Flexible AC Transmission Devices (FACT Devices Notes)

FACTS CONTROLLERS:

With the rapid development of power electronics, Flexible AC Transmission Systems (FACTS) devices have been proposed and implemented in power systems. FACTS devices can be utilized to control power flow and enhance system stability. Particularly with the deregulation of the electricity market, there is an increasing interest in using FACTS devices in the operation and control of power systems with new loading and power flow conditions. A better utilization of the existing power systems to increase their capacities and controllability by installing FACTS devices becomes imperative. Due to the present situation, there are two main aspects that should be considered in using FACTS devices the first aspect is the flexible power system operation according to the power flow control capability of FACTS devices. The other aspect is the improvement of transient and steady-state stability of power systems. FACTS devices are the right equipment to meet these challenges.

Definition of FACTS:

According to IEEE, FACTS, which is the abbreviation of Flexible AC Transmission Systems, is defined as follows:

Alternating current transmission systems incorporating power electronics based and other static controllers to enhance controllability and power transfer capability.

The basic applications of facts-devices are:

• Power Flow Control.

• Increase of Transmission Capability.

• Voltage Control.

• Reactive Power Compensation.

• Stability Improvement.

• Power Quality Improvement.

• Power Conditioning.

• Flicker Mitigation.

• Interconnection of Renewable and Distributed Generation and Storage.

Figure 1.1 shows the basic idea of facts for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different facts devices. It can be seen that with growing line length, the opportunity for facts devices gets more and more important.

The influence of facts-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.

Fig 1.1 Operational limits of transmissions lines for different voltage levels

The development of facts-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. Figure 1.2 shows a number of basic devices separated into the conventional ones and the facts-devices.

For the facts side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast controllability of facts-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the facts-devices can equally be static and dynamic.

Fig 1.2 Overview of major FACTS-Devices

The left column in figure 1.2 contains the conventional devices build out of fixed or mechanically switch able components like resistance, inductance or capacitance together with transformers. The facts-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. The left column of facts-devices uses thyristor valves or converters. These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the thyristors to simply bridge impedance's in the valves.

            The right column of facts-devices contains more advanced technology of voltage source converters based today mainly on insulated gate bipolar transistors (IGBT) or insulated gate commutated thyristors (IGCT). Voltage source converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBT’s or IGCTS. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network. The disadvantage is that with an increasing switching frequency, the losses are increasing as well. Therefore special designs of the converters are required to compensate this.