Reactive Power – Mysterious but Critical for Power System Operation

In this article, I will attempt to explain a key component of power system reliability called Reactive Power. Reactive Power is rarely discussed because it is so complicated, but as power systems evolve with higher levels of renewable energy located far away from load centres, maintaining sufficient levels of Reactive Power will become increasingly important, and having a basic understanding of Reactive Power, and Power Factor, and their importance in voltage control will be helpful. This is a complicated topic, so hopefully I can provide a basic understanding of what these concepts are for those who aren't familiar with them, and why they're important for power system operations.

The key things to know about Reactive Power are as follows:

• Reactive Power is a feature of alternating current power systems - it does not exist in direct current systems
• Reactive Power is necessary to regulate voltage on the power system
• Inverter-based resources (wind, solar, batteries) can provide Reactive Power
• Reactive Power does not "travel well", so it must be supplied near where it is needed on the power system
• Power Factor is a key generator parameter used by power system operators to manage voltage on the power system

First, the basics. Electric Power (P) is the product of voltage (V) and current (I):

P = V x I

Think of current as a flow of electrons moving along a wire and voltage as the force that moves the electrons. If we simply had a one-way flow of power (direct current) from its source (a generator of some type) to a load that was purely resistive (like an old school incandescent light bulb or your toaster) then all the power would be called Real Power and there would be no need for Reactive Power. What you pay for on your monthly electricity bill is Real Power delivered over a some period of time, which we call energy: Energy (measured in kilowatt-hours, kWh) is simply power (measured in kilowatts, kW) multiplied by time (measured in hours, h). Relatively simple.

However, because the power system uses alternating current (AC), and there are many devices connected to the grid that use or generate magnetic and electric fields, Reactive Power enters the scene and things get much more complicated.

With AC power, both voltage and current change direction, or alternate, many times each second. In North America, the frequency of that alternation on our power grids is 60 cycles per second, or 60 Hertz (Hz). When AC power moves through the grid and interacts with all the different devices connected to the grid, it must provide power for resistive loads (light bulbs, toasters, ovens), for the frictional losses involved with moving electrons along power lines, and also for loads that require and create magnetic and electric fields, such as transformers, motors, fans, pumps, LEDs, and the power lines themselves. In engineer speak, the delay, or phase shift between AC voltage and current gives rise to Reactive Power, but to keep it simple, Reactive Power arises due to the magnetic and electric fields associated with different types of devices on the AC power system.

There are three types of loads on the power system: resistive loads, inductive loads, and capacitive loads.

Resistive loads are things like an incandescent light bulb, a toaster, or an oven that just consume Real Power.

Inductive loads are things like transformers and motors that consume Reactive Power to create their magnetic fields.

Capacitive loads are things like transmission lines, buried cables, and capacitor banks that store and provide Reactive Power in their electric and magnetic fields.

Voltage plays a critical role in the transmission of AC electricity and in the operation of these inductive and capacitive devices, so the precise control of voltage is a key activity of power system operators. Because of this, the planning and control of Reactive Power is essential to grid voltage control and stability.

Another important characteristic of Reactive Power is that it does not "travel well" over long transmission lines due to high losses, so it must be supplied relatively close to the areas on the grid where it is most needed. It is a "local" grid service, in contrast to inertia which is a "global" grid service. This is an important consideration for power system planners and operators as the locations of renewable energy resources, usually far away from major load centres, will change the needs for Reactive Power on the system and a sufficient supply of Reactive Power must be provided as the power system evolves.

The main thing to understand is that AC power systems rely on Reactive Power to maintain stable voltage levels and operate properly.

This means that power generators have to produce both Real and Reactive Power in order to keep the grid operating properly. To do this, the power that generators produce is called Apparent Power, which is the total amount of power required to create both Real and Reactive Power. Therefore, there are three components of power called Real Power, Reactive Power, and Apparent Power. The "Power Triangle" defines the relationship between them:

Real Power is the basis for the energy you pay for on your electricity bill. It is measured in Watts or Kilowatts (kW). Your electronic devices use Real Power.

Reactive Power is necessary to regulate AC voltage and supply inductive and capacitive loads. It is measured in volt-amps reactive (VAR).

Apparent Power, measured in volt-amps (VA), is the total amount of power a generator must produce to provide both Real and Reactive power.?

Power Angle (Φ) is the angle between the Apparent Power and Real Power vectors.

Power Factor (PF) is the ratio of Real Power to Apparent Power. It is equivalent to the cosine of the angle between the Apparent Power and Real Power vectors. The PF of generators is extremely important in power system operations and must be precisely controlled to ensure voltage stability. It is also a measure of the efficiency of the power system.

If the Power Angle between Apparent Power and Real Power is zero, there will be no Reactive Power and the PF equals 1, which is called "unity". When a generator is operating at unity PF, Apparent Power is equivalent to Real Power with no Reactive Power, so all of the power produced by a generator is useful with no losses, and the Reactive Power in the power system is balanced between the devices that consume and provide it, so the power system is operating at a very stable and efficient state.

As the Power Angle increases or decreases, PF becomes less than 1, and a generator has to produce more Reactive Power, which decreases its efficiency and increases operating cost. In addition, losses associated with all the inductive and capacitive devices increase and power system efficiency and voltage stability decrease.

If the PF drops significantly below 1, then voltage becomes unstable, and this negatively affects the magnetic fields of transformers and motors which results in bad things like flicker, brownouts, and blackouts. For this reason, we always want the PF to be greater than around 0.9. ?

Due to its critical role in maintaining power system voltage stability, Power Factor is a key parameter used by power system operators to regulate the operation of generators on the grid.

So how does all this relate to renewable energy, or inverter-based resources (IBR)?? IBR use sophisticated power electronics to connect to the grid, and while this means they cannot supply inertia like conventional synchronous generators can, these power electronics can be designed to provide Reactive Power, which is helpful to the future grid. Currently, traditional synchronous generators (gas, steam, and hydro turbine generators) supply the majority of the grid's Reactive Power requirements, and as the resource mix of the power system changes over time to more renewables, the use of IBR to provide Reactive Power and the design of IBR controls to respond properly to power system disturbances will become increasingly important to the future reliability of the power system.?

FERC recognizes this in their Reliability Guideline for Bulk Power System Connected Inverter-Based Resource Performance as follows:

“The North American BPS and electric grids around the world are undergoing rapid changes in generation resource mix with increasing amounts of renewable generation such as wind and solar photovoltaic (PV) power plants.? These resources are asynchronously connected to the grid and are either completely or partially interfaced with the BPS through power electronics, hence referred to as inverter-based resources. The power electronics aspects of these generating resources present new opportunities in terms of grid control and response to abnormal grid conditions.”

”FERC found that, due to technological advancements, the cost of providing reactive power no longer creates an obstacle to wind power development...”

In Alberta, the Independent System Operator (the AESO) stipulates the Reactive Power operational requirements for all generators on the system (both renewable and conventional) in ISO Rule 503.3 Reactive Power as follows:

The legal owner of a generating unit, aggregated facility, or energy storage resource must ensure that reactive power capability complies with the following minimum requirements:

(a) 0.9 power factor, supplying dynamic reactive power; and

(b) 0.95 power factor, absorbing dynamic reactive power. ?

As you can see, the Power Factor should be kept above 0.9 at all times to ensure sufficient levels of Reactive Power for safe, reliable, and efficient power system operation.

Another important thing to remember about Reactive Power is that it does not travel well over long distances, as mentioned earlier. This means that the power system must be thoughtfully planned so that sufficient Reactive Power sources are placed at the appropriate locations on the grid to help regulate voltage. So, while IBR can be helpful from a Reactive Power perspective, this is another reason why careful planning and design of the power system is required as we transition to higher levels of renewable energy over time.

To summarize, here are the key takeaways for Reactive Power:

• Reactive Power is a feature of alternating current power systems - it does not exist in direct current systems
• Reactive Power is necessary to regulate voltage on the power system
• Inverter-based resources (wind, solar, batteries) can provide reactive power
• Reactive Power must be supplied near where it is needed on the power system, so it is an important consideration in the transition to net zero
• Power Factor is a key parameter used by power system operators to manage voltage on the power system

Reactive Power and Power Factor are very difficult concepts to understand because there are no clear analogies to the physical world, so hopefully my explanation was somewhat helpful and left you a little less confused ??.

And, if you are interested in a brief explanation of the importance of power system inertia with increasing IBRs, please see my article on the topic.

Optional (More Technical) Appendix - The Relationship Between Reactive Power and Voltage and Current Waveforms

To further explain Reactive Power, we must discuss the relationship between voltage (V), current (I) in an AC power system. The movement of electricity is governed by the physics of electrons moving through wires, interacting with electrical devices with electrical and magnetic fields, and encountering resistance along the way.? Remember that Power (P) is the product of voltage (V) and current (I):

P = V x I ?

Relatively simple, right?? In theory, yes. But, because the power system operates with alternating current (AC), things quickly get complicated, so I'll try to keep it as simple as possible.?

The key concept to understand with alternating current (AC) is that voltage and current follow sine wave patterns with a frequency of 60 cycles per second (60 Hz). In one cycle, a sine wave goes from zero to positive peak back to zero then to negative peak then back to zero. The voltage and current waveforms each repeat this 60 times each second - they "alternate" between positive and negative peaks at 60 Hz.?

If both voltage and current are “in phase” with each other, their wave forms would coincide so they would move through zero at the same times.? However, due to the various components within the power system with electric and magnetic fields that absorb or provide Reactive Power, the voltage and current waves are usually shifted out of phase with each other to some degree so that they move through zero at different times. This phase shift is where Reactive Power arises.

Think of Reactive Power as being like a spring that stores energy and connects voltage and current, pushing apart or pulling together the waveforms depending on conditions within the power system. In doing this, Reactive Power keeps voltage and current in balance, which helps maintain power system voltage stability so that transformers and motors operate efficiently and reliably.

The diagram below shows voltage and current waves out of phase with each other and the angle between them, Φ, is called the Power Angle. The Power Angle is the element that relates back to the Power Triangle we discussed earlier and ties everything together.

Recall that the Power Angle is also the angle between Apparent Power and Real Power in the Power Triangle discussed earlier.

If the Power Angle is zero, then Power Factor (PF) is 1 and voltage and current are "in phase". This means that Apparent Power equals Real Power and that the Reactive Power in the system is balanced between the devices that are using and supplying it so the system is at a very efficient and stable state.

If the PF is anything less than unity, then generators must produce Reactive Power. As the Power Angle increases, and the phase difference between voltage and current increases, the level of Reactive Power required also increases. If the Power Angle becomes too great, voltage becomes unstable, and this results in bad things like flicker, brownouts, and blackouts.

To keep the phase difference between voltage and current at safe levels for power system operation, power system operators require generators to operate within strict PF limits.

For those who want to dive deeper, here is a good overview of Reactive Power from FERC called Principles for Efficient and Reliable Reactive Power Supply and Consumption.