Power Electronics in Distribution Networks Thomas Frost Power Electronics Centre Imperial Open Day, July 2015
Overview Introduction Low Carbon Technologies Growth Drivers for PE in distribution systems Background Voltage Control Regulation Topologies Loads, Networks & Limits Results Increased DG penetration Increased loading due to EV/HP Comparison of regulation approaches Active power filters Conclusions
Introduction Proliferation of domestic low carbon technologies at LV has dramatically increased and is projected to continue In many cases, the limits on power delivery occur because the voltage goes outside of tolerances (UK +10/- 6%), rather than due to thermal limits With power electronics: Capacity can be released Higher efficiency is possible DNO investment can be deferred DNOs are aware of this and have started to trial PE in LV networks
Domestic LCT Growth National grid predict [1] EV s could increase demand by 13%, whilst HP s could increase demand 30%, all by 2035 Installed PV capacity could exceed 30 GW by 2035 All cases envisage a net growth in domestic electrical energy demand
Drivers for PE Solutions High cost of network reinforcement, and maintenance typically account ~40% of DNO expenditure (UKPN estimate 2.7 billion in next 8 years) The solution of operating system voltages high in the permissible band is no longer feasible with DG Network scale is very large, and as such upgrade is a continual process LCNF projects provide innovation incentives for DNOs Substation Costs ( 1000) * Voltage Type kva Min Max HV/LV Pole 100-200 11 22 HV/LV Pad < 200 21 34 HV/LV Ground 200-1000 20 57 EHV/HV Internal x2-1826 2730 Voltage Underground Cable Costs ( /m) * CSA (mm 2 ) Urban Rural Min Max Min Max LV 95 90 223 43 59 LV 95-185 100 360 55 73 LV 185 105 380 58 82 HV - 111 414 70 101 UKPN Eastern Network (3.6m customers) Voltage Level Asset Volume High Voltage Lines & Cables 38,882 km Secondary Subs 67,772 Low Voltage Lines & Cables 49,319 km
Background & Methods
LV Voltage Control System X/R ratio is well below unity in LV cable networks Active Power imparts greatest change on system voltage Reactive compensation will be of limited use for noteworthy voltage control V (RP + XQ)/V Existing voltage control finishes at the primary substation OLTC
Regulation Solutions Electronic Substations (SST) Tap Changers (OLTC) Active Power Filtering (APF) Mid feeder compensation (MFC) Point of load regulation (PoL) Soft open points (SOP) Inline voltage regulators Deregulation Reinforcement Dynamic Rating Demand Response Energy Storage Active Network Management Many more
PES Power electronic substations (PES) include primarily the solid state transformer (SST) along with remote control, protection, and monitoring functionality giving the DNO greater flexibly of the LV network High frequency isolation transformer reduces footprint, hence simplifies retrofit A modular output converter facilitates per feeder regulation, staggered LVDC implementation, and eases handling of load growth
MFC Functional as APF and DVR with a common DC link High frequency DC link transformer provides galvanic isolation with a small size Four wire installation requires processing of zero sequence components, but only a small faction of the total line power Assuming evenly distributed loading the MFC is located at 1/3 of the feeder length A compensation limit of 10% implies a rating no larger than 15kVA Decentralised control
OLTC & APF Commercially produced OLTC transformers for LV applications Successfully trialled by ENWL Compared to PES, the OLTC is a proven technology, that is efficient, economic and reliable with suitable maintenance Commercial produced APF from numerous manufactures Typically used by large end users for either protection or mitigation purposes
Loads, Networks & Limits Loads Significant increase in energy consumption with EV or HP These are of high power and typically a constant power nature with 3x peak load Run the scenarios listed on the network under study by assigning LCT to nodes Networks Study of both generic and real networks, results show for generic Automation of analysis to investigate a large numbers of real networks Limits EN50160 for voltage quality issues, G83 for DG connection, and manufacture ratings Differing monitoring, averaging and acceptable limits for PQ issues Proliferation of LCT (%) Uptake DG EV HP Low - 12.5 - Mid 30 33 30 High 80 71 60
Results & Analysis
Results with DG Networks operate at upper end of voltage range to allow for voltage drop along the feeder Substation LV voltages measured 6-8% above nominal almost 50% of the time This has a detrimental effect when DG is connected and small amounts of DG cause voltage to exceed tolerances In the generic UK network over voltages encountered with 30% nameplate rating of PV ( 35k upgrade for high PV) PV will not cause thermal limitations in the network
Results with HP/EV Networks regulator set point lowered to accommodate DG, but new loading increases power flow Voltage limits are encountered prior to current limits PES and OLTC slightly reduce thermal limits due to new constant power loads Voltage optimisation less effective at peak time as fewer large resistive loads Whole feeder upgrade cost ( 64k) removes all thermal and voltage issues
APF & MFC Compared to APF the MFC rating is smaller (12kVA vs. 55kVA) and voltage control is improved For reduction of losses the APF gives annual savings of 120 with losses at 0.06/kWh [2] In the MFC, when the series converter is not inserting a compensating voltage (or associated power is low) the shunt converter acts as an APF Improvements with APF Losses (kwh/day) Scenario Base w/ APF Reduction No LCT 16.10 14.47 1.63 (10%) High EV 60.93 55.14 5.79 (9.5%) High HP 60.82 56.45 4.37 (7.1%)
Comparisons All topologies improve regulation PES & MFC are most effective OLTC suffers due to slow response and simultaneous regulation of all feeders (residential & commercial), along with reduced tap range Permissible Scenarios Voltage Limits Regulation Over Under Thermal Overall Base 1 0 6 44 OLTC 3 2 6 69 PES 6 2 6 88 APF 2 1 6 56 MFC 5 2 6 81 For certain substation demand profiles the OLTC will be as effective as the PES, thus 3 of the regulation devices will all be able to utilise network capacity up to thermal limits.
Conclusions Power electronics at LV can improve voltage regulation, with economic benefits compared to network reinforcement The MFC and PES offer best performance, but PES are expensive Thermal limits are the ultimate barrier to network capacity, but are not easily increased by PE Harmonic limits not always exacerbated by LCT [3], but improved by APF or MFC Short term DNOs want to be able to retrofit novel PE based equipment not revolutionise LV networks [4] : A barrier to PES OLTC technology suitable for LV ready [5] MFC is shown to be small and effective
References & Notes [1] National Grid Future Energy Scenarios (FES) 2015 [2] J. Stewart, Review of WPD unit costs," tech. rep., Parsosn Brinckerhof, 2013. [3] L. Kütt, E. Saarijärvi, M. Lehtonen, H. Mõlder, J. Niitsoo, Estimating the harmonic distortions in a distribution network supplying EV charging load using practical source data case example [4] B. O. Brewin, S. C. E. Jupe, M. G. Bartlett, K. T. Jackson, and C. Hanmer, New Technologies for Low Voltage Distribution Networks [5] D. Rogers and T. Green, An active-shunt diverter for on-load tap changers, Power Delivery, IEEE Transactions on, vol. 28, no. 2, pp. 649 657, Apr. 2013. * Cost derived from Southern Electrics, Statement of Methodology and Charges for Connection to Southern Electric Power Distributions Electricity Distribution System, May 2013.
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