APS Capacitor Banks

Capacitor Selection
There are two basic types of capacitor installations: individual capacitors on
linear or sinusoidal loads, and banks of fixed or automatically switched capacitors at the feeder or substation.

Individual vs. Banked
Installations
Advantages of individual capacitorsat the load:
■ Complete control. Capacitors cannot
cause overcompensation on the line
during light load conditions
■ No need for separate switching. Motor
always operates with capacitor
■ Improved motor performance due to
more efficient power utilization and
reduced voltage drops
■ Motors and capacitors can be easily
relocated together
■ Easier to select the right capacitor
for the load
■ Reduced line losses
■ Increased system capacity
Advantages of bank installations at
the feeder or substation:
■ Lower cost per kvar
■ Total plant power factor improved—
reduces or eliminates all forms of
kvar charges
■ Automatic switching ensures exact
amount of power factor correction,
eliminates overcapacitance and
resulting overvoltages

Table Summary of Advantages/Disadvantages of Individual, Fixed Banks,
Automatic Banks, Combination

Method	Advantages	Disadvantages
Individual capacitors	Most technically efficient, most flexible	Higher installation and maintenance cost
Fixed bank	Most economical, fewer installations	Less flexible, requires switches and/or circuit breakers
Automatic bank	Best for variable loads, prevents over-voltages, low installation cost	Higher equipment cost
Combination	Most practical for larger numbers of motors	Least flexible

Selection Criteria
The selection of the type of capacitor installation will depend on advan-
tages and disadvantages of each type and several plant variables, including
load type, load size, load constancy, load capacity, motor starting methods and manner of utility billing.

Load Type
If a facility has many large motors, 50 hp and above, it is usually economical to
install one capacitor per motor and switch the capacitor and motor together.

If there are many small motors, 1/2 to 25 hp, motors can be grouped with
one capacitor at a central point in the distribution system. Often, the best solution for plants with large and small motors is to use both types of capacitor installations.

Load Size
Facilities with large loads benefit from a combination of individual load, group load and banks of fixed and automatically-switched capacitor units.
A small facility, on the other hand, may require only one capacitor at the service entrance. Sometimes, only an isolated trouble spot requires power factor correction in applications such as welding machines, induction heaters or DC drives. If a particular feeder serving a low power factor load is corrected, it may raise overall plant power factor enough that additional capacitors are unnecessary.

Load Constancy
If a facility operates around-the-clock and has a constant load demand, fixed capacitors offer the greatest economy. If load is determined by eight-hour
shifts five days a week, use switched units to decrease capacitance during
times of reduced load.

 

Load Capacity
If the load on a transformer is approaching its maximum kVA rating
and the load has a poor power factor, below 0.9 for example, capacitors may
be added to supply reactive power to the load thereby reducing loading on
the transformer. This will therefore add kVA capacity to the system.
Similarly feeder cable load current can be reduced by the addition of
capacitors, if the load requires a significant amount of reactive power.

Utility Billing
The severity of the local electric utility
tariff for power factor will affect payback
and ROI. In many areas, an optimally
designed power factor correction
system will pay for itself in less than
two years.

National Electrical Code
Requirements for Capacitors
Nameplate kvar: Tolerance +15, – 0%.
Discharge resistors: Capacitors rated at 600 V and less must reduce the charge to less than 50 V within 1 minute of de-energization. Capacitors rated above 600 V must reduce the charge within 5 minutes.

Continuous operation: Up to 135%
rated (nameplate) kvar, including the effects of 110% rated voltage
(121% kvar), 15% capacitance tolerance and harmonic voltages over
the fundamental frequency (60 Hz).

Dielectric strength test: Twice the
rated AC voltage (or a DC voltage
4.3 times the AC rating for non-
metallized systems).

Overcurrent Protection: Fusing
between 1.65 and 2.5 times rated
current to protect case from rupture.
Does not preclude NEC® requirement
for overcurrent protection in all three
ungrounded conductors.
Note: When capacitor is connected to
the load side of the motor overcurrent
protection, fused disconnects or breaker
protection is not required. Fuses are recom-
mended for all other indoor applications.

 

 

Capacitor Switching Devices
Low Voltage Capacitor Switching
Circuit breakers and switches for use with a capacitor must have a current
rating in excess of rated capacitor current to provide for overcurrent from
overvoltages at fundamental frequency and harmonic currents.

The following percent of the capacitor-rated current
should be used as a general guideline:

Fused and unfused
switches. . . . . . . . . . . . . . . . . . . . 165%
Molded-case breaker or
equivalent . . . . . . . . . . . . . . . . . . 150%
Power circuit breakers . . . . . . . . . 135%
Insulated case circuit breakers. . . 135%
Contactors . . . . . . . . . . . . . . . . . . . 150%

The NEC, Section 460.8(c)(4), requires the disconnecting means to be rated not less than 135% of the rated capacitor current (for 600 V and below).See
for more information on Low Voltage Capacitor Switching Devices

 

Medium Voltage
Capacitor Switching
Capacitance switching constitutes severe operating duty for a circuit
breaker. At the time the breaker opens at near current zero the capacitor is
fully charged. After interruption, when the alternating voltage on the source
side of the breaker reaches its opposite maximum, the voltage that appears
across the contacts of the open breaker is at least twice the normal peak line-
to-neutral voltage of the circuit. If a breakdown occurs across the open
contact the arc is re-established. Due to the circuit constants on the supply
side of the breaker, the voltage across the open contact can reach three times
the normal line-to-neutral voltage. After it is interrupted and with subsequent alternation of the supply side voltage, the voltage across the open contact is even higher.

ANSI Standard C37.06 (indoor oilless circuit breakers) indicates the preferred
ratings of APS Type VCP-W vacuum
breaker. For capacitor switching careful attention should be paid to the notes accompanying the table. The definition of the terms are in ANSI
Standard C37.04 Article 5.13 (for the latest edition). The application guide
ANSI/IEEE Standard C37.012 covers the method of calculation of the quantities
covered by C37.06 Standard.

Note that the definitions in C37.04 make the switching of two capacitors banks in close proximity to the switchgear bus a back-to-back mode of switching.
This classification requires a definite purpose circuit breaker (breakers specifically designed for capacitance
switching).

 

We recommend that such application be referred to APS.
A breaker specified for capacitor switching should include as applicable:
1. Rated maximum voltage.
2. Rated frequency.
3. Rated open wire line charging switching current.
4. Rated isolated cable charging and shunt capacitor switching current.
5. Rated back-to-back cable charging and back-to-back capacitor switching current.
6. Rated transient overvoltage factor.
7. Rated transient inrush current and its frequency.
8. Rated interrupting time.
9. Rated capacitive current switching life.
10. Grounding of system and capacitor bank.
Loadbreak interrupter switches are permitted by ANSI/IEEE Standard C37.30 to switch capacitance but they must have tested ratings for the purpose. Refer to APS Type MVS ratings.

 

Projects that anticipate requiring capacitor bank switching or fault
interrupting should identify thebreakers that must have capacitivecurrent switching ratings on theequipment schedules and contractdrawings used for the project. Manufacturer’s standard medium voltage breakers meeting ANSI are notall rated for switching capacitive loads. Special breakers are usually availablefrom vendors to comply with the ANSI and other applicable ANSI standards. The use of capacitive current rated breakers can affect the medium voltage switchgear layout, thus early identification of these capacitive loads are critical to thedesign process For example, the standard 15 kV APS 150 VCP-W 500, 1200 A vacuum breaker does not have a capacitive current switching rating; however, the 15 kV Eaton 150 VCP-W 25C,
1200 A vacuum breaker does have the following general purpose ratings:
■ 25 A rms cable charging current switching
■ Isolated shunt capacitor bank switching current ratings of 25 A to 600 A
■ Definite purpose back-to-back capacitor switch ratings required
when two banks of capacitors are independently switched from the
15 kV switchgear bus The special breakers with these
capacitive current ratings do not haveUL labels, thus UL assembly ratings
are not available. Contact APS for more details on vacuum breaker and fused load interrupter switch products with capacitive switching current ratings at medium voltages.