Non-linear equipment distorts the typical sine curve of the current and applies different frequencies to it, so that the current becomes 'dirty'. The resulting grid perturbations can also disrupt the operation of neighboring consumers in various ways - ranging from limited functionality or shortened service life of the devices to increased energy consumption of drives to grid failure or fire hazards in the power grid.

Top: Strongly distorted current curve (based on the typical current curve of an energy-saving lamp). Center: Decomposition of the signal into the harmonic components (up to the 7th harmonic). Bottom: Harmonic spectrum of the signal.

© Bender

Conventional consumers can also cause quality problems in the power supply grid - however, the most common problems are caused by harmonic components in the operating current of electronic devices. In addition to harmonics, there are other damage phenomena:

• If devices are used that require a particularly high voltage for a short time (for example industrial welding systems, arc furnaces or drives with intermittent loads), this can lead to light fluctuations, known as flicker. These are primarily just a nuisance, but can lead to faster fatigue or dizziness in humans.
• Certain industrial machines and systems often exhibit strong load fluctuations. If these are operated on networks with low short-circuit power, there is a risk of voltage fluctuations. Even if these only last a few milliseconds and are therefore barely perceptible to humans, they represent a load for electrical consumers.
• Starting large drive motors in industry requires high starting currents - especially if they have to start under high load. The effective voltage value can briefly drop by up to 90 % of the nominal voltage.
• Lightning strikes or short circuits are frequent causes of damage in the form of transients. They usually only last a few milliseconds and cause overvoltages with voltage peaks of several kV.
• The uneven distribution of single-phase loads and the operation of two-phase loads can result in an asymmetrical load on the transformers. The active load of the loads is responsible for uneven phase voltages.

Harmonics are sinusoidal components that are superimposed on the fundamental oscillation of the voltage or current. They are caused by non-linear loads in the grid. The ratio of harmonic frequency to mains frequency is characterized by the ordinal number h. High harmonic components in the operating current can cause overloading of cable and line systems: Typically, third-order harmonics occur more frequently in many electronic devices. These (and all other 3n harmonics, i.e. multiples of 3) add up in the neutral conductor. In extreme cases, the resulting current in the neutral conductor can be greater than the individual phase conductor currents. This results in an unnoticed overload of the neutral conductor and a fire hazard.

The standards situation

Power Quality and Energy Measurement (PEM) and Residual Current Monitoring (RCM) in comparison.

© Image: Computer&AUTOMATION, Source: Bender

These phenomena and their effects are already dealt with in standards. For example, DIN VDE 0100-430 (VDE 0100-430):2010-10 requires overload detection for the neutral conductor if an excessive load due to harmonics is to be expected. If the recommendations of the cited standard are implemented, destruction of the neutral conductor can be prevented by timely tripping of the protective device. In practice, however, this only shifts the problem to another area: the fire hazard and the risk of neutral conductor destruction are eliminated by switching off. Although the operation of the electrical system is safer, availability is reduced due to frequent downtimes.

Monitoring with 'Power Quality and Energy Measurement' (PEM) from Bender can help here. By permanently monitoring the power quality, potentially dangerous situations are detected and reported at an early stage, allowing sufficient time to take remedial action without having to switch off equipment or the entire system. In the example of the monitored neutral conductor, in the event of an overload, less relevant consumers can be switched off via switching signals before the entire supply is interrupted by the triggering of the protective device.

Prevent the triggering of protective devices

Residual current monitoring allows deterioration in the insulation level of a system or certain system parts to be detected before a high residual current triggers protective devices. This time saving makes countermeasures possible. Similarly, 'Power Quality Monitoring' from Bender can be used to detect harmonics, for example, to detect a risk to the system at an early stage before the protective device trips.

Power Quality Monitoring provides the basis for the energy management required by DIN EN ISO 50001: the same measuring point that checks the power quality is also an energy meter. This means that a second important function can be performed with one device: mapping energy consumption per cost center. This makes it possible to avoid high general electricity costs. At the same time, the effect of the measures introduced to improve power quality or reduce costs through power quality monitoring can be viewed from the other perspective: How much energy do implemented measures against grid perturbations 'swallow'? What impact do energy-saving measures have on the quality of the electricity grid?

Scaled to fit

Granularity is essential in a complex system. Bender therefore offers devices for different tasks; for energy management, a cost center-specific view is essential, while for 'power quality monitoring', individual faults can only be localized precisely if the monitored network area can be clearly delimited.

The digital universal measuring devices from Bender 'Linetraxx PEM735', 'PEM575' and 'PEM353' record and display electrical variables in an electricity supply network.

© Bender

A scenario could look something like this: A 'PEM735' monitors the voltage quality of the supply at the infeed in accordance with DIN EN 50160. Transients, flicker and harmonic components are recorded and high-resolution curves are recorded. Outgoing circuits in the low-voltage main distribution board (LVMD) are monitored by 'PEM575' network analyzers. The sampling rate is 12.8 kHz, which means that any events that occur can be easily recognized in the recorded curves. Energy meters can be replaced by 'PEM353' universal meters. In this way, the basis for an energy management system in accordance with DIN EN ISO 50001 can be formed and high general electricity costs can be avoided. In addition, a PEM353 provides information on the total harmonic content of the monitored system component.

A monitoring system set up in this way enables quick and uncomplicated analysis and localization if power quality phenomena occur and the availability of the system is at risk. All measurement data is collected, filtered and evaluated at a central location. Bender offers the 'Condition Monitor CP700', for example, to display this amount of information in a target group-oriented manner.

This article is based on documents from Bender, Grünberg.