Harmonic Studies help address harmonic distortion, which is a growing concern in today’s increasingly inverter-dominated electricity networks. The connection of new generation or storage to the UK distribution or transmission networks is now subject to stricter harmonic compliance requirements under Engineering Recommendation G5/5 (ER G5/5). At Blake Clough Consulting, we offer end-to-end expertise in conducting harmonic assessments in accordance with ER G5/5, helping clients avoid compliance issues, costly mitigation, and project delays.
Background: ER G5/5 and Its Importance
ER G5/5 was introduced to replace ER G5/4 and came into effect for all connection offers issued after 17 June 2020. It was developed by the Energy Networks Association (ENA) and approved by Ofgem to address the rising concern around harmonic distortion resulting from modern power electronics-based generation, such as solar PV, battery storage systems, and wind farms.
Key reasons for the update include:
- Increasing prevalence of non-linear loads and inverter-based generation
- The potential for parallel and series resonance due to widespread cable use and reactive components
- The need to consider cumulative harmonic distortion across multiple connecting generators
Harmonic studies are mandatory for all generation and storage connections that could materially impact harmonic levels on the grid, and they represent a critical path activity in the connection process.
Overview of Harmonic Study Requirements Under ER G5/5
ER G5/5 introduces a more detailed and rigorous process for evaluating harmonic emissions. It requires generators to demonstrate compliance with both incremental and total harmonic voltage distortion limits at the Point of Common Coupling (PCC).
Key Calculations:
- Incremental Harmonic Voltage – the voltage distortion resulting solely from the plant’s harmonic current injection
- Total Harmonic Voltage – the combined result of incremental distortion and pre-existing background harmonic voltages
These calculations must be conducted up to the 100th harmonic order, with each order assessed individually and in combination.
Key Compliance Principles:
- Harmonic assessments must take into account concurrent connections and how their combined emissions may interact
- Harmonic resonance phenomena, including parallel resonances that can amplify certain orders, must be investigated
Network-wide impacts must be considered – not just at the PCC but at other sensitive nodes in the system
Essential Input Data for Harmonic Studies
Accurate and meaningful harmonic assessments depend on the quality and completeness of input data provided by both the network operator (DNO/ESO) and the equipment manufacturers. The key data sets required include:
1. Harmonic Voltage Limits and Background Levels
- Incremental Limits: Defined by ER G5/5, these limits vary depending on the system voltage, network topology, and other factors
- Total Limits: Set to ensure that the total harmonic voltage distortion (including background) remains within safe limits
- Background Harmonic Voltages: Typically based on two weeks of measured data at or near the PCC, capturing variations under typical network operation
2. Harmonic Impedance Loci
The impedance loci define the envelope of possible network impedance values (both resistance and reactance) for each harmonic order. These are derived from network models under various operating conditions and contingencies. Loci may be provided as:
- Grouped Envelopes: Multiple harmonic orders bundled into a single impedance envelope
- Ungrouped Envelopes: One envelope per harmonic order, preferred for detailed and accurate analysis
These loci are critical in assessing how the plant’s harmonic injection interacts with the system, especially in identifying resonance risk areas.
3. Norton Equivalent Model of the Plant
Provided by the inverter or converter manufacturer, the Norton model characterises the plant’s harmonic behaviour:
- Norton Current Source: Defines the magnitude and phase of harmonic current injection
- Norton Impedance: Represents the internal impedance of the device at each harmonic order
This data must be available up to the 100th harmonic order for comprehensive analysis.
Identifying Harmonic Risk Ahead of Time
One of the main challenges developers face is that final harmonic data from DNOs/ESOs is often delivered late in the project cycle, frequently just months before energisation. This can create uncertainty and project risk. While definitive studies must wait for final input data, preliminary screening can still be extremely useful.
How We Identify Early Harmonic Risk:
- Frequency Sweep Analysis: We perform frequency-domain simulations of the plant to detect parallel resonances, especially in cable-heavy sites. Resonances can cause amplification of certain harmonic orders, even when current injection is low.
- Topology Review: Identifying risk factors such as long cable runs, high capacitance, or weak grid connections
- Sensitivity Studies: Using assumed or indicative impedance data to explore “what-if” scenarios
This enables us to advise clients on risk likelihood, design mitigations, and potential cost exposure well before firm DNO data is available.
What to Do If Harmonic Non-Compliance is Identified
If a plant is found to be non-compliant under ER G5/5, it does not automatically mean that connection is unachievable. Several mitigation pathways exist, each with different implications:
1. Install Harmonic Filters
- Most common solution, but often expensive and space-intensive
- Requires early design integration; late-stage additions may necessitate substation redesign
- Long lead times can delay project schedules significantly
2. Engage the DNO/ESO
- If non-compliance is minor or occurs only under narrow operating conditions (e.g. a small portion of the impedance loci), the network operator may accept the connection under monitoring or conditional approval
3. Engage the Inverter OEM
- Inverter parameters (e.g. switching strategies, controller tuning) can often be adjusted to reduce harmonic current injection
- Manufacturers may provide updated Norton models reflecting mitigated configurations
4. Request Ungrouped Loci
- More granular impedance data can reveal that non-compliance only occurs for specific orders or conditions—allowing for targeted mitigation
5. Request Frequency Sweep Data
- Understanding the scenarios behind the impedance loci may help identify that only a few contingency cases cause the issue, potentially allowing a restricted operational envelope as an alternative to physical mitigation
Our Capabilities in Harmonic Compliance Studies
At Blake Clough Consulting, we provide a full suite of services for harmonic compliance studies under ER G5/5, including:
- Early-stage feasibility assessments
- Detailed G5/5 compliance reports using client-supplied or network operator data
- Custom model development, including inverter Norton models and plant impedance profiles
- Collaborative engagement with network operators, OEMs, and developers to explore mitigations
- Design integration support for harmonic filters where necessary
Our technical experts use a combination of industry-standard tools, Python-based scripting, and custom modelling frameworks to perform fast, transparent, and repeatable studies that clients and stakeholders can rely on.
Why This Matters
Harmonic compliance is not only a technical necessity, it can be the difference between a successful energisation and costly project delays. The complexity introduced by ER G5/5 means that harmonic studies are no longer a simple formality. They must be approached with rigour, foresight, and adaptability. By working with Blake Clough Consulting, clients gain a partner who understands both the technical detail and the commercial impact and who can navigate this complexity with confidence and clarity.