The fifth revision of ENA Engineering Recommendation G5, EREC G5/5 “Harmonic voltage distortion and the connection of harmonic sources and/or resonant plant to transmission systems and distribution networks in the United Kingdom”, came into implementation on 17th June 2020. The updated revision has been published following a long review process which started in 2010 to address the key issues of:
- Increasing numbers of polluting loads connecting to the electricity networks.
- Complications around electrically near and concurrent connections.
- Concern about the fairness of a “first come first served” approach of allocating harmonic “headroom” to new connections.
Figure 1: Impedance loci at a network node for group of harmonic orders 29 to 36 inclusive, showing individual impedance points.
Figure 2: Heat map of voltage distortion results for two individual harmonic orders across impedance loci (result colour coded by percentage of allocated limit).
What has changed in the new G5 revision?
While the new revision of G5 constitutes a complete re-write of the recommendation, the fundamental principles underpinning it are unchanged. The three-stage process is retained with each stage pertaining to the same class of equipment as previously. While all three stages have been completely revised, there has been an intention to make the new document as clear and easy to follow as possible. The revised Stage 1 and Stage 2 assessments in particular are designed as linear processes presented as a series of easy to follow flow-charts.
A summary of the main changes incorporated into the new revision are as follows:
- A review of planning and compatibility levels has led to the levels for some individual orders being increased. No levels have been reduced and those for Total Harmonic Distortion remain unchanged.
- In EREC G5/4 limits and levels were specified for specific voltages. In EREC G5/5 this has been revised to use a set of voltage ranges for which the planning and compatibility levels are applicable. These voltage ranges have been aligned with typical ranges used in the U.K. This should avoid the need for user interpretation where a voltage level is used that wasn’t previously specified, e.g. 150 kV or 220 kV.
- Due to the emission spectrum of modern equipment, planning and compatibility levels are now specified up to the 100th order, and it is recommended that assessments consider distortion up to this frequency. The practicality of measuring at these frequencies has been acknowledged by allowing assessments up to only 50th harmonic at the discretion of the relevant network operator.
- A clear definition for interharmonic distortion has been presented and interharmonic limits revised in accordance with IEC 61000-34-30, IEC 61000-4-7 and IEC 61000-2-2.
- Limits on voltage notches, in terms of depth and duration, have also been revised.
- Assessment of the connection of resonant plant is now specifically included in a new section of the recommendation. Resonant plant covers equipment such as power factor correction capacitors and long cables, the connection of which may amplify existing network distortion. A simple assessment process for equipment connected at voltages below 33 kV is presented. Equipment connected at voltages of 33 kV and above is captured in the Stage 3 process.
- The Stage 3 assessment process, which covers equipment connected at 33 kV and above and that which has failed a Stage 2 assessment, has undergone extensive revision. The requirements of the assessment are now much more clearly defined. Significant additions to the Stage 3 assessment are:
- Harmonic limits for a new connection are now calculated based on an allocation of the available distortion “headroom” in the network against the planning limits. This is a departure from the previous first come first served approach, the intention is to retain “headroom” for future connections. The methodology for carrying out the apportionment is specified in the recommendation.
- There is now a requirement for the Network Owner/Operator hosting a connection to prepare a harmonic specification for the new connection. This specification details; the network impedance at the point of connection, in terms of a set of impedance loci or tabular data; the existing background harmonic level; limits for incremental distortion; and limits for the resultant total harmonic distortion at the point of connection. Incremental distortion is that distortion produced by emissions from the new connections plant. Total harmonic distortion is that resulting from the combination of emissions from the new connections plant combined with existing network distortion (modified by the impedance of the new connection). The methodology for deriving the limits has been specified in the recommendation.
- There is also a requirement for a harmonic compliance report, detailing the results of a Stage 3 assessment. This specifies the minimum set of results/data that must be included in a report to ensure consistency.
- The methodology to generate harmonic specifications for concurrent connections at the same or electrically near nodes has been specified. This should provide a benefit, by means of minimising delays, for developers.
ENA EREC G97, "Process for the connection of non-linear and resonant plant and equipment in accordance with EREC G5", complements G5 by providing guidance on aspects of the connection process that are beyond the scope of EREC G5. It provides a framework for the assessment of harmonic voltage distortion from non-linear and resonant plant and equipment as part of the connection process thus facilitating a consistent approach to connection. G97 also provides guidelines on timescales for which the Network Owners should provide harmonic data and specifications after receiving such a request.
TNEI can provide support to DNOs/TSOs in the development of sufficiently accurate harmonic models that adequately represent their networks and implement the allocation of incremental limits to individual users as per G5/5. In addition, TNEI has extensive experience in the development of harmonic impedance loci, determining the polygon characteristics and grouping of harmonic orders with similar characteristics in order to minimise any inherent risks due to various modelling assumptions.
TNEI can also support network users (including generators, demand, storage) throughout the compliance assessment process and carry out a full impedance area-based study using over 1000 individual points or using the individual profiles that the network operator provided. TNEI has developed their harmonic compliance assessment methodology to enable identification of regions on the loci that could constitute a compliance risk and support the user throughout the entire connection process. Pre- and post-energisation, TNEI has the capability to carry out power quality measurements and data review services to further support both network users and operators. Our team also have extensive international experience in design and specification of harmonic filters that have successfully resolved power quality issues as well as mitigating non-compliance risks.
If you would like to know any further information or you have an enquiry relating to any G5/5 and harmonic issues, please get in touch to discuss with TNEI’s Director of Connections, Mustafa Kayikci: firstname.lastname@example.org.
Additional graphs demonstrating simulation accuracy against measurements up to the 100th order:
Additional plots showing the impact of harmonic filters designed by TNEI:
The impact of only MV filters on the LV2 voltage during the initial start-up peak period.