A class of harmonic current injection based three-phase rectifiers with sinusoidal input current

Оvа disеrtаciја sаdrži аnаlizu tеоriјskih principа rаdа i prаktičnu rеаlizаciјu klаsе trоfаznih isprаvlјаčа bаzirаnih nа principu ubrizgаvаnjа struје. Меtоd ubrizgаvаnjа struје zаhtеvа prоcеsirаnjе mаlоg dеlа snаgе isprаvlјаčа, nе gеnеrišе еlеktrоmаgnеtnе intеrfеrеnciје i imа visоk stеpеn iskоrišćеnjа u pоrеđеnju sа kоmpеtitivnim rеšеnjimа, budući dа nеmа kоmutаciоnih gubitаkа. Izvršеnо је dеtаlјnо rаzmаtrаnjе mеtоdе ubrizgаvаnjа trеćеg hаrmоnikа i аnаlizа pојаvа kоје sе dеšаvајu u trоfаznоm diоdnоm isprаvlјаču pri ubrizgаvаnju trеćеg hаrmоnikа struје rаzličitih аmplitudа i fаznih stаvоvа. Nа оsnоvu аnаlitički dоbiјеnоg izrаzа zа ukupnо hаrmоniјskо izоbličеnjе ulаznе struје је izvеdеn zаklјučаk dа је оptimаlnа vrеdnоst аmplitudе ubrizgаnоg trеćеg hаrmоnikа 1,5 putа vеćа оd јеdnоsmеrnе struје kојоm је оptеrеćеn izlаz isprаvlјаčа, uz prеtpоstаvku dа је pоtrоšаč struјni pоnоr. Nајmаnjе tоtаlnо hаrmоniјskо izоbličеnjе kоје sе mоžе pоstići nа tај nаčin iznоsi 5,125% i pri tоmе mrеžа zа ubrizgаvаnjе struје аpsоrbuје 8,571% ulаznе snаgе. Zаtim su prikаzаnе nеkе оd mоgućih rеаlizаciја оptimаlnоg ubrizgаvаnjа trеćеg hаrmоnikа i dаје sе pоrеđеnjе nеkih tipičnih mrеžа zа ubrizgаvаnjе. Аnаlizirаnа је mоgućnоst ubrizgаvаnjа idеаlnоg tаlаsnоg оblikа struје i pоstаvlјеni prоblеm је rеšеn u pеtоm pоglаvlјu disеrtаciје, putеm rаzmаtrаnjа tоkоvа viših hаrmоnikа ubrizgаnе struје i tо kаkо nа nаizmеničnој tаkо i nа јеdnоsmеrnој strаni, nа primеru trоfаznоg diоdnоg isprаvlјаčа kојi је оptеrеćеn sа dvа boost DC/DC kоnvеrtоrа. Prikаzаni su rаzličiti putеvi ubrizgаnih pаrnih i nеpаrnih hаrmоnikа struје. Izvеdеnе su јеdnаčinе kоје dајu tаlаsni оblik ubrizgаnе struје sа kојim sе pоstižе idеаlni sinusni tаlаsni оblik struје koјu isprаvlјаč uzimа iz nаpојnе mrеžе. Zаtim је prikаzаnо nеkоlikо prаktičnih rеаlizаciја trоfаznih diоdnih isprаvlјаčа sа оptimаlnim ubrizgаvаnjеm struје, sа еkspеrimеntаlnim rеzultаtimа. Pri tоmе је prvо аnаlizirаnо оptimаlnо ubrizgаvаnjе sаmо nеpаrnih hаrmоnikа i pоkаzаnо је dа sе tаkо mоžе pоstići tоtаlnо hаrmоniјskо izоbličеnjе ulаznе struје ispоd 5% а аkо sе nа јеdnоsmеrnој strаni ubrizgаvајu i pаrni hаrmоnici, pоstižе sе tоtаlnо hаrmоniјskо izоbličеnjе оd 0%. Pоkаzuје sе dа u svаkоm slučајu mоrа dа budе disipirаnо nа оtpоrimа (ili prоcеsirаnо krоz kоnvеrtоrе kојi аktivnо еmulirајu оtpоrnоst) 8,81% ulаznе snаgе i tо 8.65% u mrеži kоја ubrizgаvа nеpаrnе hаrmоnikе а 0,16% u mrеži kоја ubrizgаvа pаrnе hаrmоnikе nа јеdnоsmеrnој strаni. Оsnоvni kоncеpt оptimаlnоg ubrizgаvаnjа struје је primеnjеn nа slučај šеstоfаznоg diоdnоg isprаvlјаčа. Pri tоmе su fаzni nаpоni pоmеrеni zа 30 еlеktričnih, čimе sе pоstižе gеnеrisаnjе kružnе struје i njеnо ubrizgаvаnjе u ulаznu fаznu struјu isprаvlјаčа, оdnоsnо niје pоtrеbnо kоristiti nеutrаlnu tаčku. Таkо је cео prоblеm pоstizаnjа sinusоidаlnоg tаlаsnоg оblikа ulаznih struја trоfаznоg diоdnоg isprаvlјаčа rеdukоvаn nа ubrizgаvаnjе оptimаlnih tаlаsnih оblikа struје nа јеdnоsmеrnој strаni. I u оvоm slučајu је pоtrеbnо dа mrеžа zа ubrizgаvаnjе аpsоrbuје približnо 2,35% izlаznе snаgе.This thesis presents the theoretical explanation of the current injection technique applied in three phase diode rectifiers as well as practical rectifier circuits using that technique. The current injection technique application needs to process a small amount of the total rectified power through current injection network; it does not generate EMI and has high conversion efficiency due to absence of the switching losses. The detailed analysis of the third harmonic injection technique is performed, regarding different amplitude and phase values of the injected current. By using analytically derived exact expression for input current THD, the optimal amplitude value of the injected current is found to be 1.5 times higher than the output rectifier current. In that case the minimum input current THD value achieved is 5.125% at the price of processing the 8.571% of the input rectifier power through current injection network. Some of the possible third harmonic current injection networks are explained and compared. The next problem tackled and the solution presented in the 5th chapter is the analysis of the possibility of the ideal current waveshape injection.. The optimal solution is derived by analyzing the current flow of higher harmonics. The base rectifier circuit which is used for the analysis is three phase diode rectifier loaded with two boost converters. Different flow paths for odd and even current harmonics are clarified and explained. To achieve the most sinusoidal waveshape of the rectifier input current, the optimal injected current waveshape is derived based on the previous analysis Besides that some practical current injection networks are presented. By current injection of the odd harmonics only, the THD achieved can be below 5%. Power flow in optimal waveshape current injection case is such that 8.81% of input rectifier power has to be processed through current injection network (8.65% in odd harmonics current injection network and 0.16% in even harmonics current injection network). In the 6th chapter the optimal current injection technique is applied on the six phase diode rectifier case. The two sets of three phase input voltages are displaced for 30 electrical degrees, thus enabling circular current generation and injection of the same current into input rectifier current. In this way all the necessary current injection is performed on DC rectifier side and the power processed through the current injection network in this case is only 2.35% of the rectifier output power

Detection of large induction motor cage failure by novel analysis of the waveform of the start up current

In the case of large induction motors, it is very important to quickly and safely identify the onset of failure. In that way one should take appropriate corrective measures to prevent significant damage. In this paper, the comparative diagnostics between two high-voltage large induction motors using the specific proposed waveform analysis of current during the direct-on-line start is presented. Using the previously recorded signals, a rotating current vector amplitudes for both machines are determined. Their shapes are analysed and the presence of the initial 100Hz stator current component, indicating the existence of a rotor failure, is observed

Coordinated reactive power-voltage controller for multimachine power plant

Abstract: This paper presents a newly developed voltage and reactive power control device, the coordinated Q -V controller (CQVC), for a multimachine steam power plant (SPP). The requirements for the new controller are defined based on identified inadequacies of manual Q-V control. The controller uses a feed-forward correction signal based on the application of the sensitivity matrix at a power plant level. It is designed and its performance initially assessed using the Matlab/Simulink environment. The performed simulations show that all the primary design objectives are met. The Q-V controller maintains the voltage at the SPP busbar with a certain droop characteristic during slow voltage changes in the power system whilst also maintaining equal per unit reactive power sharing among the SPP generators. This ensures that the power system fully benefits from all generating units during system perturbations. The performance of the designed CQVC is validated by comparing simulation results with measured responses obtained from a real CQVC installed at a SPP

PLC-based model of reactive power flow in steam power plant for pre-commissioning validation testing of coordinated Q-V controller

Abstract: The paper presents the digital realization of a model of reactive power flow (QFM) in a steam power plant using a programmable logic controller (PLC). The steam power plant (SPP) model is developed for pre-commissioning validation testing of the coordinated reactive power-terminal voltage (Q-V) control system. The SPP QFM includes a model for a synchronous generator, an excitation system, a step-up transformer, and the generator's droop characteristic modeled through the automatic voltage regulator (AVR). A QFM synthesis is based on a series of experiments performed on site. The parameters of the generator and AVR are estimated from recorded generator voltage and current time responses to a step change in voltage reference of the AVR. To get a complete QFM, transformers and network reactances are also included. In order to calculate reactive power (Q) flows more accurately, the generator Q output is adjusted by taking into account its real power output. Standard PLC hardware, as industrial grade equipment appropriate for on site testing, is used for practical QFM implementation after discretization of the continuous mathematical model. The developed QFM response is verified through a series of experiments performed in the laboratory

Analysis of step-up transformer tap change on the quantities at the point of connection to transmission grid

The analysis of a step-up transformer tap change on the quantities at the point of connection to the transmission grid is presented in this paper. The point of connection of generator TENT A6 has been analyzed, and a detailed model of this generator is available in software package DIgSILENT Power Factory. The comparison between the effect of a step-up transformer tap change on the quantities at the point of connection during automatic and manual operation of voltage regulator has been conducted. In order to conduct the analysis of the manual operation of the voltage regulator, the comparison between the different methods of modeling of these modes has been performed. Several generator operating points, selected in order to represent the need for tap change, have been analyzed. Also, previously mentioned analyses have been performed taking into account the voltage-reactive stiffness at the point of connection

Robustness of commissioned coordinated Q-V controller for multimachine power plant

Abstract: This paper presents commissioning details of a coordinated reactive power–voltage controller (CQVC) installed in a multimachine steam power plant (SPP). The CQVC regulates reactive power and voltage of a 1992MVA multimachine SPP. After briefly introducing basic control principles and relevant details about the CQVC's implementation, CQVC parameterization and tuning are thoroughly discussed. Once commissioned,the CQVC performance under various operating conditions is assessed. It is shown that the CQVC successfully maintains an HV busbar voltage and allocates reactive power to participating generators in accordance to their capability. Numerous CQVC responses recorded at the site are presented to demonstrate controller's performance under normal and extreme plant and network operating conditions

Improved hydrogenerator rotor thermal supervision

<p>The ongoing energy transition to cleaner energy involves three main changes: using less energy (energy savings on the demand side), making energy production more efficient, and using renewable and low-carbon sources instead of fossil fuels. However, relying more on intermittent renewable energy sources means we need to balance them with conventional sources for a stable electricity supply. Hydrogenerators can provide this stability, but also flexibility, by quickly increasing power when needed. They are designed for a daily average number of start-stop cycles equal to twice per day. On the other hand, they will face new challenges, as they were not designed for frequent and large load changes, which will put additional stress to the hydrogenerator parts. The continuous, safe, and reliable operation of the hydrogenator is determined by the boundaries of the capability curve (active-reactive power PQ diagram) provided by the manufacturer. Most of the limits given in the PQ diagram are isotherms indicating permitted temperatures of certain generator parts. In the inductive region, the predominant limitation is on the rotor current. If we wish to use the hydrogenerator as a flexible power source and maximize its available capacities, it is crucial to know the rotor temperature. Unfortunately, temperature sensors are not typically installed on the rotor due to its rotation and various associated issues, such as problems with the proper installation of temperature sensors because of large centrifugal forces and strong electromagnetic fields that affect them, issues with the power supply of the measuring system, and difficulties with data transmission from transmitters mounted on the rotating rotor. To ensure the safe operation of the hydrogenerator, we need to monitor the temperature of the rotor winding. The rotor winding temperature can be determined either indirectly or by direct measurements. The indirect method is widely used and is based on measurements of the rotor winding resistance, as specified in relevant standards. It is relatively easy to apply, but the following should be kept in mind: it requires precise measurements of the rotor voltage and current, which can be challenging, and provides only information about the average temperature of the rotor winding. On the other hand, the direct method requires installation of temperature sensors on rotor parts and provides information about the local temperature of rotor part on which the sensor is mounted. The accuracy of the measurement is highly dependent on the way the temperature sensor is mounted and its position. Specifically, the sensor should be mounted in such a way that it is completely isolated from the cooling medium and at the same time has good thermal contact with the part of the generator whose temperature is being measured. This paper presents a comparison of two independent systems for hydrogenerator rotor thermal supervision, along with their respective advantages and disadvantages. The results of measuring the rotor temperature (both indirect and direct) during the heat run test of a hydrogenator at the hydro power plant "Pirot" are also given. Models for comparing the two rotor winding temperature measuring systems are presented with the aim of enhancing the reliability of hydrogenator rotor thermal supervision. These models can be used for hydrogenerator asset management, planning of near-term and long-term outage activities, improved rotor thermal supervision, and more.</p&gt

Improved hydrogenerator rotor thermal supervision

<p>The ongoing energy transition to cleaner energy involves three main changes: using less energy (energy savings on the demand side), making energy production more efficient, and using renewable and low-carbon sources instead of fossil fuels. However, relying more on intermittent renewable energy sources means we need to balance them with conventional sources for a stable electricity supply. Hydrogenerators can provide this stability, but also flexibility, by quickly increasing power when needed. They are designed for a daily average number of start-stop cycles equal to twice per day. On the other hand, they will face new challenges, as they were not designed for frequent and large load changes, which will put additional stress to the hydrogenerator parts. The continuous, safe, and reliable operation of the hydrogenator is determined by the boundaries of the capability curve (active-reactive power PQ diagram) provided by the manufacturer. Most of the limits given in the PQ diagram are isotherms indicating permitted temperatures of certain generator parts. In the inductive region, the predominant limitation is on the rotor current. If we wish to use the hydrogenerator as a flexible power source and maximize its available capacities, it is crucial to know the rotor temperature. Unfortunately, temperature sensors are not typically installed on the rotor due to its rotation and various associated issues, such as problems with the proper installation of temperature sensors because of large centrifugal forces and strong electromagnetic fields that affect them, issues with the power supply of the measuring system, and difficulties with data transmission from transmitters mounted on the rotating rotor. To ensure the safe operation of the hydrogenerator, we need to monitor the temperature of the rotor winding. The rotor winding temperature can be determined either indirectly or by direct measurements. The indirect method is widely used and is based on measurements of the rotor winding resistance, as specified in relevant standards. It is relatively easy to apply, but the following should be kept in mind: it requires precise measurements of the rotor voltage and current, which can be challenging, and provides only information about the average temperature of the rotor winding. On the other hand, the direct method requires installation of temperature sensors on rotor parts and provides information about the local temperature of rotor part on which the sensor is mounted. The accuracy of the measurement is highly dependent on the way the temperature sensor is mounted and its position. Specifically, the sensor should be mounted in such a way that it is completely isolated from the cooling medium and at the same time has good thermal contact with the part of the generator whose temperature is being measured. This paper presents a comparison of two independent systems for hydrogenerator rotor thermal supervision, along with their respective advantages and disadvantages. The results of measuring the rotor temperature (both indirect and direct) during the heat run test of a hydrogenator at the hydro power plant "Pirot" are also given. Models for comparing the two rotor winding temperature measuring systems are presented with the aim of enhancing the reliability of hydrogenator rotor thermal supervision. These models can be used for hydrogenerator asset management, planning of near-term and long-term outage activities, improved rotor thermal supervision, and more.</p&gt