Uploaded by Gedrich Hombre

SHUNT-CAPACITOR-REVIEWER

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SHUNT CAPACITOR
 Shunt capacitors are widely used in primary distribution to supply reactive power to loads.
 They draw leading currents that offset the lagging component of currents in inductive loads.
 Shunt capacitors provide an economical supply of reactive power to meet reactive power
requirements of loads as well as transmission and distribution lines operating at lagging power
factor.
 They can also reduce line losses and improve voltage regulation
 Power plants can generate reactive power as needed during heavy load periods or absorb
reactive power as needed during light load periods by changing the voltage magnitude of the
generator
 Note: The voltage magnitude determines the flow of reactive power, while the angular
differences (phase angles) determine the flow of active power.
 However, the generation of reactive power (Mvar) at power plants and transmission of the
reactive power over long distances to loads is not economically feasible.
 If there were only one load on the feeder, the best location for the capacitor bank would be
directly at the load
 Shunt capacitors at distribution substations, however, can be effective in reducing 𝟏𝟐 𝑹 losses
and voltage drops on the transmission or subtransmission lines that feed the distribution
substations
 For a primary feeder that has a uniformly distributed load along the feeder, a common
application is the “two-thirds” rule; that is, place 2/3 of the required reactive power 2/3 of the
way down the feeder.
 To meet the changing reactive power requirements, many utilities use a combination of fixed
and switched capacitor banks.
 Fixed capacitor banks can be used to compensate for reactive power requirements at light
loads, and switched capacitor banks can be added during heavy load conditions.
 The goal is to obtain a close-to-unity power factor throughout the day by switching capacitor
banks on when needed and off when not needed.
DISTRIBUTION TRANSFORMER
Typical Characteristics of Distribution Substation Transformers
 Usually contain mineral oil for
insulating and cooling purposes
 Some sealed transformers have a
pressure-relief diaphragm that is
designed to rupture when the internal
pressure exceeds a specified value,
indicating possible deterioration of the
insulation.
 Sealed transformers may also have a sudden pressure relay to either provide an alarm or deenergize the transformer when the internal pressure suddenly increases above a specified
threshold.
Load Tap Changer (LTC)
 Allows for variable turn ratios to be selected in distinct steps so that it could regulate voltage
levels
 This is done by connecting to a number of access points known as taps along either the primary
or secondary winding.
 Many distribution substation transformer LTC can automatically regulate voltage level based on
loading conditions.
 Some distribution substations have distribution substation transformers with fixed taps and
separate voltage regulators
 Voltage Regulator-basically an autotransformer with taps that automatically raise or lower
voltage, operating in a similar way as LTCs on distribution substation transformers
Condensator
 A tank on the top of the transformer in which expansion and contraction of the oil takes
place
 Also, condensation of moisture and formation of sludge
 Provided with a sump pump to draw off the moisture and sludge
 MVA rating
 Continuous load that the transformers carry without exceeding a specified temperature
rise of either 55°C (for older transformers) or 65°C (for newer transformers) above a
specified ambient temperature (typically 40°C)
 Multiple MVA ratings that depends on external radiator used to dissipate heat generated by
copper and core losses.
 OA rating (passive convection with oil circulating pumps and fans off)
 FA rating (with fans on but oil circulating pumps off)
 FOA rating (with both fans and oil circulating pumps on).
Three-phase 22.9 kVD/ 4.16 kVY distribution substation transformer rated 12 MVA OA/16 MVA FA1/20
MVA FA2. The transformer has fixed taps on the high-voltage side and an LTC on the low-voltage side.
 A lower FA rating with one of two sets of fans on
 A higher FA rating with both sets of fans on
 The nameplate transformer impedance is usually given in percent using the OA rating as the
base MVA
 Read Example 4.1
 2 transformer rated 9 MVA OA/ 12 MVA FA1/ 15 MVA FA2
 Substation normally operate at or below 15 MVA
 During emergency (e.g. forced of scheduled outage of one transformer), the other
transformer can supply all the feeders
 For this conservative operating practice, emergency transformer ratings above
nameplate are not used.
 Some utilities operate their distribution substation transformers above nameplate ratings during
normal operating conditions, as well as during emergency conditions.
 ANSI/IEEE C-57.91-1995 entitled, IEEE Guide for Loading Mineral Oil-Immersed Transformer
 Identifies the risks of transformer loads in excess of nameplate rating and establishes
limitations and guidelines
 Minimize the risks to an acceptable limit
 Typically, there are two emergency loading criteria for distribution substation transformers:
1. A two-hour emergency rating, which gives time to perform switching operations and
reduce loadings.
2. A longer-duration emergency rating (10 to 30 days), which gives time to replace a
failed transformer with a spare that is in stock.
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