Download Phase angle error

CURRENT TRANSFORMERS AND
VOLTAGE TRANSFORMERS
INTRODUCTION
• Function of the protection relays
• Information to the protection relays have
to be accurate and reliable
• Currents and voltages
• How to overcome this difficulty
WHAT DOES CURRENT AND VOLTAGE
TRANSFORMERS DO…
• Insulate secondary circuits from primary
• Provide currents/voltages proportional to the primary
ROLE OF CT, PT IN POWER SYSTEM
PROTECTION
• Protection systems depend heavily on information provided
•
CTs and PTs a constituting part of the protection system
• No great difference between a measuring voltage transformer and a
protective voltage transformer
• However, Current transformer is different
BASIC PRINCIPLES OF OPERATION
• Transformer can be operated in
» Shunt mode
» Series mode
SHUNT MODE
• Voltage applied across the
primary terminals
• When secondary is on open
circuit, Induces an EMF equal
to the voltage applied
• Current flowing will be that
required to excite the core
• When secondary is loaded
Ampere turns of the primary
winding will exceed that of
secondary
• Difference is the flux needed
to excite the core.
SERIES MODE
• Primary winding is connected
in series with a circuit.
• Its current is determined by
the sources and loads.
•
Its current is determined by
its sources/load conditions
• A component of this current is
used to excite the core
sufficient to induce an EMF to
drive a current in the
secondary
COMPARISON-SHUNT MODE AND SERIES
MODE
Shunt mode
Series mode
•
Voltage applied across the primary
terminals
•
Primary winding is connected in
series
•
When secondary is on open
circuit, Induces an EMF equal to
the voltage applied
•
Its current is determined by its
sources/load conditions
•
A component of this current is
used to excite the core sufficient
to induce an EMF to drive a
current in the secondary
•
Current flowing will be that
required to excite the core
•
When secondary is loaded Ampere
turns of the primary winding will
exceed that of secondary
– Difference is the flux needed to
excite the core
STEADY STATE THEORY OF CTS
CT ACCURACY
•
If the CT is ideal
Primary AT = Secondary
AT
Secondary current ∞
Primary current
•
Every CT requires a certain AT to
induce in it the magnetic flux
•
With secondary impedance fixed
exciting AT form a larger
proportion at smaller primary
currents
Exciting current required to magnetize the core is the cause for th errors in
CTs
AC PERFORMANCE OF A CT
ES = 4.44 f A B 10-8 Volts
ES
= Secondary induced volts
f
= Frequency in cycles/second
N
= Secondary number of turns
A
= Core cross sectional area in sq.cm
B
= Flux density in lines per sqcm.
ES
= Is( Zb + Zs + Zl)
CT MAGNETIZATION CURVE
• Exciting current is
the source of errors
• Value of exciting
current depends on
the core material and
the amount of flux.
• Ankle point and knee
point,
CT EQUIVALENT CIRCUIT
OPEN CIRCUITING OF CT
• If the secondary of a CT
is kept open with
a
current flowing in the
primary, then
– No secondary current
– No MMF to oppose the flux
– Core driven to saturation
– High rate of change of flux
– High voltages
CT ERRORS
• Ratio error
(KnI s - Ip ) x 100/ Ip
Kn = Nominal ratio
I s = Actual secondary current
Ip = Actual primary current
• Phase angle error
– Angle
by
which
the
secondary current vector
when reversed differs from
the primary current.
• Composite error
– r.m.s.
value
of
the
difference between the
ideal secondary current
and the actual secondary
current.
Includes
the
current and phase angle
errors
LIMITS OF ERRORS
• Metering CTs
• Protection CTs
CT RATINGS
• Accuracy limits
– Limit up to which the specified accuracy could be retained.
Accuracy Limit Factor
• Rated short time current
– r.m.s. value of the a.c. component of the current which a CT is
capable of carrying for the rated time without being damaged
by thermal or dynamic effects
–
• CT Burden
– the load applied to the secondary of the CT. Normal practice is
to express the burden in terms of VA and power factor, the VA
being what would be consumed in the burden impedance at
rated secondary current
CHOICE OF CT RATIO
• Secondary current 5 Amps or 1 Amp
• Primary current to be equal to or greater than the normal full load
current of the protected circuit
• Maximum ratio of a CT is made about 3000/1 and secondary CTs of
20/1 are used.
TRANSIENT RESPONSE OF CT
• Transient conditions, prevail within the first few cycles of a fault.
• System conditions are very different to that under steady state
conditions.
• Power systems expansions increases fault currents and time
constants of dc transient components.
• It is mandatory for the protective equipment/circuit breakers to
clear the faults within few cycles
• This makes CT duties more onerous
• Hence, behaviour of CTs during transient conditions is of great
importance.
TRANSIENT CONDITIONS IN A POWER
SYSTEM
• Sudden application of a sinusoidal voltage to a series R/L circuit will
cause a current to flow in the circuit.
• This is equivalent to a fault condition of a power system,
Equivalent circuit