Interline Power Flow Controller: Review Paper

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1 Vol. (0) No. 3, pp ISSN Interline Power Flow Controller: Review Paper Akhilesh A. Nimje, Chinmoy Kumar Panigrahi, Ajaya Kumar Mohanty Abstract The Interline Power Flow Controller (IPFC) proposed is a recent concept for the compensation and effective power flow management of multi line transmission systems. In its general form, the IPFC employs a number of inverters with a common DC link, each to provide series compensation for a selected line of the transmission system [],[] & [3]. This paper investigates the use of IPFC, which are dc/ac converters linked by common DC terminals, in a DG-power system from an economy perspective [4]. Because of the common link, any inverter within the IPFC is able to transfer real power to any other and thereby facilitate real power transfer among the lines of the transmission system. Since each inverter is able to provide reactive compensation, the IPFC is able to carry out an overall real and reactive power compensation of the total transmission system. This capability makes it possible to equalize both real and reactive power flow between the lines, transfer power from overloaded to under loaded lines, compensate against reactive voltage drops and corresponding reactive line power and to increase the effectiveness of the compensating system against dynamic disturbances. Keywords: FACTS, Compensation, power flow, IPFC, voltage stability. I. INTRODUCTION The evolution of power industry in recent years has imposed many challenges due to the radical changes in the energy market as power demand is more than the availability [4]. Due to heavy demand of power, distribution networks are always in stress which results in reduced voltage across the load and it affect on the performance. It is necessary to improve the performance of power system to received quality power at the consumer end. Reactive power compensation is the main measure to keep power network running with high voltage stability, high power quality and minimum system loss. Flexible AC transmission system (FACTS) devices are found to be very effective controller to enhance the system performance. FACTS Technology invented in 986 by N. G. Hingorani from the Electric Power Research Institute (EPRI) USA and it is based on thyristor operation techniques. FACTS controllers are broadly classified as series and shunt, both used to modify the natural electrical characteristics of ac power system. compensation modifies the transmission or distribution system parameters, while shunt compensation changes the equivalent impedance of the load. In both the cases the reactive power that flows through the system can be effectively controlled by FACTS, which improves the overall performance of ac power system. The introduction of the Flexible AC Transmission systems has been a considerable effort in the recent years on the development of the power electronic based power flow controllers. These controllers use thyristor switched capacitors or reactors to provide reactive shunt and series compensation. Active Power Filters, Universal Power Line Conditioners, mainly Unified Power Flow Controllers and Unified Power Quality Conditioners are in stage of hard researches and increasingly applied. Their possible functions are enlarging and include power flow control, current and voltage harmonic compensation, voltage imbalance, reactive power, negative sequence current compensation. To one of the most powerful arrangements we can add so called UPFC (Unified Power Flow Controllers). Those systems are the classical series-parallel filters (or special matrix converter), which can control active and reactive powers transmitted through the line. The major purpose of the parallel filter is to keep voltage on the source element on constant value. The series filter has to inject controllable (with angle and magnitude) voltage and in this way control power flow. One of the disadvantages of this solution is need to equip every transmission line with independent UPFC system. Corresponding Author is Akhilesh A. Nimje, School of Electrical Engineering,KIIT University, Bhubaneswar 7504, India buntynimje@yahoo.co.in 550

2 Akhilesh A. Nimje et al. Interline Power Flow Controller: Review Paper CB Shunt the effectiveness of the overall compensating system for dynamic disturbances. In other words, the IPFC can potentially provide a highly effective scheme for power transmission management at a multi-line substation. Intermediate Intermediate (i) SW VSC VSC VSC VSC SW Fig.. A Two Converter IPFC FIG.. A UPFC SCHEMATIC The closing of switches and enable the two converters to exchange real power flow between the two converters. The reactive power can be either absorbed or supplied by the series connected converter. The provision of a controllable power source on the DC side of the series connected converter, results in the control of both real and reactive power flow in the line (measured at the receiving end). The shunt connected converter not only provides the necessary power required, but also the reactive current injected at the converter bus Thus, a UPFC has three degree of freedom unlike other FACTS controllers which have only one degree of freedom (controlled variable). This solution is not attractive from economical point of view. The Interline Power Flow Controller (IPFC) concept proposed in this paper addresses the problem of compensating a number of transmission lines at a given substation. Conventionally, series capacitive compensation (fixed, thyristor-controlled or SSSC based) is employed to increase the transmittable real power over a given line and also to balance the loading of a normally encountered multi-line transmission system. However, independent of their implementation, series reactive compensators are unable to control the reactive power flow in, and thus the proper load balancing of, the lines. This problem becomes particularly evident in those cases where the ratio of reactive to resistive line impedance (Xm) is relatively low. reactive compensation reduces only the effective reactive impedance X and, thus, significantly decreases the effective X/R ratio and thereby increases the reactive power flow and losses in the line. The IPFC scheme proposed provides, together with independently controllable reactive series compensation of each individual line, a capability to directly transfer real power between the compensated lines. This capability makes it possible to: equalize both real and reactive power flow between the lines; transfer power demand from overloaded to under loaded lines; compensate against resistive line voltage drops and the corresponding reactive power demand; increase A pure series reactive (controllable) compensation in the form of TCSC or SSSC can be used to control or regulate the active power flow in the line, the control of reactive power is not feasible unless active (real) voltage in phase with the line current is not injected. The application of a TCSC (or SSSC with impedance emulation) results in the reduction of net series reactance of the line. However, X/R ratio is reduced significantly and thereby increases the reactive power flow (injected at the receiving end) and losses in the line. The interline power flow controller (IPFC) provides, in addition to the facility for independently controllable reactive (series) compensation of each individual line, a capability to directly transfer or exchange real power between the compensated lines. This is achieved by coupling the series connected VSC in individual lines on the DC side, by connecting all the DC capacitors of individual converters in parallel. Since all the series converters are located inside the substation in close proximity, this is feasible. Control II. BASIC PRINCIPLE OF IPFC. Conv Conv Conv3 Optical Links Fig. 3 IPFC Comprising n Converters DC Bus 55 P a g e

3 Vol. (0) No. 3, pp ISSN An IPFC with two converters compensating two lines is similar to UPFC in which the magnitude and phase angle of the injected voltage in the prime system (or line) can be controlled by exchanging real power with the support system (which is also a series converter in the second line). The basic difference with a UPFC is that the support system in the later case is the shunt converter instead of a series converter. The series converter associated with the prime system of one IPFC is termed as the master converter while the series converter associated with the support system is termed as slave converter. The master converter controls both active and reactive voltage (within limits) while the slave converter controls the DC voltage across the capacitor and the reactive voltage magnitude. For the system shown in figure 4, the received power and the injected reactive power at the receiving end of the prime line ( ) can be expressed as: V V p V r j X V V p V r j X = - = - 3 I I V 3 3 Similar equations also apply to the support line ( ) except that V p is not independent. It is related to V p by the equation. V p I V p I = 0. (4) The above equation shows that V p is negative if V p is positive. With the resistance emulation, we have V p = -R I, V p = - R I. (5) Substitute equation (5) in equation (4), we get the constraint involving R and R as R I = - R I (6) The constraint equation (4) and (6) can limit the utility of IPFC. In such a case, an additional shunt converter (forming a GUPFC) will be useful as shown in figure 5 below: The concept of combining two or more converters can be extended to provide flexibility and additional degrees of freedom. A generalized UPFC refers to three or more converters out of which one is shunt connected while the remaining converters are series connected as shown in figure 5. I I I I Fig.4. Representation of IPFC VV p VVr P P0 sin cos X X () Shunt VSC VSC VSC VV p VVr Q Q0 cos sin X X () Fig. 5. A Three Converter GUPFC where = -, V p sin V sin P 0 and Q 0 are the real power and reactive power in the line (at the receiving end ) when both V p and V r are zero. These are expressed as: V sin V P0, Q0 X X cos (3) III. MODELLING OF MULTI CONVERTER FACTS DEVICES The studies of multi converter FACTS devices are carried out from the objectives of planning and operational analysis. The broad spectrum of the required studies is listed below with increasing order of complexity.. Power flow studies. Dynamic stability 55

4 Akhilesh A. Nimje et al. Interline Power Flow Controller: Review Paper 3. Transient analysis neglecting harmonics 4. Detailed transient analysis considering switching action in the converters. The power flow studies involve the computation of solution of non-linear algebraic equations that relate the specifications to the system state variables. The constraints are usually handled by modifying the specifications. For example, limits on the reactive current/ power are handled by changing the voltage (magnitude) specification. The dynamic stability refers to the stability of a power system influenced by various controllers (AVR, PSS and network controllers including HVDC and FACTS). There are different mechanisms of system instability. Both power flow and dynamic stability analysis are based on the single - phase models of the network. Since dynamic stability analysis involves phenomena of frequency below 5 Hz, the network variables (voltage and currents) are represented by phasors that vary slowly. However it is essential to test the controller performance using detailed three phase models to validate the simplified analysis. For example, the design of AC voltage regulator for shunt converter requires the study electromagnetic interactions that result from the network transients. In general, this is true for all fast acting controllers. The detailed transient simulation considers three phase nonlinear models of all relevant components. For the analysis and simulation of SSR, network transients (below third harmonic) need to be modeled by approximate models. For example, a transmission line can be modeled by a single equivalent model. There is no need to consider the switching action in the converters and the resulting harmonics. The FACTS controllers can be modeled using dynamic phasors or d q variables referred to a synchronously rotating reference frame. V V inj L t I Fig. 6. Model of a SVC It would be desirable to employ a common model for all types of studies. For multi-converter circuits, a converter can be modeled by a variable voltage source in series with inductive impedance as shown in figure 6. Here the voltage source is related to the voltage across the DC capacitor based on the converter topology and control action. For three phase models, the voltage source is defined instantaneously and contains harmonics. Neglecting harmonics, we can represent the voltage by d q components (dynamic phasors) that are determined by exact controller models. R t V.. V For the series converter, V inj V sh inj V se For transient or dynamic stability analysis, the converter model shown above can be represented conveniently by Norton equivalent that simplifies the network solution using the admittance matrix. For power flow analysis, a shunt converter in isolation can be modeled as synchronous condenser with the specification of bus voltage (magnitude). The two control variables V sh and are calculated from the specified voltage magnitude and the constraint equation that relates the power drawn to the losses in the converter. For the series converter, the specification in the line power flow (P) and the constraint is the power supplied by the series converter which may be assumed as zero. For the coupled converters such as UPFC, the four control variables, V sh, V se, and can be computed from the three specified variables, (say V, P, Q ) and the constraint that relates the power balance in the DC circuit. IV. APPLICATION CONSIDERATIONS The concept and basic operating principles of the IPFC are explained in this paper. In practical applications the IPFC would, in general, have to manage the power flow control of a complex, multi-line system in which the length, voltage, and capacity of the individual lines could widely differ. One of the attractive features of the IPFC is that, although it may pose engineering challenges particularly in the area of control, it is inherently flexible to accommodate complex systems and diverse operating requirements. A few relevant points to consider are briefly mentioned below. () The IPFC is particularly advantageous when controlled series compensation or other series power flow control (e.g., phase shifting) is contemplated. This is because the IPFC simply combines the otherwise independent series compensators (SSSCs), without any significant hardware addition, and provides some of those with greatly enhanced functional capability. () The operating areas of the individual inverters of the IPFC can differ significantly, depending on the voltage and power ratings of the individual lines and on the amount of compensation desired. It is evident that a high power line may supply the necessary real power for a low capacity line to optimize its power transmission, without significantly affecting its own transmission. (3) The IPFC is an ideal solution to balance both the real and reactive power flow in a multi-line system. (4) The prime inverters of the IPFC can be controlled to provide totally different operating functions, e.g., independent P and Q control, phase shifting (transmission angle regulation), transmission impedance control, etc. These functions can be selected according to prevailing system operating requirements. V The phasor inj is expressed differently for the shunt and series converters. For the shunt converter, 553 P a g e

5 Vol. (0) No. 3, pp ISSN V. CONCLUSION IPFC like other FACTS Controller contribute to the optimal system operation by reducing the power loss and improving the voltage profile. The IPFC is a kind of combined compensators, which combines at least two SSSCs via a common DC voltage link. This DC voltage link provides the device with an active power transfer path among the converters, which enables the IPFC to compensate multiple transmission lines at a given substation. This is a very attractive feature of this FACTS device. REFERENCES [] Narain G. Hingorani, Laszlo Gyugyi, Understanding FACTS Concepts and Technology of Flexible AC Transmission Systems, IEEE Press, Standard Publishers Distributors, Delhi. [] K. R Padiyar, FACTS Controllers in Power Transmission and Distribution, New Age International Publishers (formerly Wiley Eastern Limited), New Delhi. [3] Laszlo Gyugyi, Kalyan K. Sen, Colin D. Schauder, The Interline Power Flow Controller Concept : A New Approach to the Power Flow Management, IEEE Trans. on Power Delivery, Vol.4, no. 3, pp 5 3, July 999. [4] Kishor Porate, K. L. Thakre, G. L. Bodhe, Voltage Stability Enhancement of Low Voltage Radial Distribution Network Using Static VAR Compensator: A Case Study, WSEAS Transactions on Power Systems, Issue, vol. 4, pp 3 4, January 009. [5] R. Strzelecki, G. Benysek, Interline Power Flow Controller New Concept in Multiline Transmission Systems. 554