Transcript Advanced Computer Architecture

Protection of Power
Systems
3. Instrument Transformers
 There are two basic types of instrument
transformers: voltage transformers (VTs),
formerly called potential transformers (PTs),
and current transformers (CTs).
 Figure 10.2 shows a schematic
representation for the VT and CT.
 The transformer primary is connected to or
into the power system and is insulated for the
power system voltage.
 The VT reduces the primary voltage and the
CT reduces the primary current to much
lower, standardized levels suitable for
operation of relays.
Donut or Window-Type
Bar-Type
600V and 1000V Current Transformers
CT
Dead-Tank Circuit Breaker with Bushing-Type CTs
A 110 kV CT
SF6 110 kV Current Transformer
Current Transformers and Live-Tank Circuit Breakers
SF6 insulated Inductive Voltage Transformers
500 kV Capacitor Voltage Transformer
Capacitor Voltage Transformer
VTs
 For system-protection purposes, VTs are
generally considered to be sufficiently
accurate.
 Therefore, the VT is usually modeled as an
ideal transformer, where
 V is a scaled-down representation of V and
is in phase with V.
 A standard VT secondary voltage rating is
115 V (line-to-line).
 Standard VT ratios are given in Table 10.1
 Ideally, the VT secondary is connected to a
voltage-sensing device with infinite
impedance, such that the entire VT
secondary voltage is across the sensing
device.
 In practice, the secondary voltage divides
across the high impedance sensing device
and the VT series leakage impedances.
 VT leakage impedances are kept low in order
to minimize voltage drops and phase-angle
differences from primary to secondary.
CTs
 The primary winding of a current transformer
usually consists of a single turn, obtained by
running the power system’s primary
conductor through the CT core.
 The normal current rating of CT secondaies is
standardized at 5 A in the United States,
whereas 1 A is standard in Europe and some
other regions.
 Currents of 10 to 20 times (or greater) normal
rating often occur in CT windings for a few
cycles during short circuits.
 Standard CT ratios are given in Table 10.2
 Ideally, the CT secondary is connected to a
current-sensing device with zero impedance,
such that the entire CT secondary current
flows through the sensing device.
 In practice, the secondary current divides,
with most flowing through the low-impedance
sensing device and some flowing through the
CT shunt excitation impedance.
 CT excitation impedance is kept high in order
to minimize excitation current.
 Current transformers are normally equipped
with two cores—one for measurements and
one for protection purposes.
 The measurement core will give accurate
measurements during normal power system
operation, but will saturate for the much
higher fault currents.
 The protection core, on the other hand, is not
capable of providing accurate measurements
for low currents, but will not saturate for fault
currents.
 An approximate equivalent circuit of a CT is
shown in Figure 10.7, where
 The total impedance ZB of the terminating
device is called the burden and is typically
expressed in values of less than an ohm.
 The burden on a CT may also be expressed
as volt-amperes at a specified current.
 Associated with the CT equivalent circuit is an
excitation curve that determines the
relationship between the CT secondary
voltage E and excitation current Ie.
 Excitation curves for a multiratio bushing CT
with ANSI classification C100 are shown in
Figure 10.8.
FIGURE 10.8 Excitation curves for a multiratio bushing CT with a C100
ANSI accuracy classification
 Current transformer performance is based on
the ability to deliver a secondary output
current I that accurately reproduces the
primary current I.
 Performance is determined by the highest
current that can be reproduced without
saturation to cause large errors.
 Using the CT equivalent circuit and excitation
curves, the following procedure can be used
to determine CT performance.
 For simplicity, approximate computations are
made with magnitudes rather than with
phasors.
 Also, the CT error is the percentage
difference between (I + Ie) and I, given by:
 The following examples illustrate the
procedure.
 Note that for the 15-A secondary current in
(c), high CT saturation causes a large CT
error of 57.1%.
 Standard practice is to select a CT ratio to
give a little less than 5-A secondary output
current at maximum normal load.
 From (a), the 100 : 5 CT ratio and 0.5 
burden are suitable for a maximum primary
load current of about 100 A.
Homework 1
Optical CTs
 Other means of providing power system
information for protective relays are
developed and finding applications.
 One is the magneto-optic current transducer.
 This uses the Faraday effect to cause a
change in light polarization passing through
an optically active material in the presence of
a magnetic field.
 The passive sensor at line voltage is
connected to a stationary equipment through
a fiber-optic cable.
 This eliminates the supports for heavy iron
cores and insulating fluids.
 The output is low energy, and can be used
with microprocessor relays and other lowenergy equipment.
 These are most useful at the higher voltages
using live tank circuit breakers that require
separate CTs.
 In the meantime, iron-cored devices are
ubiquitous in power systems and do not
appear to be easily replaced.
 Optical CTs offer increased accuracy,
reduced size, and wider bandwidth.
 Optical CTs are also safer to work with than
conventional CTs.
 When the secondary circuit of a conventional
CT is opened while the primary current is
flowing, very high voltages can be developed
at the location of the opening.
 These voltages can be damaging to the
equipment and a safety hazard to the
personnel working in the area.
 Panel boards have been completely
destroyed when CT secondary circuits had
been inadvertently opened.
 Test personnel must take special precaution
when working on CT secondary circuits which
can be time consuming or dangerous if an
error occurs.
 It is obvious that optical sensors provide
significant operational advantages over
conventional CTs and will probably receive
greater utilization as protective systems are
converted to digital devices, fault current
levels continue to increase on power
systems, costs are reduced, and engineers
become more familiar and confident with the
technology.
 Besides, the advantages associated with
accuracy and freedom from saturation, the
fundamental characteristics of such
measuring systems can be changed by
simple program changes in the associated
software.
CCVTs
 VTs would have primaries that are either
connected directly to the power system (VTs)
or across a section of a capacitor string
connected between phase and ground
(Coupling-Capacitor VT (CCVTs )).
 VTs are used at all power system voltages
and are usually connected to the bus.
 At about 115 kV, the CCVT type becomes
applicable and generally more economical
than VTs at the higher voltages.
 Usually, the CCVTs are connected to the line,
rather than to the bus, because the coupling
capacitor device may also be used as a
means of coupling radio frequencies to the
line for use in pilot relaying.
 The metering CCVT, shown in the figure,
consists of a modular capacitive divider which
reduces the line voltage V1 to a voltage V2
(10–20 kV), with a series-resonant inductor to
tune out the high impedance and make
available energy transfer across the divider to
operate the voltage transformer which further
reduces the voltage to VM, the metering level.