new G5-5 guidance for harmonics in power systems

The new G5-5 recommendations on harmonic disturbance are a significant change to the previous G5-4. Importantly, this is a site standard rather than a product standard. This means system integrators and end-users must ensure compliance or risk withdrawal of supply by their utility company.

In a power system, most equipment not drawing current in a linear way will create harmonics. This category includes variable speed drives. Harmonic disturbance changes the current and voltage waveforms causing possible disruption to electronic controllers and overheating in switchgear. In extreme cases, it can also result in the failure of connected equipment.

Moreover, the harmonic disruption can radiate from a company’s location to other nearby sites that share the power grid. This effect may need special consideration in the design of an electric system to serve non-linear loads.

Wayne Turtill, Product Manager for Variable Speed Drives at Mitsubishi Electric UK, explains the impact of G5-5 recommendations. (Go here for the G5-5 Webinar recording)

The proliferation of connections such as small-scale power generation plants has increased distortion on power networks. The Energy Network Association (ENA), representing the electricity network, created the Engineering Recommendation (EREC) G5-5. These limit the harmonic voltage distortion levels applicable to UK distribution networks for connection to the supply.

G5-5 defines the roles and responsibilities of all parties relating to harmonic disturbance levels, including network operators and network users. The network operator sets the disturbance limits on its network and it is the user’s responsibility to follow them. As a result, the network operators can enforce the levels detailed in G5-5 and withdraw electrical supply if it is not met.

Scope of G5-5

G5-5 applies to new sites and modifications to existing sites. It also includes new planning levels and compatibility of harmonic distortion. Planning levels represent a site’s probability of disturbance, and the compatibility level is the point where equipment failures can occur. The network operator specifies voltage distortion likely to affect connected equipment based on a percentage of total harmonic distortion (THD).

G5-5 also includes a headroom calculation for further supply distortion above the background levels, but below peak permitted planning levels. The amount of distortion already generated by other users on the network impacts the headroom value allowed by the operator. Medium-sized sites with their transformer have some further allowance compared to the previous G5-4 recommendations. The level of harmonic current permitted depends on the calculated harmonic voltage distortion for a supply and fault level.

A key point is there is no compliance assessment of individual items of equipment to G5-5. It is a site standard, and the assessment is significantly more detailed than its predecessor G5-4.

Help is at hand

G5-5 affects all power systems, and end-users will now have to think about harmonic assessment when making changes. Equipment drawing greater power will need new headroom calculations as part of a harmonic survey and design. G5-5 also means that VSD suppliers must provide more information on the harmonics their devices generate to help users achieve compliance.

Mitsubishi Electric helps by providing consultancy services for end-users and system integrators to achieve G5-5 compliance. They also offer modelling and design of active harmonic filtering to meet the industry levels.


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