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Underpinning Research
Power Electronics in Distribution Networks
Power Electronics Centre Imperial Open Day, July 2015
Thomas Frost
Underpinning Research
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
Underpinning Research
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
Underpinning Research
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
Underpinning Research
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
Underground Cable Costs (£/m) *
Voltage CSA
(mm2)
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
Underpinning Research
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
Underpinning Research
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 …
Underpinning Research
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
Underpinning Research
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
● Decentralised control
● A compensation limit of 10% implies
a rating no larger than 15kVA
Underpinning Research
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
Underpinning Research
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
Underpinning Research
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
Underpinning Research
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
Underpinning Research
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
Scenario Losses (kWh/day)
Reduction Base w/ APF
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%)
Underpinning Research
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
Regulation Voltage Limits
Thermal Overall Over Under
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.
Underpinning Research
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
Underpinning Research
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.