Converter Transformer
HVDC Systems for…
• Energy Transmission over long distances
• Asynchronous coupling between AC Regional networks
Power Electronic circuits are used to convert AC to DC (Rectifier circuits) or
convert DC to AC (Inverter Circuits). Both of these circuits are also called
converter circuits.
A transformer that has one of its windings connected to one of these
circuits, as a dedicated transformer, is a Converter Transformer
Converter Transformer in HVDC system….
•Supply of AC voltages into two separate circuits feeding the
rectifier bridges with a phase shift of 30 electrical degrees for
reduction of low order harmonics esp. 5th & 7th harmonics.
•As a galvanic barrier between AC and DC systems to prevent
DC potential entering into the AC system
•Reactive Impedance in the AC supply to reduce short circuit
currents and to control the rate of rise in valve current during
commutation.
Converter Transformers for 12 pulse rectification….
Primary: Star 400 kV AC
Secondary: Two windings connected to converter (Thyristors)
connected in series to build up required level of DC voltage
Choice of transformer design is mainly
ruled by….
– transport restrictions (dimensions and weight)
– number of necessary spare transformers
– technical possible solutions for core and windings
Converter Transformers for 12 pulse rectification
Type of Connections
No. of design X No.
of units
Spares
required
3 Phase Star-Delta & StarStar
2X2
2
Single phase 2 winding
2X6
2
Singe phase 3 winding
2x3
1
Extended deltaconnection
2X2
1
3 Phase 3 winding
2X2
1
Converter Transformer....
Design
active part
Valve Winding (Y)
Tap Winding
Valve Winding (D)
Line Winding
HV-Terminal
Neutral-Terminal
Simplified connection of
line windings and tap windings
Winding and core arrangement
Converter Xmer & Normal AC Xmer…..
• Polarity Reversal
• Voltage Distribution in Oil Barrier System
• Impedance variation influence
• On load Tap Changer
• Harmonic Currents
• Losses
• DC Magnetisation
• Short Circuit Forces
• DC Bushings
Under Polarity Reversal…..
•
Beginning voltage stress distributions capacitive – Oil is stressed
more than PB
•
Successively change over to Resistive distribution – PB is
stressed more (almost all stress across solid insulation)
•
PD stresses under DC – Sporadic pulses at random intervals
-Discharges in oil gaps under rapid changes in voltage
-Discharges in cellulose insulation due to imperfections in
insulation
-Discharges at the oil-cellulose interface
……. To meet above, special oil-barrier insulation system is required
Voltage Distribution in Oil-Barrier System...
Voltage Distribution in Oil-Barrier System...
•
Main duct in HVDC transformers require more PB barriers than normal
AC transformers as DC voltage is taken mainly by PB. Voltage
distribution is by resistivity in steady state.
- Transient DC voltages:
•
Start up of converter when full DC potential from bridges
is developed almost instantaneously
- DC Voltage Polarity Reversal:
•
•
•
•
When direction of power flow is changed in HVDC system, current
direction remains the same while polarity of the voltage will be reversed.
This is done within a few number of power cycles.
A sudden change in DC voltage is capacitive.
Time constant for the transition from capacitive to resistive distribution
is about an hour.
Voltage Distribution in Oil-Barrier System...
AC
DC
Insulation Design
•
Valve windings to withstand AC voltages, superimposed DC
voltages on AC voltages, DC Voltage Polarity Reversal
•
Composite insulation of Pressboard or Paper (Solid) and
oil (liquid)
•
Voltage distribution between paper & oil under AC conditions
depends on inverse ratio of dielectric constants of
PB/ Oil (2:1)
•
Voltage distribution between oil & paper under steady state
DC voltage depends on direct ratio of resistivities (1:10 ~ 500),
depends on oil quality, moisture content and temperature
Influence of Impedance variation…
Closer Tolerance in Impedance is required between three phases and
also between upper & lower bridges (star-star, star-delta circuits)
•
•
(± 6% on special tolerance and 2-3% variation between units
(Normal transformers ± 10%)
The above is necessary :
•
•
•
To reduce distortion in DC voltage wave form
To reduce non-characteristic harmonics, thereby cost of AC filters
To reduce residual currents between three phases, which can act
as DC magnetization on the core.
To achieve close tolerance on impedance variation
•
•
•
•
Close dimensional tolerances in windings
Proper stabilization of windings before assembly
Better insulating material
Good winding machines
On Load Tap Changer….
•
Large voltage control requirements at converter & inverter ends.
Tapping range is large (25 ~ 30%) with small steps to give
necessary adjustments in supply voltage.
•
High frequency of operation – Mechanical aspects of OLTC
should be reviewed to ensure a robust design (Contacts wear,
linkages, motor, relays, contactors, interlocks). Derating
necessary compared to normal transformer applications.
•
Small step voltage permit small variation in DC voltage, valve
firing angles and reactive power demand
On Load Tap Changer…
•
There is need to compensate for the reactive voltage drop in
the conversion between AC & DC by changing taps.
•
In OLTC, switchover from one tap to another is carried out by
the diverter switch.
•
When changing over from one tap to next, the current in the leaving
tap has to be broken during the normal current zero passage.
•
In star-star connected windings, the current is zero for a long time
and the change over is smooth.
•
But in star-delta connected windings, the current change from
positive to negative is abrupt with very little time at zero current.
•
This puts strain on diverter switch.
Higher Harmonic Currents….
•
Additional losses due to harmonic currents in valve windings
•
Stray flux from harmonic currents can heat up structural
members like Yoke Clamps, Tank
•
Yoke shunts are used to contain and direct the above leakage
flux to core
•
Harmonic stray flux can induce larger currents than power
frequency stray flux
Losses….
•
No-Load losses – Depends on applied AC voltage, same as normal transformer
•
Load Losses – I2R + Stray loss from circulating currents in windings & metallic parts
from leakage flux
•
Circulating current depend on rate of change in winding current and thus leakage
flux.
•
With stepwise change in load current during commutation from one valve to
another, the induced voltages will be fairly high to create circulating currents. So
stray losses are increased compared to conventional power transformers.
•
Stray losses in windings – Increase as square of harmonic number
•
At 150 Hz, Stray losses (150/50)2 more than at 50 Hz current
•
Stray losses in metal- varies as 0.8 of harmonic number
•
High percentage of harmonic currents in the load current causes higher load losses
compared to normal transformers
DC Magnetization….
•
Due to inaccuracies in valve firing.
•
Results in a small residual DC current oscillating around zero.
•
DC components in magnetizing current lead to core saturation, which
results in high levels of vibration
increased sound level in transformers
marginal increase in no-load loss.
Short Circuit Forces….
•
A short circuit across a valve or phase to ground on a valve side terminal
can result in a completely asymmetrical current for a few cycles.
•
Resulting forces on the winding can be larger than for the normal power
transformers where the asymmetry decays rapidly.
Mechanical forces during a short
circuit may reach critical values
An inner winding buckles under
radial forces
Excessive axial force in
winding will cause tilting
of conductor
DC Bushing….
•
DC withstand voltage of contaminated insulator is 20 ~ 30% of that of AC.
•
To meet this, bushings creepage used are 40 mm/kV or more (Normal
bushings are with 25mm / kV of service maximum voltage)
•
To avoid chances of phase to ground short circuits, valve side bushings are
located inside the valve hall. This also reduces the pollution related
flashovers and consequent short circuit.
•
IEC & IEEE standards for the DC bushings.
Converter Transformer
Design
Interface: DC bushings (Basslink)
Converter Transformer
• Few Formula…..
Rectifier mode
U d = 1 . 35 • U • cos α −
3
π
ωL Id
Inverter mode
U d = − (1.35 • U • cos γ −
3
π
ωL I d )
Overlap
cos(α + µ ) = cos α −
α + µ +γ =π
2 ωL I d
U
Converter Transformer
Tests
required acc. to IEC 60076-1, ~-3, IEC61378-2
routine
tests
meas of DC wind. res.
meas. of voltage ratio and check of phase
displacement
meas. of s/c imp. and load loss
meas. of no load loss and current
at fr and Ur
dielectric routine tests (for
Um>300kV)
LI (for line & neutral, principal and extr. neg. Ut
taps)
SI
ACLD (AC long duration), sine wave>>fr,
100%Utrms(60s)
seperate source AC (applied potential
test)
seperate source DC volt. withstand incl
PD meas.
polarity reversal test
(aux.wiring IEC60076-3, cl.10: 2kV rms)
tests on on-load tap- changer (where appropriate)
meas. insulation resistance
test of magn. circuit insulation and associated ins.
IEC 60076-1, 10.2
IEC 60076-1, 10.3
IEC 60076-1, 10.4
IEC 60076-1, 10.5
IEC 60076-3
IEC 60076-3, cl.13 & 14
IEC 60076-3, cl.15
IEC 60076-3, cl.12.4
IEC 60076-3, cl.11
IEC 601378-2, cl.10
IEC 601378-2, cl.10
IEC 60076-1, 10.8
IEC 601378-2, cl.10
IEC 601378-2, cl.10
Converter Transformer
Tests
type
tests
temp.rise
test
dielectric type tests
IEC 60076-2, also 5.2.3
n.a., all tests are routine
tests
IEC 60076-3
sound
power level
sound power level of cooling
equipment
IEC 60076-10
dielectric special tests (for Um>300kV) (to be agreed
see routine tests
about)
ACSD (AC short duration), 1ph transformer: ph- gr only,
100%Utest for 60s
IEC 60076-3
IEC 60076-10
special
tests
det.of cap. windings-to-earth and between windings
det. of transient volt. transfer
characteristics
meas. of zero-seq. imp. on 3-ph.
transformers
s/c withstand test (test or calc.)
det. of sound levels
meas. of harmonics of the no
load current
meas of power taken by fan and
oil pump motors
meas. of ins. res. to earth of
windings
meas of tan(delta) of the ins. sys.
capacitances
losses and imp.(other taps)
load current
test
1-ph.:
IEC 60076-3, cl.12.2 &
U2=1.5*U 12.3
m/Wurzel
(3)
IEC 60076-1, 10.7
calc.: 4.1.2...4.1.5
IEC 60076-5
IEC 60551
IEC 60076-1, 10.6
IEC 60076-1, 10.4+10.5,
IEC 61378-1
IEC 601378-2, cl.10
Standards:
1) IEC 61378-1 (1.0) 1997-09 Converter Transformers –
Part I Transformers for Industrial Applications
2) IEC 61378-2 (1.0) 2001-02 Converter Transformers –
Part 2 Transformers for HVDC Applications.
3) IEC 61378-3 (under issue) Converter Transformers –
Part 3 Application Guide for Converter Transformers
4) IEC 62199: 2004-05 Bushings for DC Application
5) IEEE Std. C57.129-1999 General Requirements and
Test code for Oil Immersed HVDC Converter Transformers.
Standards:
6) IEEE Std. 1158-1991 (R 1996) Recommended Practice for
Determination of Power losses in HVDC converter stations.
7) IEEE Std. C57.19.03 - 1996 Standard Requirements,
Terminology and Test Code for Bushings for DC applications.
THANK YOU