Hello! Welcome to your next lesson on the UK's electrical grids.
In our last lesson, we explored the world of electrical faults and the fundamental principles of network protection: detection, isolation, and coordination. We learned that distribution networks were traditionally designed to be passive, one-way streets, delivering power from large substations down to consumers.
Today, we'll examine how the rise of modern, smaller-scale generation is fundamentally changing this design. The learning outcome for this lesson is to explain how distributed generation can cause reverse power flow and voltage rise issues in radial distribution networks.
We will cover:
- The difference between the traditional "top-down" grid and one with distributed generation.
- What "reverse power flow" is and why it happens.
- How injecting power into the local grid causes "voltage rise" and why this is a problem.
1. The Traditional Grid vs. Distributed Generation
As we've touched on, the electricity grid was historically built around a "top-down" model. Large power stations (like coal, gas, or nuclear plants) connect to the high-voltage transmission network. Power flows from this "top" level down through a series of substations, which step the voltage down until it reaches homes and businesses at the "bottom".
Most distribution networks, especially those serving residential and rural areas, use a radial design. This means the circuits branch out from a substation like the limbs of a tree, with each line having only one source of power. In this model, power only ever flows in one direction: away from the substation.
A simple radial network. Power flows from the substation (the single source) outwards to all consumers.
This one-way flow is being challenged by the growth of Distributed Generation (DG).
To get a formal definition of DG and understand how it changes the traditional grid model, please read the following brief excerpts.
International Good Practices in Renewable Distributed... & What is reverse power flow?
This document from GET.transform defines Distributed Generation (DG), while the article from Roadnight Taylor describes the traditional 'top-down' power flow that DG disrupts.
First, in the 'International Good Practices' document, please read the short section 'What is Distributed Generation and What Is this Brief About?' on page 7. Focus on the definition in the blue box.
Next, in the 'What is reverse power flow?' article, please read the section 'History of the grid'. This provides a clear description of the traditional 'top-down' power flow.
In simple terms, Distributed Generation refers to any electricity generation connected to the local distribution network rather than the high-voltage transmission network. This includes solar farms, onshore wind turbines, and even rooftop solar panels on homes.
The key point is that DG introduces power sources within the "tree branches" of the radial network, leading to new and complex behaviours that the grid wasn't originally designed for. Let's explore the first of these.
2. Reverse Power Flow
In a traditional network, power flows from the higher voltage grid to the lower voltage grid through a substation transformer. This is called forward power flow.
With DG present, what happens on a sunny, windy day when local generation (e.g., from a solar farm) is high, but local electricity demand (from homes and businesses) is low? The surplus energy has to go somewhere. It flows back up the network, from the low-voltage side of the substation transformer to the high-voltage side. This is reverse power flow.
The following reading explains this process with some excellent diagrams.
This article from Roadnight Taylor uses simple diagrams to show exactly when and how reverse power flow occurs.
Please read the section 'The rise of embedded generation'. Pay close attention to the scenarios shown in Figure 1, Figure 2, and Figure 3. Notice how the flow of power through the transformers changes from forward to reverse once the local generation exceeds the local demand.
As the article illustrates:
- Scenario 1 (No DG): All 20MW of demand is supplied from the high-voltage grid. Power flow is forward.
- Scenario 2 (Some DG): The 15MW generator supplies some of the local demand. The grid only needs to supply the remaining 5MW. Power flow is still forward.
- Scenario 3 (High DG): The 25MW generator supplies all 20MW of local demand and exports the surplus 5MW back to the high-voltage grid. Power flow is now reverse.
While the transformer itself might not mind which way the power flows, this reversal can cause significant problems for other equipment at the substation and on the wider network, which were designed assuming one-way traffic. The most immediate consequence of this power injection is its effect on voltage.
3. Voltage Rise
In any electrical wire, there is a small amount of resistance (or more accurately, impedance). As electricity flows along the wire to meet demand, some energy is lost, causing the voltage to gradually drop with distance. DNOs must manage their networks to ensure that the voltage at the furthest customer's property doesn't fall below a legally required minimum (e.g., 216.2V in the UK).
However, when a Distributed Generator injects power into the network, it has the opposite effect: it pushes the voltage up.
This creates a major challenge. On a day with low demand and high generation, the voltage can be pushed above the statutory maximum limit (e.g., 253V in the UK), a condition known as overvoltage.
The following resource provides a fantastic visual for this phenomenon.
International Good Practices in Renewable Distributed ...
This document explains the technical impacts of DG and includes a key diagram showing how DG affects the voltage profile along a distribution line.
Please read the section '3.1 Technical Considerations of DG in the Operation of Distribution Systems' (starts on page 8). Focus on the first two paragraphs and the explanation of 'Overvoltages'. Most importantly, study FIGURE 1 on page 9. This diagram is crucial for understanding the concept of voltage rise.
Let's break down what FIGURE 1 from the document shows:
- The Orange Line (High Load / No DG): This is the traditional scenario. Voltage is highest at the substation and drops steadily along the feeder as it supplies power to customers. The DNO's job is to keep it from dropping below the lower limit.
- The Green Line (Low Load / High DG): This is the new scenario. Local demand is low, but a generator at the end of the line is producing a lot of power. This power injection causes the voltage to rise along the feeder. The voltage is now lowest at the substation and highest near the generator. The DNO's job is to keep it from rising above the upper limit.
This voltage rise is a primary reason why connecting new generation to the grid can be difficult and expensive. If a DNO's analysis shows that a new solar or wind farm would cause voltages to exceed the legal limits, then either the project cannot connect, or costly network upgrades are required to manage the voltage.
Conclusion
In this lesson, we've seen how the shift from a few large power stations to many small, distributed generators is turning the old one-way grid into a dynamic two-way system.
Here are the key takeaways:
- Traditional distribution networks are radial (like a tree) and were designed for one-way power flow from substations to consumers.
- Distributed Generation (DG), like solar and wind farms, introduces power sources directly into these local networks.
- Reverse power flow occurs when local generation exceeds local demand, causing surplus power to flow "backwards" from the distribution network to the transmission network.
- Voltage rise is a direct consequence of injecting power into a radial network. This can push voltages above their legal upper limits, creating an "overvoltage" condition that can damage equipment and is a major barrier to connecting new DG.
In our previous lesson, we learned how protection systems operate under predictable, one-way fault conditions. Now that we know DG can reverse the normal direction of power flow, you can probably guess that it will have a major impact on those protection systems.
In our next lesson, we will explore exactly that: describing the impact of distributed generation on network protection schemes, fault levels, and system safety.