Transcript Wigton Wind Farm Ltd.

Wigton Windfarm Ltd.
Power Factor and VAR Control Experience
-Problems and Solutions
Presented by: Francois A. Lee, PE
Leecorp Ltd ,Wigton consultant
Background
• Wigton Windfarm started operations in April
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2004
Connected to JPS grid under the terms of the
Power Interchange Agreement (PIA)
Main Project Manager/ Design Engineer/
Contractor: RES Ltd. UK
23 Neg-Micon NM900/52 900KW turbines
installed
Total estimated capacity of wind farm: 20.7MW
Wigton single line Summary
System description summary
23 x NEG Micon NM/52 wind Turbines ,each
turbine generates at 690V,50 hz, and is
transformed up to 24KV by 1000KVA step up
transformers at the base of each turbine.
Turbines located 100M apart are connected to a
common collection point at the substation.The24
KV collection is further transformed up to 69 KV.
The point of interconnction with the local
utility (JPS) is at the terminals of the
windfarm power transformer 69KV
disconnect switch.
The windfarm main substation is
connected to JPSco’s substation rated
139/69 KV at Spur Tree via 11.315 KM of
newly built 69 KV overhead lines
Problems
• Metered MVAR demand indicates
significant consumption by the wind plant
up to 8 MVAR.(see P/Q characteristic)
• JPSCO charges for MVAR demand at a rate
equivalent to its industrial customers.
Solutions
• Consultants hired to look at scenario and
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determine strategy for way forward Nov.2004.
Consultants Findings
- Power Interchange agreements mandates
Wigton Winfarm must be atleast .9PF
and simulatenously providing its
reactive power requirements.
Consultants Findings contd.
• At 20 MW output there is a shortfall of aprox. 7.8 MVAR
inductive
• Neg-Micon turbines installed are inductive types and
consume 275 KVAr @ no load,410 KVAR @ full load.
• Neg-Micon turbines installed have capacitive
compensation up to 275 KVAR only.
Consultants Findings contd.
• Cabling and transformers add to reactive
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demand
Balance of Plant contractor has mandate to
deliver facility that fulfills PIA.
The utilities charge to Wigton for Var support is
in line with PIA
Capacitive compensation of aprox. 8 MVAR is
required at Wigton
Automatically switched banks at the 24 KV
substation point is the likely and cost effective
solution.
Implementation of solution
• QVARX Canada,specialists at Var compensation at
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medium and high voltage
identified as substation Capacitor bank system designer
/supplier
Preliminary harmonic data indicates that tuning reactors
will be required.
Preliminary design done ,meetings convened with
JPS,Wigton ,consultants and design discussed as per
PIA procedures.
QVARX commisioned to and undertakes design and
system study for 24 KV capacitor bank
System and 24 KV Capacitor Study
Highlights
• Study Objectives
1- Determine the min. no of switched
steps and suitable control strategy.
2- Determine if harmonic tuning reactors
are required to avoid harmonic
resonance problems.
3- Examine the potential for switching impact on
the LV banks when switching in a 24 KV
capacitor step.
Study Results and Findings
• A capacitor bank of rating 8 MVAR total is
required to compensate /offset the
maximum var demand of 7.8 MVAR
inductive.The capacitor bank should be
split into 3 identical steps of 2.67 MVAR,in
order to keep the voltage steps following
bank switching to less than 3% at the
24KV bus.
Study Results and Findings contd.
• The step # 1 bank should remain connected at all times
,which will be more stable operation when the wind is
very low and as it picks up.This is because at low wind
speeds ,the var demand is rapidly changing between
capacitive and inductive.The steps #2 and #3 will be
switched in and out as the var demand at the metering
point will vary.It is recommended to control these 2
steps in the capacitor PLC which will allow the most
rapid switching times as opposed to the standard PF
controller. This will keep the average inductive kvar as
low as possible in the event of sudden wind gusts.
Study Results and Findings contd.
• The step 3 capacitor will require tuning
reactors to avoid resonance problems with
the typical grid harmonic distortion. The
step 1 will be a combination of 2 tuned
banks .One tuned at the 5th harmonic and
one tuned at the 7th harmonic. This bank
will be permanently connected. The steps
2 and 3 banks will each be tuned at the
7th harmonic.
Study Results and Findings contd.
• The switching transient modeled results do not
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indicate there will be any problems of interaction
on the LV capacitor banks at the turbines when
a 24 KV step is switched on.
The control of all low voltage caps (at turbines)
should be such that they should all be connected
as the MW increases ,and should remain
connected as the 2nd and 3rd steps come on to
avoid fighting between the both capacitor
systems.
Design Study approaches
• Impedance modelling done on 69/24 KV network using ATPDRAW power simualtion
software
Modelling basis
- 24 kv cables are installed trefoil
arrangement,direct buried,ground
resistivity 36 ohm-m assumed
- cables modeled as series R,L and parallel C
to ground
- Cables are modelled as lumped impedances per identified branch
between two nodes,each node being a junction box.
-Cable splices are not considered nodes in the model as the distributed effect of cable
inductance and capacitance over the section does not have a noticeable impact on
the lower order parallel resonance ,which is the key frequency of interest.
Design Study approaches
• Transformers
- 69/24 KV, 25 MVA rating,this transformer has a
nameplate impedance of 9.09% at 25 MVA. Load losses
is assumed to be 83 KW at 25 MVA
- 24KV/690V,1000 KVA rating,the turbine transformer has
a nameplate impedance of 5.73% at 1.0MVA.
• Turbine Impedance
- The turbine at 690V is modelled as the stator
subtransient reactance which is assumed at 20%.
-Each turbine has LV capacitor bank of 275 KVAR,which
is assumed to be fully connected.
Design Study approaches
• Grid Connection
- The windfarm is connected to the 69KV grid via
a 25MVA step up transfomer.The 69 KV has a
3phase fault level of between 487 t0 544 MVA
with x/r ratio of 4.8.There are no capacitor
banks directly on the 69 KV network. There is
one main breaker controlling all collector circuits
at 24 KV.See appendix 1 for impedance diagram
of 69/24 KV network.
Design Study aproaches
• Total capacitor KVAR size
- Load flow model without substation
capacitors shows apparent load at 24 KV
of 20.8 MW and 6.1 MVAr inductive
- Load flow done with 8 Mvar capacitor
bank shows correction of max.demand
vars to 0.3 Mvar capacitive.
Design Study aproaches
• Minimum Switch ON Time Delay
- Capacitors must be discharged via internal resistors before re-energizing and also
consideration made for rapid wind gusts which would tend to drive up MVAr demand
rapidly hence minimum switch on time is critical
• Mode of switching control
- 3 options available 1st is basic control device of PFC which works on separate time
delay setpoints,2nd option is var control within the capacitor system PLC (programable
logic controller) .This option also has fixed time delay setpoints for switching on and
switching off however PLC can determine whether it is necessary to enforce time
delays.3rd option is 3 steps connected permanently which is mot advisable due to
voltage increase at low output.
• 2nd option of Var control in the PLC chosen .In this mode the PLC controller will
switch off a step when it detects the load is decreasing and the metered var is
capacitive and exceeds the MVAr size of one step (2.67 MVAr). As such the reactive
power at the metering point will be controlled aproximately in he range of 0 vars to
2700 Capacitive ,averaged over 15 min intervals with the rare possibility of inductive
vars up to 2 MVar.
Design Study aproaches
• Harmonic and transient Analyses
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-The system is modelled in the EMPT (Electro-Magnetic Transients
Program)computer simulator program.This programme is used to model the
harmonic impedances ,currents and voltages of electric power systems.
Between Jan 31- Feb.1,2005 recordings were made at the 24 KV bus,the
dominant harmonics were the 5th and the 7th.Voltage distortion at 24 KV is
in the order 2.5 to 3.5% which is acceptable.
Ratings of the Capacitors and tuning reactors
-The ratings of the capcitor and tuning reactor components need to be such
that the equipment will withstand the max.level of harmonic currents as
well as fundamental voltage magnitude without overloading.Measurements
on site of harmonic conditions were used as input for calcualtions of the
required harmonic duties and equipment ratings.See the single line idagram
of the bank step configurations in the following slide.
Final Design and equipment
selection
Site Design and construction phase
-Contract awarded Feb.05
-Contract completed Aug.05
-Contractors-QVARX,LEECORP
Plant Performance data after
capacitor project
Performance of compensation
system
• Var imports now in the order of 200KVar
peak per mth at peak (20 MW) output
• Minimal downtime experienced
• Negligible payments to JPS