Distributed Generation: Impact on Network Protection and Safety

1. A Fundamental Shift: From One-Way to Two-Way Faults

As we've learned, traditional distribution networks are radial, with power and fault currents flowing in one direction: from the substation outwards. Protection systems, like overcurrent relays, are designed with this simple logic. They are set to detect abnormally high currents and are coordinated based on the assumption that the fault is always "downstream".

Distributed Generation breaks this fundamental assumption. When a fault occurs on a network with DG, the fault is now fed with current from two directions:

• From the main grid via the substation.
• From the local DG unit(s).

This turns a simple radial network into a more complex, multi-source system during a fault. To begin, let's explore this concept and its first major consequence: the change in fault level.

Changing Fault Levels

The fault level (or short-circuit level) is the maximum current that would flow at a specific point on the network during a fault. The circuit breakers and protection relays are all sized and configured based on the expected fault level.

By providing an additional source of current during a fault, DG typically increases the fault level. This can exceed the design ratings of network equipment, but more importantly, it disrupts the careful calculations used to coordinate protection.

The following resource provides a concrete example of this increase.

As you saw in the table, the presence of DG increased the fault current by over 20%. However, the type of DG also matters. A traditional synchronous generator (like a large diesel engine) will contribute a very high fault current. In contrast, modern solar and wind farms are connected via inverters, which are power electronic devices. Their fault current contribution is limited by their internal software and hardware, creating a different set of challenges for protection engineers.

To understand this crucial difference, the following video explains what "inverter-based resources" are.

This multi-directional and altered flow of fault current leads to several specific problems with protection schemes.

2. Malfunctioning Protection: Blinding and False Trips

The changes in fault current can cause protection devices to misinterpret what is happening on the network, leading to two classic problems: blinding and false tripping.

The following reading provides an excellent overview of the potential problems DG can cause.

Now, let's dive into the two most significant issues from that list.

A. Blinding of Protection

Protection "blinding" occurs when a relay fails to see a fault that it should have detected.

Imagine a fault occurs far down a feeder. The DG unit, being closer to the fault, starts feeding a large current into it. From the perspective of the main substation relay, the current it needs to supply is now reduced because the DG is helping to "feed the fault". If this reduction is large enough, the current seen by the substation relay may fall below its trip setting. The relay is "blinded" to the fault, which remains on the system, posing a safety risk and potentially damaging equipment.

B. False Tripping (Sympathetic Tripping)

False tripping is the opposite problem: a protection device on a healthy part of the network operates incorrectly.

Imagine a fault on Feeder 1. A large DG unit on an adjacent, healthy Feeder 2 sees this fault (as a voltage dip) and begins exporting a large fault current back towards the substation to feed the fault on Feeder 1. The protection relay on the healthy Feeder 2 sees this large outward current and mistakenly concludes there is a fault on its own feeder. It then trips, unnecessarily cutting off power to all customers on the healthy Feeder 2.

To solve these issues, DNOs often need to upgrade protection to more advanced "directional" relays, which can determine the direction of the fault current, making them smart enough not to be fooled by DG.

3. Critical System Safety Impacts

Beyond confusing the logic of relays, DG can compromise fundamental safety mechanisms of the grid.

A. Unwanted Islanding

If a fault or maintenance action causes the main grid supply to a feeder to be disconnected, any DG on that feeder should also disconnect immediately. If it doesn't, it will continue to energise that section of the network, creating an island.

This is extremely dangerous for several reasons:

• Safety of Personnel: Utility workers may believe the line is de-energised and begin work on it, risking electrocution.
• Power Quality: The DG unit may not be able to maintain stable voltage and frequency within the island, potentially damaging customer equipment.
• Reconnection Damage: If the main grid supply is reconnected while the island is still running, the two systems will be out of synchronisation. This can cause massive current surges, destroying the generator and other network equipment.

For these reasons, all DG must have robust anti-islanding protection.

B. Interference with Auto-Reclosing

On overhead line networks, around 80% of faults are temporary (e.g., a lightning strike or a tree branch briefly touching the line). To improve reliability, networks use auto-reclosers—devices that automatically trip the line and then re-close it after a fraction of a second. This brief "dead time" is usually enough for the temporary fault arc to extinguish.

However, if a DG unit is present on the line, it can continue to feed current into the fault during the dead time. This prevents the arc from extinguishing, effectively turning a temporary, self-clearing fault into a permanent one that requires manual intervention.

Conclusion

In this lesson, we've seen that integrating Distributed Generation into a grid designed for one-way power flow has profound consequences for its protection systems and overall safety.

Here are the key takeaways:

• DG changes the grid from a single-source radial network to a multi-source network during faults, altering the magnitude and direction of fault currents.
• This can cause protection schemes to fail in two main ways: protection blinding (failing to detect a real fault) and false tripping (disconnecting a healthy part of the network).
• DG introduces significant safety risks, such as creating dangerous unwanted islands of power and preventing auto-reclosers from clearing temporary faults.
• These challenges mean that connecting DG often requires significant and costly upgrades to the protection system, moving from simple overcurrent relays to more intelligent, directional schemes.

In our last lesson, we saw how DG can cause voltage rise. In this lesson, we've seen how it impacts protection. There is one more major constraint: the physical capacity of the wires and transformers themselves. Just like a pipe can only carry so much water, a cable can only carry so much current before it overheats.

In our next lesson, we will explore this issue: explaining how distributed generation connections can exceed the thermal capacity of network assets like cables and transformers.