Thesis Project: Power Quality Analysis at Murdoch University eng470: Engineering Honours Thesis
Download 1.28 Mb. Pdf ko'rish
|
Abdullah2018
|
Active Filter
Passive Filter Requires an external power source Requires no external power source Made of active components Made of passive components R, L, and C Has high input and low output impedance Has low input and high output impedance Has power gain in output signal Has no signal gain Has oscillations and noise due to feedback loops for regulating active components Has low noise due to thermal noise in elements Does not consume energy of signal Consumes energy of signal Does not use inductors Uses inductors Has stable tuning, accuracy, and high immunity to EMI Has self-regulation of voltages driving loads and has no bandwidth restrictions, thus operates at higher frequencies 2.5.2 Distribution Static Compensator (D-STATCOM) Low power factor, harmonic distortion, voltage swells, and voltage sags can be enhanced using current injected by a Distribution Static Compensator (D-STATCOM) [45]. The D- STATCOM is a device that is used to limit the reactive power flow in a distribution system through compensation and can be based on a voltage source converter (VSC) [46]. A VSC is capable of generating a sinusoidal voltage of any desired phase angle, frequency, and magnitude. The VSC relies on energy storage devices such as capacitors that supply it with a DC Voltage that is then used to switch the solid-state electronics inside the VSC. Then it injects the voltage difference or completely replaces the voltage. The voltage difference refers to the difference between the actual voltage and the nominal voltage [46]. The following Figure 16 demonstrates a distribution system compensated by an ideal D-STATCOM connected in a shunt configuration. Power Quality Analysis at Murdoch University 33 From Figure 16, the D-STATCOM is functioning in current control mode and hence is represented by a current source I s. 2.5.3 Dynamic Voltage Restorer (DVR) Sensitive loads can be protected from voltage sags, surges, or disturbances using a dynamic voltage restorer (DVR). A voltage source injecting a voltage V f can be used to represent an ideal DVR for protection of sensitive loads as illustrated in Figure 17. Figure 16: Ideal D-STATCOM [20] Figure 17: Schematic of a DVR [20] Power Quality Analysis at Murdoch University 34 This device can be constructed so that it is able, or not able to supply or absorb real power. Voltage control of the DVR is done by regulation of the bus voltage to any value by measuring the terminal voltage V t . Then the balance is supplied through o f for a DVR which is designed to be able to absorb or supply real power. If the DVR is not able to absorb or supply any real power during steady state, it will perform this throughout transients [20]. 2.5.4 Unified Power Quality Conditioner (UPQC) A UPQC uses a common capacitor for DC energy storage when connected to two voltage source inverters (VSIs). One of the VSIs is connected in shunt with the AC line whereas the other is connected in series with this same AC line. A UPQC is used to compensate for harmonics, reactive power, negative sequence current, voltage imbalance, and voltage flicker [47]. Voltage imbalance and voltage flicker are eliminated from the load terminal voltage by the series VSI that injects a series voltage at the point of common coupling. A series voltage proportional to the line current can be injected into the distribution line to attenuate current harmonics [20]. The shunt VSI is employed in this device to provide a path for the real power to flow to help in the functioning of the VSI connected in series [20]. It can be seen from Figure 18, which represents a UPQC in right shunt compensation configuration. Power Quality Analysis at Murdoch University 35 2.6 Power Quality Case Studies Power Quality monitoring research has been a topic that has attracted considerable interest in recent times. Some of the consequential research includes: a study in Buffalo New York sponsored by Niagara Mohawk Power Corporation in mid- 1989. This study showed that most power quality problems originate from the end user’s equipment [48]. Moreover, a five-year study of single phase electrical disturbances in normal mode in 1990 by the National Power Laboratory (NPL), indicated the effects of capacitor switching which was observed in most locations under study with the magnitude of maximum voltage not being so severe at these sites [49]. A survey by Electrotek Concepts Inc. who were contracted by the Electrical Power Research Institute (EPRI) in 1990 showed harmonic distortions, transients, voltage sags, and short power interruptions as the most common type of power disturbances [50]. Other case studies include the Wisconsin Public Power Inc. [51] that installed power quality meters at the input of equipment to identify the cause of circuits tripping. This was later found to be capacitor banks. Filters and chokes were then added at these inputs to solve the Figure 18: UPQC Schematic [20] Power Quality Analysis at Murdoch University 36 problem. The same company installed meters to identify the cause of flickering light and computer problems for an industrial consumer. This was identified to be the several large single-phase welders in the company. This issue was solved by installing a more massive transformer with larger conductors at the industrial company. A recommendation was also provided to the company to add a second service line for the welders [51]. Power quality research was performed for a manufacturing plant [52] that had experienced a failure of a recently bought test instrument when it was plugged into a GPO (General Purpose Outlet). The problem was discovered to be the rated RMS voltage of 245 V of the test instrument that was lower than the Australian Standards RMS voltage of 253 V. This was solved by using a constant voltage transformer between the test equipment and the GPO that would both suppress the voltage spikes and lower the RMS voltage. Another case study conducted on a commercial office building involved utilizing two banks of AC motors with variable speed drives (VSDs) for control of heating, ventilation and air conditioning [1]. A 45 kVA transformer was used in order to service each of the banks. The following Figure 19 indicates the snapshot of the system operation. Power Quality Analysis at Murdoch University 37 From Figure 19, it can be seen that after reaching peak voltage, the variable speed drives demand peak current with the neutral current shown in the second figure made up of 180 Hz current that peaks above 150 Amperes. The neutral current is a 3 rd harmonic that is a common occurrence when there is a concentration of single- phase electronic loads. For this site, heating of the service transformer was the main problem due to the third harmonic. The transformer was not correctly sized for the load, therefore, calculations were done based on IEEE 519 harmonics guidelines, and it was found that there was at least 5 kVA load on the transformer above its nominal de -rated capacity. For this site, neutral current limiting devices or phase shifting transformers were not required because of the low neutral to the ground voltage and the isolated nature of the VSDs. New rated transformers for every bank was the solution to the power quality problem for this site [1]. Figure 19: System Operation Snapshot [1] Power Quality Analysis at Murdoch University 38 2.7 Costs of Power Quality Problems Quantifying the cost of power quality problems is not an easy task as there are both direct and indirect costs. Direct costs are directly related to disturbances and include production loss costs, damage to equipment expenses, environmental or financial penalties, restart of process costs, salary expenditures during downtimes, and resources and downtime that is not recoverable. In addition, it describes the human health and safety direct costs, loss of quality of semi-finished products, financial penalties due to breached contracts, and utility costs due to power interruptions. Organizations incur indirect costs, and it includes the cost of restoring a brand, loss of market share and postponement of income or revenue of an organization [53]. Various studies have been conducted to provide the quantitative economic costs of poor power quality. The cost was estimated at a few tens of billions of dollars in early 1990s, and this had grown to a few hundreds of billions of dollars by 2003 [4]. It is estimated that globally, 500 billion euros amounting to 50 % of the electricity sector global turnover is lost annually because of poor power quality [54]. Another study [55] indicates that the US economy loses between 104 billion dollars and 164 billion dollars per year due to outages alone. Moreover, 6.7 billion dollars are lost by industrial economies and the digital economy in the US due to disturbances excluding voltage sags with a further 15 billion to 24 billion dollars lost to other power quality problems. An estimated 100 billion euros is lost annually in the world due to harmonics according to a Eurelectric report [56]. Another study also showed that different industries experience different costs due to poor power quality. For instance, a telecommunications company losses up to 30 000 euros every minute due to power outages, Power Quality Analysis at Murdoch University 39 while as high as 3.8 million euros is lost by a semiconductor facility due to one power quality disturbance [57]. Power Quality Analysis at Murdoch University 40 Chapter 3: Methodology The approach used in this project involved using of actual current and voltage data in the analysis of power quality issues experienced at Murdoch University. The current and voltage data was measured using an EM133 meter installed at 1.330, in substation 12, South Street Building 330, chancellery. Data measured from the electrical systems at Murdoch University, especially from 1.330 South Street Building 330, Chancellery, and Substation T12, and it was saved in the form of spreadsheets in order to perform the analysis. Then the data was extracted from these spreadsheets and were used to plot graphs for all the measured variables in order to obtain a visual impression of the data. Calculations were performed where applicable (voltage and current unbalance) and graphs for the calculated variables were plotted to show the trend existing in the electrical network. There are two significant methods were used in the calculation of voltage unbalance. The first method involves calculation of the negative phase sequence voltage and positive phase sequence voltage from the phase voltage phasors. The voltage unbalance is then the ratio of the negative phase sequence to the positive phase sequence multiplied by one hundred. This method is based on IEC 61000 standards. The second method that is related to National Electrical Manufacturers Association (NEMA) is determined by finding the maximum deviation from the average line voltage and multiplying this deviation by one hundred. Afterward, the graphs were compared to the theoretical charts for each power quality issue to identify the PQ issues existing at the institution. After identification of the power quality issues, its effects on the other parameters such as temperature, phase, current, voltage, or Power Quality Analysis at Murdoch University 41 power were investigated. This was conducted by plotting graphs of these measured parameters throughout the occurrence of the power quality problem then the trend analysed. In other cases such as in voltage disturbance, calculations were done to indicate its effect on the temperature rise due to loading. Finally, the results of this analysis were compared with theoretical knowledge from journal publications and literature focusing on power quality problems. In order to test the power quality devices, there are three imperative objectives that have been taken into account include: 1) To determine the types and magnitude of power quality disturbances, which produced by a device connected to the power system. 2) To test power quality mitigation devices for their ability in order to reduce disturbances. 3) To test electrical devices for their performance in terms of disturbances of power quality. Power Quality Analysis at Murdoch University 42 Chapter 4: Results and Discussion of Data Analysis 4.1 Voltage Unbalance The following Figure 20 illustrates the voltages of three-phase to neutral lines. From Figure 20, it can be seen that the magnitude of voltage A-N is lower than the magnitudes of voltage B-N and C-N. Further investigation and calculations were therefore carried out to prove the level of voltage unbalance. The average voltage magnitude at the various times is given as V av = V L1 + V L2 + V L3 3 (23) Plotting this average value together with the three voltages gives the graph in Figure 21 below. 234 235 236 237 238 239 240 241 242 243 12:00:00 AM 2:24:00 AM 4:48:00 AM 7:12:00 AM 9:36:00 AM 12:00:00 PM 2:24:00 PM Download 1.28 Mb. Do'stlaringiz bilan baham: 
|