Naslov
Unified Power Flow Controller in alleviation of voltage stability problem

Autori
Dizdarević, Nijaz

Vrsta, podvrsta i kategorija rada
Ocjenski radovi, doktorska disertacija

Fakultet
Fakultet elektrotehnike i računarstva Zagreb

Mjesto
Zagreb

Datum
12.10

Godina
2001

Stranica
120

Mentor
Tešnjak, Sejid i Andersson, Goeran

Ključne riječi
power system voltage stability; load modelling; FACTS; UPFC; sensitivity analysis

Sažetak
ABSTRACT: The problem of voltage stability with voltage collapse as its final consequence is an emerging phenomenon in planning and operation of modern power systems. The increase in utilisation of existing power systems may get the system operating closer to voltage stability boundaries making it subject to the risk of voltage collapse. This possibility in association with several incidents throughout the world have given major impetus for analysing this problem with increased interest in modelling of generation, transmission and distribution/load levels of electric power systems. Since the rapid development of power electronics has made it possible to design power electronic equipment of high rating for high voltage systems, the problem resulting from transmission system may be, at least partly, improved by use of the well-known FACTS-controllers. The de-regulation (restructuring) of power networks will probably imply new loading conditions and new power flow situations. This is an additional reason to face the FACTS. In order to deal with the voltage stability problem, the solutions with FACTS-controllers must provide voltage support and/or appropriately co-ordinated control actions. Apart from several larger rating prototypes of Static Var Generator, as an improved version of Static Var Compensator, no other modern FACTS-controller application has been considered to help solve the voltage stability problem. This is considered to be a new impetus, since the most often the analyses are concentrated only on a power flow regulation and damping methods of electromechanical oscillations. Therefore, a work on this dissertation has been launched aiming to fulfil following general objectives: - set a comprehensive explanation of voltage stability problem and mechanism of voltage collapse in a multi-machine power system, - develop analytical methods useful in the problem solving by early recognition of impending voltage collapse within combined dynamic and static approach, and - develop preventative measures and actions using adequate FACTS-controllers (especially Unified Power Flow Controller, UPFC) to alleviate the problem and increase transmission throughput. The thesis goal is stated as to show how it is possible to alleviate voltage stability problem by using modern, power electronics based, FACTS device named UPFC. In the elaboration of the thesis, a computer analysis of power system voltage stability has been taken. In order to analyse dynamic phenomena, a FORTRAN computer program for mid-term time domain simulation of multi-machine power system is developed. By combining dynamic and static aspects of analysis, its prime focus is in off-line simulation of voltage emergency situations using differential-algebraic model of power system. In-house developed program is tested and verified on the basis of other commercially available software. Eventually, it could be installed in a control centre serving for voltage security evaluation of a power system. There have been several phases in the thesis elaboration. In the first phase of its development, the program for time domain simulation has been made. It has featured different generator models with generic excitation and speed governor - turbine control systems, models of static and dynamic loads, induction motors, LTCs, transmission lines and buses... Computational methods that have been included have comprised fast de-coupled transient load flow along with Gauss-Seidel and Jacobi load flows, sparse matrix techniques, and 4th-order Runge-Kutta method of solving differential equations by using constant step-size integration. In the second phase, the static aspects of linearised differential-algebraic power system model have been included, concerning properties of state and extended Jacobi matrices that are useful in general stability analysis (singular and eigenvalue decomposition, sensitivity analysis, and participation factors). Static aspects are very helpful in recognition of voltage stability problem, while modern means of voltage support are sought for should it be applied thereafter. Thus, the third phase of the thesis elaboration has been initiated turning out the need for modelling of the FACTS device. The UPFC injection model has been set up along with its control structure. The UPFC in its general form can provide simultaneous, real-time control of all basic power system parameters (transmission voltage, impedance and phase angle) and dynamic compensation of ac system. While the static aspects help the voltage stability problem being recognised through the matrix decompositions, the UPFC, heart and soul of the thesis, represents source of voltage support and power flow control. The UPFC has been studied concerning its possibilities to help voltage stability problem solved. Combined static and dynamic approach is used to analyse the control system of the UPFC injection model. It fulfils functions of reactive shunt compensation, voltage regulation, line power flow regulation, series compensation and PAR/QBT phase shifting meeting multiple control objectives. The injection model enables three parameters to be simultaneously controlled, namely the shunt reactive power, Qconv1, and the magnitude, r, and angle, gamma, of the injected series voltage. The control system is of de-coupled single-input single-output type. The selection of input/output signals depends on the predetermined control mode, which could be changed during simulation. At external level, following locally measured variables of the UPFC are controlled: - shunt side bus voltage magnitude, Vi, (by changing Qconv1), - series side bus voltage magnitude, Vj, - reactive power flow into series side bus, Qj2, - reactive power requirement of the series converter, Qconv2, or - compensating voltage magnitude, Vcomp, (by changing r), and - active power flow into the series side bus, Pj2, - active power requirement of the series converter, Pconv2, - bus voltage angle difference, fiij, or - compensating voltage angle, ficomp, (by changing gamma). In the model, the shunt side is controlled only in the voltage mode Vi-Qconv1, emphasising that Qconv1 represents reactive power loading of the shunt converter. The series side is controlled through the r-gamma pair in several different modes: bus voltage and active power flow Vj-Pj2, reactive and active power flow Qj2-Pj2, series compensation Qconv2-Pconv2(=0), phase shifting of Phase Angle Regulator (PAR) type Vj (=Vi)-fiij, and phase shifting of Quadrature Boosting Transformer (QBT) type Vcomp-ficomp(=pi/2). The variables are chosen satisfying general V-Q and theta-P de-coupling. Damping of electromechanical oscillations based on transient energy function is implemented in the model as well. The strategy is of take-over type using time derivatives of local variables from both sides of the UPFC. The shunt part of TEF block uses information from dVi/dt, whereas the series part from dthetaij/dt. The sensitivity analysis, applied in more general sense, contributes to find appropriate location and action of the UPFC. The UPFC has rather unique capability of simultaneous series control of branch power flow and shunt control of bus voltage magnitude. By correlating branch and bus participation factors obtained from the matrix decompositions, it is possible to recognise location of its installation where appropriate regulation characteristics could be extensively used during voltage emergency situations. Since the voltage support capability of the UPFC is usually achieved by using two of its parameters, there is an additional advantage of its design, which enables simultaneous control of the third parameter as well. Sensitivity analysis of the minimum singular value, total active power loss, and total reactive power generation is carried out to recognise critical points of power system during voltage collapse scenario. It serves to show capability of having all three UPFC parameters controlled simultaneously. Often, sensitivity involves the inverse of the state and the extended Jacobi matrix. Therefore, it has larger magnitudes as the critical point is approached with abrupt change to opposite sign as it is crossed. The appearance of the critical point represents a reliable sign of impending voltage unstable situation and could trigger voltage support from the UPFC. Therefore, if the sensitivity analysis is applied with respect to the set of the UPFC control parameters it is possible to define its adequate regulating action. Within the thesis, special attention is paid to load modelling, since the load is considered to be the driving force of the voltage stability problem. After being shortly disturbed with a consequence of permanently decreased voltage magnitude level, by its mid-term recovery process of re-establishing initial power consumption, the loads could bring generators under over-excitation limiting conditions and/or cause transmission system being incapable of requested power delivery. Therefore, several different load models (static and dynamic) are analysed regarding its contribution to voltage collapse. Besides static ZIP load model, there are included dynamic models of induction motors and thermostatically controlled loads in presence of the LTC transformers, and first order non-linear recovery loads. Besides to the set of the UPFC control parameters, the static aspects and the sensitivity analysis are applied to the characteristic parameters of the load models as well. This helps to conclude about the most influential loads and their critical parameters that drive the system to the collapse. Benefits of the proposed methodology are explored by analysing a multi-machine test system using in-house developed computer program. According to the established general objectives, it is possible to point out the main contribution and achievements of the thesis as follows: - development of the differential-algebraic model of the multi-machine power system useful in time domain simulation with combined dynamic and static elements of analysis in order to study dynamic phenomena of mid-term transient periods of decreased voltage security level, - development of the UPFC injection model with adequate control system in order to define the role of the device and its regulation capabilities useful in the voltage stability problem, as well as to define the methodology of rating, locating and timing of the UPFC action with simultaneous three-parameter control, and - development of the methodology aimed to recognise critical points of power system operation using matrix decompositions and functions characteristic for appearance and evolution of voltage stability problem, as well as to compute their sensitivities with respect to the load parameters and the UPFC set of control parameters. Future prospects are mostly dependable on a number of practical applications of the FACTS-controllers. Expecting increased number of their installations, the analysis could be very important in overall planning and operational procedures. By making transfer from the test-system towards larger real power system configuration, the answers of practical value could be given. To achieve that, the proposed analysis should be implemented on a regular basis within EMS paradigms in a control centre.

Izvorni jezik
Engleski

Znanstvena područja
Elektrotehnika

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