Rectifier · Glossary

A rectifier is an electrical circuit or device that converts alternating current into direct current. Alternating current periodically changes direction and magnitude, with a nominal frequency of 50 hertz in the European interconnected grid. Direct current, by contrast, always flows with the same polarity. The rectifier ensures that the current at the output flows in one direction only. Strictly speaking, rectification alone yields only a pulsating DC voltage. A smooth DC voltage or a regulated DC current is obtained only with downstream smoothing and control stages, which in practice are often housed together with the rectifier in a single device.

The simplest form is the diode rectifier. Diodes essentially allow current to pass in one direction only. A single diode (half-wave rectifier) lets only every second half-wave through. A bridge circuit of four diodes, the Graetz bridge, flips the negative half-wave upward and thus uses both half-waves. In three-phase systems, the six-pulse bridge is common: six valves alternately connect the three line conductors to the two DC terminals, and the output voltage has six pulses per mains cycle instead of two. Capacitors, chokes or actively controlled power semiconductors smooth this waveform, limit current peaks and hold the output voltage or output current at the desired value. Modern rectifiers are therefore often power electronic circuits with thyristors, IGBTs, MOSFETs or silicon carbide semiconductors rather than merely passive diodes.

At the output of a rectifier one usually measures a DC voltage in volts and a DC current in amperes. The transmitted power is the product of the two. With lossless conversion, the active power at the input would equal the power at the output. Real rectifiers, however, have losses in semiconductors, magnetic components, cables and filters. These losses become heat and help determine how large the cooling, enclosure and protection equipment must be. Efficiency is therefore no minor matter: it affects operating costs, size and reliability.

Distinction from inverter and converter

The rectifier is often confused with the inverter. Both belong to power electronics but operate in opposite directions. The rectifier turns alternating current into direct current. The inverter turns direct current into alternating current, for example in photovoltaic systems, battery storage or in frequency converters for motors. Many modern circuits can do both: an active bridge works as a rectifier or as an inverter depending on the direction of power flow.

The umbrella term converter (in German Umrichter or Stromrichter) refers to devices that convert electrical energy from one form into another. These include rectifiers, inverters and DC-DC converters. A frequency converter for an electric motor typically consists of a rectifier, a DC link and an inverter. Mains alternating current is first turned into direct current, and from this the device generates an alternating current with the frequency and voltage suited to the motor. Calling the entire frequency converter a rectifier describes only part of it.

A charger is also not the same as a rectifier. It almost always contains rectification, but it takes on further tasks: it regulates charging power, voltage and current, communicates with the battery management system, monitors temperature limits and performs protective functions. In an electric car, an AC charging point supplies mains alternating current, and rectification and voltage adaptation are handled by the charger in the vehicle (on-board charger). With DC fast charging, by contrast, the large rectifier and power electronics are located in the charging station outside the vehicle.

Why rectifiers are becoming more important in the power system

The public power grid has historically grown as an alternating current system. Alternating current can easily be brought to higher or lower voltage levels with transformers. For transmission and distribution this was long the decisive advantage. However, many new consumers, generators and storage units work internally with direct current or need a DC link. Batteries store energy electrochemically and are charged and discharged with direct current. Photovoltaic modules supply direct current, and electrolysers for hydrogen require direct current. Data centres, LED lighting and most electronic devices likewise operate internally on DC voltages.

As a result, there are more and more interfaces between the AC grid and DC applications. At these points sit rectifiers, inverters or bidirectional converters. They are more than mere adapters. They determine how cleanly, quickly and grid-compatibly energy flows between the two worlds. Their control influences power consumption, reactive power demand and voltage quality, and in the event of a fault also protection behaviour.

In large industrial plants, the rectifier can strongly shape the grid load. An electrolyser with several hundred megawatts of connected capacity is not just a consumer for the grid but a large load block coupled via power electronics. Its rectifier technology helps decide how evenly it draws power, which harmonics and what reactive power demand arise, and how flexibly the plant can respond to electricity prices, grid signals or congestion.

Grid feedback effects and harmonics

A common misconception: a rectifier draws a clean sinusoidal current at the input and delivers a smooth DC power at the output. For simple diode or thyristor rectifiers this is true only to a limited extent. With a large smoothing capacitor, current flows from the grid only when the mains voltage is above the capacitor voltage, that is, in short, high pulses around the voltage peak. This current shape deviates markedly from a sine wave and contains harmonics.

Harmonics are current or voltage components at integer multiples of the fundamental frequency. In a 50-hertz grid these are, for example, 150 hertz (3rd harmonic), 250 hertz (5th) or 350 hertz (7th). Which orders dominate depends on the circuit. Single-phase rectifiers with a capacitor mainly produce the 3rd, 5th and 7th harmonics. An ideal six-pulse three-phase bridge typically produces orders 5, 7, 11, 13 and so on. A twelve-pulse circuit of two phase-shifted bridges largely suppresses the 5th and 7th harmonics. Harmonics place additional stress on transformers, cables and capacitors, can disturb protective devices and degrade voltage quality. Larger rectifier installations therefore receive filters, line reactors, multi-pulse circuits or actively controlled input bridges.

For devices on the public low-voltage grid, European EMC standards set limit values. EN IEC 61000-3-2 (in Germany as DIN EN IEC 61000-3-2, VDE 0838-2) limits the harmonic currents of devices with an input current of up to and including 16 A per conductor. EN 61000-3-12 (VDE 0838-12) applies to devices with more than 16 A and at most 75 A per conductor. For larger installations connected to medium or high voltage, the grid operator checks the permissible feedback effects as part of the connection assessment. In Germany this is done on the basis of the technical connection rules in force in each case and the conditions at the point of connection. The specific limit values therefore depend on voltage level, grid strength and installation class.

Thyristor rectifiers have a further peculiarity: they shift the onset of current relative to the mains voltage in order to control the output voltage. The more they do so, the more reactive power they draw from the grid. Modern active rectifiers with turn-off semiconductors (active front end, PFC stages), by contrast, can make the input current nearly sinusoidal and in phase with the voltage. They thereby achieve a power factor close to one. If design and control provide for it, they can also deliberately supply or absorb reactive power. A simple conversion circuit thus becomes a controllable interface between plant and grid. Better grid compatibility generally costs more investment and often brings somewhat more switching losses. In return, it can reduce filtering effort, grid reinforcements or operating restrictions.

Controllability, power and flexibility

Rectifiers differ greatly in how well they can be controlled. An uncontrolled diode rectifier simply follows the input voltage and the connected load. A thyristor rectifier uses the firing angle to control when current flows in each half-wave, thereby changing the mean output voltage. Active rectifiers with fast-switching semiconductors control current, voltage and power consumption considerably more finely and quickly.

This controllability is becoming more important in the power system because many new loads do not have to run constantly at constant power. Charging parks, battery storage, electrolysers or industrial processes can shift or adjust their power consumption within certain limits. The rectifier is the component that makes such adjustment technically possible in the first place. Without suitable power electronics, a theoretically flexible plant remains sluggish or grid-incompatible in practice.

A controllable rectifier alone, however, does not create flexibility. Process limits, storage capability, grid connection, market rules and contracts also matter. An electrolyser can change its power only as far as the plant, hydrogen offtake, auxiliary systems and operating strategy allow. A fast-charging station can throttle its rectifier power only if user wishes, battery state and billing go along with it. Technical controllability is thus a prerequisite but does not yet describe usable flexibility.

Rectifiers in HVDC and DC grids

Rectifiers play a special role in high-voltage direct current transmission, HVDC for short. In this, alternating current is converted into direct current at one end, transmitted via overhead line or cable, and turned back into alternating current at the other end. Every HVDC system has at least one station that feeds active power into the DC circuit as a rectifier and one that draws active power as an inverter. Which station has which role depends on the direction of power flow. The converters are thus usually not fixed to a single function.

There are two basic technologies. Line-commutated converters (LCC) work with thyristor valves that can be switched on but not actively switched off. They need a connected AC grid and draw a lot of reactive power. Because the current direction always remains the same in them, the polarity of the DC voltage must be reversed to reverse the power flow. Thyristor HVDC is today built for up to 6,000 MW and ±800 kV. Self-commutated converters (VSC, voltage source converters) work with turn-off semiconductors such as IGBTs. They can supply their reactive power themselves and reverse the power flow via the current direction without changing the voltage polarity.

HVDC is used when large amounts of power are to be transmitted over long distances, through submarine cables or between AC grids that are not synchronously coupled. The rectifier is then part of a converter station with transformers, valve halls, filters, cooling, protection equipment and complex control. LCC stations in particular need large filter and compensation installations against harmonics and for reactive power demand. The converter station can control power flows very precisely but behaves differently in the grid than a classic AC line.

Local DC grids are also being discussed more often, for example in data centres, industrial sites, buildings with photovoltaics and battery storage, or in charging infrastructure. The question there is how often energy must be converted between AC and DC. Every conversion stage costs losses and money. Rectifiers therefore belong to a broader debate about electrical architecture: which voltage level makes sense, where is control exercised, where is energy stored, and which interface remains connected to the public grid?

Typical oversimplifications

The statement that a rectifier "turns alternating current into direct current" is correct but glosses over important differences. First, it says nothing about the quality of the direct current. A pulsating DC voltage, a smoothed DC voltage and a precisely regulated DC current are technically different results. A battery, an electrolyser and a converter DC link each place different requirements.

Second, it says nothing about behaviour at the grid connection point. Two rectifiers with the same output power can produce quite different current shapes, reactive power shares and harmonics. For grid operators, therefore, not only the rated power counts but also the connection characteristics, the protection equipment and the control.

Third, the rectifier is sometimes treated as a passive minor component. For small power supplies this seems plausible in everyday life. For large electrified loads, however, rectifier technology is an essential part of plant planning. It influences connection costs, efficiency, cooling requirements, electromagnetic compatibility, availability and the ability to participate in markets or to operate in a grid-supportive way.

Rectifier is thus not the name of a uniform class of devices with always the same behaviour. It denotes a conversion function that, depending on the design, has very different technical and economic consequences. In the power system it marks the point where the AC-dominated grid meets DC-based applications. Anyone who describes this point precisely can better distinguish: mere energy conversion, controllable power electronics, grid compatibility and actual operational flexibility.

Sources

DIN EN IEC 61000-3-2 (VDE 0838-2):2023-10 – Grenzwerte für Oberschwingungsströme (Geräte-Eingangsstrom ≤ 16 A je Leiter) — DKE Deutsche Kommission Elektrotechnik Elektronik Informationstechnik in DIN und VDE; 2023-10. Normenseite, Kurzdarstellung
DIN EN 61000-3-12:2025-09 – Grenzwerte für Oberschwingungsströme von Geräten mit Eingangsstrom > 16 A und ≤ 75 A je Leiter — DIN Media; 2025-09. Produktseite, Overview
Control and Protection of LCC and VSC HVDC Converter Stations — Springer; unbekannt. Abstract
Überstromschutz der Halbleiterventile und Leitungsschutz in HGÜ-Anlagen — VDE; unbekannt. Einleitung
Selbstgeführte Hochspannungsgleichstromübertragung — EnArgus (Projektträger Jülich, BMWK-Förderung); unbekannt. Glossareintrag

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Edited by eCMS on 12.11.2026 05:05