Definition of FACTS? Compare between the performance analysis of one of FACTS series and shunt types?

Definition of FACTS? Compare between the performance analysis of one of FACTS series and shunt types

                                                               Abstract

                 FACTS “Flexible AC Transmission Systems” and its definition, the meaning of series compensation and shunt compensation. The FACTS classified to Series FACTS (TCSC, SSSC, FCL), Shunt FACTS (SVC, STATCOM), Hybrid FACTS (UPFC, IPFC), SSSC is a VSC-based series FACTS device, The main objectives of the SSSC internal control, The objective of the SSSC external control, STATCOM is a shunt-connected reactive power compensation device

Introduction and Research Objectives

               FACTS “Flexible AC Transmission Systems” technology is an energy conversion, transmission and control technology that uses high power semiconductor switching devices. It is a high-tech area based on the development of high-voltage and large-current electronic switching devices. Facing electric power systems, it integrates manufacturing technology, modern control technology, and the traditional power grid technology, and has become the core of FACTS. Further development of this technology will lead to a revolutionary change in power systems, greatly improve the level of security and stability of transmission lines as well as power transmission capacity, and greatly improve system reliability and operational flexibility. It can even replace the traditional mechanical breaker power electronic switches, making the traditional power system as easy to control as electronic circuits, Based on its structure. what is FACTS, series compensation, shunt compensation, FACTS classification. a FACTS device can be classified as shunt, series, and hybrid, Comparison  between  the performance  analysis of one of FACTS series and shunt types.

  FACTS is the acronym for “Flexible AC Transmission Systems” and refers to a group of resources used to overcome certain limitations in the static and dynamic transmission capacity of electrical networks. The IEEE defines FACTS as alternating current transmission systems incorporating power-electronics based and other static controllers to enhance control ability and power transfer ability. The main purpose of these systems is to supply the network as quickly as possible with inductive or capacitive reactive power that is adapted to its particular requirements, while also improving transmission quality and the efficiency of the power transmission system^[1].

    FACTS technology:  

          The FACTS controller is defined as "a power-based electronic system and other static equipment that provides control of one or more parameters of the transmission system." FACTS could be connected to power systems in series (series compensation) and power systems shunt (shunt compensation) or both in series and power systems shunt.

·       Series compensation:

FACTS for series compensation modify line impedance: X is decreased so as to increase the transmittable active power. However, more reactive power must be provided.

·       Shunt compensation:

Reactive current is injected into the line to maintain voltage magnitude. Transmittable active power is increased but more reactive power is to be provided.      

FACTS Device Classifications:

o   Series FACTS:

            Series FACTS include the thyristor controlled series capacitor (TCSC), the thyristor switched series capacitor, the static synchronous series compensator (SSSC), the fault current limiter (FCL), the thyristor controlled phase modulator (TCPAR), etc. Taking TCSC, SSSC, FCL, e.g., to explain the basic principle as follows:

1.       The TCSC is based on the conventional compensation technology from the series. It can also dampen SSR and low frequency oscillation, and reduce line losses, when used to boost system reliability and line transmission efficiency.

2.      The SSSC has no external power source, so its output voltage vector is orthogonal to the line current, therefore independent of voltage and current regulation. The transfer of power is regulated by growing or rising inductive tension. Transient energy storage systems may be used in the SSSC and where the excess instantaneous active power is balanced to increase the dynamic efficiency of the network, Transmission line resistive voltage can often increase or decrease. There are no specific SSSC systems in the world although they have been commonly used in numerous power oscillation damping and power flow regulation experiments in the grid ^[2] .

3.      The basic FCL theory is built on the basis of a series reactor. To solve the shortcomings of traditional series reactors, the switch is closed and under regular operating conditions no reactance is inserted into the system. When a failure happens, the control is removed rapidly and the reactor is adjusted to restrict the flow of electricity. From the viewpoint of recent decades' developments, FCL control electronics is split into electronics and those focused on modern materials. Unit thyristor safety compensator (Thyristor Protected Series Compensation, TPSC), This project is designed to increase the transient reliability of EHV (extra high voltage) systems, reduce the maximum swing angle of the generator, and suppress voltage fluctuation

o   Shunt facts:

        Shunt Details include the SVC, STATCOM, magnetic shunt reactor, rating controllable shunt reactors, thyristor-controlled reactor, thyristor-converted condenser, thyristor-converted reactors and others. Taking the SVC and STATCOM, for example, one may describe the basic theory as follows:

1.       The SVC controls individual power device parameters by changing the capacitative or inductive current output. The SVC was effectively introduced to increase the transient stability of synchronous motors and became an important technological means of solving the power transmission bottleneck.

2.      2. The STATCOM is the center of the FACTS family, and is small in size relative to traditional compensation tools, with strong low voltage characteristics and rapid reaction, making it the research hotspot in the field of reactive power management. This is a static synchronous generator parallel to the grid which is able to boost the health which reliability of the power network by regulating the capacitive or inductive performance current, as well as providing tremendous economic and social advantages for the electricity industry. Centered on the damping features of the STATCOM ^[2].

o   Hybrid FACTS:

        Hybrid FACTS include the Unified Power Flow Controller (UPFC), the Convertible Static Compensator (CSC), etc. Take UPFC, IPFC as instances to illustrate the underlying theory of:

1.     The UPFC is a combination of STATCOM and SSSC which aims to achieve the two-way power flow between series and shunt sides. This will provide active and reactive current compensation via a series line without external power storage. a UPFC can independently regulate voltage, line impedance, and transmission angle by injecting a series voltage as well as selectively monitor the active and reactive power flow in the network.

2.     The IPFC is the newest advancement of integrated power electronics technologies and comprises of multiple DC / AC converters with all DC sides linked together; each converter is able to supply the DC connection capacitor with active power via linked wires. Each converter in this setup offers SSSC series compensation to linked wires, offers versatile power flow control and therefore significantly increases the power system's transmission capability and efficiency and reliability.

After this we will compare between one of shunt and series types:

First: Series FACTS (SSSC):                                 

The SSSC in (Fig. 1) is a FACTS system centered on the VSC series that injects a controllable voltage in quadrature with a power network's line current. It is analogous to providing strong, independent of line current, a controllable capacitive or inductive reactance compensation. In addition, the SSSC may also be used with a suitably built external damping device to boost damping of intermittent low-frequency power oscillations in a power grid. Ses features make the SSSC an enticing FACTS tool for regulating air-flow, damping air oscillation and enhancing transient stability. This section briefly discusses the conventional internal and external control of the SSSC ^[4].

              The main objectives of the SSSC internal control (Fig. 1) are to inject a controllable voltage (by injecting a desired compensating reactance) at the ac terminals of the inverter as well as to keep the dc terminal voltage of the inverter constant at steady state.

               The objective of the SSSC external control (Fig. 2) is to damp transient power oscillations of the system. This external device is able to rapidly adjust the SSSC 's applied compensating reactance, thereby supplying additional damping during intermittent power swings. In a functional transmitter, the option of a local signal is typically optimal. In this paper, the active power deviation on the transmission line, measured at the connection point of the SSSC, is used as the input signal to the external controller. In (Fig. 2), is passed through two first-order low-pass filters and a damping controller (consisting of a proportional damping gain and a washout filter) to form a supplementary control signal , This is then applied to a fixed set-point value for the feedback of the SSSC internal controller to shape the overall directed value of the compensating reaction. The washout filter is a high-pass filter that eliminates the dc offset, because without it, the frequent shifts in active power will alter the compensating reactance frequency. The use of two low-pass filters is based on two reasons:

 1) Filtering the electrical noise in the measurements and

 2) Phase compensation to ensure that the variations in compensating reactance are correctly phased with respect to the transient power oscillations in order to provide supplementary damping ^{[4], [5]}.

Second: Shunt FACTS (STATCOM):

          STATCOM is a shunt-connected reactive power compensation device. It is a device used to provide voltage support to the system by injecting or absorbing reactive power to/from the system. (Fig. 1) displays the three major STATCOM components: a voltage source converter (VSC) with a DC-side condenser, a coupling transformer and a control panel. The relation between the voltage of the AC network and the voltage at the side terminals of the STATCOM AC provides control of the reactive power flow. If the voltage at the STATCOM terminals is higher than the system voltage, reactive power will be injected from STATCOM to the system and STATCOM will work as a capacitor. When the voltage at the STATCOM is less than the AC voltage, STATCOM will work as an inductor, and reactive power flow will be reversed ^[6].

        The STATCOM control can be designed to maintain the bus voltage to which it is attached similar to a fixed reference value by regulating the VSC's AC side voltage using a PI control device. Under normal operating conditions, the phase change between internal and terminal voltages would be low, which would result in STATCOM consuming minimal quantities of active power to sustain internal losses ^[6].

    The STATCOM equations in d-q reference frame are summarized as follows ^[7]:

where i_sd' i_sq' V_1d, V_2d, V_1q and V_2q are the d-axis, and q-axis STATCOM current and voltage components, Rs. Xs are the resistance and leakage reactance of the coupling transformer, V_dc is the capacitor voltage, Rc represent the leakage resistance of the electronic component, and Wo is the angular frequency.

 The compensator 's reactive output power is varied to regulate the voltage at the contact point so as to maintain the voltage within the limits permitted.  The STATCOM damping controller structure is shown in (Fig. 2.)

          STATCOM can have reactive strength almost instantly by regulating the modulation index m and thereby increasing transitory stability of the device ^[5].

 

References

[1].                   https://www.electrical4u.com/facts-on-facts-theory-and-applications/

[2].                   https://www.sciencedirect.com/topics/engineering/flexible-ac-transmission-systems

[3].                   R. Mohan Mathur, Rajiv K. Varma, “THYRISTOR-BASED FACTS CONTROLLERS FOR ELECTRICAL TRANSMISSION SYSTEMS”, A JOHN WILEY & SONS, INC. PUBLICATION, United States of America, 2002

[4].                     4] W. Qiao and R. G. Harley, “Indirect adaptive external neuro-control for a series capacitive reactance compensator based on a voltage source PWM converter in damping power oscillations,” IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 77–85, Feb. 200

 

[5].                    W. Qiao, R. G. Harley, and G. K. Venayagamoorthy, “Fault-tolerant optimal neurocontrol for a static synchronous series compensator connected to a power network,” IEEE Trans. Ind. Appl., vol. 44, no. 1, pp. 74–84, Jan./Feb. 2008

 

[6].                    G. Cakir and G. Radman, "Placement and performance analysis of STATCOM and SVC for damping oscillation," 2013 3rd International Conference on Electric Power and Energy Conversion Systems, Istanbul, 2013, pp. 1-5, doi: 10.1109/EPECS.2013.6713076.

 

[7].                    K. R. Padiyar, and AM. Kulkarni, "Design of reactive current and voltage controller of static condenser," International Journal of Electrical Power & Energy Systems, vol. 19, pp. 397-410, 1997.