Download CT VT CVT

CT,VT,CVT
Connections of a CT and a PT to supply, load and relay.
CT, VT, CVT
The voltage transformers and current transformers continuously
measure the voltage and current of an electrical system and are
responsible to give feedback signals to the relays to enable them to
detect abnormal conditions
CTs and VTs are the eyes and ears of the protective system.
They extract information from the power system and form an
important link between the high-voltage high-current power system
and the low-voltage low-current protective system. CTs and VTs have
two important roles to play, namely:
1 Electrically isolate the protective relays from the high-voltage
power system for, the purpose of safety of the operating personnel.
2 Step down the current and voltage to standard values of 1 A, 5 A,
110 V so that the design of relays can be standardized irrespective of
the actual primary voltage, and current.
Current Tranformer
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There are two types of current transformers:
1. Wound primary type
2. Bar primary type.
Bar primary
Wound primary
• The wound primary is used for the smaller currents, but it can only be
applied on low fault level installations due to thermal limitations as well as
structural requirements due to high magnetic forces.
• For currents greater than 100 A, the bar primary type is used
• Protection CTs are most frequently of the bar primary, toroidal core with
evenly distributed secondary winding type construction. In such CTs, the
secondary winding can be conveniently put inside the high voltage
bushings.
Current transformer
• The basis of all transformers is that:
Amp-turns on the Primary = Amp-turns on the
secondary
e.g. 100 A × 1 turn = 1 A × 100 turns
• The primary current contains two components:
• An exciting current, which magnetizes the core
and supplies the eddy current and hysteresis
losses, etc.
• A remaining primary current component, which
is available for transformation to secondary
current in the inverse ratio of turns.
• The exciting current is not being transformed
and is therefore the cause of transformer
errors.
• The amount of exciting current drawn by a CT
depends upon the core material and the
amount of flux that must be developed in the
core to satisfy the output requirements of the
CT. that is, to develop sufficient driving voltage
required, pushing the secondary current
through its connected load or burden. This can
be explained vectorally as shown in figure
Vector diagram for a current transformer
The transition from the unsaturated to
the saturated region of the opencircuit excitation characteristic makes a
CT not to produce equivalent primary
current beyond certain point. This
transition is defined by ‘knee-point’
voltage in a CT, which decides its
accurate working range.
Equivalent circuit of CT
Phasor diagram of CT
CT errors
Phase angle error
• The conventional CT core saturates during transient causing delay
in high speed relay operation
• Secondary resistance
• The secondary resistance of a CT is an important factor, as the CT
has to develop enough voltage to push the secondary current
through its own internal resistance as well as the connected
external burden. This should always be kept as low as possible.
• Burden :circuit connected to secondary winding. Expressed in voltampere at rated secondary current at rated power factor.
CT specification
• A current transformer is normally specified in terms of:
• A rated burden at rated current
• An accuracy class
• An upper limit beyond which accuracy is not guaranteed (known as
the accuracy limit factor, ALF), which is more vital in case of
protection CTs.
• In terms of the specification a current transformer would,
for example, be briefly referred to as 15 VA 5P20 if it
were a protection CT or 15 VA Class 0.5 if it is a metering
CT. The meanings of these figures are as below:
Open circuits of CTs
• Current transformers generally work at a low flux
density. Core is then made of very good metal to give
small magnetizing current.
• On open-circuit, secondary impedance now becomes
infinite and the core saturates. This induces a very
high voltage in the primary up to approximately system
volts and the corresponding volts in the secondary will
depend on the number of turns , multiplying up by the
ratio (i.e. volts/turn  no. of turns).
• Since CT normally has much more turns in secondary
compared to the primary, the voltage generated on the
open-circuited CT will be much more than the system
volts,leading to flashovers.
• HENCE AS A SAFETY PRECAUTION, NEVER
OPEN-CIRCUIT A CURRENT TRANSFORMER ON
LOAD!!!
Voltage (potential)Transformer VT or PT
• Voltage transformers are much like power
transformers operating on very light load.
• There are basically, two types of voltage
transformers used for protection equipment.
• 1. Conventional two-winding, electromagnetic
type (commonly referred to as a VT)
• 2 Capacitive voltage divider type(referred to
as a CVT).
The electromagnetic type is a step down
transformer whose primary (HV) and secondary
(LV) windings are connected as below
• The number of turns in a winding is directly proportional to
the open-circuit voltage being measured or produced
across it. The above diagram is a single-phase VT.
• In the three-phase system it is necessary to use three VTs
at one per phase and they being connected in star or delta
depending on the method of connection of the main power
source being monitored.
• This type of electromagnetic transformers are used in
voltage circuits upto 110/132 kV.
Capacitive voltage transformer (CVT)
• Capacitive voltage transformer consists of a capacitive voltage divider as
shown in Figure A.6(a).
• However, a simple capacitor divider cannot be used as a CVT because the
output voltage will depend on the current drawn by the burden.
• Hence, a tuning coil is used so that it resonates with the equivalent
capacitance seen looking into the capacitor potential divider.
• This makes the output voltage, a constant fraction of line voltage and
independent of the current drawn.
• In EHV national grid networks of utilities, the CVTs are commonly used for
both protection and communication purposes.
• Difficulty : Ferro resonance
CT Testing