Power Systems--Flexible AC Transmission Systems

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1. Introduction

The electric power system is one of the largest and most complex man-made systems, encompassing billions of components, tens of millions of kilometers of transmission lines, and thousands of generators serving a diverse, huge number of consumers. The function of a power system is to generate electric energy economically and with the minimum ecological disturbance and to transfer this energy over transmission lines and distribution networks with the maximum efficiency and reliability for delivery to customers at virtually fixed voltage and frequency. The conventional power system structure is highly hierarchical where power flows are typically unidirectional (i.e., from generating plant to end user).

Most of the electricity is still generated in large, centrally managed power plants, due to economies of scale and location of resources (e.g., coal, water), and transported in bulk to the areas through a meshed transmission grid (with built-in redundancy to increase security and availability) and finally delivered to the consumers through passive typically radial distribution systems. The end users are typically nonresponsive consumers and do not participate in system operation.

The constant growth of demand for electricity over the years resulted in the appearance of "bottle necks" in electrical power transmission corridors (i.e., the amount of power that could be transferred from one point in the network to the other started to become more and more limited due to physical capacities of transmission lines) and in increasing difficulty to ensure appropriate regulation and control of key attributes of the electrical power transfer that would meet increasing customer demands for high quality of electricity supply (i.e., efficiency of power transfer, reliability of supply, and delivery to customers at almost fixed voltage and frequency). It was soon realized that relying solely on the construction of new primary plants to solve the problem is not a viable option for economic, environmental, and public acceptance reasons. The investments in new generating plants and transmission lines are extremely expensive and take years to complete. The transmission lines, generating plants, and large storage facilities have a visual and environmental impact that will limit their acceptance by the public.

The task of delivering the electricity in a reliable, secure, and controllable manner relied in the past, and still does to a large extent, on the supervisory control over the transmission system. This has been achieved largely by means of control equipment such as the tap-changing transformers, shunt and series reactors, the capacitor banks, the protection apparatus and systems, etc. With the growth of the transmission system, due to increased interconnections among different (geographical) regions and demand for operating the system under more stressed conditions in order to satisfy the growing and more versatile demand in more and more environmentally and economically aware surrounding, the ability of the conventional equipment to control the system became limited and the need for fast and frequent self-operating equipment that introduces additional degrees of freedom in system operation appeared.

The flexible alternating current transmission system (FACTS) devices, often referred to as flexible alternating current transmission systems (FACTS) represent a dependable solution to the task of advanced control of transmission systems in a new operating environment. The terms "FACTS" and

"FACTS device" will be used interchangeably in the rest of this section as this is often the case in published literature. The Electric Power Research Institute (EPRI) introduced this technology originally during the 1980s, since then, it has been constantly evolving. The FACTS technology is largely based on application of the high-voltage power electronic switches enabling, through fast and sophisticated control, modulation of key parameters that govern the operation of transmission systems including series and shunt impedances, currents, voltages, phase angles, and real and reactive power flows. In addition to their advanced control capabilities they are also environmentally friendly. They are built from safe materials and do not produce any kind of emissions or waste during the operation that may pollute the environment.

2. Basic FACTS Technology

Two different technical approaches influenced the development of FACTS devices. The first group of devices employs reactive elements or a tap-changing transformer with thyristor switches as controllable elements. The second group uses self-commutated static converters as controlled voltage sources.

Ever since the first thyristor has been developed, the main design objectives for power semiconductors were low switching losses, high switching rates, and minimal conduction losses. Subsequent innovations in FACTS technology were mainly driven by those objectives.

2.1 Power Semiconductors

The most widely used power semiconductors in FACTS technology are a conventional thyristor, a gate turn-off (GTO) thyristor, and an insulated-gate bipolar transistor (IGBT).

The conventional thyristor is a device that can be triggered (turned on) with a pulse at the gate and afterward remains in conducting mode (turned on) until the next current zero-crossing. Therefore, only one switching per half cycle is possible. This property limits the controllability of the device.

Conventional thyristors have the highest current and blocking voltage among conventionally used power semiconductors, therefore, fewer semiconductors are required for an application. They are used as switches for capacitors or inductors and are still the preferred devices for applications with the highest voltage and power levels. Thyristors are an essential part of the most frequently used FACTS devices including the biggest high-voltage DC (HVDC) transmission systems with voltage levels exceeding 500 kV and power ratings of several thousands MVA.

GTO thyristors are devices that can be switched off with a current pulse at the gate. They were developed to increase the controllability of conventional thyristors. This technology has grown very rapidly, and high-power GTOs are now available. The latter, are nowadays replaced by insulated-gate commutated thyristors (IGCT), which combine the advantage of a conventional thyristor, i.e., low conducting losses, with a low switching losses.

The IGBT can be switched on with a positive voltage signal and switched off by removing the voltage signal. Therefore, a very simple gate drive unit can be used to control the IGBT. It is becoming more and more important for FACTS technology. The voltage and power level of applications are being increased to 300 kV and 1000 MVA, respectively, for an HVDC transmission with voltage-sourced converters (VSCs). The capabilities of modern IGBTs make them applicable in the wide range of power system applications.

2.2 Thyristor-Based FACTS Devices

In high-power applications semiconductor elements are used primarily as switches. To accommodate switching in an AC system, two unidirectional conducting devices are connected in an antiparallel configuration. This group of FACTS controllers employs conventional thyristors. Most of them have a common characteristic that the necessary reactive power, required for compensation, is generated or absorbed by a traditional capacitor or reactor banks. The thyristor switches are used only to control the combined reactive impedance that these banks present to an AC system, as shown in FIG. 1.

FIG. 1 Schematic representation of a three-phase thyristor controlled reactor.

2.3 Converter-Based FACTS Devices

The second group of FACTS devices employs self-commutated VSCs. A VSC basically represents rapidly controllable static synchronous AC voltage source. Compared to the first group of FACTS devices, the VSC-based devices generally have superior performance characteristics. FIG. 2 illustrates the basic scheme of a two-level three-phase VSC consisting of six power transistors with a parallel power diode connected in reverse and a capacitor on the DC side. A suitable switching pattern must be defined for the switch on and switch off capability. The simplest solution is the combination of a triangular voltage with a reference voltage as control variables, i.e., pulse width modulation (PWM).

Three stages of an output voltage (plus, minus, and zero) can be achieved with a three-level converter.

Whereas, increasing frequency of switching not only reduces harmonics injected into the network, but also increases the switching losses. A compromise between harmonic injection (and consequent requirement for output harmonic filters) and switching losses must be found for practical applications.

In high-power applications more complex (multi-pulse) converters are used. In these converters, the number of used semiconductor elements increases the cost of devices more than reduction in switching losses, or harmonic injection, would justify.

FIG. 2 Basic scheme of a two-level voltage sourced converter.

cont to part 2 >>

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