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Harmonising a classic

16 September, 2020

The UK electricity has issued a new edition of its G5 harmonics recommendations. ABB’s Martin Richardson outlines the main changes between G5/4-1 and G5/5. In a future column, he will examine the impact of the recommendations on variable-speed drive installations.

On 17 June, the Energy Networks Association (ENA) issued a revised edition of its UK harmonics recommendation. The Engineering Recommendation G5 Issue 5 2020 (G5/5) is not only a major revision, but a complete technical rewrite of the long-standing G5/4-1. 

The new recommendation aims to tackle the increasing number of loads connecting to electricity networks, and the interactions between these connections. 

Some users had previously raised concerns over the “first come, first served” principle of allocating emission rights. The new recommendation provides clearer and more defined guidance and assessment methods for connecting harmonic generating loads, and allocates harmonic headroom in a more transparent and fair manner. Basically, it ensures new harmonic sources do not generate excessive harmonic levels across the network.

The ENA’s intentions remain the same – stipulating permissible levels of harmonic voltage distortion, and the connection of harmonic sources and/or resonant plant to UK transmission systems and distribution networks. 

Major changes in the new edition include:

•  Planning and compatibility levels   Planning levels for total harmonic distortion (THDv) are either the same or higher than those in G5/4-1. G5/5 also aligns the voltage ranges to which the planning and compatibility levels are applicable to typical UK voltages. Because of the emission spectrum of today’s technology, the planning and compatibility levels see an extension to the frequency range from the 50th harmonic (2.5kHz) in G5/4-1 to limits now reaching up to the 100th harmonic (5kHz) in G5/5. However, the challenges of measuring and modelling accurately at these higher frequencies are acknowledged by allowing assessments up to only the 50th harmonic at the discretion of the network operator.

• Inter-harmonics  Measurement of inter-harmonics is more clearly defined in G5/5, with new limits based on those specified in IEC 61000-4-30 and IEC 61000-4-7. 

• Voltage notch limits  Limits for voltage notches are revised to consider the depth and duration of the notch on the voltage waveform. Voltage notching is caused by rectifier commutation when two phases of the supply are short-circuited. 

• Changes to connection processes  The three-stage connection process is retained with each stage relating to the same class of equipment as before.

Stage 1 has been rewritten completely so that users unfamiliar with the standard should be able to relate more readily to the process of connection.

Stage 2 is designed for connection at PCC voltage levels below 33kV and assessments that have failed Stage 1. The Stage 2 process is now linear with a defined assessment process flow.

Stage 3 covers equipment connected at 33kV and above, or that which has failed a Stage 2 assessment. In Stage 3, harmonic limits for a new connection are calculated using the available distortion “headroom” in the network against planning limits. This aims to retain “headroom” for future connections.

 




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