Ensuring grid code harmonic compliance of wind farms

The prevalence of cabled infrastructure associated with wind farms, combined with the increase in power-electronic interfaces in wind turbines and reactive power plant, has highlighted the pervasive nature of harmonics on electricity networks.

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Jul 17, 2017
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The prevalence of cabled infrastructure associated with wind farms, combined with the increase in power-electronic interfaces in wind turbines and reactive power plant, has highlighted the pervasive nature of harmonics on electricity networks. Insufficient consideration is given to the effects of harmonics through the low prioritisation of power quality in the Front-End Engineering and Design stage and a lack of awareness of the obligations of the respective parties involved. The resulting designs are incompatible with the requirements of the relevant Grid Codes. This can lead to delays, commercial exposure, restricted network access and subsequent loss of revenue. The obligations of the Transmission Owner (TO), System Operator (SO) and wind farm developer are discussed with respect to Grid Code requirements and the responsibilities of each party in ensuring compliance. The technical aspects of assessing harmonic compliance are described in the context of the design considerations which are made at the various stages of the project development. These include the shift in resonances within the host network, the modification of existing harmonic distortion and the propagation of injected harmonics into the network and through to the extra high voltage, high voltage and medium voltage substations. The ensuing challenges associated with ensuring compliance through filter design are discussed in the context of relevant international standards, including UK ER G5/4-1, IEEE 519, EN 50160 and IEC 61000-3-6. The results indicate that shifts in resonances are more problematic to resolve, compared to the propagation of injected harmonics: injected harmonics are readily absorbed through local filtering and thus the emissions are typically low; in contrast, the effects of shifts in resonances must be transferred back to the point of common coupling, resulting in complex local filtering. Early selection by the developer of appropriate transmission infrastructure and technology can lower the filtering requirement necessary to meet the harmonic specification issued by the host TO.


  • LV: low voltage ( U n ≤ 1 kV)
  • MV: medium voltage (1 kV < U n ≤ 36 kV)
  • HV: high voltage (36 kV < U n ≤ :  EN 50160 : 150 kV  IEC 61000: 230 kV)
  • EHV: extra high voltage ( U n > HV)


Wind farm capacities have traditionally been in the order of tens of megawatts with relatively small associated cable infrastructure to export the power to the electricity supply network. However, challenging national renewable energy targets coupled with advancements in wind farm technology have led to a significant increase in the deployment and generating capacities of wind farms. This has resulted in a corresponding increase in the extent and operating voltages of the associated cabled offshore transmission infrastructure. This presents new challenges to the host TO in controlling the levels of harmonic voltage distortion, not just at the point of common coupling (PCC) but at substations deeper within the transmission and distribution system.

This article details three complementary aspects of harmonic compliance on transmission networks:

  • Roles and responsibilities describe the roles and responsibilities of the parties involved in wind farm developments.
  • Standards presents the relevant standards which are applicable to the field of harmonic compliance in order to identify what guides and procedures exist to assist network owners in ensuring harmonic compliance.
  • Finally, Technical aspects of compliance details an illustrative desktop study into the effects of a large offshore wind farm on the onshore transmission and distribution system. This study deals with four areas of harmonic compliance: Firstly, it presents the propagation of harmonic voltage emissions from the PCC through to substations deeper within the onshore network; secondly, it illustrates the effect of the export infrastructure on the network impedances and how this modifies the existing harmonic voltage distortion at the PCC; thirdly, it describes the shifts in resonances within the network and how this affects the levels of background harmonic voltage distortion at remote substations; finally, a summary is presented of the objectives of harmonic filter design in light of the issues highlighted by the preceding analysis.

Roles and Responsibilities

The responsibilities for ensuring harmonic compliance are embodied in the codes of the relevant country. The main parties involved in the compliance of wind farms in the UK are shown in Fig 1 along with an indication of the contractual mechanisms through which harmonic compliance is secured.

Fig 1: Contractual relations between the main parties involved in wind farm developments

The responsibilities for harmonic compliance in the UK are set out in the Grid Code [1] and the SO/TO Code (STC) [2]. The Grid Code applies to users of the system and to the SO, which is National Grid Electricity Transmission plc (NGET). The wind farm developer/owner is a user and the two are bound by a Bilateral Connection Agreement (BCA) through which NGET places requirements on the user to comply with the Grid Code. The STC is the mechanism through which the SO places obligations upon each TO in order to ensure that the Grid Code requirements are met.

Harmonic compliance is secured at the planning and design stage of a user connection by reference to the planning levels of Engineering Recommendation (ER) G5/4-1 [3]. These planning levels are either equal to or more stringent than the compatibility levels, also documented in ER G5/4-1, with which the SO is obliged to comply within operational timescales.

For harmonic compliance, Section K of the STC stipulates that, when planning and developing its transmission system, a TO shall ensure that their system complies with the minimum technical, design and operational criteria and performance requirements set out, or referred to, in the Grid Code Section CC 6.1.5; this in turn refers to ER G5/4-1. Through this mechanism, the responsibility for compliance within planning timescales is transferred to the host TO with ER G5/4-1 established as the standard against which compliance is assessed.

The host TO must perform a harmonic assessment, based upon the plant details submitted by the user, to ensure that the system is planned in compliance with the planning levels set out in ER G5/4-1. The users are obliged by the Grid Code to submit their plant data to enable the host TO to carry out the necessary system studies. The host TO can, but is not obliged to, adopt the methodology set out in ER G5/4-1 to ensure that planning levels are not exceeded by the connection. This responsibility extends beyond the PCC into all other transmission and distribution networks to the extent that they are affected by the connection. To exemplify this point, an Offshore TO (OFTO) is required to study the effect of a wind farm connecting to its network on the onshore transmission and distribution system, as well as other offshore networks, to ensure the continued compliance of all affected substations. Alternative methodologies must be agreed with the SO because the operational compliance is dependent on appropriate planning standards being followed.

On completion of the harmonic assessment, the host TO will submit a corresponding harmonic performance specification to the SO to complement the relevant BCA. The SO may either adopt the specification directly or modify the specification according to its own criteria so as not to prejudice existing or future connections while being fair to the user. The user is obliged to comply with the specification which might imply a need for harmonic filtering at the PCC. It is normal practice for the host TO to carry out harmonic monitoring prior to and post energisation of any wind farm installation. Pre-energisation monitoring is carried out to establish a baseline of distortion for any subsequent investigation into non-compliance on the system.

Case Study

As part of the UK offshore transmission regime introduced by OFGEM in 2009, a scenario has arisen where the wind farm developers are responsible for the construction of the associated offshore transmission assets, with a view to transfer them to an OFTO upon completion through a tendering process. In this instance, the wind farm developer acts as the host TO in a role referred to as an Offshore Transmission System Development User Works party and is thus bound by the same codes as any other TO.


There are numerous harmonic standards available to provide guidance to SOs, TOs and wind farm developers in the control, mitigation and monitoring of harmonic distortion. This section outlines the more commonly employed standards.

EN 50160 is the prevailing European Standard for supply voltage characteristics of public networks up to 150 kV [4]. It is a broad standard dealing with the frequency, magnitude, waveform and symmetry of the line voltages and thus does not solely focus on harmonic distortion. It provides expected harmonic voltage distortion levels on low voltage (LV), medium voltage (MV) and high voltage (HV) public supply networks up to the 25th harmonic order. From a TO perspective, its scope is limited as it does not provide guidance on the assessment of non-linear connections and is restricted to below 150 kV.

The American National Standards Institute (ANSI) recognised IEEE 519 is an American national guide which gives recommended practices for the management of harmonic voltage and current distortion in electrical power systems [5]. For the allocation of harmonic current distortion limits, it provides limits which are then scaled according to the ratio of system fault current to user's rated current. It provides harmonic voltage distortion limits at the PCC; for HV connections it permits a 1% limit across all harmonics, which is onerous from a system perspective. It describes a general methodology for evaluating new harmonic sources, providing guidance on modelling techniques, obtaining measurements, simulations and mitigation measures. However, it does not provide clarity to the TO in terms of modelling system impedance and thus fails to consider the modification of background harmonic distortion because of resonance effects.

Lessons Learned

The application of standards like IEEE 519, which base the harmonic current limits on idealised harmonic impedance representations of a system, should not be used where there is a high probability of resonances. This includes assessments close to highly capacitive cable infrastructure and/or nearby shunt capacitive reactive compensation.

IEC 61000-3-6 is an international technical report on the assessment of emission limits for the connection of distorting installations at voltage levels above LV [6]. The distinctive principle behind the report is the allocation of harmonic distortion limits according to the ratio of rated plant power to the capacity of the system at the PCC, known as ‘equal rights allocation’. The emission limits depend upon the agreed power of the connectee and the system characteristics. The report provides compatibility levels for LV and MV connections and indicative planning levels for MV, HV and EHV.

The report outlines three stages of assessment which can be used sequentially or in conjunction with one another with varying degrees of complexity:

  • Stage 1 is a simple calculation based on the ratio of agreed power to the short-circuit power at the PCC. It deems a ratio of 0.2% acceptable for connection without further examination.
  • Stage 2 offers a more detailed examination taking into account the actual system characteristics when the criteria for a Stage 1 assessment are not met. For HV and EHV connections, it sets out a more complex assessment than Stage 1. The assessment calculates the total available power capacity at the PCC and then apportions the planning levels at the PCC using a general summation principle documented in the standard. It considers the effects of resonances because of varying network conditions and thus can lead to PCC limits being set below planning levels.
  • Stage 3 addresses connections which do not qualify under Stage 2 criteria but may be permitted access on a conditional basis after considering factors such as phase aggregation and operational diversity.

ER G5/4-1 is the national standard in the UK written by the Energy Networks Association [3]. It provides harmonic voltage distortion planning levels for the connection of non-linear equipment and compatibility levels for operational time scales for all voltage levels up to 400 kV. It details three stages of assessment with varying degrees of detail to balance the degree of assessment complexity with the risk of connection:

  • Stage 1 specifies the maximum size of converters and regulators which may be connected at LV without further assessment.
  • Stage 2 facilitates the connection of equipment below 33 kV, which is too large for a Stage 1 assessment, through a simple study of the connection using typical data.
  • Stage 3 applies to any connection at or above 33 kV, or beyond the scope of a Stage 2 assessment. It is a detailed assessment involving background harmonic measurements at the PCC and surrounding substations. It requires details of the connected plant along with harmonic emission profiles and impedances. Detailed modelling and simulations are indicated resulting in a specification which includes characteristic system impedances and harmonic voltage distortion limits at the PCC. A Stage 3 assessment procedure according to ER G5/4-1 takes into account other substations within the supply system to ensure they do not exceed planning levels. Similar to IEC 61000-3-6, this can lead to PCC limits being set below planning levels.

A significant difference between ER G5/4-1 and IEC 61000-3-6 is that ER G5/4-1 does not seek to allocate headroom but rather aims to ensure that the planning levels are not exceeded by appropriately limiting the connection of non-linear equipment and installations.

Lessons Learned

In accordance with a user's BCA with the SO, the user's harmonic voltage distortion limits and equivalent harmonic impedance representation of the host TO network are defined in a harmonic performance specification issued by the SO. This ensures that, under the network conditions encapsulated by the harmonic impedances, the connection shall remain below the stipulated harmonic voltage distortion limits.

ER G5/4-1 does not specify how these limits are defined; it only presents a connection assessment procedure which allows the responsible TO to ensure that the system is planned such that the SO can discharge its Grid Code responsibilities. The SO will use its own criteria to develop the harmonic performance specification without prejudicing existing or future connections while being fair to the user; these limits are informed by the responsible TO's assessment. The SO will allocate the headroom that is required by the connection, subject to these not exceeding planning levels, not the full headroom available at the site. In this way, the SO facilitates the user's connection but does not restrict further grid access.

A singular connection has no right to the planning levels in ER G5/4-1; these are applicable to the system as a whole. Each individual connection is entitled only to the limits defined in the relevant agreement with the SO, which is typically a BCA for customer connections.

Technical Aspects of Compliance

In the initial Front-End Engineering and Design (FEED) of a wind farm development, some fundamental design choices will affect future harmonic compliance. These decisions are often required before financial closure is obtained and are therefore finalised before detailed design has concluded. It is imperative that these decisions are considered in the context of Grid Code harmonic compliance because failure to do so might result in delays in the detailed design, procurement and commissioning of wind farm installations.

Structural and technological decisions which will affect the harmonic compliance include:

  • The use of AC or DC export infrastructure between the wind farm and the grid.
  • The use of voltage-source or line-commutated HVDC converters where DC infrastructure is used.
  • The export system voltage level.
  • The choice of dynamic compensation equipment.

The combination of technology and structure must be evaluated in light of the following possible consequences:

  • Propagation of the emitted harmonics through the grid.
  • Modification of the background harmonic voltage levels at the PCC.
  • Modification of the background harmonic voltage levels at remote substations.

To demonstrate these consequences, consider Fig. 2 which is used to investigate the effects of a 250 MW wind farm connection using 132 kV AC export infrastructure. The model is derived using typical values based on the transmission system of England and Wales, the data for which is presented in the Appendix.

Fig 2: System model under investigation with the PCC at Substation 3

Propagation of emissions

The wind farm installation, including all compensating plant, is designed to limit the incremental harmonic distortion at the PCC such that the wind farm does not materially affect the network by introducing additional harmonic content. Therefore, the emissions are typically low, as documented in Table 1 of the Appendix.

Table 1: Wind Farm Harmonic Emissions [% of rated current]

OrderWind farmLoadOrderWind farmLoad

Fig 3 illustrates the percentage voltage total harmonic distortion evident at each of the remote substations resulting from these wind farm emissions alone. This illustrates the propagation of harmonics from the wind farm installation through to the remote substations. The incremental distortion at the PCC is 0.090% while the maximum transfer is 0.117% through to Substation 1. A significant transfer through to a remote substation will necessitate a limitation on the incremental individual harmonic voltage distortion (IHDv) permissible at the PCC, based on a limiting aggregate harmonic distortion at the remote node.

Fig 3: Propagation of wind farm harmonics through the network

Lessons Learned

IEC 61000-4-30 [7] defines a minimum accuracy factor for a Class A power quality monitor of 0.1% so as to provide a consistent metric for power quality monitors. However, this does not mean that power quality monitors cannot be more accurate than this (acknowledging that transducers introduce additional errors). Therefore, an appropriate accuracy factor should be introduced in line with the equipment which is available to the SO or TO.

This accuracy factor must not be applied separately to both the incremental harmonic distortion and the modified background harmonic distortion levels so as not to introduce superfluous margin; it should be applied only at the end of the full assessment to the aggregate harmonic distortion level.

Case Study

Many wind farms and HVDC interconnections use voltage-source converter power electronic interfaces. With this technology, inappropriate design of the control loops can result in a DC offset on the AC side of the converter. When applied to the step-up transformer, the magnetisation of the transformer shifts such that the upper and lower half cycles saturate differently. This results in a waveform which is not half-wave symmetrical and introduces even order harmonics onto the AC network. This is particularly onerous and should be remedied in the control system; it is not practical to filter low order even harmonics.

Modification of PCC background levels

Prior to the connection of the wind farm, the other connected users will have built the appropriate mitigation measures to limit their emissions such that the system harmonics are below planning levels. After the wind farm is connected, the system impedances change such that the transfers of the load harmonic emissions through the system are modified.

Fig 4 illustrates the simulated pre- and post-connection voltage IHD at the PCC, without the emissions from the wind farm being considered. This represents the modification of the existing background through the modification of the system impedances.

Fig 4: Modification of background voltage IHD at substation 3 (PCC)

For all harmonics below the 50th order, the pre-connection scenario is compliant with ER G5/4-1; post-connection, the 17th harmonic order is non-compliant at the PCC for which the planning level is 0.5%. For the specific modification shown for h = 17, (1) shows the corresponding percentage change in the reduced bus impedance matrix. This relates the PCC to the substations at which harmonic current emissions are known (or would be measured on a physical system). From this matrix, it is possible to inspect which transfers are most affected by the connection of the wind farm.


For the purposes of the modification of the PCC harmonic compliance, inspection of the row indicated by shows a 340% increase in the transfer impedances from harmonic injections emanating from substations marked by , where # represents the substation number. Using both the impedance and harmonic current phasors, it can be shown that this impedance matrix modification at the 17th harmonic order corresponds well to the pre- and post-connection levels of 0.32 and 1.11%, respectively, as shown in Fig 4 .

Lessons Learned

For AC connected offshore wind farms, the background modification is typically more prominent than the propagation of harmonics from the turbines; for DC connected offshore wind farms, the propagation of harmonics from the HVDC converter becomes dominant.

Modification of background levels at remote substations

Although most assessments of background modification only consider the modification at the PCC, the connection of the wind farm does significantly affect the system impedance matrix such that the emissions transfer through the system differently at each harmonic and to each substation.

Fig 5 illustrates the simulated pre- and post-connection IHD at remote Substation 5, without the emissions from the wind farm being considered. From the plot, it is evident that the 5th harmonic order is significantly affected at Substation 5, even though the connection was at Substation 3. The increase is from 1.2 to 2.7% which exceeds the 2.0% planning level of ER G5/4-1.

Fig 5: Modification of the background voltage IHD at substation 5

(2) shows the corresponding percentage change in the reduced bus impedance matrix. This relates Substation 5 to the busbars at which harmonic current emissions are known (or would be measured on a physical system).


With the exception of ΔZ55−5ΔZ5−55 , which is ineffective because there are no emissions at Substation 5, the weighted average (according to load magnitude) of all the remaining values in the row indicated by is 159% which, when considering that this simple average does not take into account the impedances’ phases, corresponds well to the 125% increase shown in Fig. 5 .

Case Study

A large offshore wind farm was connected to the grid using large, HV AC submarine cables. The capacitance of these cables modified the system impedances such that a remote node became susceptible to high seventh-order harmonic voltages under certain outage conditions including those of adjacent customer connections.

Filter design objectives

Following suitable analysis of the preceding effects relating to the connection of a wind farm, filter designers might aim to:

  • Absorb emitted harmonic content to prevent their propagation through the network.
  • Modify the harmonic driving point impedance at the PCC to reduce the background harmonic voltage levels.

For both of these objectives, designers typically have a defined harmonic profile to target, which is developed in conjunction with a variety of network harmonic impedances as defined in the harmonic performance specification from the SO. To this end, the designer is effectively considering the current or voltage division between the harmonic filter and the network harmonic impedance under all operational eventualities. Both objectives, when applied to the measurement at the PCC, are readily achieved using well understood methods.

In contrast, the effect shown in modification of background levels at remote substations, relating to the modification of remote background harmonic levels, has largely been ignored. The full procedure required to address this effect is beyond the scope of this present paper, although its essence is in the investigation of the bus admittance and impedance matrices.

With reference to the example in modification of background levels at remote substations, the bus admittance matrix for the fifth harmonic order is constructed using the actual values for each element, and replacing the admittance at the PCC according to Y'3−3=Y3−3−YfY3−3′=Y3−3−Yf , where Y f represents the harmonic admittance of the filter. The inverse Z'busZbus′ matrix thus includes elements as functions of Y f . Consider (2) from where it is apparent that the greatest changes are seen at elements Z 5–4, Z 5–6, Z 5–12 and Z 5–13; Z 5–5 is ignored because there is no harmonic current injection at Substation 5. By considering the defined current injection at the various substations, and analysing the elements Z 5–4( Y f ), Z 5–6( Y f ), Z 5–12( Y f ) and Z 5–13( Y f ), filter designers can optimise their designs to limit the remote harmonic voltage distortion as a function of Y f , according to the following equation


Lessons Learned

Although it might not be possible to eliminate the full effects of remote background modification through local filter optimisation without compromising on the required performance at the PCC, designers can limit the effect on remote nodes to within tolerable limits.


This article has presented the responsibilities of the SO, TO and wind farm developer in ensuring Grid Code harmonic compliance. The SO is ultimately responsible for compliance in operational time scales, but delegates the planning for compliance to the TO through the STC.

The TO has a number of guidelines with which to assess compliance, although in the UK the primary guideline is ER G5/4-1. These guidelines provide a TO with the procedures to assess a connection in light of the effects illustrated in this paper, namely:

  • The propagation of harmonics through the network.
  • The modification of the background harmonic voltage distortion levels at the PCC.
  • The modification of the background harmonic voltage distortion levels at remote substations.

With the increased penetration of wind energy lowering the system fault levels, harmonic impedances will increase as a result of the reduction of linear harmonic impedances traditionally provided by synchronous generation. The host TO will need to be ever more diligent in assessing harmonic compliance through the network.

Although the TO is responsible for compliance planning, this article has shown that careful consideration needs to be given to the harmonic compliance by the developer during the initial FEED phases of a wind farm development. Appropriate selection of transmission infrastructure and technology can radically affect the modification of background harmonic levels. Insufficient consideration to these effects in the early stages of development, and before financial closure, might result in delays and commercial exposure during detailed design, procurement and commissioning as the technical obligations on the developer to meet the host TO's harmonic performance specification become apparent.


This work was presented at the 11th Wind Integration Workshop and at the IET ACDC 2012 International Conference on AC and DC Power Transmission and is published in the respective proceedings. The author would therefore like to thank Peter Haigh and Joseph McCullagh who were co-authors of those papers on which this work is based. Furthermore, the author would like to thank Mark Perry of National Grid Electricity Transmission plc for his support in preparing this work.


  1. National Grid Electricity Transmission plc. The Grid Code, August 2012. Issue 5 Revision 0.
  2. National Grid Electricity Transmission plc. System Operator-Transmission Owner Code, August 2012. Section K.
  3. Energy Networks Association. Planning Levels for Harmonic Voltage Distortion and the Connection of Non-Linear Equipment to Transmission Systems and Distribution Networks in the United Kingdom, October 2005. ER G5/4-1.
  4. CENELEC. Voltage Characteristics of Electricity Supplied by Public Distribution Networks, March 2009. EN 50160.
  5. Institute of Electrical and Electronic Engineers. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, June 1992. IEEE Std 519-1992.
  6. International Electrotechnical Commission. Electromagnetic Compatibility (EMC) – Part 3-6: Limits – Assessment of emission limits for the connection of distorting installations to MV, HV and EHV power systems, February 2008. IEC 61000-3-6 Ed. 2.0.
  7. International Electrotechnical Commission. Electromagnetic Compatibility (EMC) – Part 4-30: Testing and measurement techniques – Power quality measurement methods, October 2008. IEC 61000-4-30 Ed 2.0.

First published on Engineering &  Technology Reference 2014


Fig 6: Harmonic impedance model for loads

Table 2: Circuit data – R, X: [Ω/km], B: [μS/km]

Circuit length voltageAll OHL Fig.  2 400 kVSUB3-WFA 0.13 km 400 kVWFB 12 km 132 kVWFC 48 km 132 kV
R 10.01740.00910.03140.0664
X 1j0.2783j0.2022j0.1460j0.1223
B 1j4.1200j72.2600j75.9900j64.1100
R 00.10290.07920.22000.2246
X 0j0.7884j0.5541j0.1000j0.1021
B 0j2.4798j72.2600j75.9900j64.1100

Table 3: Transformer and reactor data


Qty × S [MVA]

Voltages [kV]


Vector group


1 × 600





2 × 240





3 × 120





1 × 420





1 × 750





2 × 120





1 × 240





2 × 600





1 × 420





2 × 240





2 × 300





2 × 240





2 × j40


Table 4: Generator data


Table 5: Load data

SUB127510020Fig.  6
SUB413210020Fig.  6
SUB613210020Fig.  6
SUB913210020Fig.  6
SUB1213210020Fig.  6
SUB1340050050fixed R– L

Table 6: 33 kV harmonic RLC impedance of the wind farm

OrderR, ΩL, mHC, µF


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Danson Joseph

Power supply energy storage manager, Jaguar Landrover

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