Trystan Casey
AC electrical loads are referred to as either linear or non-linear loads depending on how they draw current from the mains power supply. Linear loads are loads that draw a current that directly follows the voltage waveform the source provides. In other words, the current waveform is a re-scaling of the voltage waveform. The voltage and current sine waves look the same. With an increase in the applied voltage, the current flowing through the circuit increases, and vice versa. Linear loads are modelled as conventional PQ and PV buses at the fundamental frequency. Non-linear loads, on the other hand, draw currents in abrupt short pulses, which distort the current waveforms and generate harmonics that can lead to power problems.
| Characteristics | Values |
|---|---|
| Current | Proportional to voltage |
| Waveform | Looks like voltage |
| Current and voltage | Increase and decrease in sync |
| Examples | Incandescent light bulb, electric baseboard heater, gas discharge lighting, thyristor (SCR) controlled loads |
| Harmonics | Not generated |
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What You'll Learn
Linear vs Non-Linear Loads
AC electrical loads are referred to as either linear or non-linear depending on how they draw current from the mains power supply waveform.
Linear Loads
With a linear load, the relationship between the voltage and current waveforms is sinusoidal, and the current at any time is proportional to the voltage (Ohm's law). When the applied voltage is increased, the current flowing through the circuit also increases, and the same applies in the reverse case. The current waveform is a re-scaling of the voltage waveform. Examples of linear loads include incandescent light bulbs, electric baseboard heaters, transformers, motors, and capacitors.
Non-Linear Loads
Non-linear loads, on the other hand, draw current in a non-sinusoidal manner, and the current is not proportional to the voltage. Instead, it fluctuates based on the alternating load impedance or resistance. Non-linear loads draw currents in abrupt short pulses, which distort the current waveforms and generate harmonics. These harmonics can lead to power problems, such as distortion of the mains supply voltage, equipment overheating, and tripping of circuit breakers. Common examples of non-linear loads include rectifiers, variable-speed drives, computers, printers, TVs, servers, and telecom systems that use SMPS power conversion technologies.
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Voltage and Current Relationship
The term "linear" in electrical engineering refers to the relationship between voltage and current in a circuit. In a linear load, the current drawn from the power source is directly proportional to the applied voltage, resulting in a smooth and synchronous waveform. This relationship is described by Ohm's law, represented by the equation V=IR, where V is voltage, I is current, and R is resistance. If the value of R remains constant over time, the load is considered linear.
In a linear load, the voltage and current waveforms are typically sinusoidal. As the voltage waveform rises and falls, the current waveform follows suit, creating a synchronous and proportional relationship. This means that at any given time, the current flowing through the circuit is directly related to the applied voltage. For example, if the voltage in an AC circuit oscillates above and below zero, the current used in the circuit will rise and fall in sync with those changes, maintaining a linear relationship.
Linear loads are characterised by a smooth and steady current flow. When the voltage increases, the current increases, and when the voltage decreases, the current decreases accordingly. This is in contrast to non-linear loads, where the current does not follow the voltage waveform and can fluctuate or create abrupt pulses. In non-linear loads, the current may be influenced by factors such as alternating load impedance or resistance, resulting in a distorted waveform.
Examples of linear loads include incandescent light bulbs, electric heaters, transformers, motors, and capacitors. In these devices, the current drawn is directly proportional to the voltage applied. For instance, in a simple resistive load bank, there is no inductance or capacitance, resulting in a steady current draw that immediately rises to a steady state.
It's important to note that the behaviour of linear and non-linear loads can have significant implications for power distribution systems. Non-linear loads can generate harmonics, which can lead to power problems, equipment failures, and distribution system issues. On the other hand, linear loads provide a more stable and predictable current flow, making it easier to manage and design electrical systems.
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Waveform and Harmonics
In a normal alternating current power system, the current varies sinusoidally at a specific frequency, usually 50 or 60 hertz. When a linear time-invariant electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage, though not always in phase with the voltage.
Harmonics are higher frequency waveforms superimposed onto the fundamental frequency, which is the frequency of the circuit. Harmonics are sufficient to distort the fundamental wave's shape. The amount of distortion applied to the fundamental wave depends on the type, quantity, and shape of the harmonics present. Harmonics are integer multiples of the fundamental frequency, and they can sometimes propagate outwards from nonlinear loads, causing problems elsewhere on the power system.
Non-linear loads create distortion in the pure sinusoidal voltage waveform supplied by the utility, and this may result in resonance. Non-linear loads draw current in abrupt pulses rather than a smooth sinusoid, indicating a distorted response. Examples of non-linear loads include rectifiers, computers, printers, battery chargers, variable-speed drives, and electronic ballasts.
The presence of harmonics in electrical systems means that current and voltage are distorted and deviate from sinusoidal waveforms. Harmonic currents are caused by non-linear loads connected to the distribution system. The flow of harmonic currents through system impedances creates voltage harmonics, which further distort the supply voltage.
The classic example of a non-linear load is a rectifier with a capacitor input filter, where the rectifier diode only allows current to pass to the load when the applied voltage exceeds the voltage stored in the capacitor. Non-sinusoidal complex waveforms are constructed by "adding" together a series of sine wave frequencies known as "Harmonics".
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Power Problems
Nonlinear electrical loads can cause power problems. They draw currents in brief and rapid pulses, which distort the current waveforms and generate harmonics. These harmonics can lead to issues such as distortion of the mains supply voltage, equipment overheating, nuisance tripping of circuit breakers, and misfiring of variable-speed drives. These power problems can affect distribution system equipment and any loads connected to them.
The power factor also plays a role in power quality. An inappropriate power factor can lead to increased power outages and costs.
Nonlinear loads can create current and voltage distortion as the current is pulled through the system. This distortion can impact distribution system equipment and any connected loads.
Additionally, nonlinear loads can generate harmonics on the neutral wire, which accumulate rather than cancel out. These harmonics can cause issues with three-phase systems, where they line up and add together, potentially exceeding the wire's ampacity.
Understanding the difference between linear and nonlinear loads is essential for managing power quality and preventing power problems.
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Load Distribution
The demand for electricity, in the form of appliances, lighting devices, and equipment, creates an electrical load—the burden on a T&D system to deliver power to the consumer. Load distribution is an essential aspect of meeting customer needs and ensuring grid stability.
Linear Loads
In a linear load, the current is proportional to the voltage. When the applied voltage increases, the current flowing through the circuit also increases, and vice versa. The waveform of the current matches that of the voltage. An example of a linear load is an incandescent light bulb or an electric baseboard heater, where the current used to generate light or heat rises and falls in sync with the voltage change.
Non-Linear Loads
In a non-linear load, the current is not proportional to the voltage and fluctuates based on the alternating load impedance or resistance. The current wave is out-of-sync with the voltage wave, resulting in abrupt short pulses that distort the current waveforms. These distortions generate harmonics, which can lead to power problems affecting distribution system equipment and connected loads. Examples of non-linear loads include electronic devices such as computers, printers, TVs, and servers.
Load Management
Load management strategies, such as peak shaving and demand-side management (DSM), aim to reduce electricity consumption during peak demand periods. Dynamic load management optimizes energy usage by monitoring the grid connection load and adjusting power distribution to multiple charge points, ensuring capacity limits are not exceeded.
Sector Coupling
Integrating electricity with heating and mobility, as seen with heat pumps and electric vehicles (EVs), is a trend known as sector coupling. This approach enhances efficiency and sustainability. By controlling the usage of flexible assets like heat pumps and EVs, their consumption can be shifted to minimize costs and relieve the grid during peak demand. Additionally, these assets can store and feed surplus energy back into the grid.
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Frequently asked questions
A linear load is when the current and voltage are proportional to each other. In other words, the current and voltage waveforms look the same. When the voltage increases, so does the current, and vice versa.
A non-linear load is when the current and voltage are not proportional to each other. The current waveform is different from the voltage waveform. Non-linear loads draw currents in short, rapid pulses, which can cause harmonics and power problems.
An incandescent light bulb or an electric baseboard heater is an example of a linear load. Examples of non-linear loads include rectifiers, variable-speed drives, and electronic devices such as computers and printers.
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