Power Quality: The quality of electrical energy (Grid Code EN50160)
What is Power Quality? Power Quality is a collective term for the quality of electrical energy. Just as power can be described as current and voltage, Power Quality can be described as current and voltage quality. Problems with the quality of electricity lead to poor Power Quality.
If you are considering buying a car, it is usual that you will want to establish its quality. You want a good quality car that lasts, has low maintenance costs and is economical to run. For a less tangible product like electrical energy, we are less aware of the importance of monitoring its quality aspects. The quality of electrical energy is described by the term Power Quality and is safeguarded by standards and guidelines.
The costs of poor Power Quality are usually paid for out of the maintenance budget and are seen as unavoidable. At the same time, Power Quality is coming under increasing pressure due to the increase in electronic equipment and decentralised energy generation. By providing insight into Power Quality and managing this data, costs are saved and the quality of the organisation and end product is increased.
In this white paper we discuss the Power Quality phenomenon and describe the different responsibilities and standards. We also discuss the Power Quality phenomena and their consequences. Finally, we will discuss how to make Power Quality transparent and how to improve it.
Poor quality of electrical energy (= Power Quality) leads to:
- higher failure rate of equipment;
- higher maintenance costs;
- A reduction in the lifespan of equipment;
- Increased energy consumption;
- The expiry of guarantees;
- The possibility of fines or claims.
Power Quality and "Ohm's law".
Pure sinusoidal
The voltage (U) is expressed in Volts (V) and in the Netherlands ideally has an effective value (RMS) of 230 V (low voltage), a frequency of 50 Hz and is sinusoidal. This voltage is generated in the power plants and is available to the end user via the national and local distribution network. If the voltage offered is purely sinusoidal and the load is linear, the current will be purely sinusoidal throughout the installation.
Voltage and current influence each other through the impedance of the installation" - Ohm's law
The figure below shows a motor connected behind a mains transformer. The mains, the system and in particular the mains transformer form an impedance. The motor has a linear behaviour and will draw a sinusoidal current. Therefore, after the impedance the voltage is slightly lower (voltage drop) and also slightly shifted in phase (inductance), but it is still sinusoidal.
Voltage drop due to impedance
Contaminated voltage
If the current drawn is not sinusoidal, due to electronic equipment such as LED lighting, drives or electric car charging points, the impedance of the installation will pollute the voltage. This is because with a non-sinusoidal current, the voltage drop across the impedance is not sinusoidal. This leads to a polluted voltage.
Voltage drop due to impedance
Now, if a motor with linear behaviour is connected behind the impedance from the above example, the motor suffers from the dirty voltage (which is caused by the impedance and the non-linear behaviour of the electrical equipment).
Power Quality and Responsibilities (Grid Code EN50160)
In order for equipment to work properly, responsibilities lie with three parties:
The system operatorThe grid operator is responsible for a supply of voltage of sufficient quality. This quality can be negatively influenced by a weak network and the influence of the customer. At the transfer point - the point of common coupling (PCC) - the quality must comply with the Grid Code, which is derived from the European standard EN50160. EN50160 specifies boundary conditions for voltage. |
The manufacturerEquipment manufacturers ensure that their equipment operates correctly within the applicable standards and requirements. The equipment is hereby immune to voltage contamination that occurs within the limits of the standard. This means that the equipment functions correctly and will achieve the expected economic life. If the contamination is outside the norm, there is a high chance that the equipment will not function properly or will even fail. Equipment also has standards for maximum emission of pollutants to other consumers. For example, equipment up to 75 A has special standards for maximum permissible harmonic currents. The emission of harmonic currents for equipment with higher capacities is not laid down in standards. In this case, the responsibility lies with the designer of the installation. |
The person responsible for the installation (the customer)The customer is responsible for ensuring that the electrical installation meets the requirements, that the equipment is installed according to the installation manual and that it is used in accordance with the user manual. If the grid operator and the manufacturers fulfil their responsibility, it is expected that the quality of the voltage offered to the equipment at the PCC will meet the IEC61000-2-4 is sufficient. It lays down the preconditions for voltage in non-public networks. However, this is not a guarantee. If the voltage at the PCC does not meet this standard, the warranty for the connected equipment will lapse and the expected technical service life of the equipment will be greatly reduced. This makes the IEC61000-2-4 the explicit responsibility of the installation manager. |
Power Quality and standards
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.
|
Measured value |
Limit value |
Table 2: Limit values for voltage and individual harmonic components according to IEC61000-2-4 In the EN61000-2-4 three classes can be distinguished for the design and monitoring of a non-public electrical installation. These classes (Class 1, 2 and 3) determine the degree of permissible contamination within an electrical installation. When designing an installation, one must take this into account and ensure that all connected equipment functions correctly within the chosen class. Class 2 is the standard, where standard equipment can be used. Class 1 is mainly used for sensitive equipment in data centres, hospitals and laboratories. Class 3 is mainly used for special (dedicated) systems in heavy industry. This class allows a high degree of contamination, which means that all connected equipment must be immune to this high degree of contamination. |
||
| Minimum | Maximum | |||
|
Tension L1-N |
207 V |
253 V |
||
|
Tension L2-N |
207 V |
253 V |
||
|
Tension L3-N |
207 V |
253 V |
||
|
Frequency |
49 Hz |
51 Hz |
||
|
THD-U L1-N |
|
8% |
||
|
THD-U L2-N |
|
8% |
||
|
THD-U L3-N |
|
8% |
||
|
Imbalance |
|
2% |
||
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)
Power Quality phenomena
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 |
|
|
- (inter-)Harmonics |
- Voltage peaks |
|
- Flickering |
- Power interruption |
|
- Asymmetry |
- Transients |
|
- Frequency |
- Tone frequency signals |
We will briefly discuss some of the phenomena.
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
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
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 flickeringFlicker 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 |
Power Quality Management
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
|
|
|
Step 2: Analyse
|
|
|
Step 3: Improve
|
White papers
Read more about specific substructures in one of the white papers below. On the corresponding page, you can request the white paper free of charge as a PDF.
- Voltage dips and current dips
- Active dynamic filtering
- Measurement accuracy and measurement uncertainty
- Cos-phi compensation
- Higher harmonics and pollution
- Reducing and compensating reactive power
