Harmonics

We asked Peter Haigh, from National Grid’s Network Engineering team: How can National Grid help customers to manage their risks from harmonic distortion when they connect to our network?

In the last decade, the electricity supply industry has changed dramatically. Offshore and onshore wind generation has been connecting at an ever-increasing rate and fossil-fired synchronous plant is closing as we embrace the decarbonisation of our electricity supply. With these changes, power quality has increasingly been at the top of the agenda for grid integration of renewables. 

Renewable technologies such as wind farms are non-linear devices, they inject harmonic current into the network. This distorts the voltage waveform that all customers see and from which they draw their power. 

In order to control the quality of the voltage waveform in the UK, National Grid as system operator is obliged to ensure that all transmission networks are compliant with the applicable standards for harmonic voltage distortion. 

As System Operator, National Grid discharges this responsibility by requiring all network owners that host non-linear connections to perform harmonic assessments to ensure that they limit the impact of the connection on the quality of the system voltage. 

It’s a lot like water quality, and renewables are, ironically, polluting loads. Passive filters are the most common mitigation technique to clean the voltage waveform.

The process is different according to which network the non-linear connection is connecting to:

Onshore Wind Farm, Interconnector and Electric Traction Power Supplies

National Grid’s transmission system is the onshore extremely high voltage (EHV) system in England and Wales. When we host a connection, we do a harmonic assessment and set limits and impedance loci that inform the design of mitigation, usually in the form of a passive harmonic filter. 

  • Onshore wind farms - National Grid does a harmonic assessment as part of its license conditions, and the limits that result acts as an input for the analysis to see if the connection is compliant, and for subsequent design of a harmonic filter if required. 
  • Interconnectors and electric traction supplies - for interconnectors and electric railway (traction) connections the same thing applies, National Grid does an assessment and sets limits, and the connecting party decides if they need mitigation to be compliant. If required, then mitigation is designed and built as part of the project. 

How can National Grid help out more than we already do for these customers?

  • It is the responsibility of the developer to study the impact of their connection against our limits and to design any mitigation. 
  • National Grid’s harmonics experts are available to do this task on behalf of the developers, offering the following distinct services:
  • We can construct a suitable model of the developer’s connection based on supplied manufacturer’s data.
  • We can determine whether the connection will be compliant without mitigation.
  • We can produce conceptual design for an efficient mitigation solution.

Offshore Connections 

Offshore wind is a slightly different story. Since Ofgem introduced the enduring offshore regime and offshore competition, the wind farm developer opted to build both the offshore network and the offshore wind farm at the same time. Acting as the Offshore Transmission Owner (OFTO), the developer has the same responsibilities that National Grid would have if the connection was to their network onshore and so they must set limits on their connection to the National Grid system.

In order to discharge their responsibilities, a process has been developed between National Grid System Operator and the OFTO, summarised as:

  • OFTO submits an Outage Study Specification to NGET SO, which sets the parameters for the harmonic assessment.
  • NGET SO runs the network study and provides the raw output to the OFTO.
  • OFTO uses this data to produce harmonic limits and impedance loci.
  • OFTO models the connection of OFTO network and wind farm.
  • OFTO determines whether mitigation is required.
  • OFTO produces conceptual design of filter if required, then builds as part of the project.

How can National Grid help out more than we already do for these customers?

  • It is the responsibility of the developer to define the study that is used to set the limits and loci for the connection of the OFTO network, and then to study the impact of their connection against these limits and to design any mitigation. 
  • National Grid’s harmonics experts are available to do these tasks on behalf of the developer, offering the following distinct services:
    • We can produce the Outage Study Specification for submission to NGET SO.
    • We can set the harmonic limits and impedance loci for the connection of the OFTO network. 
    • We can construct a suitable model of the OFTO and wind farm connection based on supplied manufacturer’s data.
    • We can determine whether the connection will be compliant without mitigation.
    • We can produce conceptual design for an efficient mitigation solution.

If you wish to discuss any aspect of our harmonics consulting services, please contact the Commercial Engineering Services (CES) team on: +44 (0)800 783 9228 or send us your enquiry.

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