Voltage sag

In case of short circuits, the rotational speed of induction motors (IM) of auxiliary means in the heat-and-power station (HPS) decreases, which can lead to the shutdown of the technological equipment of power plants and disconnection of generators from the electric network.
The aim of the research is to determine the effect of voltage dips appeared as a result of short circuits at different electrical distances on the possibility of self-starting of IM.
The study was carried out on the example of a 0.4 kV power supply for the auxiliary means of HPS No. 1 in the city of Dushanbe, Republic of Tajikistan.
Calculation of the run out and self-starting of the induction motor was carried out taking into account not only the voltage drops on the buses of the auxiliary devices of HPS, but also taking into account the change in the phase sequence of the EMF of the IM with respect to the network voltage [1]. When the EMF of the IM shifted by 180° with respect to the auxiliary mains voltage, when the voltage was recovered after the short circuit was eliminated, the IM draws a current, which exceeded its starting current by more than 2 times [2]. The change in the angle between the EMF vector of the IM and the network voltage vector up to 180° occurs in 0.3-0.5s, and the value of the residual voltage on the buses of the auxiliary means, at that time, was about 0.5-0.7 of its nominal values. The maximum value of the electromagnetic shock torque, exceeds in 1.5-2 times the torque during a short circuit performed at switching angles of 180°± (45°-75°). From this point of view, it is desirable to have a sufficiently long period of time before restoring the voltage to ensure a decrease in the residual voltage and acceptable value of the shock current and torque when the short circuit is eliminated. The voltage on the buses of the auxiliary devices almost completely attenuates in 2s [2].
Using ETAP program (OTI USA company), the maximum allowable duration and magnitude of voltage dips on the buses of the auxiliary devices caused by arc short circuits at different electrical distances were investigated and determined for the most important systems of auxiliary means, taking into account the on-hold characteristics of magnetic starters and contactors.

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This work presents a case study on technology assessment for power quality improvement devices. A system compatibility test protocol for power quality mitigation devices was developed in order to evaluate the functionality of three-phase voltage restoration devices. In order to validate this test protocol, the micro-DVR, a reduced power development platform for DVR (dynamic voltage restorer) devices, was tested and the results are discussed based on voltage disturbances standards.► We present a case study on technology assessment for power quality improvement devices. ► We developed a system compatibility test protocol for power quality mitigation devices. ► The proposed test protocol evaluates the functionality of three-phase voltage restoration devices. ► In order to validate this test protocol, the micro-DVR, a reduced power development platform for DVR (dynamic voltage restorer) devices, was tested. ► The results are discussed based on voltage disturbances standards.

Voltage Sags and RVC, rapid voltage change, limits are becoming a connection requirement for many electric utilities. There are very good ways to determine this, but they require detailed information and a site simulation study to come up with this value and are expensive as well as time consuming. There is now a "Back of the Envelope" approach to estimating this with a minimum of information. It takes only a minute to do this simple estimation to determine if mitigation is likely to be needed. Now-Drum Roll Please……….. R(Critical) = System Voltage x Local Fault Current / Solar Farm Transformer MVA If R >= to 50-Then inrush is likely to be within than most interconnection requirements. If R is lower, then some method to reduce inrush current is likely to be needed! That's it! An example: V phase to phase in kV = 25kv I fault current at site in kA = 3kA Solar Interconnection Transformer MVA = 3MVA Result R = 25kV * 3kA / 3MVA = 25 Site needs Inrush Control Yes, there is a lot more theory and evaluation that is needed to accurately come up with the exact numbers but this is a simple "Rule of Thumb" can quickly ballpark the need for a site to estimate if inrush control is needed. This technique was developed by plotting the results of many solar site studies on distribution circuits. The site variables were plotted against the desired result, RVC (voltage sag). The figure below shows this simple relationship along with upper and lower limits. The statistical significance of the correlation coefficient has an R 2 of 87%. Limits are shown two standard deviations above and below the average curve. If this ratio is less than 50, then the site is possible to have an RVC more than 6%, the common limit for interconnection agreements with utilities. If the ratio is less than 30, then it is very likely to exceed this limit. These limits come from IEEE 1547, Standard for Interconnection. Some utility requirements have an RVC limit as low as 3%. The numbers for the acceptance of this requirement needs a ratio greater than 45 to be a concern and 75 to be relatively sure that there is not an issue. Try it out on the link below: https://www.southernstatesllc.com/applications/connecting-solar-to-the-grid Ratios smaller than these limits are likely to need inrush current control. There are many techniques to limit the RVC however most are complicated to install. The TranSwitcher® from Southern States is effective and simple to install, maintain and allows saving no load losses by turning off at night.

Voltage sags, transients, and momentary interruption of power together constitute 92% of the power quality problems encountered by typical industrial customers. The series-parallel-connected dynamic sag corrector (DySC) provides statistically significant protection at greatly reduced cost. The DySC is rated from 1.5 kVA one phase to 2000 kVA three phase and features a patented single-stage power conversion circuit with minimal stored

The power quality is a major issue in the distribution system. There will b e problem occurs regarding reactive power transfer in distribution system due to large power angle even with substantial voltage magnitude gradient. Here a DSTATCOM is used as a FACTS device wh ich can compensate reactive power. D-STATCOM is three phase voltage source converter used to compensate voltage and make the system stable by absorbing and generating reactive power. Here simulin k model and control system is designed in MATLA B simu lin k environ ment. Here modeling of the D-STATCOM is also given.

The power quality becomes one of the most important factor in our present day economy. The concept of true power mainly depends on how correctly the issues like voltage sag and voltage swell are mitigated. Among all the power quality problems, like voltage sag andvoltage swell mentioned are of key importance. This is because of the most widely use of newly invented as well as the outdated sensitive equipments connected to the load. Sensitivity is the main cause of the above power quality problems and it cannot be eliminated completely as it has many other operating properties. So for solve voltage sag and swell problem caused by the sensitive equipments connected to the faulty loads solve by connect some other system like DVR. The occurrence of sag and swell varies with equipment, environment, process operations, desired control schemes etc. by referring IEEE standards, the power quality events will last only for small instants. Thus the real time detection becomes a really tough task and even a small instant variation will cause a series of problems to the process industry. The dynamic voltage restorer is a special type of power device used for providing consistent and reliable supply power to the load devices. Dynamic voltage restorer uses a vector control strategy for mitigating power quality problems by automatically detecting and injecting the voltage components through an injection transformer. Repeated occurrence of sag and swell will cause these equipments to malfunction

Abstrak Pada penelitian ini akan disimulasikan pemulihan jatuh tegangan yang terjadi akibat gangguan hubung singkat pada saluran distribusi tiga fasa dengan menggunakan pemodelan Dynamic Voltage Restorer dengan pengendali PI. Untuk mendeteksi sistem jatuh tegangan menggunakan Park Transform dan menerapkannya dalam regulator tegangan sebagai fungsi sistem kontrol yang mendeteksi amplitudo tegangan pada beban sensitif secara terus menerus. Simulasi dilakukan pada beban aktif 280 KVA dan reaktif 50 KVAR di sistem distribusi PT. Dian Swastatika Sentosa Serang Power Plant dengan gangguan voltage sag sebesar 0,6 pu. Hasil simulasi menunjukkan DVR dapat mengembalikan jatuh tegangan ke 1 pu dengan waktu pemulihan 0,25 s. Kata kunci: Kualitas Daya, Jatuh Tegangan, DVR, Simulink 1. PENDAHULUAN Kualitas daya listrik menjadi masalah yang penting, karena daya listrik yang disalurkan ke konsumen harus benar-benar yang berkualitas baik. Gangguan pada sistem tenaga listrik dapat terjadi sewaktu-waktu dan tidak dapat dihindari. Adapun gangguan-gangguan tersebut bisa berasal dari internal maupun eksternal, dan dapat menyebabkan masalah jatuh tegangan. Jatuh tegangan dapat mengurangi kualitas daya pada sistem tenaga listrik[1,2]. Adanya gangguan dapat mengakibatkan penurunan kualitas daya sistem tenaga, masalah kualitas daya adalah: jatuh tegangan, flicker, ketidakseimbangan tegangan, pemutusan dan masalah harmonisa. Peralatan-peralatan yang sensitif seperti komputer, rele, Programmable Logic Controller (PLC), penggerak motor listrik dan sebagainya, sangat peka terhadap perubahan tegangan yang diakibatkan oleh gangguan yang terjadi pada bagian lain pada sistem[3]. Jatuh tegangan didefinisikan sebagai penurunan nilai rms tegangan yang dapat terjadi dari 10 ms sampai ke 1 menit dengan kedalaman jatuhnya tegangan sebesar 0,9 pu sampai 0,1 pu dari 1 pu nominal berdasarkan standar IEEE 1159-1995 [4]. Pada sistem kelistrikan PT. Dian Swastatika Sentosa Serang Power Plant, sering terjadi penurunan tegangan dan salah satu faktor penyebabnya adalah adanya gangguan hubung singkat yang berakibat pada kinerja driver motor pada mills di generator unit 6. Pada penelitian ini akan dilakukan pemodelan sistem tiga fasa yang mengalami penurunan tegangan, dari hasil pemodelan akan dianalisis pemulihan jatuh tegangan yang terjadi dengan beban sensitif yang akan dilindungi oleh DVR dari pengaruh jatuh tegangan, sehingga akan diperoleh suatu pola besar tegangan DC PWM inverter yang dibutuhkan oleh DVR sesuai dengan kebutuhan beban sensitif yang akan dilindungi[5]. Dynamic Voltage Restorer merupakan peralatan yang digunakan untuk memulihkan tegangan atau memperbaiki mutu tegangan di sisi beban dan posisinya dipasang secara seri antara sumber dan beban. DVR dirangkai secara seri dengan sistem distribusi untuk melindungi peralatan sensitif terhadap terjadinya jatuh tegangan [6,7]. Fungsi dasar DVR adalah mendeteksi terjadinya jatuh tegangan yang terjadi pada saluran sistem daya, kemudian menyuntikkan tegangan untuk mengkompensasi jatuh tegangan yang terjadi. Oleh karena itu DVR ditempatkan dekat dengan beban sensitif yang dilindungi[8].

This paper presents a study to investigate response of adjustable speed drives due to voltage sags. A small ASD of 0.39 kW, 50/60 Hz, and 400-480 V was used for testing how drives
will respond to voltage sag types; A, B, C and D. The drive response is influenced by sag magnitude, sag duration and phase angle shift. Behavior of the diodes rectifier for conducting is influenced by sag magnitude and phase angle shift represented by sag of types C and D. The diodes rectifiers are reverse biased when it was supplied by voltage sags. Even though there are no charging currents to dc bus capacitor during voltage sags, the drives did not directly trip to certain
sag magnitude and sag duration. Sensitivity of the ASD to voltage sag types is presented in voltage tolerance curve. Sag of type A produces highest sensitivity on the ASD and it trips for sag magnitude of 62.5% and 250 milliseconds in duration.

Abstract- This work presents a control strategy of a dynamic voltage restorer (DVR) to improve the doubly fed induction generator (DFIG) based wind turbine in case of fault. The application of a dynamic voltage restorer (DVR) connected to a doubly fed induction generator (DFIG) based wind-turbine-driven is investigated.Voltage in faulty line can be compensated using DVR, while nominal operation of DFIG wind turbine is continued as demanded in actual grid codes.

Power quality is a pressing concern and of the utmost importance for advanced and high-tech equipment in particular, whose performance relies heavily on the supply’s quality. Power quality issues like voltage sags/swells, harmonics, interruptions, etc. are defined as any deviations in current, voltage, or frequency that result in end-use equipment damage or failure. Sensitive loads like medical equipment in hospitals and health clinics, schools, prisons, etc. malfunction for the outages and interruptions, thereby causing substantial economic losses. For enhancing power quality, custom power devices (CPDs) are recommended, among which the Dynamic Voltage Restorer (DVR) is considered as the best and cost-effective solution. DVR is a power electronic-based solution to mitigate and compensate voltage sags. This paper provides a thorough discussion and comprehensive review of DVR topologies based on operations, power converters, control methods, and applications. The review compares the ...

The present power system is a complex network and specially the power
distribution network is very intricate. Voltage sag is the most severe problem among
all power quality issues. Voltage sag can cause critical power interruption in the
distribution network. Generally, voltage sag occurs with the fault in the power system.
In this paper, the working of dynamic voltage restorer (DVR) for the compensation of
source voltage sag is presented. Here, a voltage sag in the source voltage due to three
phase fault has been compensated by the photo voltaic (PV) voltage through voltage
source inverter (VSI) with a DVR control scheme. The pre-sag open loop feed forward
control scheme has been used for this purpose.

Voltage sags and unbalance pose a serious power quality issue for the electric power industry. The induction motor has widely used the machine in process industries (around 70%). In this paper, the performance of the Induction Motor Drive (IMD) under the condition of voltage sag and voltage unbalance is studied. The drive model is modelled in MATLAB/Simulink environment and the simulation results are obtained for time varying load. The performance of the IMD under different indexes viz. variation in current, speed and torque is investigated under varying sag depths and durations. The ripples in speed and torque are also investigated under unbalance conditions