APPENDIX 60
Supplementary memorandum by the Energy
Networks Association
CLUSTERING OF
MICRO-GENERATION
The current volume of microgeneration connected
to distribution networks is very small. Consequently, the impact
of microgeneration on networks has not been a significant issue
to date. There have been no clustering problems in existing networks
as a result of customers choosing to install microgenerators either
as a new device or as a replacement, for example, of a previous
heating system. In the future, however, local authorities may
require developers to install smaller generators on new buildings
as a requirement of obtaining planning consent. Where there are
new housing developments that contain microgeneration, the network
will specifically be designed to cater for the technical issues,
but a scenario that could create a challenge is where there is
a high penetration of, for example, domestic CHP on existing networks
causing reverse power flows during periods of low demand and high
generation (eg mornings).
The existing low voltage (LV) distribution networks
have been designed to cater for a unidirectional energy flow from
the distribution substation to customer installations. The connection
of multiple microgeneration units can have an adverse effect on
network operation. The main issues that need to be considered
include:[12]
1. Impact on voltage levels, both over and
under-voltage.
1. Impact on Voltage levels
Overvoltage
Export from a microgeneration unit will tend
to increase the voltage profile on the LV networks. Under low
load conditions, where voltages are at the upper end of the statutory
voltage range the voltage level within the installation could
increase above statutory limits and this could lead to nuisance
tripping of the microgeneration unit.
Undervoltage
There is a risk that transient disturbances
on the HV system could lead to voltage dips on the LV system which
in turn could result in widespread tripping of microgeneration
units by operation of the undervoltage element within the interface
protection. The original disturbance will then be compounded by
the sudden increase in demand as the system supports the load
that was previously supplied from the microgeneration units.
2. Short circuit levels
The connection of a microgeneration unit or
units to the LV networks could result in the prospective short
circuit current, from the source and the microgenerator, exceeding
the short circuit rating of equipment connected to the network.
The short circuit contribution from a microgenerator must be considered
to ensure that the single and three phase fault duty imposed on
Distribution Network Operator (DNO) and customer's equipment does
not exceed the equipment rating. The contribution to short circuit
currents will depend upon the microgeneration technology.
3. System loading
Low voltage distribution networks have been
designed with the assumption that power flows from the substation
out to the customers. With the wider introduction of microgeneration
units, the direction of power flows may reverse. The DNO may need
top make an assessment of the magnitude and direction of power
flow to ensure equipment thermal ratings are not exceeded.
4. Voltage unbalance
For multiple installations of single-phase microgeneration
units (eg housing estates), balancing the unit generation evenly
against the load on the phases will need to be considered.
The connection of significant quantities of
microgeneration units in the LV networks could also have an adverse
impact on the 11kV and higher voltage networks. The issues are
similar to those for the low voltage network. In particular, the
impact of voltage changes and effect on the primary voltage control
needs to be carefully considered.
It is worth pointing out however that the microgeneration
penetration level at which any of the issues mentioned above appears
will depend upon the generator and network characteristics.[13]
Across the UK there is a range of distribution network designs
and operating practices and thus the impact will vary accordingly.
Mitigation could take the form of more sophisticated control systems,
plant and switchgear upgrade but network augmentation will often
be required. Realising the full potential of lower voltage networks
in particular will require further clarification of the requirements
under ESQCR that would potentially permit a wider (LV) voltage
operating range within specified parameters and hence permit a
wider penetration of microgeneration into LV networks. Additional
governmental support for a few mixed microgeneration based installations
could also be helpful here to provide much needed learning experiences
for all stakeholders. The network operators in particular would
benefit from practical real tests of appropriate scale where the
effects of microgeneration technologies can be properly planned
for, assessed and tested.
CONNECTION OF
SMALL-SCALE
EMBEDDED GENERATORS
(UP TO
16A PER PHASE)
AND SAFETY
ISSUES
The procedures for connection of microgeneration
installations delivering electricity have been described in the
Engineering Recommendation G83/1Recommendations for the
connection of small-scale embedded generators (up to 16A per phase)
in parallel with public low-voltage distribution networks (2003).[14]
The procedures are designed to facilitate the connection of microgeneration
units whilst maintaining the integrity of the public low-voltage
distribution network, both in terms of safety and supply quality.
The purpose of this Engineering Recommendation
is to simplify and standardise the technical requirements for
connection of small scale embedded generators (SSEGs) by addressing
all technical aspects of the connection process from standards
of functionality to site commissioning. The ER G83/1 was drawn
up by an all stakeholder group (including DNOs, Generators and
taking input from DTI and Ofgem) and drafted and approved in accordance
with the Ofgem agreed rules on the governance of standards.
The Engineering Recommendation provides sufficient
information to allow:
SSEG Manufacturers to design and
market a product that is suitable for connection to the public
low voltage distribution network.
Users, manufacturers and installers
of SSEG units to be aware of the requirements that will be made
by the local distribution network operator before the SSEG installation
will be accepted for connection to the DNO's network.
DNOs to confirm that the SSEG installation
is compliant with the relevant international, national and industry
standards.
The Engineering Recommendation outlines generic
requirements related to the connection, installation and network
design requirements for all types of SSEG unit. The specific requirements
for each different type of technology of SSEG (eg domestic CHP,
photovoltaic, fuel cells, microhydro) are defined in the annexes.
The annexes also describe a methodology for testing the particular
types of SSEG in order to demonstrate compliance with generic
requirements of this Engineering Recommendation. By satisfying
the test conditions in the relevant annex the SSEG can be considered
as "Approved" SSEG for connection.
In accordance with the Electricity Safety, Quality
and Continuity Regulations (ESQCR) 2002, Regulation 22(2)(c) the
Installer is to ensure that the DNO is made aware of the SSEG
installation at or before the time of commissioning. The DNO may
not refuse to accept the connection providing the installation
complies with the requirements of ESQCR Regulation 22. However
under the terms of ESQCR Regulation 26 the DNO may require a SSEG
unit to be disconnected if it a source of danger or interferes
with the quality of supply to other consumers.
In the case of a single microgeneration installation,
in addition to the notification required under ESQCR the Installer
shall provide the DNO with the commissioning form (which provides
a template for all the technical details required by the DNO)
within 30 days of the SSEG unit being commissioned.
In the case of a planned installation project
where the proposal is to install multiple SSEG units in a close
geographic region (eg part of a new housing development, housing
refurbishment programme in the same street etc) it is strongly
recommended that the Installer discusses the installation project
with the local DNO at the earliest opportunity. The Engineering
Recommendation provides a template for initial application. The
DNO will need to assess the impact that these connections may
have on the network and specify conditions for connection. The
confirmation of commissioning will need to be made within 30 days
of commissioning (ie as in the case of single installation).
Each micro-generator installation must also
comply with BS 7671 (IEE Wiring Regulations). All wiring between
the supply terminals and the SSEG shall be protected by a suitably
rated protective device; and shall be of suitable size and type
for the rating of the SSEG. The SSEG shall be connected directly
to an isolation switch where for single-phase machines the phase
and neutral are isolated and for multi-phase machines all phases
and neutral are isolated. In each instance the manual isolation
shall be capable of being secured in the "off" (isolated)
position; this switch is to be located in an accessible position
within the Customer's installations.
The interface protection is also required so
as to ensure that the connection of a SSEG unit will not impair
the integrity or degrade the safety of the distribution network.
The protection function can either be incorporated within the
SSEG or afforded by separate devices. In either case the interface
protection shall meet the requirements of IEC 60255-5, or equivalent
standard and comply with all other relevant standards as described
in the annex for specific technologies.
The Installer shall provide labelling at the
Supply terminals (Fused Cut-Out), meter position, consumer unit
and at all points of isolation within the customer's premises
to indicate the presence of a SSEG. In addition to the safety
labelling, the Engineering Recommendation requires the following
information to be displayed at the point of interconnection with
the DNO network:
A circuit diagram showing the circuit
wiring, including all protective devices, between the SSEG and
the DNOs fused cut-out. This diagram should also show by whom
all apparatus is owned and maintained.
A summary of the protection settings
incorporated within the equipment.
It is worth mentioning that the ER G83/1 has
received international acclaim for its innovative approach to
facilitating the connection of micro-generation.
12 "ENA Engineering Recommendation G83/1-Recommendation
for the connection of small-scale embedded generators (up to 16A
per phase) in parallel with public low-voltage distribution networks",
September 2003. Back
13
"The Impact of SSEG on the Operating Parameters of Distribution
Networks", K/EL/00303/04/01, URN 03/1051. Back
14
According to the ESQCR, any unit above 16A per phase requires
permission to connect from DNO. The network operator would carry
out a proper assessment of each connection application before
granting a permission. The Engineering Recommendation G59/1 covers
the connection requirements for generation up to 5MW by specifying
the generic requirements for the connection of generation to public
distribution networks. Back
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