Understanding Grid Code Compliance: A Practical Guide for Developers

Grid code compliance is one of the most misunderstood aspects of connecting generation to the UK grid. Whether you're developing a solar farm, a wind project, or a battery storage facility, understanding what's required — and why — can save months of delay and significant cost.

What is G99?

G99 is the Engineering Recommendation published by the Energy Networks Association (ENA) that governs the connection of generating plant to electricity distribution networks in the United Kingdom. It replaced G59 and applies to generating units with a capacity greater than 16A per phase.

The standard sets out the requirements that generators must meet to ensure their plant does not adversely affect the stability, quality, or safety of the wider power system. It covers everything from voltage ride-through capability to reactive power export limits.

The Connection Process in Practice

The journey from application to energisation typically involves several stages that developers often underestimate in complexity.

Pre-application Assessment

Before formally applying for a grid connection, it's worth conducting a preliminary feasibility study. This involves:

• Identifying the most appropriate connection point based on proximity and available capacity
• Estimating the likely network reinforcement costs
• Assessing whether the site can meet the technical requirements for that voltage level

The Formal Application

Once you've identified a viable connection point, you submit a formal application to the Distribution Network Operator (DNO). The DNO will then carry out a Connection Impact Assessment (CIA), which can take anywhere from 13 to 65 working days depending on the complexity.

Common Compliance Challenges

In my experience working with developers across transmission and distribution projects, the most common issues arise in three areas.

Voltage Ride-Through Requirements

G99 requires generating plant to remain connected during voltage disturbances within specified limits. Many developers are caught out by the Low Voltage Ride Through (LVRT) requirements, particularly for older inverter-based technologies that were not designed to meet the current standard.

Power Factor and Reactive Power

Generators connected at 11kV and above are typically required to operate at a power factor between 0.95 leading and 0.95 lagging at the point of connection. This sounds straightforward, but achieving this across the full operating range — particularly at low output levels — requires careful system design.

Protection Coordination

The protection settings required by G99 must be coordinated with the DNO's own network protection to ensure that faults are cleared correctly without causing unnecessary disconnection. Getting this wrong can lead to failed energisation or, worse, equipment damage.

Practical Advice

The key to navigating grid code compliance successfully is to engage early — with the DNO, with your equipment suppliers, and with a competent protection engineer. The standards exist for good reason: they protect the integrity of the grid that everyone depends on.

Don't treat compliance as a box-ticking exercise. Understand what the requirements are trying to achieve, and you'll be in a much better position to design a system that meets them first time.

If you're working on a grid connection project and need technical support, feel free to get in touch.