Power Quality Aspects
of
Electric Traction
Sh. R.N Lal
Sr. Executive Director/TI
&
Sh. S. Bhatnagar
Jt. Director/ TI
RDSO/Luckow
Out Line of the presentation
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What is total power quality ?
International power quality standards.
Statutory requirements for IR.
Power quality control techniques.
Harmonics & voltage unbalance control.
Scenario on IR.
Internationally followed practices on total
power quality system.
Conclusion and recommendations.
Power Quality Definition
The term power quality is used to
describe the extent of variation of the voltage,
current and frequency on the power system.
The variation of voltage and current can either
be in terms of magnitude or waveform
shape/distortion.
Power Quality Standards
• IEEE 519, IEC 61000-3-2/4 for THD voltage and TDD
i.e. current distortion limits at PCC for different power
system voltages & fault levels and for different
equipments respectively. IEEE 1159 describes
Recommended practices for monitoring of power
quality.
• Performance standards like EN 50160 followed by EC,
voltage un balance, harmonic distortion have been fixed
for utilities. In UK for connection of non-linear
equipments, ENA engineering guide G5/4-1 specifies
harmonic limits.
• For PCC > 33KV, to determine the distortion of the local
network, system impedances data is used to construct a
computer model, to show the effect of consumers’
network with local supply network.
Comparison of standards
Parameter
IEEE 519
Harmonics
THD voltage
1.5-5.0%
TDD current
5-20% at PCC
NA
2%
Voltage
Unbalance
IEC 610002-2(for
equipments
)
THD<8%
EN 50160
(only up
to 35 kv)
Individual
harmonics
limit 0.56.0%
2-3%
Responsibility of power quality
improvement
Regulatory commissions Frame supply/grid codes,
Performance standards for
transmission and
distribution licensees, tariff
structures.
Decides disputes.
Utilities,
distribution,transmission
licensees.
Consumer/industrial user
Enforce above codes,
standards. Penalties and
incentives.
Source of problems like
generation of harmonics,
voltage sags etc.Required to
follow all rules and
regulation.
Main features of electric traction
•
•
•
•
Traction load is nonlinear and prominent source
of generation of odd current harmonics (16-20%
THD).
Wide variation of load in short duration leading
to voltage flickers and fluctuations (swell and
sags in power quality terminology) and also
resulting into poor voltage regulation.
Load is inductive resulting in poor power factor
in the range of 0.7 to 0.8.
Essential nature, high level of reliability with
minimum interruptions is desirable.
Statutory requirements for IR.
• Indian Electricity act 2003 section 79 (i) Para (h)
and (i) specifies obligation for transmission and
distribution licensees to enforce the standards
w.r.t. Quality, Continuity, and Reliability as
mentioned in grid codes issued by regulatory
commissions.
• Bureau of energy efficiency constituted vide
Energy conservation act 2001in its thrust area
No.2 specifically mentions about demand side
management.
• Railways is mentioned at sr.no.10 of list of energy
intensive industries.
Aspects of Power Quality relevant to
Indian Railways
1. Compensation of reactive power to improve
power factor.
2. Compensation or elimination of harmonics in the
system, for improvement in voltage form factor
and reduction of line losses.
3. Correction of voltage imbalance in 3 phase
supply system.
4. Correction of short duration voltage disturbances
i.e. sag , swell and flickers.
Compensation of reactive power to improve
power factor
Reactive power compensation Methods to improve PF
• Using HT capacitors with 13% detuned reactors. It also
act as passive filters for filtering of harmonics.
• Static VAR compensators (SVC) : It uses switching
devices to control capacitive or inductive energy into the
system. Available SVC configurations are Thyristor
switched capacitors (TSC), Thyristor controlled reactors
with passive filters(TCR).
Self commutated VAR generators: The fast switching
power electronic devices are used to act either as voltage
or current fed inverter to inject required amount of
leading/lagging reactive KVAR into the system.
Applications of different PQ techniques
PF
Harmonic
correction Filtering
Voltage
unbalance
correction
(If 3-ø
available)
No
Complete
Harmonic
cancellation
by injection
No
No
Yes
No
Yes (with Yes
fixed
Shunt
capacitors)
Yes
Fixed shunt Yes
Capacitors
Yes
(partially)
TSC
Yes
TCR+TSC
+FILTERS
Yes
Yes
(partially)
Yes
VSC/STAT Yes
CON
No
Effects of harmonics in the system
• Form factor deterioration, i.e. ratio between the
RMS and the Average value of the rectified
traction voltage. Increase of form factor means
reduction in ability of loco to absorb power from
system.
• Harmonics are also non useful reactive energy and
cause poor PF.
• Increase in line current and losses.
• Limitations of signaling track circuits.
• Interference to telecommunication.
• Limitations of harmonic injections into public
utility.
Standards and guidelines on harmonics
Indian scenario
SN
1
•
State
UP
Supply/Distribution Code summary
• THD< 5% for HT, 4% for EHT
• Licensees shall publicize for installation of
harmonic filters.
• Time limit of One year.
• Licensee to monitor harmonic level at supply
point to the users.
• Some type of loads which produce harmonic
to be monitor.
2
Rajasthan
3
Maharastra •
Distribution licensee shall control harmonics
level at PCC as described by IEEE 519-1992
Standards and guidelines on harmonics
Indian scenario
4
ASSAM =>Individual harmonic voltages shall not exceed 1%.
=>Harmonic measurement shall confirm to IEC
61000-4-7 or IEEE std. 519.
=>Measurement of harmonic on sample basis on sixmonth interval.
=>Licensee will compile list of all metering point
prone to harmonic generation
5
MP
=>For 132 kV EHT consumers cumulative THD in
voltage at PCC limited to 3%
=>Licensee to monitor harmonic level at strategic point at
regular interval.
=>Licensee can measure the harmonics generation of any
customer.
Standards and guidelines on harmonics
Indian scenario
6
Tamil Nadu
=> Additional charges on harmonic dumping, the
consumer shall provide adequate harmonic
suppression units to avoid dumping of harmonics
into distribution system.
=>Licensee may provide suitable metering
equipments to measure harmonic level, consumer
failing to provide harmonic suppression shall be
liable to pay compensation.
7
Gujrat
=> Licensee to monitor harmonic level at strategic
point at regular interval.
=>Licensee can measure the harmonics generation
of any customer.
=>Licensee shall follow the harmonics distortion
limits as laid down by grid connectivity standards
IEEE std 519 as applicable to Railways
as a consumer
• Philosophy of developing harmonic limits in the
recommended practice is to
--Limit the harmonic current injection from
individual consumers so that to prevent
unacceptable voltage distortion levels.
--Limit the overall harmonic distortion of the system
voltage supplied by the utility.
• Current distortion limits--- TDD (Total Demand
Distortion) is harmonic current distortion in % of
max. demand load current.
• Railways traction supply PCC is at 132KV or
above, hence table 10.4 for 69 to 161 KV shall be
applicable.
IEEE std 519 as applicable to Railways
as a consumer
• Limits are as per ratio Isc/IL i.e. short circuit current at
PCC and maximum fundamental load current.
• The load current be calculated as the average current of
the maximum demand for preceding 12 months.
• For most of the 132/25KV TSS with short circuit
power of 1000 MVA this ratio will be between 1001000, wherein TDD of 7.5% is prescribed.
• Larger customers have more stringent limits because
they represent a larger portion of the total system load.
• The standard vide Para 12 recommends methodology
for evaluating any new Harmonic source before
connecting to the system
IEEE std 519 as applicable to
Railways as a consumer
• The harmonic voltage distortion on the system is a
function of , total injected harmonic current and
system impedance at each harmonic frequency.
• Utilities are recommended to maintain voltage
distortion limits as per the voltage levels
mentioned at 11.1.
• For a TSS, PCC at 132KV max voltage THD of
2.5% is mentioned.
• Utilities can force identified customers to take
corrective steps if above limits are not maintained.
• The limits mentioned are for “worst case” &
normal operation conditions lasting > 1 Hr.
Controlling Harmonic currents
• Shunt filters. One or more tuned series L-C
circuits. On IR presently HT capacitor with 13%
reactor acts as harmonic filter.
• Harmonic compensation or injection using Voltage
source converters or active filters.
• Phase Multiplication. The basic 3-Phase converter
is a six pulse unit. Phase shifting transformers are
used to combine 6-pulse bridges to construct
12/24 pulse converters.
Voltage unbalance
• Railways take different phase to phase voltage in
consecutive TSS to limit imbalance at PCC
• Various regulatory commissions have fixed voltage
unbalance limit for transmission licensee to 3% only
• In European countries it is generally fixed at 1-2%
• To maintain this limit transmission licensees may
enforce Railways to deploy unbalance correction
equipments by taking 3- phase supply at a
nominated TSS.
Voltage disturbances
• Long term variations of > 1 minute are called
under/over voltages, Railways monitor this.
• Short term variation of < 0.5 to 1.0 cycle are
called sag, swell and flickers, this level of
monitoring is not done in Railways.
• Voltage sag, swell and flickers affect sensitive
electronic equipments.
• Series active filters or dynamic voltage restorers
are used for correction.
Over voltages in traction supply system
• IEC 61374 refers to over voltages in traction systems
and contains comparative data from China, France, UK,
USSR, Chezkoslovakia and Japan.
• The over voltages of traction system are classified into
Temporary over voltages, switching / lightening over
voltages.
• Temporary over voltages are of few seconds to minutes,
causes may be earth faults on the 3 phase system supply
to Railways, load dropping in conjunction with supply
stations, resonance of filter banks, over compensation
etc.
• Switching over voltages of 250 to 5000 micro seconds
may occur due to switching operations and faults.
Over voltages in traction supply
system
• The over voltages tolerances, limits in different
Railway systems world over vary widely because
of differences in system characteristics e.g.
• Line impedances and configurations like single
line/ double line, BT/AT systems, transformer
types and connections, parameters of supply
system, type of shunt/series compensation and
filtering techniques adopted, load and protection
coordination etc.
Power factor improvement on IR
• 1975 onward different electricity boards specified
the minimum PF to be maintained.
• IR started providing fixed capacitor banks by
1982.
• IR was paying penalties for low PF up to 11.83
crores in year 1990-91.
• Penalties on account of low PF are now reducing.
• Other incidental benefits accrued were reduction
in MD, Harmonics, line current and voltage drop.
Limitations with Fixed Capacitors
• Not feasible to achieve PF higher than 0.9 due to
problems of high voltages under light load
conditions and possibility of parallel resonance at
a harmonic frequency causing distortion of voltage
waveform.
• Not possible to avail benefit of incentives on PF
above 0.95 and Unity.
• Frequent switching on/off of CB may results
system instability, damage to breakers.
• States like MP, TN and Chatisgarh have intended
to calculate apparent power by adding the leading
KVAR generated by Fixed capacitors hence under
light load conditions. Average chargeable PF shall
be less while KVAh MD shall increase.
DRPC in Indian Railways
C. Rly, TSS at
Lasalgaon,
Pimperkeda, and
Nagpur division
IGBT base voltage
fed
PWM
converter
(STATCOM)
of
total
installed
Capacity
1800
kVA.with
fixed
capacitor
and
reactor.
-It is used for
compensating
reactive power to
achieve PF near
unity.
Harmonic
compensation by
fixed capacitor &
reactor.
C.Rly,
W.Rly. TSC based reactive Reactive
power
Bhadli,
Maxsi, power
compensation
to
Mohammad kheda compensation.
achieve near unity
PF
Performance of DRPC systems
As reported by CR Average power factor has
improved from 0.79 to 0.93 for Lasalgaon ,0.76 to
0.89 for Bhadli, 0.78 to 0.92 for Pimperkhed TSS
(Expected power factor is near Unity, Deciding
correct ratings of fixed and variable compensation is
very important )
Reduction in THD Voltage Harmonics at Lasalgaon
TSS reported from 12.36% to 7.86% at max load.
(Harmonic Filtering is done by Fixed capacitor bank
branch of STATCON, Present design in use does not
have harmonic cancellation feature )
Performance of DRPC systems
Reduction in MD
NAME OF TSS
Before
Installation
After Installation
Average
Average
Reduction in
MD in KVA MD in KVA
KVA
Demand
Lasalgaon
12703
10764
1991
Pimperkhed
16354
15990
1260
Bhadli
16100
13920
2100
Selection of DRPC systems
Selection of DRPC system should be based
on applicable tariff conditions to the
Railways e.g.
• Leading PF being charged.
• Substantial rebates/ Incentives on PF above
0.95.
• MD exceeding CD.
International examples of Power Quality
correction for traction applications
World Railways Technology
Purpose
Channel
Tunnel Rail link
(CTRL)
phasephase/2x25
kv
supply
arrangement.
Static
VAR
Compensator
- 45 MVAR each
Capacity Fixed
capacitor &
reactors and TCR
based.
Maintaining unity
PF
To balance the
unsymmetrical
load up to 1%.
-Filtering
of
harmonics.
TokidoSinkansen bullet
MW load each. 3phase
154KV/2x25 KV
single phase.
Fixed shunt
capacitor 6 MVA,
shunt Active filter
GTO based,
voltage fed inverter
16x3 = 48 MVA.
Voltage
impact
drop compensation.
-Variation
and
imbalance of 3.6%
is corrected to 1%.
International examples of Power Quality
correction for traction applications
SPL – London
Underground – 5
substation
60 MVAR capacity Maintaining unity
each SVC using
PF
combinations of
To balance the
TSC, TCR, filters unsymmetrical
load up to 1%.
-Filtering
of
harmonics.
EDF/SNCF –SVC
Light Evron
36 MVAR at 90
KV capacity SVC.
- Maintain unity
PF,
Compensate
for
voltage
fluctuations
and
unbalances as well
as active filtering
Example of Performance Guarantee
What needs to be done
• Select an appropriate PF correction method
according to applicable tariff conditions.
• HT capacitors can not provide PF above 0.9
and are not advantageous if leading VAR are
added to calculate apparent power, Use of
DRPC shall be beneficial in such condition.
• Tariff, supply code and other directives of
regulatory commissions on harmonics, voltage
parameters etc must be watched.
What needs to be done
• Measure harmonic distortion, and monitoring of
voltage surges by providing suitable power
analyzers with data loggers for generating a data
base for sufficiently long duration.
• Develop a total power quality correction strategy
which should include PF, Harmonics and voltage
unbalance correction (by taking 3-ø supply at
TSS).
• Conduct trials for the same at one or two TSS on
IR for extended period.