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 2004 Connected to JPS grid under the terms of the Power Interchange Agreement (PIA)
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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
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
- Power Interchange agreements mandates
Wigton Winfarm must be atleast .9PF
and simulatenously providing its
reactive power requirements.
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.
identified as substation Capacitor bank system designer /supplier
1- Determine the min. no of switched
steps and suitable control strategy.
2- Determine if harmonic tuning reactors
are required to avoid harmonic
3- Examine the potential for switching impact on
the LV banks when switching in a 24 KV
- 24 kv cables are installed trefoil
resistivity 36 ohm-m assumed
- cables modeled as series R,L and parallel C
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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.
-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.
-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