Power Quality: The quality of electrical energy (Grid Code EN50160)

In order for equipment to work properly, responsibilities lie with three parties:

Boundary conditions for voltage in public networks

The minimum quality of the voltage in the Netherlands is laid down in the Grid Code. This is derived from the EN50160. The European standard EN50160 states, for example, that the voltage may not deviate more than 10% from the nominal value during a one-week period of consideration. The limit values within the Grid Code are stricter than those in EN50160.

Variable	Minimum percentage
Frequency 99,5% 99.5% of the 10 sec. values over 1 year, within +/- 1% of 50Hz	99,5%
Frequency 100% All 10 sec. values, within +4/-6% of 50Hz	100%
Supply voltage unbalance 95% of 10 min. average values over 1 week, between 0 and 2%	95%
Voltage variation 99% 99% of the 10 min. average values over 1 week, within +/- 10% of U Nominal	99%
Voltage variation 100% All 10 min. average values, within +/- 15% of U Nominal	100%
Flicker 95% of the values over 1 week, less than or equal to 1 (long-term flicker)	95%
THD 95% of the 10 min. average values over 1 week, less or equal to 8%	95%
Table 1: Example of limit values EN50160 >1kV and <35kV

 

Boundary conditions for voltage in non-public networks

The standard IEC61000-2-4 describes the limit values for voltages in non-public networks up to 35 kV. The quality of the voltage supplied to an equipment must comply with the standard IEC61000-2-4. If the standard values are exceeded, the machine or installation may fail (at an accelerated rate) and warranty regulations may no longer apply. With this, the standard IEC61000-2-4 has also become an immunity directive for machine builders.

Preconditions for the flow

• EN 61000-3-2 standard: Maximum harmonic currents (I<16 A per phase)
• EN 61000-3-12 standard: Maximum harmonic currents (16>I<75 A per phase)

Within the Power Quality standards, we distinguish a number of properties of the voltage and current. In the Power Quality standards that are important for the consumer and grid operator, the term voltage quality is often used, because the voltage is influenced by the current.

When it comes to voltage quality, we distinguish between continuously occurring phenomena and temporary phenomena.

Continuous	Temporary (events)
- Voltage level	- Voltage dips
- (inter-)Harmonics	- Voltage peaks
- Flickering	- Power interruption
- Asymmetry	- Transients
- Frequency	- Tone frequency signals

 

 

 

 

 

 

 

We will briefly discuss some of the phenomena.

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Harmonic pollution

Normally, mains voltage is purely sinusoidal. If a linear load (such as a light bulb, motor or capacitor) is connected to this, the current flow will be purely sinusoidal. When a non-linear load is connected to a linear voltage, it will draw a non-linear current. This current is called a "polluted current" or "non-linear" current. For example: a non-linear current can look like figure 4. Dirty currents can be found at pumping stations, on ships, in hospitals, data centres and low-energy office buildings.

 

A non-linear flow represented as signal and in Fourier analysis

 

As described in "Power Quality and Ohm's Law", a non-linear current will cause a non-linear voltage drop across impedances such as lines and transformers. This creates voltage pollution. The total voltage contamination is expressed with the THD-U and the total current contamination is expressed with the THD-I.

Fourier analysis

Periodic sinusoidal signals can be composed of several sinusoidal signals of different frequency. This is represented in a harmonic spectrum, also called fourier analysis. This analysis is used to identify power quality problems.

The short-term or immediate effects of harmonic pollution are:

• Unintentional tripping of protective devices.
• Abnormal noises and frequencies in distributors and transformers.
• Failure of capacitors due to thermal overload.
• Causing resonances.
• Energy losses in cables in transformers due to eddy current losses.

When norm values are exceeded, this results in the occurrence of a problem:

• Accelerated ageing of equipment and loss of production capacity;
• Accelerated ageing of capacitors in, for example, VSAs;
• An ageing of cables and transformers and therefore overloading;
• accelerated ageing of motors due to pulsating current (vibrations);
• the failure of motors due to pulsating current (vibrations)

 

 Read more about higher harmonics in the white paper

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Voltage dips

Voltage dips are the most costly in practice. According to the European Standard EN-50160, a voltage drop is defined as: "A sudden reduction of the effective value of the voltage to a value between 90% and 1% of the agreed value, immediately followed by a recovery of this voltage. The duration of the voltage drop is between half a period (10ms) and one minute." If the effective value of the voltage does not fall below 90% of the agreed value, this is considered a normal operating situation. If the voltage drops below 1% of the agreed value, this is an interruption. A voltage drop can occur due to various phenomena in the grid. Most voltage dips are caused by switch-on phenomena or breakdowns and closures in the medium-voltage grid.

Classification of voltage variations

Causes of voltage drop:

• Inrush currents of large loads
• Closures in the low-voltage grid
• Disconnection or faults in medium voltage grid
• Closure or failures in the high-voltage grid

Although voltage dips often go unnoticed, they can cause the failure of critical processes or lead to quality problems in production. A voltage dip can also lead to a peak current when the voltage drops (electronic load) or a power-on peak  when the voltage returns. This peak current can lead to the activation of safety devices.

 

Read more about voltage dips in the white paper

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Flickering

Flicker is a periodically changing voltage that translates into flickering displays and lighting. To determine flicker, a measurement with a measuring algorithm described in EN 61000-4-15 must be performed. The values for flicker are defined by Pst (10 min, short term) and Plt (2 h, long term). Usually limits of "1" are used for maximum value for the Plt. Causes of flickering Flicker is often caused by repetitive switching on and off of large loads such as hammer mills, spot-welding machines and electric arc drives. Solar parks and wind turbines can also be a cause. Because the grid impedance has a major influence on the degree of flicker, in most cases flicker is a joint problem of the grid operator and the consumer. Flickering of lighting is the main and most visible effect of flicker. At a flashing frequency of about 8 Hz, this can even lead to medical complaints in people who experience this regularly. This flickering of lighting occurs particularly with incandescent lamps. Electronically controlled lamps are less affected. Electronics can also age rapidly due to the change in the top value of the sine wave.

Flicker is a periodic variation of the top value

The costs of poor power quality are not always visible and are often paid for out of maintenance budgets. When integrating energy measurement systems for energy saving purposes, it is certainly worthwhile to take the Power Quality aspect into account and integrate it into the measurement concept. This relatively small additional investment makes it possible to proactively monitor Power Quality, improve it where possible and monitor responsibilities. 

Power Quality Management is a continuous improvement process of measuring, analysing and improving with the aim of reducing maintenance costs and increasing availability. The fact that this also saves energy is a nice bonus. We continuously go through three steps: measure, analyse and improve.

	Step 1: Measuring Choosing the right meter is essential   For a continuous improvement process it is necessary to measure continuously and properly. This seems easier than it is. 30% of measuring instruments are connected incorrectly. How do you ensure that you measure properly? determine what you want to measure and at what level in the installation choose the right meter and measuring transformers place the measuring instruments (analysers) in the right place connect the measuring instruments correctly  ensure the correct configuration of the measuring instruments Read more about "Choose the right energy meter"
	Step 2: Analyse Draw the right conclusions   Only by properly interpreting the measurement data can we draw the right conclusions. We want to be able to view measurement data, be alerted at the right moments and generate periodic reports. This requires well-designed software. Both software on the meter (decentralised) and software in which all the measuring points can be installed (centralised). Read more about analysis and reporting software
	Step 3: Improve Solving the PQ problem In case of Power Quality problems, we prefer to solve the problem at the source. It may be that the manufacturer of the equipment is not fulfilling its obligation or the installation needs to be built differently. In some cases, the problem lies with the grid operator. If this does not work, we will have to intervene with a passive solution (filter or compensation) or an active solution such as an active dynamic filter.