Abstract
List of contents
- Introduction
- 1. Review of standards
- 2. Voltage fluctuations
- 3. Flicker
- 3.1 Generalities
- 3.2 Dose of flicker
- 3.3 Flickermeter
- Conclusion
- References
INTRODUCTION
The problem of the quality of electrical energy is one of the most important in electrical systems and it defines reliability and efficiency of consumer’s operation. The power of non-linear asymmetrical and abruptly variable loads is growing constantly and it causes degradation of electrical energy quality. It can also be a reason of economical losses.
Quality of electrical energy is a combination of its general properties, that are estimated by power quality criteria, which characterize the level of interference in power grid numerically.
Power quality criteria are standardized by GOST 13109-97 [1]:
- — voltage deviation;
- — voltage fluctuation;
- — distorted sine wave;
- — asymmetry of voltage;
- — frequency deviation;
- — electromagnetic interferences.
To prevent the decrease of power quality there are some protective measures [2]:
1) Methodic, that are necessary for interference limitation, operating in different energy systems conditions by regulating frequency and voltage, providing the reliability of electrical systems; control and analysis power quality and automated measuring of power quality criteria;
2) Organizational – developing and using regulatory and lawful basis to support the conditions of power energy criteria provisioning by enhancing positions on the wholesale market of electrical energy;
3) Technical, which includes using different equipment to regulate voltage and provide it’s quality, systematic controlling the quality of electrical energy, creating unified apparatuses for measurement and control power quality; creating automated systems for management of the power quality.
1 REVIEW OF STANDARDS
In the Russian Federation requirements to the quality of electrical energy are regulated by GOST 32144-2013 «Electrical energy. Electromagnetic compatibility. Norms of power quality in the general-purpose power supply systems» [3].
In the Ukraine there is used GOST 13109-97 «Norms of power quality in the general-purpose power supply systems» [1], that is outdated in the Russian Federation.
Also for regulating the quality of electrical energy in low-voltage power supply systems there is adopted GOST 30804.3.3-2013 «EMC. Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ?16 A per phase and not subject to conditional connection» [4].
Norms and technical requirements to flickermeter are stated by GOST R 51317.4.15-2012 «EMC. Testing and measurement techniques. Flickermeter. Functional and design specifications» [5].
All of the currently used standards are equal to international standards IEC of the IEC 61000 «Electromagnetic compatibility (EMC)» series. Specifically IEC 61000-3-3-2017 «EMC. Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤16 A per phase and not subject to conditional connection» [6] and IEC 61000-4-15-2010 EMC. Testing and measurement techniques. Flickermeter. Functional and design specifications» [7].
2 VOLTAGE FLUCTUATIONS
The level of voltage in load centers of electrical system can deviate from normal, that is caused by balance of reactive power, load diagrams, voltage dropping on different sections of the network and power schedule of compensating installations. Therefore, stable voltage deviation is the deviation of voltage from it’s normal value in the steady-load conditions of electrical network, that is averaged over a certain interval.
There can be negative consequences for the network when power consumers are working with big voltage deviation for a long time.
When voltage is higher than normal:
- — danger of overheating stators of induction motors;
- — shortening the life of filament lamps;
- — increasing of non-load current of the power transformers, that causes overheating it’s magnetic cores;
- — increasing of consuming reactive power by valve invertors.
When voltage is less than normal:
- — dropping of the driving torque of inductive motors;
- — overheating of rotors of inductive motors, degradation of its detent and breakdown torque, that can cause disruption of the production process at the factories;
- — decreasing of the light quantity of the filament lamps;
- — excessive consumption of the electricity and disruption of technological process in electric furnaces.
Voltage fluctuations are caused by the same processes, that cause voltage deviation, but it happens much more frequently. So, voltage fluctuations is a fast variation of it’s current values [8]. According to [1] voltage fluctuation are characterized by two parameters: range of voltage variation and the dose of flicker.
The range of voltage variation (δUt) is a measure that equals the difference between value Ui and Ui+1 of the two extremums which are following each other or the extremum and horizontal region of the envelope of meansquare values of mains frequency voltage, defined on each alternation in percent from standard:
If the coefficient of distortion of the sine wave if less than 5% It’s acceptable to evaluate the range of voltage variation according to expression:
where Uampi и Uampi+1 are values of the two extremums which are following each other or the extremum and horizontal region of the envelope of meansquare values of mains frequency voltage (fig. 2.1), Δt is peak-to-peak interval.
The total maximum allowable value of the voltage range and its steady-state deviation should be no more than 10% at the points of connection to the 0.4 kV network in normal modes.
3 FLICKER
3.1 Generalities
Fluctuation and other fast voltage changes (<1 min) cause to noticeable annoying changing of the illumination of filament lamps, that can be a reason of physiological weariness. Changing of luminous power of lamps δΦt is proportional to voltage swing and frequency of voltage changing.
Flicker is a subjective perception of fluctuations in the luminous power of artificial lighting sources by a person that is caused by voltage fluctuations in the electrical network [9].
Flicker has a cumulative effect, which grows when the range of oscillation and the frequency of its repetition grows. This is true until some critical value (≈17,5 Hz), ), after the passing of which light flickering is merging (25-30 Hz) and eyes no more can differentiate it. Allowable range of voltage changing if increasing due to this. Also, with a high frequency of changes the thermal inertia of the filament of the lamp appears, which reduces fluctuations in illumination.
3.2 Dose of flicker
The dose of flicker determines the measure of a person's susceptibility to exposure to flicker for a certain period of time [9]. In contrast to the voltage swing and frequency of its fluctuations, the flicker dose estimates integral cumulative perception of the light flux by a person.
There are short-term (10 min) PSt and long-term (120 min) PLt flicker severity, that are measured at the point of transmission of electrical energy.
Methods for assessing the dose of flicker are established by GOST R 51317.4.15 [5]. Measurement at an interval of 10 minutes (TS) allows you to determine the short-term dose of flicker PSt. First, the flicker levels P(%)2 are found, corresponding to the integral probability 0,1; 0,7; 1,0; 1,5; 2,2; 3,0; 4,0; 6,0; 8,0; 10,0; 13,0; 17,0; 30,0; 50,0; 80,0 %. Then the smoothed flicker levels Ps are determined with an integral probability equal to 1,0; 3,0; 10,0; и 50,0 %:
Using the obtained values, the short-term flicker dose PSt over the time interval TS calculates according to the formula:
The long-term flicker dose PLt is determined over a time interval TL equal to 2 hours according to the formula:
where PStk is a short-term flicker dose on the k-th time interval TS during the observation period TL.
If the characteristic of the relative voltage change d(t)is known, then the computer simulation method can be used to estimate the short-term flicker dose.
For the above-mentioned criteria of power quality the following norms are established [4]:
- — the value of PSt shall not be greater than 1,0;
- — the value of PLt shall not be greater than 0,65;
- — Tmax, the accumulated time value of d(t) with a deviation exceeding 3,3% shall not exceed 500 ms;
- — the maximum relative steady-state voltage change dc shall not exceed 3,3%;
- — the maximum relative voltage change dmax, shall not exceed:
- 1) 4% without additional conditions;
- 2) 6% for equipment which is:
- а) switched manually;
- б) switched automatically more frequently than twice per day, and also has either a delayed restart (the delay being not less than a few tens of seconds), or manual restart, after a power supply interruption;
- 3) 7% for equipment which is:
- а) attended whilst in use (for example: hair dryers, vacuum cleaners, kitchen equipment such as mixers, garden equipment such as lawn mowers, portable tools such as electric drills);
- б) switched on automatically, or is intended to be switched on manually, no more than twice per day, and also has either a delayed restart (the delay being not less than a few tens of seconds) or manual restart, after a power supply interruption.
3.3 Flickermeter
The flicker dose is measured with a flickermeter - a device that reproduces a mathematical model of the channel of the impact of this process, formed by a light source, on the human eye and brain [2]. The visual appearance of the device is shown in fig. 3.1.
The flickermeter architecture is described by the block diagram of Figure 3.2. It can be divided into two parts, each performing one of the following tasks:
- — simulation of the response of the lamp-eye-brain chain;
- — on-line statistical analysis of the flicker signal and presentation of the results.
The first task is performed by blocks 2, 3 and 4 as illustrated in Figure 3.2, while the second task is accomplished by block 5.
Let's take a closer look at how each block works:
1) Block 1 – Input voltage adaptor. This block contains a voltage adapting circuit that scales the input mains frequency voltage to an internal reference level. This method permits flicker measurements to be made, independently of the actual input carrier voltage level and may be expressed as a percent ratio.
2) Block 2 – Squaring multiplier. The purpose of this block is to recover the voltage fluctuation by squaring the input voltage scaled to the reference level, thus simulating the behavior of a lamp.
3) Block 3 – Weighting filters. Block 3 is composed of a cascade of two filters, which can precede or follow the selective filter circuit. The first low-pass filter eliminates the double mains frequency ripple components of the demodulator output.
The high pass filter (first order, ?3 dB at 0,05 Hz) can be used to eliminate any DC voltage component.
The second filter is a weighting filter block that simulates the frequency response of the human visual system to sinusoidal voltage fluctuations of a coiled filament gas-filled lamp (60 W / 230 V and/or 60 W / 120 V).
4) Block 4 – Squaring and smoothing. Block 4 is composed of a squaring multiplier and a first order low-pass filter. The human flicker perception, by the eye and brain combination, of voltage fluctuations applied to the reference lamp, is simulated by the combined non-linear response of blocks 2, 3 and 4. The output of block 4 represents the instantaneous flicker sensation Pinst.
5) Block 5 – On-line statistical analysis. Block 5 performs an on-line analysis of the flicker level, thus allowing direct calculation of significant evaluation parameters.
A suitable interface, either with analog signals or digital data transfer, allows data presentation and recording. The purpose of this block is to derive flicker severity indications by means of statistical analysis. This statistical analysis, performed on-line through block 5, shall be made by sampling the instantaneous flicker signal level and subdividing these samples into a suitable number of classes.
CONCLUSION
The concept of the quality of electric energy, as well as its indicators, which require strict regulation, were considered.
The main technical documentation is considered, which is aimed at setting standards for the considered criteria.
Also familiarization was made with such phenomena as voltage fluctuations and deviations and the consequences that they can cause, in particular the flicker effect.
Methods for estimating the dose of flicker in general-purpose electrical networks and the established norms for the dose of flicker were considered.
Also, the structure and principle of operation of the device designed to simulate and measure the dose of flicker was studied.
The results obtained can be used in the study of the flicker dose in general-purpose electrical networks.
REFERENCES
- ГОСТ 13109-97 «Нормы качества электрической энергии в системах электроснабжения общего назначения (МЭК 868, 1000-3-2, 100-4-1)». – Введ. 01.01.1999г.
- Карташев И.И., Управление качеством электроэнергии / И.И. Карташев, В.Н. Тульскмй, Р.Г. Шамонов и д.р.; под ред. Ю.В. Шарова. – Издательский дом МЭИ, 2006 – 320 с.
- ГОСТ 32144-2013 «Электрическая энергия. Совместимость технических средств электромагнитная. Нормы качества электрической энергии в системах электроснабжения общего назначения. (EN 50160:2010, NEQ)». – Введ. 01.07.2014г.
- ГОСТ 30804.3.3-2013 «Совместимость технических средств электромагнитная. Ограничение изменений напряжения, колебаний напряжения и фликера в низковольтных системах электроснабжения общего назначения. Технические средства с потребляемым током не более 16 А (в одной фазе), подключаемые к электрической сети при несоблюдении определенных условий подключения (IEC 61000-3-3:2008 EMC)» - Введ. 01.01.2014г.
- ГОСТ Р 51317.4.15-2012 «Совместимость технических средств электромагнитная. Фликерметр. Функциональные и конструктивные требования (МЭК 61000-4-15:2010)» - Введ. 01.01.2013 г.
- IEC 61000-3-3-2017 «Electromagnetic compatibility: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤ 16 А per phase and not subject to conditional connection» - Введ. 05.2017г.
- IEC 61000-4-15-2010 «Electromagnetic compatibility: Testing and measurement techniques – Flickermeter – Functional and design specification» - Введ. 08.2010г.
- Подгурская, И.Г., Управление качеством электроэнергии: методические указания к практическим работам / Подгурская И.Г., Ротачева А.Г., Наумов И.В., - Благовещенск: АмГУ, 2015 – 71 с.
- Ананичева, С.С., Качество электроэнергии. Регулирование напряжения и частоты в энергосистемах: учебное пособие для вузов / С.С. Ананичева, А.А. Алексеев, А.Л. Мызин.; 3-е изд., испр. – Екатеринбург: УрФУ, 2012 – 93 с.