[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

RCF
x
Bx
Bt
RCF t – RCF 0 cos t – x t – 0 sin t – x
Bx
x
t – 0 cos t – x - RCF t – RCF 0 sin t – x
Bt
where
RCF x = RCF of new burden
RCF t = RCF of known burden
RCF 0 = RCF at zero burden
x = phase error of new burden, radians (to obtain x in minutes, multiply value from
Equation 2.6.7 by 3438)
t = phase error of known burden, radians
0 = phase error at zero burden, radians
B x = new burden
B t = known burden
x = new burden PF angle, radians
t = known burden PF angle, radians
(2.6.9)
(2.6.10)
2.6.3.1 Overvoltage Ratings
The operating flux density is much lower than in a power transformer. This is to help minimize the losses
and to prevent the VT from possible overheating during overvoltage conditions. VTs are normally
designed to withstand 110% rated voltage continuously unless otherwise designated. IEEE C57.13 divides
VTs into groups based on voltage and application. Group 1 includes those intended for line-to-line or
line-to-ground connection and are rated 125%. Group 3 is for units with line-to-ground connection
only and with two secondary windings. They are designed to withstand 173% of rated voltage for 1 min,
except for those rated 230 kV and above, which must withstand 140% for the same duration. Group 4
is for line-to-ground connections with 125% in emergency conditions. Group 5 is for line-to-ground
connections with 140% rating for 1 min. Other standards have more stringent requirements, such as the
Canadian standard, which defines its Group 3 VTs for line-to-ground connection on ungrounded systems
to withstand 190% for 30 sec to 8 h, depending on ground-fault protection. This also falls in line with
the IEC standard.
2.6.3.2 VT Compensation
The high-voltage windings are always compensated to provide the widest range of performance within
an accuracy class. Since there is compensation, the actual turns ratio will vary from the rated-voltage
ratio. For example, say a 7200:120-V, 60:1 ratio is required to meet 0.3 class. The designer may desire to
adjust the primary turns by 0.3% by removing them from the nominal turns, thus reducing the actual
turns ratio to, say 59.82:1. This will position the no-load (zero burden) point to the bottommost part
of the parallelogram, as shown in Figure 2.6.10. Adjustment of turns has little to no effect on the phaseangle
error.
2.6.3.3 Short-Circuit Operation
Under no normal circumstance is the VT secondary to be short-circuited. The VT must be able to
withstand mechanical and thermal stresses for 1 sec with full voltage applied to the primary terminals
without suffering damage. In most situations, this condition would cause some protective device to
operate and remove the applied voltage, hopefully in less than 1 sec. If prolonged, the temperature rise
would far exceed the insulation limits, and the axial and radial forces on the windings would cause severe
damage to the VT.
2.6.3.4 VT Connections
VTs are provided in two arrangements: dual or two-bushing type and single-bushing type. Two-bushing
types are designed for line-to-line connection, but in most cases can be connected line-to-ground with
reduced output voltage. Single-bushing types are strictly for line-to-ground connection. The VT should
never be connected to a system that is higher than its rated terminal voltage. As for the connection
between phases, polarity must always be observed. Low- and medium-voltage VTs may be configured in
delta or wye. As the system voltages exceed 69 kV, only single-bushing types are available. Precautions
must be taken when connecting VT primaries in wye on an ungrounded system. (This is discussed further
in Section 2.6.3.5, Ferroresonance.) Primary fusing is always recommended. Indoor switchgear types are
often available with fuse holders mounted directly on the VT body.
2.6.3.5 Ferroresonance
VTs with wye-connected primaries on three-wire systems that are ungrounded can resonate with the
distributed line-to-ground capacitance (see Figure 2.6.11). Under balanced conditions, line-to-ground
voltages are normal. Momentary ground faults or switching surges can upset the balance and raise the
line-to-ground voltage above normal. This condition can initiate a resonant oscillation between the
primary windings and the system capacitance to ground, since they are effectively in parallel with each
other. Higher current flows in the primary windings due to fluctuating saturation, which can cause
overheating. The current levels may not be high enough to blow the primary fuses, since they are generally
sized for short-circuit protection and not thermal protection of the VT. Not every disturbance will cause
ferroresonance. This phenomenon depends on several factors:
• Initial state of magnetic flux in the cores
• Saturation characteristics (magnetizing impedance) of the VT
• Air-core inductance of the primary winding
• System circuit capacitance
One technique often used to protect the VT is to increase its loading resistance by (1) connecting a
resistive load to each of the secondaries individually or (2) connecting the secondaries in a deltaconfiguration
and inserting a load resistance in one corner of the delta. This resistance can be empirically
approximated by Equation 2.6.11,
FIGURE 2.6.11 VTs wye-connected on ungrounded system.
R delta = (100 L A )/N 2 (2.6.11)
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC