Passive Reactive Power Compensation: Capacitors and Reactors

1. The "Confusing" Part of the Grid: What is Reactive Power?

In a simple DC circuit, power flows in one direction. In an AC grid, it's more complex. The voltage and current are constantly alternating, and certain types of electrical equipment can cause them to fall out of sync with each other. This "out-of-sync" energy is known as reactive power.

To get an intuitive feel for this, let's watch a video that uses an oscilloscope to show what's happening on the wires.

As the video explained, we can break down the total power flowing in the grid into two types:

• Real Power (P): This is the "working" power that lights our homes, spins motors, and heats our water. It is measured in watts (W).
• Reactive Power (Q): This is the power that sloshes back and forth, required to create the magnetic and electric fields needed by devices like motors and transformers. It doesn't do any useful work itself, but it is a necessary byproduct. It is measured in volt-amps-reactive (VAR).
• Apparent Power (S): This is the vector sum of real and reactive power, representing the total power the grid infrastructure must be able to handle. It is measured in volt-amps (VA).

The ratio of real power to apparent power is called the power factor. A power factor of 1 (or 100%) is ideal, meaning all power is real power. A low power factor means there is a lot of reactive power in the system, which is inefficient. It's like paying to ship a large, half-empty box—the shipping company has to handle the full size of the box, even though much of it is empty space. Similarly, the grid's wires, transformers, and generators must be sized to handle the apparent power (the whole box), even though only the real power (the contents) is being used.

2. The Two Problems and Their Passive Solutions

Reactive power issues generally come in two opposing forms. The grid needs to be able to solve both.

Problem 1: Too Much Inductive Load (Lagging Power Factor)

Most of the equipment connected to the grid—motors in factories, air conditioners, and even the transformers in the grid itself—are inductive. Inductive loads "consume" reactive power to create their magnetic fields. This causes the current to lag behind the voltage, resulting in a low, lagging power factor.

The Solution: Shunt Capacitors

To counteract this, we can connect large banks of capacitors to the grid. Capacitors do the opposite of inductors: they "supply" reactive power. By adding capacitors in parallel (or "in shunt") with the inductive load, we can provide the needed reactive power locally, rather than having it slosh all the way from the power station.

Let's read a more detailed explanation of how these work.

In summary, by installing shunt capacitors, grid operators can:

• Improve the power factor, bringing it closer to 1.
• Reduce the total current flowing through transmission lines and transformers.
• Reduce power losses (since losses are proportional to the current squared).
• Support the voltage on the grid, preventing it from sagging under heavy load.

Problem 2: Too Much Capacitive Load (Leading Power Factor)

While most loads are inductive, the transmission lines themselves can create the opposite problem. Long, high-voltage overhead lines and underground cables act like giant capacitors. During periods of low demand (like overnight), there aren't enough inductive loads to absorb the reactive power generated by the lines' own capacitance.

This causes the current to lead the voltage, resulting in a leading power factor and, more critically, a significant voltage rise at the end of the line. This phenomenon is known as the Ferranti Effect.

The Solution: Shunt Reactors

To solve this, we need a device that absorbs reactive power. The solution is a shunt reactor, which is essentially a large inductor connected in parallel with the line. It counteracts the line's capacitance, absorbing the excess reactive power and preventing the voltage from rising to dangerous levels.

The following resource provides a concise explanation of a reactor's job.

So, a shunt reactor's main purpose is to absorb reactive power to control over-voltages on long transmission lines during periods of light load.

3. Switched Operation

Capacitors and reactors are called "passive" devices because they don't contain sophisticated power electronics to control their output. They are simply large capacitors and inductors. However, their effect is not always needed.

• A capacitor bank that helps support voltage during peak demand in the afternoon would cause over-voltage if left on overnight when demand is low.
• A shunt reactor needed to control voltage overnight would cause the voltage to drop too much if left on during peak demand.

For this reason, both shunt capacitors and reactors are often installed with large switches (circuit breakers, which we learned about last lesson) that allow the grid operator to connect or disconnect them as system conditions change.

The following video clip explains this switching principle for capacitor banks.

Summary Table

Here is a table to summarize the key differences between these two important devices:

Conclusion

In this lesson, we've explored the essential but often misunderstood concept of reactive power and the passive devices used to manage it.

Key Takeaways:

• Reactive power is required to energize the magnetic and electric fields in grid components. While necessary, it does no useful work and leads to higher currents and energy losses if unmanaged.
• Shunt capacitors are used to supply reactive power. They are primarily used to counteract the effect of inductive loads (like motors), which improves a lagging power factor and helps support voltage levels during high demand.
• Shunt reactors are used to absorb reactive power. They are primarily used to counteract the natural capacitance of long transmission lines, which prevents dangerous over-voltages during periods of low demand.
• Both are considered passive devices but are often switched in and out of the circuit by grid operators to match changing grid conditions.

Preview of the Next Lesson

We've seen how these passive, switched devices work. However, the modern grid requires faster and more precise control. In the next lesson, we will explore active reactive power compensation devices, such as STATCOMs, which use advanced power electronics to provide dynamic and instantaneous voltage support.