AIR CORE REACTORS
Shunt Reactors in Power Systems
Tech News
AREVA T&D’s Expertise FORWARD During normal operation of an electrical power system, the transmission and distribution voltages must be maintained within a small range, typically, from 0.95 to 1.05 pu of rated value. Due to the load variations, shunt reactors and capacitors have been applied in power systems to compensate excess reactive power (inductive for heavy load conditions, and capacitive for light load conditions). Shunt reactors are commonly used to compensate the capacitive reactive power of transmission and distribution systems and thereby to keep the operating voltages within admissible levels. The purpose of this document is to present some information about the application and specification of shunt reactors for electrical power systems.
AREVA T&D’S TECHNOLOGY AND EXPERIENCE AREVA T&D has more than 30 years of experience in designing and manufacturing air-core dry-type reactors (ACR) for various market segments around the world, including power generation, transmission and distribution systems, industrial plants, OEM and electrical test laboratories. Reactive shunt compensation is one of the most common applications of air-core reactors. Air-core shunt reactors are applicable to system voltages up to 72,5 kV and typically they are connected to tertiary winding of large power transformers. Due to the required ratings, the ACR for this application are designed in fiberglass encapsulated technology. In fiberglass encapsulated technology, the reactor’s winding consists of numerous insulated aluminum connected in parallel. These conductors are mechanically immobilized and encapsulated in epoxy impregnated fiberglass filaments forming cylinders. Depending on the reactor’s ratings, one or more of these cylinders are connected in parallel between the aluminium spiders. The individual cylinders are separated by fiberglass spacers, which form the cooling ducts of the coil. The benefits in using AREVA T&D air-core shunt reactors are: > Maintenance free and environmentally friendly > Conservative temperature rise for extended service life > Customized space saving solutions for installation in compact areas > Surface treatment for protection against UV radiation and pollution > High mechanical strength to withstand elevated short-circuit forces > Low noise level for sensitive applications
In the following sections general information will be presented about the application of shunt reactors to electrical power systems and two different ways to specify the required ratings of the equipment (MVAr ratings).
APPLICATION OF SHUNT REACTORS The calculation of optimum ratings and points of connection of shunt reactors is generally done by means of extensive load flow studies, taking into account all possible system configurations. One approach for a single line is presented below. Depending on system voltage, shunt reactors may be inserted directly connected to station busbars (Pos. 1), to transmission line terminations (Pos. 2) or connected to tertiary winding of large power transformers (Pos. 3), as shown in the picture 1.
Pos.2
Pos.2
Pos.1 Pos.3
Picture 1 – Shunt reactor application in power systems
The majority of shunt reactors for system voltages of 72,5 kV or above are in the 30 to 300 MVAr range (threephase power) and they are normally connected directly to high voltage busbar or transmission line ending. For these voltage levels, reactors are most commonly oil-filled type. Future reactors in the range 72,5 to 145 kV will tend to be air-core dry-type coil units. Shunt reactors rated below 72,5 kV are either oilfilled or air-core dry-type units and they are normally connected to the tertiary winding of large power transformers. The range of reactive power varies from a few MVAr to 100 MVAr. The winding connection of three-phase reactors or a bank of three single-phase units can be either wye (most common configuration) or delta. Typically, for system voltages of 72,5 kV or above, the reactors are wye connected with the neutral grounded directly or through a neutral reactor (also named “four reactor scheme”). For system voltages below 72,5 kV, the reactors are wye connected with the neutral ungrounded.
XR
XR X NEUTRAL
W y e c o n nec t i o n (Fo ur R eact ors Schem e )
D e l ta co nn ec t io n
Picture 2 – Winding connection of shunt reactors
MAIN CALCULATION OF SHUNT REACTORS For the calculation of the positive sequence reactance and the current requirements of a shunt reactor, it is necessary to know only the rated three-phase reactive power and the rated system voltage and frequency, as summarized in the table below. RATING
WYE CONNECTION
DELTA CONNECTION
Reactance
Rated Current
Maximum Continuous Current (Design Current)
Parameters
> rated reactance per phase (positive sequence) > rated three-phase reactive power > rated reactive power per phase > rated system voltage > maximum system operating voltage > rated current > maximum continuous current
The zero sequence reactance (X0) depends on the winding connection and grounding of the shunt reactor. For air-core drytype units, it can be calculated as follow:
Inserting shunt reactors at the receiving ending, the ABCD parameters of the line are changed, as described below:
> Wye connection with neutral directly grounded > Wye connection with neutral grounded through a reactor
+
> Wye connection with neutral ungrounded
So, the relation between the ending voltages of the transmission line is given by:
> Delta connection
Neutral reactors generally are used in shunt reactors installed in transmission line terminations to provide a faster extinguishing of the secondary arc current and, therefore, to allow the automatic reclosing of the transmission line after a fault elimination (typically, the reclosing time varies from 0.5 to 1.5 seconds).
OPEN-CIRCUIT OPERATION OF RADIAL TRANSMISSION LINES The operation of a lossless radial transmission line, which is energized by a generator at the sending ending (V1) and is opencircuited at the receiving ending (V2), can be represented in the matrix form by the ABCD parameters, where I2 = 0.
Application Example Consider a lossless radial transmission line, frequency 60 Hz, length ℓ = 350 km, and parameters z = j 0,32886 Ω/km and bC = j 5,097 µS/km. To estimate the reactive power of shunt reactors to be installed in the transmission line to provide a maximum operating voltage of 1.05 pu at the open-circuited terminal (receiving ending), when the line is energized with 1.0 pu in the sending ending. Solution: > Total impedance and admittance of the non-compensated transmission line
> Parameter A:
reactor to provide a required voltage variation in the busbar can calculated through the short-circuit power of system at the busbar where the reactor will be connected.
S CC
V2
> Operating voltage at the receiving ending of the noncompensated transmission line
V1
> Calculation of the shunt reactor reactance:
Picture 3 – Practical circuit for voltage control analysis The shunt reactor rating is given by:
Application Example
> Calculation of the three-phase reactive power of the shunt reactor:
To estimate the reactive power of shunt reactors to installed in the 34.5 kV busbar in order to reduce the voltage level from 1.02 to 0.99 pu, considering a fault current of 25 kA (or short-circuit power of 1495 MVA). Solution:
MVAr
> Calculation of the three-phase reactive power of the shunt reactor:
MVAr > Calculation of the compensation degree of the transmission line: In order to make possible line energization on both sides, it is recommended to install shunt reactors with similar ratings in their two terminations. > The line charging of the transmission line is:
Remark: > In the analysis above, it is not considered the on tap changer (OLTC) operation of power transformers near to the point of connection of the shunt reactors, which occurs in a few minutes after the shunt reactor switching.
MVAr CONCLUSIONS So, compensation degree is:
BUSBAR VOLTAGE VARIATION AFTER SWITCHING OF SHUNT REACTORS Typically, the voltage variation at the high voltage busbar after switching of a shunt reactor shall not be higher than 2 to 3% of rated voltage. A practical circuit is used to simplify the analysis of voltage control (see picture 3). The determination of the shunt
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AREVA T&D is able to supply air-core shunt reactors to provide reactive power compensation for electrical systems with rated voltages up to 72,5 kV and three-phase ratings up to 100 MVAr. AREVA T&D’s air-core shunt reactors are maintenance free, environmentally friendly and maintain a conservative temperature rise offering an extended service life. To request technical information, please contact us by e-mail:
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