.

Give appropriate system modeling, it would be interesting to see the effect of per-phase series air-core reactors for limiting capacitor-current outrush.

Longland’s Power Capacitor Handbook chapter 8 suggests consulting the J&P Switchgear Book for switching components, but seems to suggest that vacuum interrupters be limited to 11kV applications. {My copy of Longland is copyright 1984.}

 

[berrycnsk]

Thanks.

DanDel - The circuit breakers in question are those associated with the 33kV circuits which connect to both load nodes and capacitor bank nodes.

busbar - You are correct in assuming that the capacitors would normally be disconnected when the network is lightly loaded. However during maintenance outages they would be energised to support voltage and this would involve the capacitors supplying reactive power via existing circuit breakers.

I have been told air-cored 33kV reactors are expensive, physically very large and create magnetic fields around them that can interfere with communications and protection equipment. Where could I find out more about these issues?

 

[busbar]

It's an interesting (and valid) question. The reactor suggestion was just a wild idea.

Does IEE have online search capability? If so, it seems like that would be a reasonable starting point.

 

[jbartos]

Suggestion: The RC harmonic filters often use a disconnect switch to control them, and fuses. Although circuit breakers are often used for load switching, their intent is to protect. The switches or disconnect switches are used to switch loads in addition to contactors.
Visit

http://www.joslynhivoltage.com/PDFFiles/770-515Application.pdf

http://www.lke-electric.com/mv0.htm

C37.22-1997 American National Standard Preferred Ratings and Related Required Capabilities for Indoor AC Medium Voltage Switches used in metal-Enclosed Switchgear

for medium voltage switches.

 

[berrycnsk]

High voltage capacitors are installed both on the customer and the utility side of the meter and switched in and out of service by an adequately designed and rated breaker.

The point of my original post is that it is unlikely that the adjacent circuit breakers in the customer or utility networks are designed for also switching capacitive current. There must be scenarios where the existing breakers can 'see' a capacitive current and I just wonder how big an issue this really can be as it does not seem to have been considered in the installations I know about.

 

[jbartos]

Suggestion to berrycnsk (Electrical) Mar 8, 2003 marked by ///\\High voltage capacitors are installed both on the customer and the utility side of the meter and switched in and out of service by an adequately designed and rated breaker.
///Yes, this is often done and it is least expensive, at least to some. However, the breaker is supposed to protect only, not to be switching load as for example in some premises fluorescent lighting fixtures and circuits do not have switches. That is an ultimate saving in the electrical power distribution and perhaps even an ultimate greed.\\\
The point of my original post is that it is unlikely that the adjacent circuit breakers in the customer or utility networks are designed for also switching capacitive current. There must be scenarios where the existing breakers can 'see' a capacitive current and I just wonder how big an issue this really can be as it does not seem to have been considered in the installations I know about.
///If a safety or disconnect fusible switch is used for the capacitive load, then the capacitive current upstream is much overcompensated by inductive currents in favor of inductive currents that are larger. Therefore, there is no problem with circuit breaker ratings and deratings.\\

 

[DanDel]

berrycnsk, I'll restate from my first post: "A C/B at a load doesn't know that the reactive power it's load is using (and that it may have to interrupt) is coming from a capacitor or the utility. "

 

[berrycnsk]

DanDel - thanks for your input. Hopefully the following diagram will clarify my concern. The diagram shows part of a 33kV distribution network where two 132/33kV transformers 60km apart are electrically interconnected via a 33kV circuit. The 33kV circuit provides supplies to 11kV and Low Voltage customers via a number of 33/11kV transformers.
Due to the length of the 33kV circuit, at times of higher loading, the 33kV voltage is too low at the mid-point of the 33kV circuit. It is therefore proposed to install a 33kV capacitor. The capacitor will inject reactive power and this will flow from the capacitor towards each 132/33kV transformer. The 33kV circuit is mainly overhead line with a high X/R ratio and the flow of reactive power through the reactance of the line causes a voltage boost. The 33kV circuit breakers are shown as 'X' in the diagram and the unit protected circuit sections are shown 'C'. The new circuit breaker associated with the capacitor will be no problem. Your logic will apply to the existing circuit breakers remote from the capacitor as the load inductive reactive power will be greater than the injected capacitive reactive power. However the existing circuit breakers adjacent to the capacitor installation will 'see' capacitive currents.

I hope this clarifies the situation. Please comment.

132-33kV
transformer (Tx)
¦
X
C
C
C
X
33/11kV Tx -+
X
C
C
C
X
33/11kV Tx -+-X- Proposed 33kV Capacitor
X
C
C
C
X
33/11kV Tx -+
X
C
C
C
X
¦

132-33kV
transformer (Tx)

 

[DanDel]

Maybe there is a misunderstanding on my part in this post. I will attempt to clarify my point. Please correct me if I am not 'on the same page'. First of all, I'm assuming that there is a utility source at each of the 132kV transformer primaries.

Reactive power typically flows back and forth between inductive and capacitive components in an AC circuit at all times. The power going to the load does not change when a capacitor is connected in the system. There are the same components of real and imaginary power to and from the load with or without added capacitance in the system, the only thing that changes is where the reactive power comes from.

Most C/Bs are tested for interrupting ratings at 15 or 20% power factor, and, during the interruption of a fault or an overcurrent to a load, this C/B does not know that the reactive power it is interrupting is coming from the Utility generator or a capacitor bank.

As long as the capacitor is still connected to a source(at the other end of the system), you will simply be interrupting the same currents always used by the load.

I believe the 'capacitive currents' you are concerned about may only cause a problem upon de-energization of the capacitor bank.

 

[THD]

Do not use VCB's for capacitor switching. I have not read through all the posts, but I would imagine that this should of been pointed out. The new standard is to use SF6 breakers, as vacuum breakers do not seem to cope with restrike, unless they are overrated

 

[RajT]

I would disagree with THD in that VCB should not be used.
Modern VCB technology ensures that restrikes are eliminatred.When considering use/applications of VCB vs SF6, it is interesting to note that sales of VCB are rising and SF6 falling (for MV equipment that is).
Refering to the main thread question of wether or not to use VCB, it is perfectly acceptable to use them on capacitor banks as just like fuses they are rated interuptors.