Document Number: EDS 08-5010
Date: 12/08/2016
ENGINEERING DESIGN STANDARD
EDS 08-5010
ENERGY STORAGE
Network(s):
EPN, LPN, SPN
Summary:
This standard provides guidance of on the application of energy storage.
Author:
Jose Barros
Date:
12/08/2016
Approved By:
Barry Hatton
Approved Date:
01/09/2016
This document forms part of the Company’s Integrated Business System and its requirements are mandatory throughout UK
Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset
Management. If you have any queries about this document please contact the author or owner of the current issue.
Applicable To
UK Power Networks
External
All UK Power Networks
G81 Website
Asset Management
Contractors
Capital Programme
ICPs/IDNOs
Connections
Meter Operators
HSS&TT
Network Operations
UK Power Networks Services
Other
THIS IS AN UNCONTROLLED DOCUMENT, THE READER MUST CONFIRM ITS VALIDITY BEFORE USE
Version: 1.0
Energy Storage
Document Number: EDS 08-5010
Version: 1.0
Date: 12/08/2016
Revision Record
Version
1.0
Review Date
01/09/2017
Date
12/08/2016
Author
Jose Barros
New document to cover energy storage
Version
Review Date
Date
Author
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Contents
1
Introduction ............................................................................................................. 5
2
Scope ....................................................................................................................... 5
3
Glossary and Abbreviations ................................................................................... 5
4
Key Concepts .......................................................................................................... 6
4.1
Energy Storage Fundamentals and Technologies ..................................................... 6
4.1.1
Power Rating (MVA or MW) ...................................................................................... 6
4.1.2
Energy Rating (MWh) ................................................................................................ 6
4.2
Different Purposes of Installations ............................................................................. 8
5
Applications ........................................................................................................... 10
5.1
Identifying a Suitable Location for an Application ..................................................... 10
5.2
How Applications are assessed by UK Power Networks .......................................... 11
5.2.1
New Standalone Storage ......................................................................................... 11
5.2.2
New Storage + Generation Proposed at a Known Site ............................................ 12
5.2.3
Storage Additions .................................................................................................... 12
5.2.4
Storage in Locations suggested by UK Power Networks ......................................... 13
5.2.5
Storage in Flexible DG Areas .................................................................................. 13
5.2.6
Notes on LV Connected Storage ............................................................................. 14
6
Specific Network Design and Operation Requirements ..................................... 16
6.1
Thermal Rating of Equipment .................................................................................. 16
6.2
Voltage Regulation .................................................................................................. 16
6.2.1
Steady State Voltage ............................................................................................... 16
6.2.2
Step Voltage Change .............................................................................................. 17
6.3
Power Factor of the Installation ............................................................................... 20
6.4
Security of Supply Regulations (EREC P2/6 Compliance) ....................................... 21
6.5
Power Quality .......................................................................................................... 21
6.6
Additional Requirements for Applications above 50MW and Below 100MW ............ 22
6.7
European Network Codes – Requirements for Generators (RfG) ............................ 22
7
References ............................................................................................................. 23
7.1
UK Power Networks Standards ............................................................................... 23
7.2
National Standards .................................................................................................. 23
Appendix A – ENA Additional Information Form for Storage ........................................ 24
Appendix B – Profiled based connections ...................................................................... 24
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Figures
Figure 4-1 – Power Duration Curves for Two Different Energy Ratings ................................. 7
Figure 4-2 – Power characteristics of the SNS energy storage system ................................. 8
Figure 5-1 – Example import/export Limits and "Base Case" Power Flow ........................... 12
Figure 6-1 – Recommended Limit for the Size of Step Voltage Changes with Respect to
the Time between each Change (source: EREC P28) ....................................... 18
Figure 6-2 – Multiplication factor for Deriving the Minimum Time between Ramp-type
Voltage Changes from the Step Change Limit of Figure 4 (source: EREC P28) 18
Figure 6-3 – Reduced Model for Step Voltage Change Calculations ................................... 19
Tables
Table 4-1 – General Response Time of each Energy Storage System by Technology .......... 7
Table 4-2 – Different Routes for Applications including Energy Storage ................................ 9
Table 5-1 – Power quality requirements for equipment connected to public low-voltage
mains electricity supply systems ........................................................................ 15
Table 6-1 – Steady State Voltage Limits According to ESQCR 2002 .................................. 16
Table 6-2 – Typical Network Equivalent Parameters and the Impact on Voltage of 6MVA
swings (X/R=4) .................................................................................................. 20
Table 6-3 - Revised wording in Connection Agreements regarding power factor of
installations ........................................................................................................ 20
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1
Introduction
This standard provides guidance on the application of energy storage. Energy Storage has
been highlighted as a key technology in the development of smart grids and covers a broad
range of technologies, scales and operating models.
In 2012 UK Power Networks launched the Smarter Network Storage (SNS) project1, an
innovation project to trial the operation of large-scale battery energy storage and novel
commercial arrangements in order to explore and resolve some of the non-technical,
commercial and economic challenges around storage adoption under different business
models.
Over the duration of the project, the energy storage market has been rapidly maturing, and
there is now significant interest from developers and investors looking to take advantage of
growing opportunities in the deployment and operation of storage, under similar models to
that developed and trialled in SNS.
The purpose of this document is to:

Provide sufficient background and guidance to assist customers with their application
requests involving energy storage;
Propose “good connection practices” for applications involving energy storage in order to
guarantee compliance with License Conditions and relevant Engineering
Recommendations.

2
Scope
This standard applies to the connection of energy storage devices to all operating voltages
on UK Power Networks distribution networks.
3
Glossary and Abbreviations
Term
Definition
PCC
Point of Common Coupling
SNS
Smarter Network Storage
TSO
Transmission System Operator
UK Power Networks
UK Power Networks (Operations) Ltd consists of three electricity
distribution networks:



Eastern Power Networks plc (EPN).
London Power Network plc (LPN).
South Eastern Power Networks plc (SPN).
1
Detailed information on the SNS project is available on the following website:
http://innovation.ukpowernetworks.co.uk/innovation/en/Projects/tier-2-projects/Smarter-Network-Storage-(SNS)
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4
Key Concepts
4.1
Energy Storage Fundamentals and Technologies
Energy storage can come in a wide variety of forms, ranging from the large-scale pumped
hydro schemes that provide national balancing, to the very small-scale electro-chemical
storage which is typically used in laptops and mobiles. The storage medium can be either in
the form of electricity, such as in supercapacitors, electrochemical form, such as in batteries,
or even heat or gas.
All of these technologies have different operating characteristics, response times, and
advantages/disadvantages, which make them more or less effective for different applications
and at different scales. However, most can be simply characterised by their fundamental
ability to deliver power and energy to the network. There are two key ratings that determine
the overall characteristics of storage:
4.1.1
Power Rating (MVA or MW)
This describes the maximum power at which the storage can push (or pull) power onto (or
from) the network, and drives the maximum import and export capacity required at the
connection point to operate at full power, (an analogy being the size of the engine of a car,
or how fast it can go).
Most storage control systems allow for the import and export to be controlled at any rate
up to the maximum power rating. Hence, if the storage has a 3MW rating it can both
charge and discharge at any power level up to a maximum of 3MW.
4.1.2
Energy Rating (MWh)
This describes the energy duration or energy capacity of the storage device, and can be
used to determine how long the storage could import (or export) for at a given power (the
analogy being the size of the petrol tank, or how long it can run for until empty).
Different storage technologies scale differently in terms of power and energy rating, with
some being expensive to add additional energy duration, whereas others may be more
costly to add additional power capability.
The duration for which a storage system can sustain export (or import) to (or from) the
network at maximum power, can be determined by dividing energy rating by power rating.
For example, the SNS system, which is rated at 6MW / 10MWh, can provide 10/6 = 1.67
hours or around 100 minutes of import (or export) at maximum power of 6MW. This is
illustrated further in the example power duration curves of Figure 4-1, which show the
duration for which a response can be provided (import or export), at a given power level for
two different example rated systems.
An “Energy to Power” ratio of 1 or below (i.e. energy rating similar or lower than rated power)
means the energy storage device will be more suited for power delivery of one hour or less
with full power, due to limited capacity availability. Conversely, an “Energy to Power” ratio
above 1 means the delivery of power may be sustained for longer periods.
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Duration
(Hours)
6 MW / 3MWh System
Power Duration Curves
6MW / 10MWh system
18
16
14
12
10
8
6
4
2
0
0
1
2
3
4
5
6
Power
(MW)
Figure 4-1 – Power Duration Curves for Two Different Energy Ratings
The general response time of each energy storage system by technology is provided in
Table 4-1.
Table 4-1 – General Response Time of each Energy Storage System by Technology
2
2
Battery Technology
Response Time
Pumped Hydro Storage (PHS)
minutes
Compressed Air Energy Storage (CAES)
minutes
Flywheel
<second
Lead Acid
<second
Nickel Cadmium (NiCd) Vented Sealed
<second
Nickel Metal Hydride (NiMH) sealed
<second
Lithium ion (Li-ion)
<second
Zinc Air
<second
Sodium Sulphur (NaS)
<second
Sodium Nickel Chloride (NaNiCl)
<second
Vanadium Redox Flow Battery VRFB
<second
Hybrid Flow Battery HFB
<second
Hydrogen
Seconds/minutes
Synthetic Natural Gas (SNG)
minutes
Double Layer Capacitor (DLC)
<second
Superconducting Magnetic Energy Storage (SMES)
<second
Liquid Air
Minutes
IEC, Electrical Energy Storage, White Paper: http://www.iec.ch/whitepaper/pdf/iecWP-energystorage-LR-en.pdf
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Energy storage systems that are connected to the distribution network via a power
electronics interface are able to vary their reactive power output independently of their active
power output in all four quadrants. Figure 4-2 illustrates the power capabilities of the SNS
energy storage system and the relationship between reactive power (Q), active power (P)
and apparent power (S) capabilities.
+Q
Slimit
+Qlimit
-P
+Plimit
-Plimit
+P
SNS characteristics
-Qlimit
7
-Q
Slimit=7.5MVA
Plimit=6MW
Figure 4-2 – Power characteristics of the SNS energy storage system
Reactive power can be provided continuously from power electronics interfaced systems,
although it is constrained by the active power output in every instance and the apparent
power capability of the converter, as described in the equation (1):
𝑄 = √𝑆 2 − 𝑃2
4.2
Equation (1)
Different Purposes of Installations
Energy storage systems can fulfil multiple purposes, an indicative list is provided as follows:


Peak shaving: energy storage may be used to export energy during peak load (or import
energy during peak generation) periods to flatten the load (or generation) profile. This
application can be provided via exporting active or reactive power, or a combination of
both. The level of effectiveness of each method depends on the power factor at the point
of storage connection. The SNS project showed that it can be more effective to export
reactive power to provide peak shaving3.
Frequency Response: energy storage can be used to support the transmission system
operator (TSO) with frequency stabilisation. The TSO procures commercial products for
this and energy storage systems that are very fast responding can participate in these
markets and be suitable to provide:

Firm frequency response: energy storage systems can import or export energy
very fast (within 10 seconds) and sustain their response for up to 30 minutes to
provide primary (up to 30 seconds) and secondary response (up to 30 minutes)
either in a continuous manner (i.e. dynamic response) or when a loss of
generation plant or large customer causes a significant frequency excursion
(static response);
3
http://innovation.ukpowernetworks.co.uk/innovation/en/Projects/tier-2-projects/Smarter-Network-Storage(SNS)/Project-Documents/SDRC+9.7+Successful+Demonstrations+of+Storage+Value+Streams+LoRes+v1.pdf
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





Enhanced frequency response: this type of response requires the provider’s
plant to respond even faster (within a second) and sustain the response for 15
minutes;
Reserve: after a major event that caused a large frequency excursion,
frequency response resources sustain their delivery for 30 minutes by which
time reserve resources will have started providing sustained energy for up to
two hours. Energy storage systems with large capacity tanks are also able to
provide this service to the TSO.
Voltage control / reactive power support: The active and reactive power controllability
of energy storage systems can be used to provide voltage control to the DNO and
improve the efficiency of the distribution networks (i.e. reducing losses) by minimising
reactive power flows.
Market arbitrage: energy storage can be used to arbitrage on energy prices through
energy trading agreements. Effectively the storage can be used to import and store
energy at low price periods and export the stored energy at higher price periods.
Behind the meter installations: energy storage can be used to improve the economics
of existing connections by optimising energy use and procurement as well as minimising
costs arising from electricity network usage.
TRIAD: this is a mechanism operated by the TSO in which resources that are able to
reduce the national peak load demand during the highest demand periods in a year, are
remunerated. Energy storage may be used to export energy during these periods and
attract revenues from this service.
Although the scale and connection voltages will be different, the approaches to UK Power
Networks can broadly be categorised according to whether customers are considering
existing or new connections, and based upon whether storage is ‘standalone’ or paired with
renewables (or other generation) technology. This is summarised in Table 4-2 and each type
is further described in the Section 5.
Table 4-2 – Different Routes for Applications including Energy Storage
Storage paired with generation
Standalone
Storage Additions
Existing
Connection
Customers with existing generation
(e.g. PV / Wind) looking to add storage
on the same site
New Storage + Generation
New
Connection
New customers, with a specific
location already identified, looking to
connect storage combined with
generation (e.g. PV/Wind)
N/A – As for new connection
New Standalone Storage
New customers looking to connect
standalone storage (no other onsite
generation) at a specified location
Information for applications in Flexible DG areas is provided in Section 5.
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5
Applications
5.1
Identifying a Suitable Location for an Application
Customers have the following means at present to identify suitable locations for their power
requirements ahead of an application:


Generation interactive heat map, available at http://dgmap.ukpowernetworks.co.uk
(registration required).
Long Term Development Statement (LTDS), available at:
http://www.ukpowernetworks.co.uk/internet/en/about-us/regulatory-information/longterm-development-statement.html (registration required).

Distributed Generation Surgeries (running on a monthly basis on both EPN and
LPN/SPN, pre-registration required via:
http://www.ukpowernetworks.co.uk/internet/en/our-services/list-of-services/electricitygeneration/events).
The generation interactive heat map provides an understanding of the level of export
constraint throughout the network.
The Long Term Development Statement provides a rough indication of the level of import
constraint on individual substations. This is available on tab “Table 3 – Load Data” of the
spreadsheet that accompanies the network single line diagrams. In “Table 3 – Load Data”,
the customer can estimate the import headroom by subtracting from column “Firm Capacity
MW” the “Forecast (Maximum Demand) MW” for the year he is looking to energise. It must
be reiterated that this provides only an indicative view, as:



Tables only show recorded and forecasted values. They do not include unused
contracted capacity and accepted but not yet connected capacity;
Interaction between sites needs to be taken into account (e.g. existing capacity at a
given primary site is dominated by the lack of capacity of the upstream grid site);
Potential restrictions introduced by security of supply regulations (P2/6) are not included.
These would be identified by UK Power Networks following an application.
The load information included in the Long Term Development Statement and other reports
UK Power Networks is obliged to provide (e.g. Load Index report to Ofgem) will serve as
basis for the “load interactive heat map”. This heat map is likely to be implemented in similar
fashion to the “generation interactive heat map” and will provide customers with a visual
representation of the level of import constraint. However, the information provided will be
subject to the same caveats mentioned above for the Long Term Development Statement.
Distributed Generation surgeries provide customers with the opportunity to discuss (face to
face or via web/teleconference) project viability in advance of an application.
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5.2
How Applications are assessed by UK Power Networks
The assessment of applications including energy storage requires information in addition to
that supplied by the existing generation application form (commonly known as “ENA form”).
This is due to the varying operational regimes of energy storage (as described in
Section 4.2), which will correspond to different power output profiles. Therefore, UK Power
Networks created in September 2015 a “supplementary information form” specific to energy
storage applicants. The aim was to identify the customer’s specific requirements to ensure
an accurate engineering assessment and offer the most efficient connection solution. This
supplementary information form was superseded in April 2016 by the “Further Information
Request” form produced by the ENA - see Appendix 1.
Guidelines for assessing applications including energy storage are provided in Sections
4.2.1 to 4.2.6. Note that Sections 5.2.1 to 5.2.5 are more directed to EREC G59 applications
and Section 5.2.6 is typically targeted to EREC G83 applications.
5.2.1
New Standalone Storage
The following high-level planning guidelines are in place to assess a new standalone energy
storage application in all License Areas of UK Power Networks:
1. Analyse customer requirements, per ENA form and “Further Information Request” form.
2. Identify the least costly overall connection solution that satisfies the following checks,
both on import (demand) and export (generation):










Capacity limits on circuits/switchgear/transformers on intact and contingent operation
scenarios. Seasonal limits to be considered where applicable. Ratings of circuits as
defined in EREC P17 and EREC P27 are considered.
Protection limits (e.g., directional overcurrent, overvoltage).
Voltage regulation, particularly voltage step change. Compliance with EREC P28 and
statutory limits, as stipulated by ESQCR 2002, to be guaranteed at all times.
Consideration of network complexity regulations, as defined on EREC P18.
Consideration of unused contracted capacity of existing customers.
Consideration of capacity reserved for accepted and not yet connected
generators/demand customers.
Compliance with Security of Supply regulations (EREC P2/6).
Interaction with planned network interventions / reinforcements.
Consideration of forecasted load growth.
Consideration of forecasted microgeneration growth.
An example of a planning scenario for the connection of energy storage to a primary
substation is presented in Figure 5-1. The import is limited by the firm capacity of the
substation. The export is limited by the directional overcurrent protection settings.
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IMPORT
Power flow on
primary substation
transformer
firm rating (winter)
transformer
firm rating (summer)
“Base case” power flow
(with first transformer
outage (N-1))
EXPORT
1 year
time
Directional overcurrent
protection setting
Figure 5-1 – Example import/export Limits and "Base Case" Power Flow
5.2.2
New Storage + Generation Proposed at a Known Site
New applications for a combination of generation and energy storage will be assessed on
the basis of the specific customer requirements, as specified on the ENA form and “Further
Information Request” form.
General guidelines as described in 5.2.1 should apply.
For multiple technologies interfaced by power electronics, unless the customer can
demonstrate that the multiple technologies are coupled at the DC bus, then customers will
be required to demonstrate means to comply with the requested maximum import and export
capacities as set in EREC G1004 - “Technical Requirements for Customer Export Limiting
Schemes”. This will need to be witnessed by UK Power Networks under the normal EREC
G59 process until manufacturers are able to produce type tested export limiting schemes
compliant with the future EREC G100. For combinations of other technologies, the same
EREC G100 and witnessing requirements need to be complied with.
In cases where UK Power Networks believes the risk to the network following excursions
from the agreed import and/or export capacities is high (e.g. if installed capacity exceeds the
thermal rating of the circuit upstream of the point of connection), then additional provisions
may be requested to the customer (including additional monitoring and/or control, such as
reverse power flow relays).
5.2.3
Storage Additions
Addition of energy storage to connected generation sites will be dealt with accordance with
the principles outlined in the current UK Power Networks Management of Capacity
document. Any site/project that seeks to change the operating profile from generation only to
one comprising both generation and demand shall be treated as a material change requiring
a new application.
The application will then be assessed according to the same principles outlined in
subsections 5.2.1 and 5.2.2.
4
Currently under consultation, yet to be published.
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5.2.4
Storage in Locations suggested by UK Power Networks
UK Power Networks is not providing at present any direct indication of suitable locations for
energy storage or developments of any other technology. Instruments made available to
customers for them to identify suitable locations for their power requirements (as described
in Section 4.1) are only aimed to steer applications to areas of less constraint.
Commercial frameworks to enable UK Power Networks to contract response from customers
are being developed, as part of our Distribution System Operator readiness programme.
This follows the learning from our Low Carbon London innovation project, specifically the
trial of Demand Side Response5.
5.2.5
Storage in Flexible DG Areas
Active network management (ANM) systems used to date in Flexible DG areas monitor
selected network locations. Upon inadequate capacity conditions, generators are asked to
reduce their output to bring the network to acceptable operation. Generators have been
assigned to date on the basis of a priority list (“last-in, first-off”) or a shared effort (“pro rata”
or “quota”). Generators are brought to normal operation when a reset threshold (typically a
fraction of the constraint trigger) is reached.
In order to produce curtailment estimates, an operation profile for energy storage
(import/export profile) needs to be defined. This import/export profile has been constructed in
ANM deployments to date as follows:


Analyse customer requirements, as per ENA form and “Further Information Request”
form.
Where a specific import/export profile could not be determined, a conservative profile for
a standalone energy storage system was set as follows:


Import not considered (if no load-related constraints have been identified);
0.5 p.u. “flat” export all year round, with the exception of November to February.
Between November and February, Monday to Friday between 16.00 and 18.30
hours, 1 p.u. flat export is assumed. This is to capture interest in targeting
TRIAD periods.
This conservative profile is intended to represent an “allowance” to cater for the various
operating regimes of energy storage (generally export-related), whilst trying to minimise
impact on curtailment estimates for other customers. An indication of the level of constraint
and time of constraint suffered by the customer is provided.
In order to provide more flexibility to ANM, a market-based approach to regulate the access
to network capacity is in the first stages of conception. One of the expected outcomes is to
explore the flexibility of dispatchable energy resources (as energy storage) to utilise more
efficiently the existing network capacity.
5
Details on Low Carbon London – Demand Side Response available on:
http://innovation.ukpowernetworks.co.uk/innovation/en/research-area/demand-side-response
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5.2.6
Notes on LV Connected Storage
According to Part VI of the “Electricity Safety, Quality and Continuity Regulations (ESQCR)
2002”, no person shall install or operate a source of energy which may be connected in
parallel with a distributor’s network unless certain criteria are met. These criteria are covered
off in EREC G83 and EREC G59.
A charged domestic storage system is a source of energy. If it is connected in parallel with a
distribution network then it shall comply with either EREC G83 or EREC G59 (the only
exception would be if it is used as an alternative source of energy and is never paralleled
with the network). To comply with EREC G83, the aggregated capacity of all sources of
energy within a single customer premises should be 16A per phase or below and use type
tested equipment. Where the aggregated capacity is greater than 16A per phase or the
equipment is not type tested then it falls under EREC G59. EREC G59 differentiates
between <50kW type tested systems and >50kW or non-type tested systems. EREC G59
allows for either EREC G59 or EREC G83 type tested inverters to be used for the interface.
If the network is not capable of accepting an EREC G59 installation without reinforcement,
then it will be necessary to ensure export is limited by some approved means. Currently no
type tested EREC G100 scheme exists; therefore turndown schemes may need to be
demonstrated to UK Power Networks. Some manufacturers do incorporate export limiting
logic within their existing inverter systems which have many of the functions required by
EREC G100.
UK Power Networks is keen to work with manufacturers and installers to ensure that
equipment is designed and installed in such a way as to minimise the cost of installation,
whilst at the same time balancing the requirements of existing customers and ensuring
compliance with our licence conditions. We would expect that manufactures will develop
schemes that can have type approval. EREC G100 makes a differentiation in that witnessing
of sub-50 kW Export Limitation Schemes would be at the discretion of the DNO. Once
customers are able to demonstrate that export limitation schemes are fail-safe, the
requirement to witness these small sites should significantly reduce.
5.2.6.1 Multiple Energy Sources within a Single Installation
Where the total aggregated capacity of an installation with multiple energy sources within a
single installation (including energy storage) is less than 16A per phase, no prior approval is
required and the installation would be treated as a normal EREC G83 application. In line with
EREC G83, the installed capacity when an inverter is used is deemed to be the inverters
steady state rating and not the capacity of the generation connected to the DC side. Below is
an example:


If 4kW PV and 2kW energy storage devices are both connected to the DC side of one
3.68kW G83 type tested inverter, then the site complies with G83.
If the same 4kW PV and 2kW energy storage devices are each connected through
individual 3.68kW and 2kW inverters respectively, and connected together on the AC
side of the installation, then the total installed capacity is assessed as 3.68 + 2 = 5.68kW
and must therefore be treated as an EREC G59 application. This should be the case
even if it is not intended to operate both energy sources at the same time. If the
engineering assessment concluded that the local LV network could not support 5.68kW
of export then an approved export limiting scheme (as per EREC G100) would need to
be installed.
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5.2.6.2 Multiple G83 Applications in a Close Geographic Area
For multiple G83 applications in a close geographic area, an engineering assessment in
accordance with EREC G83-2 section 5.2.1 is required to make sure the local LV connection
can support them. This may also drive the need for export limiting schemes per EREC G100.
5.2.6.3 Standalone Energy Storage Applications
For import/export less than 16A per phase, standalone energy storage devices do not
require prior approval and the installation would be treated as a normal EREC G83
application. Should import/export requirements exceed 16A per phase, the installation would
be treated as a normal EREC G59 application.
5.2.6.4 Power Quality Requirements
It is not UK Power Networks’ intention to require individual power quality assessments for all
LV connected sites. Installed equipment is expected to comply with British Standard BS EN
61000 Part 3, see Table 5-1. This Standard deals with the limitation of harmonic currents,
voltage changes, voltage fluctuations and flicker injected into the public low-voltage mains
electricity supply system by electrical and electronic equipment. For installations outside the
remit of BS EN 61000 Part 3, the power quality requirements indicated in section 6.5 should
apply.
Table 5-1 – Power quality requirements for equipment connected to public low-voltage mains
electricity supply systems
Harmonic Standards
Voltage/Flicker Standards
BS-EN 61000
3-2
3-12
3-3
3-11
Power quality
issue
harmonic current
emissions
harmonic current
emissions
voltage changes,
voltage
fluctuations and
flicker
voltage changes,
voltage
fluctuations and
flicker
Equipment
rating
input current ≤ 16A
per phase
input current >
16A and ≤ 75A per
phase
rated current ≤
16A per phase
input current ≤
75A per phase
not subject to
conditional
connection
subject to
conditional
connection
5.2.6.5 Additional Information
Witnessing of the installation is not required for EREC G83, per normal practice, and it is not
normally required for EREC G59 below 50kW three phase or <17kW single phase.
Witnessing is required for EREC G59 applications above 50kW.
For any changes to an existing G83 installation, notification of the change shall be provided
to UK Power Networks.
For EREC G59 applications, where limiting export in a constrained part of the network is
required an export limiting scheme shall be installed. This scheme operation will need to be
witnessed until manufacturers are able to produce type tested export limiting schemes
compliant with the future EREC G100.
In all instances, customers shall notify UK Power Networks of the addition of storage devices
to their installations.
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6
Specific Network Design and Operation Requirements
A number of technical considerations are taken into account when assessing new energy
storage applications to ensure that safe and reliable network operation will be maintained.
Network design and operation requirements are more likely to be satisfied when connections
are made directly at primary/grid substations, depending on power requirements, and at
higher voltage levels. Also, consideration of different import and export power requirements
(depending of the intended purpose(s) for the energy storage device) may facilitate the
connection, particularly on the point of view of thermal rating of equipment, voltage
regulation and security of supply.
Information on the most relevant technical aspects for an energy storage application
assessment is covered in sections 6.1 to 6.7.
6.1
Thermal Rating of Equipment
Distribution network equipment is designed to be operated within specified loading limits.
The loading limits of each asset, e.g. cable, transformer, etc, are a function of the heat that
equipment may withstand. These limits vary with installation conditions. UK Power Networks
operates its assets as recommended in relevant Engineering Recommendations, such as
EREC P17 (underground cables) and EREC P27 (overhead lines), and as set in internal
operating standards as EOS 04-1020 for transformer ratings6.
If a customer only wishes to utilise the network beyond a defined power for limited periods of
time (e.g. up to a few minutes per hour) and risk to network operation is perceived to be
acceptable, then appropriate commercial agreements and monitoring/control solutions (such
as export limiting schemes and timers) may be implemented to accommodate this. Another
option currently being trialled is the use of profiled connections (see Appendices).
6.2
Voltage Regulation
6.2.1
Steady State Voltage
According to the Electricity Safety, Quality and Continuity Regulations (ESQCR) 20027, the
steady state voltage in different parts of the distribution network should be maintained within
pre-defined limits to ensure safe and reliable operation of the network and equipment
connected to it. These limits are provided in the Table 6-1.
Table 6-1 – Steady State Voltage Limits According to ESQCR 2002
Voltage Level
Upper limit (% above
nominal)
Lower limit (% below
nominal)
LV (<1000V AC or 1500V DC)
10%
6%
HV (<132,000V)
6%
6%
EHV (>132,000V)
10%
10%
6
http://library.ukpowernetworks.co.uk/library/en/g81/Installation_and_Records/Transformers/Documents/EOS+04
-1020+Transformer+Ratings.pdf
7
http://www.legislation.gov.uk/uksi/2002/2665/pdfs/uksi_20022665_en.pdf
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Voltage is affected by the load/generation balance in every instant. When energy storage
devices export electricity, the voltage at their terminals is increased. The opposite occurs
when energy storage devices import electricity. Specific studies using load flow software are
conducted to evaluate whether the operation of the energy storage device will impact the
steady state voltage of the distribution network.
6.2.2
Step Voltage Change
Single variations of the rms value or peak value of the supply voltage are called step
changes. The step changes in voltage are dependent on sudden changes in the
demand/supply balance. Step changes caused by a customer are measured at the point of
common coupling.
EREC 28 prescribes limits for step and ramp voltage changes with respect to the time
between each change, as shown in Figure 6-1 and Figure 6-2. The 3% limit illustrated in
Figure 6-1 is flexed to 10% only for unplanned events, such as faults, that are not expected
to occur more frequently than once a year.
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Figure 6-1 – Recommended Limit for the Size of Step Voltage Changes with Respect to the Time
between each Change (source: EREC P28)
Minimum time between voltage step changes= TIME (from Figure 4) x TIME MULTIPLICATION FACTOR
Figure 6-2 – Multiplication factor for Deriving the Minimum Time between Ramp-type Voltage
Changes from the Step Change Limit of Figure 4 (source: EREC P28)
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For example, a 2% step voltage change is only allowed approx. every 200 seconds (see
Figure 6-1) but if ramped over 1 second, a multiplication factor of approx. 0.015 would apply
(see Figure 6-2). Therefore, the minimum allowable time between 2% ramps lasting one
second each is 200 seconds x 0.015 = 3 seconds.
Should these limits be exceeded, a detailed analysis of existing severity levels needs to be
carried out. Level of background flicker at the point of common coupling is to be collected by
means of a power quality recorder. The impact created by the new customer is then
superimposed on the background flicker severity data. Ultimately, decision on whether to
allow the connection to proceed will be dependent on compliance with the severity levels
indicated in EREC P28, Section 7, Table 1.
The greatest impact of energy storage devices in terms of step voltage change is likely to
occur following a sudden change from import to export (or the opposite). Such events can
occur, for example, when the energy storage management system initiates recovery of stateof-charge, i.e. returns to a pre-defined charge. If no ramping mechanism is in place, a swing
doubling the maximum power rating of the device (in the worst case) is likely to occur well
before network voltage control mechanisms are able to respond. Ramping mechanism are
therefore recommended to comply with EREC P28 requirements and minimise impact to
other customers in the network.
Voltage step change at a location of interest (e.g., point of common coupling for a new
connection) can be readily calculated through a method involving network reduction in the
point of common coupling, see Figure 6-3.
“Rest of the network”
(equivalent)
S’’
X/R
Point of common
coupling
-I
I
V0
Energyimport
storage/
Battery
import
/ export
export
Figure 6-3 – Reduced Model for Step Voltage Change Calculations
The maximum fault level 𝑆’’ and reactance to resistance ratio X/R at the point of common
coupling are required. Step voltage change following power swings at that location is then
calculated according to equation (2), where all values are in p.u. 𝑉0 is the voltage at the point
of common coupling before the connection (assumed 1 p.u. in the calculations below –
Equation 2). Current 𝐼 is obtained from the apparent power output of the energy storage
device. In Table IV are presented voltage step changes caused by 12 MVA power swings
with typical parameters for 𝑆’’. X/R is maintained at constant value of 4 throughout all
calculations for clarity in results.
∆𝑉(%) = |𝑉0 + (𝑅 + 𝑗𝑋) ∙ 𝐼| × 100%
© UK Power Networks 2016 All rights reserved
Equation (2)
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Table 6-2 – Typical Network Equivalent Parameters and the Impact on Voltage of 6MVA swings
(X/R=4)
6 MVA swings from zero output
S’’ (MVA)
150 MVA
(typical fault level at 11kV
substation busbar)
750 MVA
(typical fault level at 33kV
substation busbar I)
1200 MVA
(typical fault level at 33kV
substation busbar II)
6MVA export swing at
unity power factor – full
active export (-Pmax)
(∆𝑽(%))
6MVA export swing at 0.95
leading power factor (-P,
+Q) (∆𝑽(%))
6MVA export swing at zero
power factor – full reactive
export (-Qmax) (∆𝑽(%))
+0.91%
-0.38%
+3.96%
+0.20%
-0.06%
+0.79%
+0.12%
-0.04%
+0.48%
Note that the higher the fault level, the lower the impact of swings on step voltage.
The impact in step voltage is mitigated when reactive power output is designed to reduce the
impact of the swing (e.g. importing reactive power when exporting active power).
The impact of a zero power factor swing (zero active power output, full reactive power
output) in step voltage is very significant in lower fault levels (that correspond to the lower
voltage networks).
First outages leading to reduction in network fault levels will increase the impact of sudden
changes in energy storage output.
6.3
Power Factor of the Installation
UK Power Networks Connection Agreements are being modified to include the wording in
Table 6-3.
Table 6-3 - Revised wording in Connection Agreements regarding power factor of installations
Active Import
Unity to 0.95 lagging [nominal operating point: 0.xx power factor]
Active Export
Unity to 0.95 leading [nominal operating point: 0.xx power factor]
As part of the energy storage engineering assessment, the full swing between 0.95 lead
(generation) and 0.95 lag (demand) at full apparent power output is assessed. If the step
change assessment results are not satisfactory, then a more specific power factor range is
defined and included in the Connection Agreement.
UK Power Networks is currently developing an operational P / Q envelope to specify the
range the developer needs to comply with and demonstrate during the initial operational
notice period. This will provide flexibility to reactive power output once the active power
drops below a fraction of the apparent power.
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6.4
Security of Supply Regulations (EREC P2/6 Compliance)
UK Power Networks has an obligation to satisfy security of supply standards as set on
EREC P2/6. DPC 4.2.1 of the Distribution Code states that 'DNOs shall plan and develop
their DNOs Distribution Systems to a standard not less than that set out in DGD Annex 1
Item 5, Engineering Recommendation P2/6 – ‘Security of Supply’ or such other standard of
planning as DNO’s may, with the approval of the Authority, adopt from time to time'.
Normal levels of security of supply requirements for different ranges of group demand are
indicated in EREC P2/6, Table 1. Consideration of first and second outages is made for
selected ranges of group demand.
Whilst a single customer does not constitute a “group” for the purposes of EREC P2/6, the
impact of a customer connected to a demand group requiring second circuit outage
capability could be material in determining compliance. This applies also for energy storage
applications when assessing import requirements. Each application needs individual
consideration in relation to how the customer’s contractual security arrangements impact on
the P2/6 assessment. Contractual security arrangements may include automatic demand
transfers or utilisation of generation embedded in the customer’s network (export capability,
in the case of energy storage) in operating scenarios to be agreed with UK Power Networks.
6.5
Power Quality
UK Power Networks has license obligation to maintain levels of quality of supply to all its
customers. A comprehensive background on power quality considerations is provided in UK
Power Networks’ planning guidance for disturbing loads8.
All installations must comply with the power quality requirements defined in:



EREC P28 (Planning Limits for Voltage Fluctuations Caused by Industrial, Commercial
and Domestic Equipment in the United Kingdom);
EREC P29 (Planning Limits for Voltage Unbalance in the United Kingdom);
EREC G5 (Planning Levels for Harmonic Voltage Distortion and the Connection of NonLinear Equipment to Transmission Systems and Distribution Networks in the United
Kingdom).
Upon acceptance of their offer, customers will be expected to demonstrate compliance with
the aforementioned Engineering Recommendations before their connection can energise.
Data is available via UK Power Networks’ Long Term Development Statement (LTDS) to
enable the customer to construct their harmonic impedance model. Also included in the
LTDS is maximum demand information and fault levels for network intact conditions. Further
to these the Network Parameters report will include an indication of minimum and maximum
fault level at the Point of Common Coupling (PCC).
Background harmonic data will be provided at the nearest available measurement point
within an agreed timescale. Upon receipt of this the customer will be expected to provide a
G5/4-1 compliance report in a timescale that enables its validation, and amendment as
required, prior to their energisation date. If the report indicates that planning levels are likely
to be exceeded then it may be necessary to include suitable mitigation within their design.
Where compatibility limits are exceeded this will certainly be the case. Similarly a P28/P29
compliance report will also be required.
8
http://library.ukpowernetworks.co.uk/library/en/g81/Design_and_Planning/Planning_and_Design/Documents/ED
S+08-0132+Planning+Guidance+for+Disturbing+Loads.pdf
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6.6
Additional Requirements for Applications above 50MW and Below 100MW
Energy storage has not yet been classified in the Grid Code. UK Power Networks takes the
view that, until further notice, the notes made in this section about generation should also
apply for energy storage.
Where a generation site has a registered capacity of 50 MW or more but less than 100MW
and is not subject to a Bilateral Agreement with National Grid, then in addition to the
Distribution Code, further Grid Code requirements will also apply. These sites are classified
as Licence Exempt Embedded Medium Power Stations or LEEMPS.
Customers wishing to connect a generation site to the DNO network within this range of
powers should make themselves aware of their obligations under the Grid Code9 in terms of:



Data provision requirements;
Technical, design and operational criteria;
Compliance, testing and monitoring requirements that the generation site must fulfil
before being allowed to connect and generate.
Note that Grid (and Distribution) Code requirements are to be updated shortly, following the
passing of the European Regulation for Requirements for Generators. For more detail refer
to Section 5.7.
6.7
European Network Codes – Requirements for Generators (RfG)
A series of European Network Codes are being produced to provide harmonisation across
products and markets in Europe. There are 11 Codes in total in three areas – Grid
Connections, System Operation and Market Operation. One of these Codes is the
Requirements for Generators (RfG).
RfG entered into force on 17 May 201610 and will apply technical requirements to new
generators who procure their main plant items after a two year transitional period of
implementation at member state level (17 May 2018). RfG uses four incremental type bands
(‘A’ to ‘D’) which set a sliding scale of generator technical capabilities to support System
Operation. Some of the new technical capabilities were not normally requested at distribution
network level in Great Britain (e.g. fault-ride through capability for generation above 1MW of
installed capacity).
At the moment only pump-storage is included in the legislation, but other forms of energy
storage are expected to be incorporated soon. Customers are advised to make themselves
aware of advances in the implementation of RfG into the GB system.
9
Full details of requirements can be found on the National Grid website under Grid Code
(http://www2.nationalgrid.com/UK/Industry-information/Electricity-codes/Grid-Code).
10
Commission Regulation EU 2016/631, available at: http://eur-lex.europa.eu/legalcontent/EN/TXT/?uri=OJ:JOL_2016_112_R_0001
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References
7.1
UK Power Networks Standards
EDS 08-0132
Planning Guidance for Disturbing Loads
EDS 08-0145
EHV Design Standard
EOS 04-1020
Transformer Ratings
7.2
National Standards
ESQCR (2002)
Electricity Safety, Quality and Continuity Regulations (2002)
ENA EREC P17
Current Rating Guide for Distribution Cables
ENA EREC P18
Complexity of 132kV Circuits
ENA EREC P2/6
Security of Supply
ENA EREC P27
Current Rating Guide for High Voltage Overhead Lines Operating in the
UK Distribution System
ENA EREC P28
Planning limits for voltage fluctuations caused by industrial, commercial
and domestic equipment
ENA EREC G83
Recommendations for the Connection of Type Tested Small-scale
Embedded Generators (Up to 16A per Phase) in Parallel with Low-Voltage
Distribution Systems
ENA EREC G59
Recommendations for the Connection of Generating Plant to the
Distribution Systems of Licensed Distribution Network Operators
ENA EREC G5/4-1
Planning Levels for Harmonic Voltage Distortion and the Connection of
Non-Linear Equipment to Transmission Systems and Distribution Networks
ENA EREC G100
Technical Requirements for Customer Export Limiting Schemes
The Distribution Code (http://www.dcode.org.uk/)
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Appendix A – ENA Additional Information Form for Storage
http://www.ukpowernetworks.co.uk/internet/en/ourservices/documents/2016_Energy_Storage_System_-_Futher_Information_Request_V15.docx
Appendix B – Profiled based connections
The diversity of the enquiries and ultimately the connections made to our network has
changed significantly over the last five years. This is likely to continue. We recognise and
acknowledge that we need to innovate to be able to continue making connections to our
network. Therefore we will soon be introducing profiling of connection agreements which will
include the trialling of timed connections, where applicable. Their introduction will allow us to
continue our work on offering both quicker and cheaper connections. Working with
customers we will be able to determine when access to our network is available and when
customers can utilise any latent capacity. This will require an element of flexibility both by us
and our customers. We will ask customers to indicate their anticipated usage profile by
completing a table for 48 half hourly periods similar to the one below. Their requested profile
will be used to design the connection and will be reflected in their connection agreement.
Time of day, week, month or year use of import capacity or export capacity
The Maximum Import Capacity and/or Maximum Export Capacity shall be subject to the
following restrictions:
UK Clock
Time
Maximum Import (kVA)
Maximum Export (kVA)
Winter
Winter
Working
Day
Summer
NonWorking
Day
Working
Day
NonWorking
Day
Working
Day
Summer
NonWorking
Day
Working
Day
NonWorking
Day
00.00-00.30
through to
23.30-24.00
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