Resincap Journal of Science and Engineering
Volume 4, Issue 4, April 2020 ISSN: 2456-9976
978
UPFC Using a Power Electronics Integrated Transformer
Bhagyashri D Nimbalkar PG Student
Electrical Engg.Dept.
SSGBCOET Bhusawal, [email protected]
Prof. Girish K Mahajan Associate Professor Electrical Engg.Dept SSGBCOET, Bhusawal [email protected]
Prof.Ajit P. Chaudhari Associate Professor Electrical Engg.Dept.
SSGBCOET Bhusawal, [email protected]
ABSTRACT
This paper presents a Unified Power Flow Controller (UPFC) application of the Custom Power Active Transformer (CPAT); a power electronics integrated transformer which provides services to the grid through its auxiliary windings.
The Custom Power Active Transformer (CPAT) which integrates both series and shunt power conditioning through power electronics in a single transformer. The CPAT structure integrates three single-phase transformers into one shunt- series combining transformer. Unified Power Flow Controller (UPFC) is used to control the power flow in the transmission systems by controlling the impedance, voltage magnitude and phase angle. This controller offers advantages in terms of static and dynamic operation of the power system. The CPAT equipped with a power converter can be utilized in distribution systems to control grid-current and load-voltage waveforms while operating as a step-up or step-down transformer between the grid and load. The CPAT’s capability to provide UPFC services which includes power flow control, reactive power compensation, voltage regulation and harmonics elimination. The Unified Power Flow Controller (UPFC) is the most versatile among a variety of Flexible AC Transmission System (FACTS) devices, which can be used for power flow control, enhancement of transient stability, damping system oscillations and voltage regulation.
Keywords
UPFC, UPFC using CPAT, Control Mechanism of CPAT.
1. INTRODUCTION
The increased demand for distributed generation to facilitate momentous contributions to the grid has faced several challenges and technical issues. Owing to the intermittent behavior of renewable generation and the ever-growing need of electrical energy, the construction and operation of substations has undergone several developments to address these challenges. The increase in peak customer demand of electrical energy, use of non-linear loads and increased line congestion, all have eventually affected power quality, system reliability, system stability and energy price. In this regard, FACTS devices propose a high-performance and cost- effective solution for power system compensation demands.
The key objective of FACTS is to get better the power transfer, thermal and limit potential of present transmission lines near the thermal limit and to effectively control the power flows in the selected corridors. Such devices have proved to enhance and achieve better exploitation of transmission and distribution facilities. In such compensation systems, the transformer is an essential element to adapt voltage levels between the power converter and electrical grid, as well as isolating both systems to avoid using complex power electronics structures. Flexible AC Transmission Systems (FACTS) have proven their capability in providing services to effectively support the transmission and
distribution systems, increasing their reliability, quality and stability. Among the FACT devices, the Unified Power Flow Controller (UPFC) is the most versatile and powerful device in reduction of line congestion and increasing existing lines capacity. Unified Power Flow Controller (UPFC) application of the Custom Power Active Transformer (CPAT); a power electronics integrated transformer which provides services to the grid through its auxiliary windings. The CPAT structure integrates three single-phase transformers into one shunt- series combining transformer. The CPAT’s capability to provide UPFC services which includes power flow control, reactive power compensation, voltage regulation and harmonics elimination. Connection of power electronics converters to provide UPFC services have either been achieved through bulky isolation transformers, complex multilevel topologies or back-to-back converters handling the rated line power. The CPAT is a monolithic transformer core structure that integrates series and shunt power electronics converters to a distribution transformer. A CPAT is comparable to a Sen Transformer in the case of combining multiple transformers into a single unit. However, the CPAT carries several advantages over a Sen Transformer which is mainly due to the presence of power electronics converters in a CPAT as opposed to the step response of a Sen Transformer.
The CPAT has been presented to provide shunt services such as reactive power compensation, harmonics elimination and inrush current mitigation. The high-power transformers are an essential element in a power system to match the voltage level between different buses, it would be interesting to integrate in such a transformer series and shunt auxiliary connection to power electronics converters. Integration of both series and shunt transformers in a single power transformer would facilitate areas of the power system and the transformer itself with services in a single structure. Hence the structure would provide an isolated connection of fractional power converters to the system as well as reduce the footprint of the entire system, manufacturing cost and number of auxiliary equipment (tanks, bushings and protection), while providing the grid with series and shunt services.
2. LITERATURE REVIEW
Tanushree Kaul, Pawan Rana et. al. 2013 In the recent years ecological concerns and high installation costs have put constraints over construction of new plants and overhead lines in many countries, thereby forcing existing system to be used more efficiently rather than constructing new lines, industry has tended towards the development of technologies or devices that increase transmission network capacity while maintaining or even improving grid stability. Our main objective is to meet the electric load demand reliably while simultaneously satisfying certain quality constraints imposed on the power supply.[1] Sadjad Galvani, Mehrdad Tarafdar Hagh et. Al. 2014 unified power flow controller (UPFC) operation which can accurately reflect the impact of UPFC on power system steady security. Economic benefit of installing
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an static synchronous series compensator (SSSC) and adding a new transmission line to power system considering equipment availability and load uncertainty. In addition, application of flexible alternative current transmission systems (FACTS) controllers is obvious and has been considered in various aspects of system operation and planning problems such as system reliability increasing, fuel cost and loss minimization, improvement of system load ability, voltage stability increasing.[1]
In (2015) Shantha Soruban et, al. proposed an ANN based control scheme for a UPFC to be used as an active power filter. The objective is to guarantee power to the load at the required power quality. The ANN control unit monitors the voltage at the point of common coupling. UPFC enables improved power quality by maintaining power factor nearer to unity rapid response time, the ability to provide reactive power at low voltage and to provide voltage compensation can be obtained. For unbalanced voltage compensation, two unbalanced controllers using the phase voltage amplitude and negative sequence component are proposed [2].
In (2013) Vaibhav S Kale et, al. proposed the real, reactive power and voltage control through a transmission line by placing UPFC at the sending end using computer simulation.
The control scheme has the fast dynamic response and hence is adequate for improving transient behaviour of power system after transient conditions [2].
Eldamaty et al (2005) presents a new control method based on fuzzy logic technique to control a unified power flow controller (UPFC) installed in a single-machine infinite-bus power system. The objective of the fuzzy logic based UPFC controller is to damp power system oscillations. Phillips- Herffron model of a single-machine power system equipped with a UPFC is used to model the system. The fuzzy logic based UPFC controller is designed by selecting appropriate controller parameters based on the knowledge of the power system performance. Simple fuzzy logic controller using mamdani-type inference system is used. The effectiveness of the new controller is demonstrated through time-domain simulation studies. The results of these studies show that the designed controller has an excellent capability in damping power system oscillations. By using power electronic controllers a flexible ac transmission system (FACTS) can offer greater control of power flow, secure loading and damping of power system oscillations. The FACTS refers to power electronic systems such as the static VAR compensator (SVC), thyristor controlled series capacitor (TCSC), static condenser (STATCON) and unified power flow controller (UPFC). A unified power flow controller is a power electronic system which can provide VAR compensation, line impedance control and phase angle shifting. The UPFC consists of two fully controlled power electronic converters as illustrated in Figure (2.2.1) Converter2 is connected in series with the transmission line by transformer T2, whereas Converter1 is connected in parallel with the transmission line by transformer TI. The real and reactive power flow in the transmission line can be quickly regulated by changing the magnitude and phase angle of the injected voltage produced by Converter. The basic function of Converter1 is to supply the real power demanded by Converter2 through the common dc link Converter1 can also generate or absorb controllable reactive power [2],[12].
2.1 UPFC
UPFC is such a controller which can independently or simultaneously control the exchange of real and reactive powers with ac system, which in turn take care of power flow control effectively to improve the power system performance.
The character of FACTS equipment is physically moderately nonlinear. The execution of these devices may be deteriorated whenever these are coupled to a power system. Therefore to deal with the dynamic variations in power system, nonlinear intellectual regulator based UPFC is essential to be modelled.
The UPFC is a centralized control device with the aim of attaining a better service quality, in terms of all nods voltage regulation and power loss minimization in loop distribution system, simultaneously[11].
The general structure of the UPFC contains two “back-to- back” voltage sourced converters using isolated gate bipolar transistor (IGBT) with a common DC link Figure2.2.1. First converter is connected as parallel and another converter as series with the transmission line. The shunt converter is used to provide active power demanded of the series converter through a common DC link. The series converter provides the main function of the UPFC by injecting an AC voltage with controllable magnitude and phase angle.
Fig.No.1 Schematic Diagram of Three Phase UPFC The transmission line current flows through this converter and therefore an active and reactive power exchange with the AC system. Since the converters are connected to a common DC link, they exchange only active power and there is no reactive power flow between them. It means that reactive power could be controlled independently at both converters.
In the parallel branch of UPFC the active power exchanges with the phase angle of the converter output voltage. In the series branch of UPFC the active and reactive power flows in the transmission line are influenced by the amplitude as well as the phase angle of the series injected voltage. Therefore, the active power controller can significantly affects the reactive power flow and vice versa. The energy storing capacity of this dc capacitor is generally small. Therefore, active power drawn by the shunt converter should be equal to the active power generated by the series converter. The reactive power in the shunt or series converter can be chosen independently, giving greater flexibility to the power flow control.
The fundamental theory of UPFC is that, the phase angle affects flow of real power and the magnitude of voltage affects flow of reactive power. As a result in transmission
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lines, to manage the real power flow the series regulator of UPFC modifies the series injected voltage angle, at the same time as the amount of series injected voltage regulates the reactive power flow. Thus the real power regulator can appreciably have an effect on the level of reactive power flow.
The reactive power regulator modifies the value of series voltage added to the system which further alters real power flow. Therefore these two regulators are responding to other’s effects.
The powerful, hitherto unattainable, capabilities of the UPFC summarized in terms of conventional transmission control concepts, can be integrated into a generalized power flow controller that is able to maintain prescribed, and independently controllable, real power P and reactive power Q in the line. Within this concept, the conventional terms of series compensation, phase shifting etc., become irrelevant;
the UPFC simply controls the magnitude and angular position of the injected voltage in real time so as to maintain or vary the real and reactive power flow in the line to satisfy load demand and system operating conditions.[3]
2.2.1 Control Strategy of UPFC
UPFC series and shunt converter control is the core of the control strategy of the UPFC, including DC voltage control, AC voltage control, power flow adjustment, start control.
Regardless of the topology adopted by a UPFC, it is always a combination of a series converter and a shunt converter connected back to back via a common DC bus. The shunt converter absorbs active power from the grid, provides it to the series converter and compensates the active power loss for circuit devices, avoiding system collapse caused by a DC capacitor voltage drop. The shunt converter also provides reactive power for the grid via a shunt transformer to maintain the voltage stability of the UPFC connecting point. Besides, the series converter injects a voltage with adjustable amplitude and phase via a series transformer, in order to control the flow in the line. Therefore, the shunt converter aims to maintain a constant DC bus voltage and provide reactive power for the grid, while the series-side converter aims to achieve active and reactive power regulation of the line by changing the amplitude and phase of the inverter output voltage.
3. UPFC using CPAT
Fig.No 2 Basic circuit arrangement of the Unified Power Flow.
DC building microgrids promise significant efficiency The Unified Power Flow Controller (UPFC) was proposed for real-time control and dynamic compensation of ac transmission systems, providing the necessary functional flexibility required to solve many of the problems facing the utility industry. The Unified Power Flow Controller (UPFC) is able to control both the transmitted real power and, independently, the reactive power flows at the sending and the receiving-end of the transmission line. The unique capabilities of the UPFC in multiple line compensation are integrated into a generalized power flow controller that is able to maintain prescribed, and independently controllable, real power and reactive power flow in the line.
The Unified Power Flow Controller consists of two switching converters, which in the implementations considered are voltage sourced inverters using gate turn-off (GTO) thyristor valves, as shown in Figure (2). These inverters, labelled
“Converter 1” and “Converter 2” in the figure, are operated from a common dc link provided by a dc storage capacitor.
This arrangement functions as an ideal ac to ac power converter in which the real power can freely flow in either direction between the ac terminals of the two inverters and each inverter can independently generate (or absorb) reactive power at its own ac output terminal.
The basic function of Converter 1 is to supply or absorb the real power demanded by UPFC consist of two back to back converters named converter1 and converter2, are operated from a DC link provided by a dc storage capacitor. These arrangements operate as an ideal ac to ac converter in which the real power can freely flow either in direction between the ac terminals of the two converts and each converter can independently generate or absorb reactive power as its own ac output terminal. One converter is connected to in shunt to the transmission line via a shunt transformer and other one is connected in series through a series transformer. The DC terminal of two converters is coupled and this creates a path for active power exchange between the converters. Converter can provide the main function of UPFC by injecting a voltage with controllable magnitude and phase angle in series with the line via an injection transformer. This injected voltage act as a synchronous ac voltage source. The transmission line current flows through the voltage source resulting in reactive and active power exchange between it and the ac system. The reactive power exchanged at the dc terminal is generated internally by the converter. The real power exchanged at the ac terminal is converted into dc power which appears at the dc link as a real power demand. And converter1 is to supply or absorb the real power demanded by converter2 at the common dc link to support real power exchange resulting from the series voltage injection. This dc link power demand of converter2 is converted back to ac by converter1 and coupled to the transmission line bus via shunt connected transformer.
In addition, converter1 can also generate or absorb controllable reactive power if it is required and thereby provide independent shunt reactive compensation for the line.
Thus converter1 can be operated at unity power factor or to be controlled to have a reactive power exchange with the line independent of the reactive power exchanged by converter1.
Obviously, there can be no reactive power flow through the UPFC dc link.[3]
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A three-phase back-to-back converter was connected to the shunt and series windings of the CPAT. As in a typical UPFC, the shunt converter operated as a current controlled voltage source inverter (CCVSI) equipped with an LCL filter, to attenuate switching frequency harmonics. The filter parameters L1sh, Csh and L2sh were selected based on the required attenuation of switching frequency harmonics and resonance frequency. Damping of filter resonance was achieved through the shunt damping resistance (Rsh). The converter was connected in a three-phase 4-wire topology to facilitate the capacity to inject triplen harmonic current in the shunt windings. The magnetizing harmonic currents required by the transformer were evident. Therefore, injection of such harmonic current components through the shunt winding would eliminate their requirement from the grid. The shunt converter controller maintains a constant DC bus voltage (Vdc1, Vdc2) over each DC bus capacitor (Cdc) and controls the shunt converter current (ip2sh). Primary voltage (Vp1) and current (ip1) were measured to synchronize the shunt converter voltage (Vp2) with the Vp1 and to provide the required services to ip1. The output PWM signals of the shunt converter controller drove the converter switches of the shunt converter to control the shunt current according to the required reference.
The series inverter operated as a voltage source inverter, equipped with an LC filter to attenuate switching frequency harmonics of the output voltage (vp3). Similarly, the filter parameters Lser, Cser and Rser were selected based on the required attenuation of switching frequency harmonics and resonance damping. The secondary voltage (vp4) and current (ip4) were measured to control the series voltage (vp3) according to the required services provided to ip4. The output PWM signals from the series converter controller drove the series inverter to achieve the required reference series voltage.
As shown in Figure 3, each phase of the primary, shunt, series and secondary winding were linked in a common CPAT core, resulting in a three-phase CPAT configuration.
Fig.No 3.Back-to-back converter topology for the three- phase CPAT
3.2 STATCOM Controller
The STATCOM controller shown in Figure 4. A phase-locked loop (PLL) that synchronizes on the positive-sequence
component of the three-phase primary voltage, V1. The output of the PLL (angle Θ=ωt) is used to compute the direct-axis and quadrature-axis components of the AC three-phase voltage and currents (labelled as Vd, Vq or Id, and Iq on the diagram). Measurement systems measuring the d and q components of the AC positive-sequence voltage, currents to be controlled, and the DC voltage Vdc. An outer regulation loop consisting of an AC voltage regulator and a DC voltage regulator. The output of the AC voltage regulator is the reference current Iqref for the current regulator, where Iq is the current in quadrature with voltage that controls reactive power flow. The output of the DC voltage regulator is the reference current Idref for the current regulator, where Id is the current in phase with voltage that controls active power flow. An inner current regulation loop consisting of a current regulator. The current regulator controls the magnitude and phase of the voltage generated by the PWM converter (V2d and V2q) from the Idref and Iqref reference currents produced respectively by the DC voltage regulator and the AC voltage regulator (in voltage control mode). The current regulator is assisted by a feed-forward-type regulator that predicts the V2 voltage output (V2d and V2q) from the V1 measurement (V1d and V1q) and the transformer leakage reactance
Fig.No.4 STATCOM Controller
4. PERFORMANCE ANALYSIS
A Simulink model is used to simulate its performance and simulated results are presented in detail. The model proposed here was developed in Matlab/Simulink and Matlab/Simscape. The source is of 11kV, trnsmission line of 150 kM & load is of 10MW and 2MVAR.
4.1 Simulation of UPFC
Fig.No 5. Simulation of UPFC
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The Simulink model of UPFC. UPFC consists of all the features of STATCOM, SSSC. Static Synchronous Series Compensator (SSSC) is a series connected controller, which is capable of providing reactive power compensation to a power system. It injects an almost sinusoidal voltage with variable amplitude. It is equivalent to an inductive or a capacitive reactance in series with the transmission line. The STATCOM is connected in shunt. The magnitude of bus voltage depends on reactive power flowing from shunt inverter. Thus, by controlling the reactive power in shunt inverter, the magnitude of bus voltage can be controlled.
with DC bank capacitor is shown in figure 5. The amount of losses in a power system is in primary and secondary distribution lines. While transmission and sub-transmission lines account for only about 30% of the total losses.
Therefore, the primary and secondary distribution systems must be properly planned to ensure within limits. The unexpected load increase was reflected in the increase of technical losses above the normal level Losses are inherent to the distribution of electricity and cannot be eliminated.
The 11 KV and 415 volt lines in rural areas are extended over long distances to feed loads scattered over large areas. Thus, the primary and secondary distributions lines in rural areas are largely radial laid usually extend over long distances. This results in high line resistance and therefore, high I2R losses in the line. To overcome the transmission line losses most versatile FACTS device is used i.e. UPFC. FACTS Controller referred to as the Unified Power Flow Controller (UPFC). It utilizes the synchronous voltage sources to provide comprehensive control of power flow in transmission systems.
Installing the UPFC can improve power transfer capability.
The UPFC is an advanced power system device capable of providing simultaneous control of Instantaneous speed of response voltage magnitude, active and reactive power flows in an adaptive fashion.
The UPFC is the most versatile device which designed based on the combination of both series and shunt FACTS devices.
Shunt FACTS device (Shunt Converter) or STATCOM (Static Synchronous Compensator) is used to control the reactive power flow in the transmission line. Series FACTS device (Series Converter) or SSSC (Static Synchronous Series Compensator) is used to control the series injecting voltage and phase angle which is injected to the transmission line through series transformer. These two converters are combined with a common dc link. This capacitor has small energy storing capacity. So, active power generated by the series converter should be equal to the active power flow through the shunt converter.
4.2 Simulation of UPFC using CPAT
The Simulink model of UPFC using CPAT is shown in figure 6 Simulations of the proposed CPAT structures are evaluated for power flow control applications and elimination of transformer magnetizing current harmonics from the grid.
Furthermore, experimental results on a stiff grid shows the capability of a CPAT to control power flow through its series winding as well as provide harmonics and reactive power compensation through its shunt winding. A three-phase back- to-back converter was connected to the shunt and series windings of the CPAT. The converter was connected in a three-phase 4-wire topology to facilitate the capacity to inject triple harmonic current in the shunt windings.
A three-phase CPAT is compared to a three-phase compensation system consisting of a three-phase shunt transformer, three-phase series transformer and three-single phase shell-type power transformers. The CPAT combines a shunt, a series and an isolation transformer into a single transformer. The main advantage of the CPAT over a conventional UPFC solution, based on using multiple transformers, can be summarized in terms of reduction of core material, winding and manufacturing cost. The CPAT can replace distribution transformers to provide services that will help maintain power system stability and reliability.
Fig No 5 Simulation of UPFC using CPAT
4.3 Result
4.3.1 Result of UPFC:
Fig.No 6 Result- Source Voltage & Current
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Fig.No 6 Result- Load Voltage & Current
Fig.No 7 Result- Load Active & Reactive Power Figure (5) & (6) the performance of the system when UPFC is connected to the transmission line. Figure (5) shows the source voltage @ the current while figure (46) same for the load. The source voltage is of 11kV while the voltage at sending end is dropped to 10kV. Fig 8 shows the Active &
Reactive Power consumed by the load. Active power received is 6MW and reactive power is of 0.75MVAR.
Fig.No 8 Result- Source Active & Reactive Power Fig 8 shows the Active & Reactive Power by the source.
Active power transmitted is 8MW and reactive power is of -
0.75MVAR. we can observed that the reactive power is compensated by the UPFC.
4.4 Result of UPFC using CPAT
Fig.No 9 Result- Source Voltage & Current.
Fig.No 10 Result- Load Voltage & Current Figure (9) & (10) the performance of the system when UPFC using CPAT is connected to the transmission line. Figure (9) shows the source voltage @ the current while figure (10) same for the load. The source voltage is of 10kV while the voltage at sending end is increased to 11kV.
Fig.No 11Result- Source Active & Reactive Power Fig 11 shows the Active & Reactive Power by the source.
Active power transmitted is 12.74MW and reactive power is of -1MVAR. We can observe that the reactive power is compensated by the UPFC.
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Fig.No 12 Result- Load Active & Reactive Power Fig 12 shows the Active & Reactive Power consumed by the load. Active power received is 12MW and reactive power is of 3.5MVAR.
CONCLUSION
The Unified Power Flow Controller, from the viewpoint of conventional transmission compensation and control, is an apparatus that can provide simultaneous, real-time control of all or any combination of the basic power system parameters (transmission voltage, line impedance and phase angle) which determine the transmittable power. This paper has presented the CPAT-UPFC consisting of three single-phase CPATs equipped with a back-to-back converter. Through the available shunt and series windings in a CPAT, several services can be supplied to the grid such as grid harmonic currents elimination, reactive power compensation and power flow control. The presented control architecture has been evaluated through simulations and an experimental prototype demonstrating the ability of a CPAT to operate as a UPFC.
The analysis, simulation and experimental results confirm the CPAT-UPFC ability to provide the required services.
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