Abstract

Introduction

Settings Description

Settings Comparison and Consequences

Proposed Methodology

Result and Discussion

Conclusion

References

Abstract

     To meet the rapid load increasing in Bangladesh, the government incorporates large power transformers (132/33kV, 80/120MVA) to serve the demand. Due to their low equivalent impedance, it significantly impacts the distance relays zone reach settings, especially in zone-3. This paper presents the effect of large power transformer on zone-3 reach settings, protection zone overlaps, the adjoining long line outside of the protection zone, and their resulting consequences. This paper proposes a systematic approach for overcoming the distance relays zone setting reach problem, ensuring zone overlapping on distance protection relays for the adjoining long line, and incorporating large power transformers into distance protection zone with proper relay coordination. For simulation, the DIgSILENT PowerFactory 2021 software is used. The short-circuit calculation is performed using the IEC 60909 method. For illustrating impedance seen by the distance relay in different fault conditions and locations, the 7SA522 line distance relay of SIEMENS is utilized. Simulations results (R-X diagram) show the effectiveness of the proposed approach, which can save the transformer from firing and minimize its damages in Bangladesh.

Introduction

     The power system of Bangladesh is expanding rapidly and getting complex day by day. The load of the grid substation is also increasing with time due to the government’s hundred percentage electrification target. The government made power system master planning in 2010 and revised it in 2016 [1]. The maximum generation of Bangladesh significantly changed after 2009. In the year 2009-10, the maximum power generation was 4606 MW, and in the year 2018-19, the maximum power generation was 12,893 MW, as shown in Fig. 1 [2]. Due to the pandemic situation (Covid-19), the 2019-2020 generation was near about the same as 2018-2019. Due to the rapidly increasing demand, power evacuation from generation to distribution has become a challenge. A vast infrastructure needs to develop for power evacuation throughout the country. The government has already taken initiatives to establish the infrastructure. Many projects are ongoing at the generation, transmission, and distribution levels. Some projects are finished, and some projects will be completed soon. Due to the rapidly increasing demand, the power sector of Bangladesh passed through a transition period.

Conclusion

     Large power transformers are greatly impacted on distance relay zone reaches. The zone-3 reach of the PGCB method is conservative, which does not cover the next long line in most cases. Therefore, if the proposed philosophy can be adapted in distance protection, it will provide backup protection for the adjoining long line and transformers. The effectiveness of the proposed methodology is shown by quadrilateral impedance diagram of SIEMENS line distance relay (7SA522) using the DIgSILENT PowerFactory 2021 software at different fault conditions at 132kV and 33kV voltage levels. This paper illustrated how distance zone overlapping was missing due to the PGCB settings philosophy and how the proposed methodology effectively ensures the zone overlapping and secures the tripping with maintaining proper relay coordination at 33kV and 132kV voltage levels. This paper also explains the leading cause of the increasing damages of failure transformers in Kushtia and Mymensingh and how damages can be minimized. It is revealed that the proposed methodology can provide fast fault clearing time in case of the circuit breaker or DC failure with proper selectivity, which will significantly reduce the damage of equipment and quick restoration of power systems. In the future, the research will extend for formula estimation of critical length at 33kV level where 33kV and 132kV relays need to coordinate for different transformer conditions and formulating chart for critical 33kV length in Bangladesh perspective.

1- Introduction

2- Set-up description

3- Verification of the PD measuring system

4- PD measurements

5- Improved PD measurements

6- Conclusions

References

Abstract

A partial discharge (PD) measuring system has been deployed in order to identify and measure PD in a high voltage (HV) cable joint under impulse and superimposed voltages under laboratory conditions. The challenge is to enable the detection of PD during the impulse conditions. The method of measurement has been investigated by introducing an artificial defect in the cable joint in a controlled way to create conditions for partial discharges to occur. Next the HV cable system is subjected to AC, impulse and superimposed voltage. Two high frequency current transformers (HFCT) installed at both ends of the cable joint were used to identify PD from the cable joint and to separate PD from disturbance. Transient voltage suppressors and spark gaps are applied to protect the measuring equipment. Band pass filters with selected characteristics are applied to suppress transient disturbances and increase the chance to detect PD during the impulse. PD signals are separated from transient disturbances during data post processing and by means of pulse polarity analysis. The developed system enables the detection of so-called main and reverse discharges respectively occurring during the rise and tail time of the superimposed impulse. The measurement results obtained show the effectiveness of the presented PD measuring system for investigating the effects of voltage transients on a HV cable system in laboratory conditions.

Introduction

Partial discharge measurements provide a useful tool to obtain information about discharging defects in high-voltage equipment. In power cables, PD occurs at insulation defects in particular in cable joints and terminations, especially at interfaces [1]. Therefore, PD measurement on cable systems can be considered a useful tool to diagnose insulation condition for both laboratory application and on-site application [2–4]. PD in power cables is normally measured under AC voltage by using the conventional technique defined by IEC 60270 [5]. In practice, power cables are not only subjected to AC operating voltage, but also to transient voltages such as lightning and switching impulses, which occasionally will be superimposed on the normal AC voltage. Those transient voltages will have an additional stress on the cable insulation. In that regard it is important to investigate PD under impulse and superimposed voltages. One of the challenges in measuring PD under impulse and superimposed voltages concerns the suppression of the disturbances caused by the transient voltages. In laboratory tests, the applied impulse voltage causes currents in the cable under test that disturb the PD measurement. So the PD measurement system needs to have a strong suppression of the disturbance. In such a case, the conventional PD technique is not suitable anymore. The unconventional method based on the measurements of electrical signals in MHz range is of more interest as a better alternative for these conditions [4,6–10]. Three circuits for PD detection under impulse are provided in [11] with a measurement frequency from hundreds of MHz to GHz, namely: the high frequency current transformer (HFCT) with multipole filter, the coupling capacitor with multipole filter, and the electromagnetic couplers. HFCTs or other sensors are commonly used with wide/ultrawide bandwidth together with filters and a digital oscilloscope to detect PD in insulation specimens or models under impulses [12–17]. A coupling capacitor was used to measure PD in material samples in cases where only impulse [18] and square wave voltage were applied [5]. Those PD measuring systems were able to detect PD during the impulse even during its front time. For superimposed impulses, PD was detected in laminated paper using a current transformer and a high-pass filter by Hayakawa et al. [19]. Nikjoo et al. [20] used a wideband detection system consisting of a coupling capacitor, a detection impedance and a low-pass filter to measure PD in oil-impregnated paper. However, in both works, PD was measured during AC cycles before and after impulses instead of during the impulses. Moreover, PD measurements in the above-mentioned works were performed on material specimens.

1- Introduction

2- An analytical electromagnetic model of the “Sen” transformer

3- Simulation case studies

4- Conclusions

References

Abstract

In flexible AC transmission systems, the “Sen” transformer (ST) is an economical and effective method for voltage regulation and power flow control, which can achieve independent control of active and reactive power. The ST is a three-phase multi-winding transformer based on the on-load tap-changing technology. In order to reveal the internal characteristics of the ST, this paper proposes an analytical electromagnetic model considering the multi-winding coupling in the ST with the three-phase three-limb structure. Firstly, according to the electromagnetic coupling relationship in the ST, a magnetic equivalent circuit model consisting of self-inductance coefficients and mutual inductance is derived via the unified magnetic equivalent circuit (UMEC). Secondly, an electric circuit model for the internal voltage and current of the ST is established based on the electrical connection between the ST and the external system. Finally, an analytical electromagnetic model considering multi-winding coupling for the ST is obtained by combining the magnetic equivalent circuit model and the electric circuit model. Two case studies have been carried out on a three-phase three-limb model of the ST with the help of MATLAB. The effectiveness of the proposed model is verified by comparing the obtained analytic results with the existing results from time-domain simulation about the series compensating voltage of the ST. In addition, the voltages and currents of windings in the ST in the conditions of different transformer core structures and ferromagnetic materials are investigated for evaluating the influence of magnetic coupling on the results.

Introduction

With the large-scale integration of renewable energy sources, it has been becoming more and more difficult for the voltage regulation and the optimal power flow control in complex power networks with UHV (Ultra High Voltage) transmission lines as the backbones[1,2]. The unified power-flow controller (UPFC) is an important way to solve the problems of voltage regulation and power flow control [3,4]. However, the high installation and operation costs prevent the UPFC from widespread applications. The “Sen” transformer (ST), as proposed in [5], has the ability of controlling active and reactive power flow independently. Compared with UPFC, the ST is lower in cost, higher in reliability and efficiency [6–8], although reducing the accuracy and dynamic performance [9]. The ST is a typical electromagnetic unified power flow controller, and is usually based on a three-phase multiwinding transformer. By changing the position of secondary winding taps, the ST can generate different voltages, achieving the adjustment of power flow. The adjustment performances between the “Sen” transformer and phase angle regulator (PAR) were compared in [10,11]. The results showed that the overall reactive power of the ST was lower than that of the PAR for enhancing the required amount of active power flow. Therefore, the “Sen” transformer is expected to be a device that will have widespread applications in flexible AC transmission systems (FACTS). As a matter of fact, the ST is a technology based on transformers and tap-changers. From the perspective of the development of electrical topologies and structural variants, many researchers have optimized the topology of the ST for the purpose of improving its power flow control performance. For example, some novel topologies of the ST were proposed in [12–15] for the sake of achieving continuous power flow control of existing transmission lines and improving the utilization rate of existing transmission lines. These topologies include the hybrid electromagnetic unified power flow controller (HEUPFC), the improved hybrid unified power flow controller (IHUPFC) and the power transistor-assisted “Sen” transformer (TAST) which consists of a high-capacity ST and a small capacity UPFC.

1- Introduction

2- Problem formulation

3- Proposed models

4- Experimental results

5- Conclusions

References

Abstract

A factor that directly impacts the lifespan of a power transformer is the hot-spot temperature, and its monitoring is vital to prevent faults, reduce costs, keep the safety, and provide a reliable service to consumers. In this paper, we propose two forecasting models to predict the hot-spot temperature of power transformers. The first is the implementation of Set-Membership in the evolving Participatory Learning with Kernel Recursive Least Squares. And the second is a combination of the evolving Participatory Learning with Kernel Recursive Least Squares and the improved version of the Set-Membership concept, named Enhanced Set-Membership. Both Set-Membership and the Enhanced Set-Membership approaches are implemented to update the rate of change of the arousal index, which is a parameter that controls the creation of rules. A data set collected from an experimental transformer is adopted to evaluate the model’s performance. The obtained results are compared with the performance of the original evolving Participatory Learning with Kernel Recursive Least Squares and with the performance of other classical models suggested in the literature. The proposals have lower errors and a competitive number of final rules, suggesting that the models are efficient approaches to modeling complex data with high accuracy.

Introduction

The power transformer is a critical equipment in power distribution [1]. It is responsible for stepped-up the voltage before to be transmitted over long distances to reduce waste, and stepped-down the voltage to provide the energy to consumers safely [2,3]. Due to the composition of a power transformer, it is the most expensive apparatus in energy distribution [4]. In the case of a power transformer’s failure, when the recovering process is possible, it is slow and inefficient [5]. Thereof, monitoring is vital to prevent faults, reduce costs, keep the safety, and provide a reliable service to consumers [6,7]. The annual spent on power transformers’ monitoring hardware will increase more than $ 642 million in eight years until 2020, according to [8,9], indicating the importance of the power transformers in power distribution. Internal failures are about 10% of the total faults, and, among them, winding and bushing defects represent approximately 44% [10]. The bushing is a fragile component constituted of four parts: insulation, conductor, connection clamp, and accessories[11,12]. In the present work, we considered power transformers composed of Resin-bonded paper bushings (RBP) [13]. The principal factor in bushing failures is the hot-spot temperature, representing 32% of the total causes [11]. The hot-spot temperature is the highest temperature near to the top of the power transformers highvoltage (HV)/low-voltage (LV) windings [7,14] and represents the main limiting factor in the load capacity of the transformer [14], since increases in this temperature reduces the lifespan of the insulation and may determine the end life of the power transformer [15]. As the estimation of the hot-spot is a complex task, many models have been proposed in the literature with the purpose of estimating the hot-spot temperature of power transformers. Among them, the most commonly used in practice is the model based on the IEEE Standard C57.91-2011 [16] which is based on transient heating equations and specific thermal characteristics and parameters of power transformers. This deterministic model is imprecise due to assumed simplifications, and consequently, the power transformer must operate below the maximum capacity to prevent damages [7].

This paper analyzes the impact of COVID-19 pandemic situation on residential loads and local distribution transformer. The transformers’ operating condition in terms of hottest winding temperature, top oil transformer, loss of life (%LOL) are analyzed considering different loads and harmonics content in the transformer currents. Based on these analyses, safe operating conditions of transformer are derived. Six types of consumers having different consumption patterns and different type of residential loads are considered for the analysis. In order to mitigate the adverse impacts of COVID-19 on loads and distribution transformer, the use of behind-the-meter (BTM) photovoltaic (PV) source, battery energy storage and electric vehicle, load scheduling, less utilization of loads that have distortion factors during office hours are proposed. A performance comparison has been made without and with considering the proposed BTM solution, load scheduling and loads having distortion factors. Simulation results show that, due to lockdown situation of COVID-19 issue, the residential loads consumptions increase during office hours and hence the local distribution transformer gets affected. Also, the proposed BTM solution, load scheduling and less utilization of loads having harmonic distortion can cope up with the increased energy consumptions during office hours.

I. INTRODUCTION

THE novel Corona virus (COVID-19) has impacted every aspect of human life not only in USA but also all over the world. The number of deaths due to COVID-19 has been piling up each day. Fig. 1 shows the COVID-19 statistics from March 2020 to October 27th 2020 in Shelby county, Memphis, Tennessee, USA [1]. Many states in USA have declared emergency and imposed orders to be in partial/fully lockdown conditions. In order to remain safe, people were forced to stay at home, work from home wherever possible and the educational institution, public places were closed in Memphis from March 24th, 2020. Because of people’s staying at home, the energy consumption patterns of residential buildings have changed. Specially, during the office hours, the load consumptions in buildings have increased dramatically. Fig. 2 shows the average load consumptions per half an hour of a house located at Memphis city for the consecutive two months (March and April 2020). The data indicates that significant amount of load consumptions increased during the office hours (specially from about noon to 6 pm) due to the lockdown situation in April 2020.

1- Introduction

2- System modelling

3- Surge protective device

4- Stochastic analysis

5- Conclusion

References

Abstract

Vacuum circuit breaker operation causes transient overvoltages that may lead to severe damages to transformer insulation. Since the parameters affecting these overvoltages have stochastic nature, a statistical analysis may provide detailed insight into the overvoltages from the point of view of insulation coordination applications. In this paper, a statistical analysis of the overvoltage variations of the step-up transformer during vacuum circuit breaker operation is conducted. Some variables, including switching angle, current chopping, high-frequency current quenching capability, and wind turbine power, are assumed as random variables. Besides, some indicators representing the switching overvoltage characteristics such as the amplitude, the number of restrikes, and the rate of rise are extracted. By performing a probabilistic analysis, the destructiveness due to switching overvoltages on typical transformer insulation can be investigated. The sensitivity of different surge protective devices and their effectivities are also analyzed and statistically evaluated. The applied methodology is very useful because of the stochastic nature of the switching overvoltages. In this work, the impact of protective equipment on each of the overvoltage indicators is analyzed. It is also found that considering only the amplitudes of transient overvoltages is not enough to draw conclusions about the safety of the transformer when exposed to frequent switching surges.

Introduction

VCBs are commonly used in offshore wind farms due to their ability to eliminate high fault currents and their low maintenance costs. Due to wind speed variations, numerous switching actions (energization and de-energization) commonly occur in wind farms every day. VCB switching transients reach the step-up transformer through the short cable connected to the wind turbine. In case, the amplitude of a switching transient voltage rises above the BIL; it can lead to transformer winding failures. Previous statistical studies related to VCB opening were only focused on the amplitude of transient overvoltages regardless of their rise times [1–4] However, some transformer failures due to VCB operations have indicated that the leading cause of the failure was the high du/dt [5]. Recently performed detailed research showed that due to fast transients and during resonance conditions, the voltages with high amplitudes are non-linearly distributed along the windings [6], and the high du/dt accelerates failure mechanisms because of high interturn overvoltages in the transformer windings; causing coil-to-coil failures in the top or in the middle of the winding. Besides, the critical voltage of the transformer BIL remarkably decreases with the reduction of the rise time [7,8] and the amplitude [7] of the transient voltage applied. Moreover, the experimental studies confirm the effects of lightning impulses on oiled paper [9] and very fast transients on cast-resin insulation [10]. Accordingly, the rate of rise and the number of transient surges should be considered alongside their amplitude in a stochastic analysis in order to accurately assess the effects of the switching overvoltages on transformer insulation lifetime. The characteristics of the transient overvoltages occurring due to VCB operation depend on various parameters. The current chopping phenomenon in VCBs being one of the key parameters that contribute to the reignition overvoltages. Experimental tests conducted in Ref. [11] however, showed the low dependency of this parameter on the operation instant.

1- Introduction

2- Deposition techniques for 3D printing of biomaterials

3- Materials for 3D printing of biomaterials

4- Applications of 3D printed biomaterials

5- 4D printing and its applications for biomaterials

6- Conclusions and future perspectives

References

Introduction

Three-dimensional printing (3D printing) or additive manufacturing (AM) promises to produce complex devices formulated from biomaterials and bioinspired materials from designs that are patient-specific. Since its advent as presurgical visualization technique and fabrication of tooling molds, 3D printing has evolved to yield bespoke devices, implants, scaffolds, diagnostic devices, and drug-delivery systems. As a result of recent public interest and affordable printing platforms, there is the desire to utilize this exciting technology for personalized health care. However, before AM can become a ubiquitous tool for the regeneration of complex tissues, such as bone, cartilage, muscles, vessels, nerves, and complex organs with multifaceted three-dimensional microarchitecture (e.g., kidneys, lymphoid organs), a number of technological limitations must be overcome. The development of advanced bioinks for 3D and 4D printing through the rational design of novel materials and their implementation as functional biomaterials through ever-expanding deposition techniques has improved the diversity and functionality of printed constructs outside of the traditional biofabrication window [1]. While there is the necessity for a compromise between suitability for purpose and biocompatibility, recent developments in materials science and bioink formulations have resulted in an improved range of solutions for biofabrication platforms. New biologically relevant printing formulations with highly consistent and accurate deposition parameters, attractive mechanical properties, high cytocompatibility, and the ability to modulate cellular functions are now being developed and offer prospects as biomaterials for use in a multitude of different applications; however, further work is needed before these novel materials become common place. In this chapter, we explore some of the approaches being pursued to achieve these targets in common 3D and 4D printing techniques, including an overview of the state-of-the-art technologies and formulations used in novel materials and bioinks, which are currently being developed, the techniques which are being utilized to produce 3D and 4D constructs, and the applications for which these new materials may be employed in. Key examples are highlighted to demonstrate the progress of this emerging technology and limitations to its development identified.

1. Introduction

2. Benefit of a thermal criterion in planning

3. Thermal Criterion: Limits in practice

4. Linear thermal criterion

5. Conclusion

Declaration of Competing Interest

Appendix A. Simplification of the thermal model

Appendix B. Linearization

Appendix C. Change of the considered thermal limit

References

Abstract

It has already been shown in the literature that power transformers may be more accurately sized by their thermal limits than by their rated power limit. In practice however, thermal limits are usually considered only in operations; but not at planning stage, where the more usual notion of rated power is used. This paper proposes a novel method to take into account (and benefit from) thermal limits directly at planning stage. This is made possible by quantifying separately the impact of each generator and load on the temperature of the distribution transformer. Decoupling the effect of individual generators and loads is achieved by linearizing and rewriting the analytical expression of the hot-spot transformer temperature. The practical value of the method is assessed using a real-world dataset, by estimating the increase in the hosting capacity of the considered transformer for additional generators and loads. Significant gains are obtained when the transformer is sized by generation, in particular when photovoltaic (PV) generators are involved.

Introduction

Distribution transformers and power (HV/MV) transformers are predominant assets in a distribution grid, and sizing them well is a crucial task for distribution system operators (DSOs). When replacing, reinforcing or adding a transformer, DSOs should find a compromise between the high capital cost of oversizing it, and the risks (in terms of endangering the reliability of electrical energy supply) of undersizing it.

The typical steps of a planning study aiming at sizing a transformer are the following. Some assumptions must first be made regarding future loading conditions, in order to generate a set of “extreme” scenarios for load and generation that will be used as stress-tests to size the transformer. For transformers sized by generation (not consumption), a typical example of such a scenario, currently used some DSOs, is to consider that all generators will output their rated power and that the load will reach its lowest possible value [1], while taking into account forecasts of the future evolution of load and generation.

After loading scenarios have been defined, one of the following two categories of methods may be used [2]:

• The first and simpler one is to simply define physical limits, typically on instantaneous active power but possibly on other criteria, and to size the equipment so that these limits are not violated in any of the stress-tests.

• And the second, more elaborate one, is to perform a so-called “lower cost optimization” that aims at finding an economical trade-off between a smaller transformer that will be heavily loaded and undergo accelerated aging and increased losses, and a larger one that will cost more but not suffer from increased aging and losses [3,4,15].

1- Introduction

2- Frequency response analysis

3- Measurement of transformer winding deformation based on frequency response method

4- Frequency response analysis based on mathematical statistical indicators

5- Conclusion and future work

References

Abstract

Since the power transformers are indispensable in power systems, their reliable operation are of significance. Winding deformation is one of the most common failures within transformers. At present, many methodologies have been proposed to detect these faults, and frequency response analysis (FRA) technique is most frequently used because of its low cost, convenience, simplicity and effectiveness. However, there is no standard and reliable code to interpret the mechanical deformations from FRA traces as so far. In this study, an actual transformer experimental platform was used to simulate variable winding deformation faults in order to standardize the interpretation of winding fault degree from FRA data. The measured FRA raw data are processed from the perspective of statistics, and some numerical indices are found to be more suitable for the diagnosis of transformer winding deformation degree.

Introduction

The stable, reliable, safe and high quality power supply concerns the lives of hundreds of millions of people. As one of the most important pieces of equipment in power system, the stable operation of power transformer plays a critical role in the safe operation of the power system [1,2]. Statistics indicates that 25 percent of transformer faults were caused by winding mechanical deformation. Since the low voltage impulse (LVI) method was first introduced by Lech and Tyminski from Poland in 1966, many methodologies have been successively proposed to detect transformer winding deformation, for instance, the short circuit impedance (SCI) method, frequency response analysis (FRA) method and vibration method. SCI is based on the comparison and analysis of two short circuit impedance tests. In FRA, the frequency response traces are compared with its fingerprint to diagnose the occurrence of internal faults within power transformers. The vibration diagnosis method utilizes the characteristics that the transformer tank vibration will change when the winding fails [3]. Recently, some scholars present other improved techniques, namely, the sweep frequency impedance (SFI) method, which is the combination of SCI and FRA [1], the online impulse frequency response analysis (online IFRA) [4], the electromagnetic wave method [5] and the Lissajous locus method [6]. Above all, FRA is an effective and economic diagnostic technique for detection of mechanical deformations inside a power transformer [7]. FRA is sensitive to failures in the windings and iron core. What’s more, it can provide reliable information about the geometry of active parts of the power transformer without any needs to dismantle the unit. FRA uses input voltage as the input signal and another voltage measured at the other terminal of transformer winding as the output signal, besides, the measured current could also be used as output signal. In most cases, the output voltage is used. However, a standard and widely accepted code for interpretation of FRA traces can be improved; at present, experienced personnel are still needed to diagnose winding faults by using visual inspection [8], which is still a matter of great concern. At present, there are variable FRA interpretation methods, for instance, the equivalent electric model simulation method [9], the artificial intelligence method [10], and the mathematical statistics method [11]. In the first method, finite element analysis (FEA) of transformer winding is often used to build its equivalent electric model [12], to aid in interpreting the FRA curves. N. Hashemnia has studied the impact of winding axial displacement and radial deformation fault on equivalent electric parameters and FRA curves based on FEA [8,13], to improve the detection of winding faults. Besides, Z. W. Zhang has built a hybrid model based on FEA and multi-transmission line (MTL) to study the FRA curve beyond 1 MHz [14]. In the artificial intelligence method, M. Bigdeli has applied support vector machine (SVM) algorithm to classify the winding faults [15].

1- INTRODUCTION

2- DESCRIPTION OF MODEL

3- CONCLUSIONS

REFERENCES

Abstract

When a transformer works under the operating mode an emergency short circuit may occur. This is a dangerous emergency mode, in which the transformer currents are many times higher than their rated values. Such currents may lead to isolation overheating, reliability decreasing or even to transformer failing. In order to prevent damage during the critical mode and increase the transformer reliability by using a protector, it is necessary to determine the short circuit current value. The article considers the problem of optimal accurate design by the method of mathematical transformer modeling by determination of short circuit current value in emergency mode.

INTRODUCTION

Computer technology progress has contributed greatly to the mathematical modeling development in all areas of science. The mathematical modeling allows to research a model of interest with minimizing the utilized resources. Modeling are used when a real experiment performing is impossible, dangerous or difficult and expensive [1]. The main task in modeling is the adoption of an adequate model which properties will be sufficiently close to the real simulated properties.

Now the problem of electromechanical compatibility of electric power system facilities is particularly acute. In [8-11], the authors touched on this important topic for the nominal operation mode of an induction motor. Of particular interest are emergency modes, such as short circuits.

A wide-band, frequency-dependent five-terminal model is developed that represents one HVDC transformer unit in the French-English IFA2000 HVDC interconnection. Three such interconnected 1-ph units constitute one 3-ph transformer bank needed in 12-pulse conversion. The model is obtained via admittance frequency sweep measurements on the transformer’s terminals, including common-mode measurements to capture the high-impedance coupling to earth at lower frequencies. The data set is modified to reduce the magnetizing current to a realistic level by a novel eigenvalue scaling procedure. The final data set is subjected to modeling by a stable and passive rational model while utilizing a mode-revealing transformation to retain the accuracy of the small eigenvalues that are related to the high-impedance coupling to earth. The paper describes details related to the measurements and modeling steps as well as the many intermediate accuracy validations that were done. Also is described challenges in the measurements that resulted due to interference of the measurements by a nearby 400 kV overhead line. The model is applied in a time domain simulation of the complete HVDC link in normal operation where voltage waveforms resulting from line commutations are compared against those by a classical model with added stray capacitances.

Introduction

HVDC transformers are together with thyristor valves the fundamental components in any LCC-type HVDC converter. The protection of these transformers is therefore essential for the reliable operation of the HVDC link. Overvoltages due to switching events, lightning impulses and in-station flashovers on the AC side result in steep-fronted overvoltages that stress the transformer AC side insulation parts, and voltages are also transferred to the DC-side where they stress and interfere the thyristor components [1]. Conversely, line faults and lightning strokes to a DC overhead line may result in overvoltages that impose stresses on the transformer DC-side terminals. The protection of HVDC substation against overvoltages therefore requires insulation co-ordination studies to be performed to determine the appropriate location of surge arresters [1]. Insulation co-ordination studies requires the use of models with adequate accuracy up to several hundred kHz in order to represent the actual waveshapes of the overvoltages. The representation of the HVDC transformer is a main obstacle in such studies since frequency-dependent transformer models are in general not available. Therefore, simplified models are usually employed, often consisting of a 50 Hz model in combination with lumped capacitors.

1- Introduction

2- Proposed new solutions

3- Protection testing

4- Summary and recommendations

References

Abstract

This paper presents a new approach to differential protection of power transformers with use of the integral principle. The required criteria signals are calculated directly from the operational and restraining currents in particular phases. Additional support of the second harmonic is also used. The proposed integral principle is simple and exhibits superior performance against other commonly applied algorithms. The theoretical investigations are followed by testing with simulation runs as well as recorded signals.

Introduction

The investigations related to differential protection of power transformers are presented in this paper, whereas special attention was put to relay performance for: – external fault cases with CT saturation, – magnetizing inrush due to transformer energization, – internal transformer faults. The problem of differential protection behavior for power transformer energization as well as for external faults with CT saturation is not new. There have been a lot of cases reported when transformer differential protection maloperated despite of the use of typical characteristics and stabilization approaches. Apart from the percentage differential characteristic, the commonly accepted solution for transformer protection stabilization is the second harmonic ratio principle, which is applied in products of most relay manufacturers, e.g. [1].