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

Background & Methods

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 & Analysis

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.

Underpinning Research

Thanks for your attention

Questions?