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Wigton Windfarm Ltd.

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Wigton Windfarm Ltd.

Power Factor and VAR Control Experience

-Problems and Solutions

Presented by: Francois A. Lee, PE

Leecorp Ltd ,Wigton consultant

- Wigton Windfarm started operations in April 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

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 JPScoâ€™s substation rated 139/69 KV at Spur Tree via 11.315 KM of newly built 69 KV overhead lines

- 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.

- Consultants hired to look at scenario and 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.

- 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.

Cabling and transformers add to reactive 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.

- QVARX Canada,specialists at Var compensation at 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

- 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.

- 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.

- 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.

- 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.

- The switching transient modeled results do not 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.

- 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.

- 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.

- 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.

- 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.

- 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.

- Harmonic and transient Analyses
-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.

Var imports now in the order of 200KVar peak per mth at peak (20 MW) output

Minimal downtime experienced

Negligible payments to JPS