Have you ever wondered how your home’s lights, appliances, and gadgets all stay powered without sudden flickers or failures? It might be tempting to think it’s all about the electricity we actively use – the kind that heats our water or charges our phones.

Yet there’s another, often overlooked, player in the power grid: Reactive power. While it doesn’t directly light our bulbs or power our motors, it’s absolutely essential for maintaining voltage levels and keeping our entire electrical system humming along.

In fact, with the rise of renewable energy and decentralized power generation, managing reactive power has taken on new urgency. If we ignore it, we risk instability, grid congestion, or even blackouts. 

This article explores the concept of reactive power: why it exists, its interaction with different types of electrical loads, and its impact on power systems – especially in the context of grid congestion caused by distributed energy resources (DERs).

I. Reactive power explained

What is Reactive power?

Let’s start with a quick recap.

Electric power consists of two components: Active power and Reactive power.

• Active power (P) is measured in watts (W) and is responsible for doing actual work in a power system. It powers appliances, charges devices, and drives machinery. Essentially, active power converts electrical energy into forms like light, heat, or motion that we can use directly.
• Reactive power (Q), measured in volt-amperes reactive (VAr), does not directly perform useful work but is critical for the operation of equipment that relies on magnetic or electric fields.

The combination of active and reactive powerThe relationship between these components is expressed mathematically as: S = √(P² + Q²). is called Apparent power (S), the total power in the system.

To visualize this better, consider the well-known beer analogy:

• The beer itself represents Active power – the useful component that quenches your thirst and provides satisfaction.
• The foam represents Reactive power – necessary to deliver the beer smoothly but doesn’t directly contribute to quenching your thirst.
• The total liquid in the glass (beer + foam) represents Apparent power.

Why does Reactive power happen?

Reactive power is an inherent characteristic of AC power systems due to the oscillatory nature of voltage and current waveforms.

In an ideal system, these voltage and current waves would be in phase, meaning that the energy transfer is entirely active power. However, real-world electrical loads introduce a phase shift between voltage and current due to energy storage in magnetic (inductive) or electric (capacitive) fields.

One moment, some energy is “borrowed” from the system to build up and maintain an electromagnetic field; the next, that energy is handed back. This back-and-forth exchange in each cycle is what we define as Reactive power.

While its presence is unavoidable in AC systems, effective reactive power management ensures grid efficiency, voltage control, and stability.

Interaction of Reactive power with different load types

Electrical loads interact with Reactive power differently based on their properties:

1. Ohmic (Resistive) loads

Purely resistive loads, such as incandescent bulbs and electric heaters, consume only active power.

Since voltage and current remain perfectly in phase, no reactive power is involved in these devices.

2. Inductive loads

Inductive loads, such as motors and transformers, require magnetic fields to operate.For instance, motors use magnetic fields to generate torque and rotate shafts, while transformers use them to transfer energy between windings and step voltage up or down.

This causes them to “absorb” Reactive power, which means temporarily storing a part of the energy of the system to establish and maintain the magnetic fields, and then returning it back.

This causes the current to lag behind the voltage, increasing the grid’s reactive power demand.

3. Capacitive loads

Capacitors, on the other hand, store energy in electric fields.

Instead of “absorbing” reactive power, capacitive loads “inject” it, causing the current to lead the voltage. This means that capacitive effects can counteract the lagging effect of inductive loads, and help stabilize the system.

4. Harmonic loads

Power electronics such as inverters in solar and wind farms do not draw current in a smooth sinusoidal manner, but operate in short pulses, and distort the ideal AC current waveform.

This non-sinusoidal waveform can be broken down mathematically into a series of sinusoidal components, each operating at a frequency that is a multiple of the fundamental frequency (usually 50 Hz). These components are known as harmonics.

The fundamental frequency, or first harmonic, is 50 Hz; the third harmonic operates at 150 Hz, the fifth at 250 Hz, and so forth.

While the first harmonic delivers Active power, other harmonics demand extra current and heighten reactive power needs.

Voltage harmonics, on the other hand, do not originate directly from non-linear loads. They are generated when the current harmonics flow through system impedances, such as source and line impedances, causing harmonic voltage drops.

The consequences of Reactive power imbalances

How much does a Reactive power imbalance really matter?

Quite a lot.

• Deficit of Reactive power: If the grid doesn’t have enough reactive power support, voltage can plummet, potentially leading to a system collapse or blackoutThe 2003 Northeast US blackout, which affected 50 million people across eight US states and parts of Canada, vividly demonstrated the critical role of reactive power in maintaining grid stability.
  
  The event was triggered by a software bug in FirstEnergy’s alarm system which prevented operators from being aware of the need to redistribute load, but it was exacerbated by insufficient reactive power support.
  
  During peak loads, the power system experienced steep voltage declines as reactive demand exceeded supply. This inadequacy led to a cascading effect of voltage instability, causing progressive and uncontrollable voltage drops. The situation rapidly deteriorated, forcing the shutdown of at least 265 power plants and resulting in a widespread blackout.
  
  This incident underscored the importance of maintaining adequate reactive power reserves to prevent voltage collapse and ensure the resilience of power systems. . Generators also have to work harder to compensate, wasting energy and reducing their lifespan.
• Surplus of Reactive power: Too much Reactive power causes overvoltages that stress insulation, damage equipment, and again reduce overall transmission efficiency.

Whether surplus or deficit, the result is the same: less efficient power transfer.

However, when managed properly, reactive power becomes a valuable tool for addressing grid challenges – especially with the integration of renewable energy sources.

II. Energy transition and Reactive power management

Today’s electrical landscape is changing fast, with solar panels, wind turbines, and other distributed energy resources (DERs) popping up everywhere. This transformation is exciting – who doesn’t want cleaner, more localized energy?

But it also introduces new headaches for grid operators, particularly grid congestionTo better understand grid operator challenges, we recommend you to take a look at the following article: “Understanding grid balancing & congestion management”. , where the flow of electricity surpasses the safe physical limits of cables and transformers.

Reversal of power flows (or bidirectional power flows)

Historically, electricity flowed one-way: from centralized power plants through high-voltage substations to end consumers. Now, it’s a two-way street. On a sunny weekend, a neighborhood loaded with rooftop solar panels can send excess power back up the line, from the distribution grid to substations, flipping the usual direction of flow. Without proper control, these reversals can:

• Cause voltage rises at feeder ends, sometimes exceeding operational safety limits.
• Lead to overvoltage conditions, damaging transformers, cables, and electrical appliances.
• Force automatic disconnection or curtailment of renewable generators to prevent damage.

Implementing Reactive power absorption strategies can effectively help managing voltage and enhancing grid stability. These strategies include:

• Smart inverters that absorb excess reactive power at feeder ends.
• Static VAR Compensators (SVCs) & Static Synchronous Compensators (STATCOMs)SVCs & STATCOMs are power electronics-based devices used for real-time voltage regulation and reactive power compensation. SVCs use thyristor-controlled components to adjust reactive power, while STATCOMs employ voltage source converters for faster and more precise voltage control. to dynamically regulate voltage.
• Capacitor banks to balanceCapacitor banks are typically used for reactive power compensation (“injecting” reactive power) rather than absorption. reactive power in weak areas of the grid.

Localized overloads

During peak generation periods, such as sunny afternoons for PV systems or windy conditions for wind turbines, large amounts of power are injected into specific, local areas of the grid. Transformers and distribution lines can become overloaded, leading to:

• Excessive heating, reducing transformer lifespan and increasing failure risks.
• Frequent voltage fluctuations, affecting system stability and connected devices.
• Higher transmission losses, as excessive reactive power occupies capacity that could be used for active power delivery.

To mitigate these effects, operators can in addition to the previously cited strategies:

• Implement dynamic voltage control through reactive power compensation at critical nodes.
• Deploy demand-side management strategies.

These measures allow for better balancing of Active and Reactive power, reducing stress on grid infrastructure while maximizing renewable utilization.

Harmonics

As previously mentioned, power electronics such as inverters used in DERs, introduce harmonicsHarmonics introduced by DER inverters can increase total harmonic distortion (THD), which negatively affects power quality. According to IEEE 519-2014 and EN50160 standards, acceptable voltage harmonic levels should remain below 8% THD for low-voltage systems. For medium and high-voltage, the IEEE 519-2014 set even stricter standards. into the grid which compound congestion challenges:

• Harmonic currents increase reactive power demand, reducing available system capacity.
• Distorted voltage waveforms degrade power quality, potentially damaging sensitive equipment.
• Higher resistive (I²R) losses occur, leading to additional heat generation and inefficiencies.

These harmonics force grid components to handle additional reactive power, increasing total current flow through transmission lines and transformers. This occupies capacity that could otherwise be used for active power transfer, leading to further congestion. To address these issues, the following methods can be applied:

• Installing active power filters (APFs) and passive harmonic filters to absorb and neutralize harmonic currents.
• Utilizing phase-shifting transformers, which reduce harmonic resonance effects.
• Optimizing reactive power dispatch to compensate for harmonic-induced reactive power demand.

III. Managing Reactive power to relieve congestion

Reactive power management isn’t just an afterthought, it can make or break the stability of a modern power grid. These strategies help regulate voltage, improve efficiency, and welcome more renewable energy onto the system without constantly resorting to curtailment.

Yet there’s a central question here: How do you manage what you’re not measuring?

Monitoring: the first step toward optimized control

Continuous measurement and monitoring of reactive power are critical for ensuring the stability, efficiency, and cost-effectiveness of electrical systems. A reliable monitoring platform, such as the Withthegrid’s Asset Monitoring Platform (AMP), provides the (near) real-time data you need to see the full picture. This actionable insight paves the way for better decision-making, proactive maintenance, and more efficient use of your grid’s capacity.

Here’s how monitoring with the AMP can play a decisive role in reactive power management:

Accurate measurement

Reactive power is dynamic and can fluctuate significantly over time. Without real-time, precise tracking, operators can miss critical fluctuations that lead to inefficiencies and higher costs. The AMP uses advanced sensors and analytics to detect sudden changes and guide power factor correction strategies that keep voltage stable and losses in check.

Condition-based maintenance

Over time, wear and environmental factors can alter the reactive power profile of transformers, capacitors, and other components. The AMP automates the detection of these changes, like drifting power factors, so you can service equipment before it fails. No  manual analysis required.

Cost optimization

Imbalances in reactive power can inflate energy bills or trigger penalties. By fine-tuning power factor correction, operators cut unnecessary charges and align with regulatory standards (EN50160). The result? A leaner, more cost-effective operation.

Continuous grid capacity insights

Want to know when your transformer is nearing its limit? The AMP delivers real-time capacity updates, making it easier to schedule expansions, support new loads, or integrate more renewables.

Conclusion

Although it’s often overshadowed by its more visible counterpart – Active power –, Reactive power is crucial for voltage stability and efficient energy transfer. In an era of rapid renewable energy adoption, managing reactive power is even more urgent to prevent congestion and maintain reliable service.

Advanced monitoring solutions like the AMP offer a window into the world of reactive power, allowing you to monitor it, optimize compensation strategies and keep your grid running smoothly.

For more information on how Reactive power management can optimize your operations, visit the Withthegrid Asset Monitoring Platform page or get in touch with us directly.