Instrument rans ormers
Sr. Faculty Member
Introduction Theory of Current Transformer Transformer
Theory and Selection of Voltage Transformer Conclusion
Introduction Theory of Current Transformer Transformer
Theory and Selection of Voltage Transformer Conclusion
Introduction
Pro rote tect ctio ion n Sys yste tem m Analo Analogy gy Brain Relay Eyes, Ears, Nose & Skin , Hands & Le s Circuit Breakers
3/4/2013 7:29:24 PM
4
Pro rote tect ctio ion n Sys yste tem m Analo Analogy gy
Sensed by Instrument Transformers & communicated to Relay Relay Issues Trip Command To Breaker Breaker Trips & Clears Fault 3/4/2013 7:29:24 PM
5
Instrument Transformers • A Vital Part of the Protection and Metering System
Transformer transforms the the high current or • Instrument Transformer transforms high hi gh vol olttag age e co conn nnec ectted to th thei eirr pr prim imar ary y wi wind ndin ings gs to the standard low values in the secondary within the required accuracy limits which feed the metering and protect on apparatus • Provide insulation Provide insulation against against High voltage (isolation) • Protect personnel Protect personnel and apparatus from high voltages
Provi vide de po poss ssib ibil ilit itie iess of of standardizing the the relays and • Pro nstruments 3/4/2013 7:29:24 PM
6
Classification of Instrument Transformers
• Metering • Based on use‐ • n oor • Outdoor • Types o Instrument Trans ormer • Current Transformer (CT) • Voltage Transformer (VT) • Electromagnetic Voltage Transformer (EVT) • Capacitive Voltage Transformer (CVT) 3/4/2013 7:29:24 PM
7
Theor of Current Transformers
What is Current Transformer • Direct use of high current (in the tune of 100A or more) is not possible as protective relays and metering devices are not designed to handle such huge amount of current
• Current Transformer is an instrument transformer level, such as, 1000A/1A (CT ratio) i.e. transforms curren rom
3/4/2013 7:29:24 PM
e eve o
n o curren o
eve
9
Why Current Transformer is required •
ystem as two as c requ rements metering of energy sourced or consumed Protection of the electrical system from faults and disturbances
• Types of Current Transformer (CT) • Measuring CTs • Protection CTs • Protection CTs for s ecial a
3/4/2013 7:29:24 PM
lications
10
Current Transformer Theory •
u w y winding, that current creates a MMF which results in a EMF in the primary winding and in any other winding wound on, or linked with, the core
3/4/2013 7:29:24 PM
11
How Current Transformer is connected wound on core of magnetic material
• Metering and Protection devices are connected to the secondaries of the CT • Primary winding connected in series and transforms the line
P R IM A R Y W IN D I N G O R B U S H IN G MAGNETIC CORE
W IN D IN G
3/4/2013 7:29:24 PM
12
How Current Transformer is connected • In current operat on or ser es mode, the primary winding is connected in series with the power system whose relatively high impedance determines the current which is independent of the secondary winding load
• The current transformer has assigned rated output termed as burden in VA which are invariabl small as against the high outputs in KVA or MVA of power
3/4/2013 7:29:24 PM
13
Current Transformer Theory •
or a s ort‐c rcu te relation holds good ‐
trans ormer t e
o ow ng
Primary Ampere turns (I1N1) = Secondary Ampere Turns (I 2N2)
I1 I 2
• •
=
N 2 N 1 ‐
transformer where the secondary terminal voltage is zero and the magnetizing current is negligible minimum when the secondary is short circuited and maximum when open circuited
Current Transformer Theory
• Simplified CT equivalent circuit Ip
Is
Ie
p
s
E
p
Im
≠ Ip
Iw
s
Z
∠
Current Transformer Theory
Phasor Diagram
3/4/2013 7:29:24 PM
16
Current Transformer Theory • Primary current has two components, first is secondary current which is transformed in inverse of the turns ratio and an excitin current which su lies the edd & hysteresis losses and magnetize the core • Exciting current is not transformed and causes errors accuracy that can be achieved with a current transformer
Ip = Ie + Is , or Is = Ip
‐ Ie
Current Transformer Theory The error in the reproduction will appear both in amplitude and phase. The error in amplitude is called curren or ra o error an e error n p ase s ca e phase error or phase displacement
3/4/2013 7:29:24 PM
18
Current Transformer Theory
I
Primary
p
Kn Kn.Is s
Kn = 3/4/2013 7:29:24 PM
Ip Is 19
Current Transformer Theory , error δ could be directly read in percent on the axis ( δ = 1% = 1 centiradian = 0.572 dgree = 34.4 minutes)
• the current error is positive if the secondary current is too high, and the phase error is positive if the secondary current is
3/4/2013 7:29:24 PM
20
Current Transformer Accuracy Why at all CTs are inaccurate? The culprit is core loss and magnetizing current, which
• • The secondary current which we get is not true reflection of its primary current. for example, for a CT with CT ratio of 1000/1 amps, if we get 0.99 amps in secondary leading primary current by 15 minutes (0.25 , has ratio error of (0.99‐1)/1 x 100= ‐ 1% and phase error of 15 minutes 3/4/2013 7:29:24 PM
21
Current Transformer Theory • T e exc t ng current Ie ntro uces rat o error, w c s e ne as the difference in magnitude of the primary and secondary current expressed as percentage of primary current
Current ( Ratio) Error =
K n . I s
− I p
× 100
p
Kn= Rated transformation ratio p = ctua pr mary current Is = Actual secondary current
• The Phase angle error is the phase angle difference between the primary current and the reversed secondary current
Current Transformer Theory • •
•
ompos e error Under steady‐state conditions, the r.m.s. value of the difference between the instantaneous values of the actual primary current, and the instantaneous values of the actual secondary current multiplied by the rated transformation ratio, integrated over one cycle including the effects of phase displacement and harmonics of excitation current ompos e error s genera y expresse as a percen age o r.m.s. va ue o primary current according to the formula T c =
2
ε
I p
T 0
K nis − i p dt
Kn is the rated transformation ratio Ip is the r.m.s. value of the primary current ip is the instantaneous value or the primary current is is the instantaneous value of the secondary current T is the duration of one cycle 3/4/2013 7:29:24 PM
23
Knee Point Voltage (KPV) •
‐ characteristic
(plot
between
secondary
z applied
at which an increase of 10% in exciting e.m.f. produces an increase of 50% in the exciting current
3/4/2013 7:29:24 PM
24
Burden of Current Transformer • The external load (e.g. meters, transducers, relays etc) connected to the secondary of a CT is called the burden The burden can be ex ressed in volt‐am eres or in ohms VA = I2 x Z Z = Total CT secondary impedance I = Secondary current (Generally 1A or 5A)
• Total burden is the sum of . ev ce trans ucer, meter, re ay etc ur en ‐ urn s e y the manufacturer 2. Burden of Interconnecting Leads ‐ can be calculated by using the above formula use conductor resistance total to the device and back) for Z 3. Internal Burden of CT Windings ‐ This is so small that it can generally be ignored or specified by manufacturer
3/4/2013 7:29:24 PM
25
Magnetization Curve • The excitation of CT depend on a) Cross‐sectional area b) Length of magnetic path of core c) Number of turns in the winding d) Magnetic characteristics o t e core
3/4/2013 7:29:24 PM
26
Current Transformer Theory Es = 4.44 * Bm * Aeff * f * Ns m‐
m
eff
Aeff ‐ Core effective area f ‐ frequency s‐
Es ‐ Induced voltage in the secondary
•
A component of primary current excites the core to the flux density . .. secondary current through total impedance of secondary circuit
•
Hence core flux density is dependent on the magnitude of primary current Es is decided by the total burden Es= Total burden (VA + lead burden + sec. winding burden) * Is y e ra o .e. p, p, s s s ec e Ns= NpIp/Is 3/4/2013 7:29:24 PM
27
Current Transformer Theory t constant ur en, core ux ens ty var es rect y as t e secondary current, hence, as the primary and consequently secondary current increases, a point is reached when core material start saturating and exciting current becomes excessive, thus resulting in excessive current error m
.
m
accuracy. Lower the Bm lower is the excitation current yielding better accuracy u arger core area Rewriting the equation A = E /(4.44 * B * f * N ) Higher core area is required for • better accuracy (lower Bm , lower Ie‐ Excitation current ), ‐ s , • and higher burden (higher Es) 3/4/2013 7:29:24 PM
28
Effect of Secondary Open Circuiting • E.M.F. induced in secondary winding is that required to drive secondary current through total impedance of secondary difference between primary and secondary m.m.f. (ampere turns)
• With secondary open circuited, there are no secondary ampere turns to oppose those due to primary current and whole of rimar m.m.f. act on the core as an excessive excitin force, which drive core into saturation on each half wave of the current •
s g ra e o c ange o ux n e reg on o pr mary curren zero induces an e.m.f., Es of high peak value in the secondary winding 3/4/2013 7:29:24 PM
29
Effect of Secondary Open Circuiting
• With rated primary current, peak value may be as low as few hundred volts in small measuring CT with 5A secondary winding, but it might reach many kilovolts, in the case of, say 2000/1A protective CT with a large core section • With system fault currents flowing in primary, even higher voltages would be induced and not only constitute hazard to nsu a on o se an connec e ns rumen s, re ays an associated wiring, but also to life 3/4/2013 7:29:24 PM
30
Selection of Current Transformers
Current Transformer Secondary Rating
• Choice of CT secondary rating – 5A Secondary . . switchgear cubicles with closely located relays) • Preferred where primary current ratings are very high • Comparatively low peak voltage when secondary gets open • Fine turns ratio ad ustment is not ossible when rimar rating is low
– 1A Secondary • re erre w en s are ou oor an ea ur en are g • Comparatively high peak voltage when secondary is open • Fine turns ratio adjustment possible 3/4/2013 7:29:24 PM
32
Current Transformer Accuracy • Measuring CTs are required to be accurate over normal working range of current, while protective CTs of the rated current • Metering if we want to measure current for metering purpose, we desire that whatever
current we measure, that should be very
purpose
• Accuracy Class A designation assigned to a current transformer, the errors of which remains within specified limits under 3/4/2013 7:29:24 PM
33
Classification of Current Transformer • Metering Class CTs 0.1 class : High precision testing 0.2 class : Laboratory class . 1.0 class : First grade indicating wattmeter 3.0 & 5.0 class : For general use WTI
• Protection Class CTs – 5P, 10P, 15P – 3/4/2013 7:29:24 PM
34
Measuring Current Transformer •
es gnat on o
eter ng
s
Metering CTs are specified in terms of – at o, ccuracy c ass, ur en (Instrument Security Factor) , . , , –
rat ng ,
• – • The errors are specified between 5‐120% of rated current and 25‐100% of rated burden connected • Higher errors are permitted at lower currents
3/4/2013 7:29:24 PM
35
Current Transformer Accuracy Limits
Metering Cores •
‐
Class
5% of rate
20% of rate
100% of rate
120% of rate
.
.
.
.
.
0.5
1.5
0.75
0.5
0.5
3/4/2013 7:29:24 PM
36
Current Transformer Accuracy Limits • IEC60044‐1 has laid down standards on this
3/4/2013 7:29:24 PM
37
Instrument Security factor (ISF) • T e nstruments connecte to t e secon ary o a T s ou e protected from getting damaged during primary fault condition, when primary current is many times higher than the rated value, the core should get saturated • For this purposes, Instrument Security Factor (ISF) for Metering • The CT cores should be such that it saturates at its instrument security factor (ISF) for safeguarding the instrument from getting amage un er au t current con t on
• ISF is defined as the ratio of rated instrument security • ISF is expressed as 3,5,7 or 10 (it shall be chosen as small as possible) 3/4/2013 7:29:24 PM
38
Protection Current Transformer • Protection Class • During fault condition, value of primary current may be 10 to 20 times the rated primary current • Here, main requirement is ability of CT to faithfully trans orm t e pr mary current ur ng au t con t on • At such high level of primary current, if CT is not proper y es gne , may sa ura e an re ay w receive very less current and, therefore, would not
3/4/2013 7:29:24 PM
39
Protection Current Transformer •
es gnat on o
rotect on
s
Protection CT are specified in terms of – at o, ccuracy c ass, ur en rat ng , Limit Factor) , ,
ccuracy
• – Error Class 5P, 10P, 15P – ALF 5, 10, 15, 20, 25, 30 – VA rating 5, 10, 15, 30
• The errors are specified at rated current and ALF times 3/4/2013 7:29:24 PM
40
Protection Current Transformer
.
Current Transformer Accuracy Limits
Protection Cores Accuracy
5
Current Error
Phase displacement
Composite Error
Primary Current
Primary Current
limit (ALF) Primary Current
±
±
3/4/2013 7:29:24 PM
±
m n
-
± . centiradians
-
±
±
42
Accuracy Limiting Factor (ALF) • Un e measur ng Ts, w c are requ re to e accurate over the normal working range of currents, protective CTs are usually required to maintain their ratio up to several times the rated primary current • At some value of primary current above the rated value, core , current error • Protection Class CTs cores should not get saturated below its ccuracy m t ng actor up to w c t e pr mary current should be faithfully transformed to the secondary side, maintaining the specified accuracy
• ALF is defined as the ratio of the rated accuracy limit primary current to the rated primary current 3/4/2013 7:29:24 PM
43
Protection Current Transformer •
, , , connected burden is less than rated then ALF would increase
• Applications of this CT are Over current relay, Inverse relay, earth fault protection, Phase fault protection etc. • While selecting 5P10 class CT for IDMT O/C or Earth fault relays – CT should have optimum ALF/VA rating, so that they do not saturate up to at least 20 times current rating (either by appropriate high value)
– high secondary currents during severe faults (in excess of 20 times setting) that may cause thermal stressing of relay curren co s an even ua a ures 3/4/2013 7:29:24 PM
44
Protection Current Transformer • Designation of Protection CTs for special applications For protection like circulating current differential, restricted . between associated CTs with close tolerance Special class Protection CT of are specified in terms of – at o 2) Accuracy class 3) Knee Point Voltage (Vk) 4 CT Secondary winding resistance RCT corrected to75 C 5) Excitation current (Ie) usually at Knee Point Voltage or a stated percentage thereof Example ‐ 200/1, PS Class, Vk > 200V, RCT < 2.0 ohms, Ie < 30mA at Vk/4
• The turn ratio error are limited to +0.25% which helps in maintaining balance between the protection system during maximum through fault condition 3/4/2013 7:29:24 PM
45
Which Current Transformer is connected
Substation to b e protected
External/Through Fault
Internal Fault
Which Current Transformer is connected
I1
I1 I2
i1 ta s ng Resistance Operating relay
i1 – i1’ i1 + i2
i1’ i2
Which Current Transformer is connected • For Unit Type Protection • Here, requirements are rather stringent as we their mutual faithfulness, moreover, our aim is that the protection must be stable for even worst through fault condition and fast acting for internal fault condition • or spee y opera on o re ay, s usua y es ra e o make the knee point voltage of the CT magnetizing curve not less than twice the rela o eratin volta e
Which Current Transformer is connected •
or g mpe ance c rcu a ng curren scheme
V K
eren a
≥ 2 I f ( RCT + 2 Rlead )volts
RCT= CT secondary winding resistance lead
group = should remain stable (referred to CT secondary)
Which Current Transformer is connected • • •
• • •
or rans ormers, f = ax mum roug au curren limited by leakage impedance of transformer , to switchgear breaking capacity For Generator, I = Maximum through fault current limited by sub‐transient reactance (Xd”) of the generator or otor, f = ax mum start ng current a out x load current for DOL Motors) , f reactor For Short feeders, I = Maximum throu h fault current for fault at busbar
Which Current Transformer is connected •
or
ase
K R=
eren a re ay
R
y
CT
lead
u
K = Constant specified by the manufacturer usually chosen to ensure positive operation of highest differential unit on severe internal fault with extreme CT saturation)
Which Current Transformer is connected •
or
s ance ro ec on
X K
R
f
relay
CT
lead
= y y account for the DC component of the fault current) f
reach point relay
RCT= CT secondary winding resistance lead
Outdoor Current Transformer
• Outdoor CTs are basically of 3 types of Construction – Dead Tank with U (Hair Pin) shaped – Dead Tank with Eye Bolt primary –
3/4/2013 7:29:24 PM
53
Dead Tank Current Transformer CB
Insulator
Primary winding Core Secondary winding
3/4/2013 7:29:24 PM
S S
Terminal Box 54
Dead Tank Current Transformer 1) Eye Bolt Type
3/4/2013 7:29:24 PM
2) Hair Pin Type
55
Dead Tank Current Transformer
P1
P2
CORE 1
CORE 5
CORE 2 CORE 4 CORE 3
3/4/2013 7:29:24 PM
56
Dead Tank Current Transformer
3/4/2013 7:29:24 PM
57
Outdoor Current Transformer
3/4/2013 7:29:24 PM
58
CT Secondary in Progress
Shell Preparation & Assembly
CT Tank Assembly & Welding
Dead Tank Current Transformer • This type construction, cores situated in a tank close to the ground, the primary ‐
shaped (eye bolt) • low centre of gravity & high earth quake withstand • using heavy cores without stressing the
• Oil circulation in the primary conductor (tube) ives even tem erature and not hot s ots
3/4/2013 7:29:24 PM
62
420 kV dead Tank CT (Hair Pin Design)
The heat in CT is mainly produced in the primary winding. The thick insulation encircling hair pin primary prevents heat transfer from most of this tube
P1
P2
orce a n cas ng prevents eat trans er to air as it is a bad conductor of heat. The oil tube to top where cooling takes place restrained. Due to limited cooling area at the top, raising the CT current rating is difficult 3/4/2013 7:29:24 PM
CORE 1
CORE 5
CORE 2 CORE 3
CORE 4
63
Live Tank Current Transformer
3/4/2013 7:29:24 PM
64
Live Tank Current Transformer
3/4/2013 7:29:24 PM
65
short conductor length of primary winding. The heat transfer is easily done at the top chamber level as compared to present Hair pin design , adequate for smaller losses. Raising the CT current rating is much easier
All over the world CTs > 3000 Amps are produced with Inverted design
3/4/2013 7:29:24 PM
66
420 V ea Tan CT (Hair Pin Design) 3/4/2013 7:29:24 PM
420 kV Live Tank CT
67
Theor of Volta e Transformers
What is Voltage Transformer • Voltage Transformer is an instrument transformer which transforms voltage rom one eve o ano er eve suc as 400KV/ √3:110V/ √3 (VT ratio) i.e. 400KV/ √3 into voltage of 110V/ √3 level
• the tune of 3.3kV or more) is not possible as devices used for measurement of voltage are not designed to handle such high level of vo age 3/4/2013 7:29:24 PM
69
Why Voltage Transformer is Required • System has two basic requirements me er ng o energy sourced or consumed electrical system from disturbances
3/4/2013 7:29:24 PM
70
Why Voltage Transformer is Required • Faults can be of many kinds, some faults such as O/C can be detected solely on current measurement, but curren oes no prov e scre on a ou na ure an location of the fault • , current during faults, we can in a way compute power or impedance of system along with its direction • Moreover O/V, U/V, O/F, U/F and over fluxing protections are also configured from VTs • Voltage signal also used for synchronizing, Disturbance recorders and event logs 3/4/2013 7:29:24 PM
71
How Voltage Transformer is connected • VT has a primary and one or more secondary windings •
e er ng an ro ec on the secondaries of the VT
•
v u , y winding is connected in parallel with the power
ev ces are connec e
o
volts suitable for the meter or relay
3/4/2013 7:29:24 PM
72
Voltage Transformer Theory • For a transformer in no load the following is valid Voltage transformation is proportional to the ratio of primary
E1 2
=
N 1 2
• An ideal voltage transformer is a transformer under no‐load conditions where the load current is zero and the voltage drop is only caused by the magnetizing current and is thus negligible
Voltage Transformer Theory
• Simplified VT equivalent circuit Ip
Is
Ie
p
s
E
p
Im
≠ Ip
Iw
s
Z
∠
Voltage Transformer Theory Vp Ip Rp p
Is Rs
Es
Vs
IeRp Ip s
Ie 0 Phasor di agram wit h referance to vol tage error
3/4/2013 7:29:24 PM
75
Voltage Transformer Theory • Ratio error, which is defined as the difference in magnitude of the primary and secondary voltage V s .K n
Volta e Ratio Error =
− V p
V p
×100
Kn= Rated transformation ratio Vp = Actual primary voltage Vs = Actual secondary voltage
• Phase Angle error is the difference between the
Voltage Factor •
o tage actor eterm nes t e max mum operat ng vo tage or vo tage transformers expressed in per unit of rated voltage, which in turn dependent on the system and voltage transformer earthing conditions
•
s use n non‐e ec ve y ear e sys em ave g vo age ac or s nce n the event of an earthed fault in one of the phases, the healthy phase voltage may rise to phase to phase value Voltage Factor VF
Duration
1.2
Earthing conditions V.T. primary winding
System
Continuous
Non‐earthed
Effectively or non‐effectively earthed
1.5
30 s
Earthed
Effectively earthed
1.9
30 s
Earthed
Non‐effectively earthed with automatic E/F tripping
1.9
8h
Earthed
Isolated neutral or resonant earthed without automatic E/F tripping
3/4/2013 7:29:24 PM
77
Protection of Voltage Transformer across its secondary terminal is achieved by incorporating fuses or MCB in secondary circuit located near to transformer as possible
• Normal secondary current is not more than 5A and short circuit , • Short circuit on secondary winding gives only a few amperes in primary winding and is not sufficient to rupture a high voltage fuse at primary side (HRC fuses on primary side up to 66kV) • Hence high voltage fuse on primary side do not protect , circuit on the primary side • CVT invariably solidly connected to the system so that there is no primary protection 3/4/2013 7:29:24 PM
78
Voltage Transformer Accuracy • As stated for CT, we need it for Metering voltage measurement, energy, power measurement Protection for distance protection, O/V, U/V, O/F , , ‐ • For metering VTs we need high accuracy in the voltage measurement durin stable conditions i.e. 80% to 120% of nominal system voltage with burdens from 25% to 100% of rated burden at power factor of 0.8 • Combination of magnitude and phase error depends
3/4/2013 7:29:24 PM
79
Voltage Transformer Accuracy • IEC 60044‐2 and 60044‐5 defines this as
3/4/2013 7:29:24 PM
80
Voltage Transformer Accuracy • For Protection VTs we need faithfulness of voltage such as from value as low as 2% of by rated voltage factors such as 1.2, 1.5, 1.9 at 0.8 pf lagging
3/4/2013 7:29:24 PM
81
Vol oltt age Trans Transff or ormer mer Accu Ac curacy racy • IEC 60044‐2 and 60044‐5 defines this as
3/4/2013 7:29:24 PM
82
Vol oltage tage Trans ransff or ormer mer Con Connect nectii on ons s three types types of connecti connections ons • There are three – V‐V connection – Star/Star connection – Star/Open delta connection
•
‐
connec on
– Used for measurement and for those protections which do – Primary of VTs is connected in V (one VT primary across R ‐Y phase and other across Y‐B phase) with identical V connection for the secondary – In this connection zero sequence voltage can not be
3/4/2013 7:29:24 PM
83
Vol oltage tage Trans ransff or ormer mer Con Connect nectii on ons s • Star/Star connection Eith ther er 3 se sepa parrate si sing ngle le ph phas ase e – Ei s or a s ng e p ase, m is used
– Both rimar and secondaries are connected in star with both star neutrals solidly grounded – ac pr mary p ase m s us connected between phase to eart ea rth h of th the e su supp pply ly ci cirrcu cuit it an and d replicate similar phase to earth voltage on the secondary
3/4/2013 7:29:24 PM
84
Voltage Transformer Connections •
tar
pen De ta connect on
– Primary windings are connected in star with star neutral solidly grounded and the secondaries are connected in series to form an o en delta connection – This type of connection is called residual connection and require single 3 phase 5 limb VT
– This residual connection is used for po ar s ng rect ona eart au t relays or for earth fault detection in non‐effectively grounded or isolated neutral system 3/4/2013 7:29:24 PM
85
Types of Voltage Transformer • Types of Voltage Transformer VT • Electromagnetic Voltage Transformer (EVT) •
apac ve o age rans ormer
M
P
P P
INDUCTIVE VOLTAGE TRANSFORMER 3/4/2013 7:29:24 PM
CAPACITIVE VOLTAGE TRANSFORMER
86
Types of Voltage Transformer •
ec romagne c o age rans ormers s m ar o a sma power transformer and differs only in details of design that control ratio accuracy over the specified range of output, cooling ou pu no more an , nsu a on es gne or ‐ system impulse voltage level) and mechanical aspects transformer is high, due to prohibitive cost of insulation, hence, at 132 kV and higher voltages, CVT may be more
capacitors can serve also for carrier current coupling (PLCC), but may be inferior in transient performance g requency s gna s onto • apac tors a ow t e n ect on o the power line conductor to provide end‐to‐end communications between substations for distance relays, te emetry supervisory an voice communication 3/4/2013 7:29:24 PM
87
Capacitive Voltage Transformer De n t on A CVT is a voltage transformer comprising of capacitor divider interconnected that the secondary voltage of the electromagnetic unit is substantially proportional to and in unit (IEC 186)
What does a CVT do?
• Inputs to measuring and protection devices • Galvanic isolation Main Parts of a CVT
• Capacitor Part • Electromagnetic Unit 3/4/2013 7:29:24 PM
Capacitor Stack, Insulator
‐ ‐
PT, HV Choke, FR circuit 88
Capacitive Voltage Transformer
Primary Terminal
Capacitor Part
Electromagnetic Unit
HF Terminal Sec. Terminal Box
3/4/2013 7:29:24 PM
89
CVT Internal Components Tank PT F R
Resistor Capacitor
k t.
FR Choke
HV Choke
3/4/2013 7:29:24 PM
90
CVT Internal Components
3/4/2013 7:29:24 PM
91
Why intermediate PT is required • Assuming the intermediate potential transformer is absent
• Expression for Us U s U s
U Z
=
Z 1 + Z 2
Up C2 Burden R
s
U KR
=
R +
1
jω C
= R and Z = 1 C = C 1 + C 2
Z
K =
C C 1 + C 2
3/4/2013 7:29:24 PM
92
Why intermediate PT is required • The per unit error is ε
Considering On simplifying
=
P= P=
KU p
− U s
KU p U s
2
R
2ε U s
2
C 1 / K
ω
• This leads to the conclusion that for iven error the ower output is proportional to – Secondary output voltage Us – 1
3/4/2013 7:29:24 PM
93
Why intermediate PT is required • As the output voltage U s is usually constant, very large
capacitance (C1) is required to get sufficient power output • This is economically unacceptable • Two modifications required to improve the situation – Introduction
of
an
intermediate
stepped
potential
transformer to boost Us , it can be 20 kV primary, the burden is connected at its secondary at
110 3
Volts
– E m nat on o t e ma n source o p ase ang e error ue to
the capacitance C(=C1+C2 ) by a series inductance tuned to
3/4/2013 7:29:24 PM
94
Why HV Choke is required • L s var a e inductive choke used for phase angle error correction • It is tuned to
C1 Up
Us
(=C1+C2) at nominal power frequency
• Wound PT is used to increase the available output power, for a given maximum error limit and C1
3/4/2013 7:29:24 PM
95
Equivalent Circuit Diagram of CVT • Leq is the sum of choke inductance and leakage n uc ance o e wound PT • Ma netizin inductance of the PT is neglected • It can be seen that the c o ce o a su ta e value of L tends to reduce the phase angle error
3/4/2013 7:29:24 PM
I1 1
1
Leq
Up
I
+U -
U2
C2 I2
R
Us
Wound PT
96
CVT Under Steady State I
Up decreases until it is in phase with Us and then increases • At the frequency when L and C are resonant and cancel each other, the circuit will behave, under , conventional transformer
2
U2
φ
UL I
Us
U U2 U1 over‐volta
e • If the burden is short circuited a considerable appears across C2, due to resonance of L and C • This value of U2 is however, usually limited by spark gap 2
at currents above the rated value 3/4/2013 7:29:24 PM
97
Ferro-resonance , wound PT which is having exciting impedance of non‐linear characteristics • enever a capac or an non‐ near n uc or are connec e n series, there is a danger of non ‐linear energy interchanges at sub‐harmonic frequencies and causes sustained oscillation and consequently large overvoltage in the circuit • Such oscillations are less likely to occur when the losses in the •
impair the transient response) • To avoid Ferro‐resonance the operating flux of iron parts is kept u ux y, w v high exciting currents during circuit transients • Alternately a special provision for damping the oscillations is provided 3/4/2013 7:29:24 PM
98
Capacitive Voltage Transformer
Coupling Capacitor • In Power Line Carrier Communication (PLCC), Coupling Capacitor (CC) is used as coupling device between power ne an carr er accessor es o a ow g frequency (40‐500KHz.) carrier signals into/out of . • Some times, the capacitor part in CVT is used as CC in PLCC • When CVT is used as CC the terminal HF will be connected to carrier accessories (carrier coupling unit) instea o groun ing it
3/4/2013 7:29:24 PM
100
Power Line Carrier (PLC) equipment C1 Wave Trap
L1
Carrier oscillator
C3
>500KHZ NOISE PICKUP <30KHZ-HARMONIC LIGHTENING,CORONA
L3
C4 Coupling capacitor C2
L2
Matching
VT
Transmitter and receiver f a = 30kHz to 500 kHz 3/4/2013 7:29:24 PM
101
