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.

Impacts Of Power Quality Problems On End Users

The causes of power quality problems are generally complex and difficult to detect. Technically speaking, the ideal ac line supply by the utility system should be pure sine wave of fundamental frequency (50/60 Hz). In addition, the peak of the voltage should be of rated value. Unfortunately the actual ac line supply that we receive everyday departs from the ideal specifications. Table 1.1 lists various power quality problems, their characterization methods and possible causes. There are many ways in which the lack of quality power affects customers. Impulsive transients do not travel very far from their point of entry. However an impulsive transient can give rise to an oscillatory transient. The oscillatory transient can lead to transient over voltage and consequent damage to the power line insulators. Impulsive transients are usually suppressed by surge absorbents. Short duration voltage variations have varied effects on consumers. 

Voltage sags (also known as dips) can cause loss of production in automated processes since a voltage sag can trip a motor or cause its controller to malfunction. For semiconductors manufacturing industries such a loss can be substantial. Voltage sag can also force a computer system or data processing system to crash. To prevent such a crash, an uninterrupted power supply (UPS) is often used, which, in turn, may generate harmonics. The protective circuit of an adjustable speed drive (ASD) can trip the system during a voltage swell. Also voltage swells can put stress on computers and many home appliances, thereby shortening their lives. A temporary interruption lasting a few seconds can cause a loss of production, erasing of computer data etc. The cost of such an interruption during peak hours can be hundreds of thousands of dollars. The impact of long duration voltage variations is greater than those of short duration variations. 

A sustained over voltage lasting for few hours can cause damage to household appliances without their owner knowing it, until it is too late. The under voltage has the same effect as that of voltage sag. In the case of sag the termination of process is sudden. But normal operation can be resumed after the normal voltage is restored. However in the case of sustained under voltage, the process cannot be started or resumed. A sustained interruption is usually caused by faults. Since the loss to customers due to any sustained interruption can be in the order of millions of dollars, it is necessary for the utility to have a good preventive maintenance schedule and to have agreements or regulations to encourage high supply reliability.

Voltage imbalance can cause temperature rise in motors and can even cause a large motor to trip. Harmonics, dc offset and notching cause waveform distortions. Harmonics can be integer multiples of fundamental frequency, fractions of the fundamental frequency (sub harmonics) and at frequencies that are not integer multiples of the fundamental frequency (Inter harmonic). Unwanted harmonic currents flowing through the distribution network can causes needless losses. Harmonics also can cause malfunction of ripple control or traffic control systems, losses and heating in transformers, electromagnetic interference (EMI) and interference with the communication systems. Ripple control refers to the use of a 300 Hz to 2500 Hz signal added to distribution lines to control switching of loads such as hot water heaters or street lighting. Inter harmonic voltages can upset the operation of fluorescent lamps and television receivers. They can also produce acoustic noise in power equipment. DC offsets can cause saturation in the power transformer magnetic circuits.

A notch is a periodic transient that rides on the supply voltage. It can damage capacitive components connected in shunt due to high rate of voltage rise at the notches. Voltage flickers are caused by arc discharge lamps, arc furnaces, starting of large motors, arc welding machines etc. Voltage flickers are frequent variations in voltage that can cause the light intensity from incandescent lamps to vary. This variation is perceived as disturbing by human observers, particularly in the range of 3 to 15 times per second. The voltage flicker can have adverse effects on human health as the high frequency flickering of light bulbs, florescent tubes or television screen can cause strain on the eyes resulting in headaches or migraines. The voltage flicker can also reduce the life span of electronic equipment, lamps etc. We can therefore conclude that the lack of standard quality power can cause loss of production, damage of equipment or appliances or can even be detrimental to human health. It is therefore imperative that a high standard of power quality is maintained.

Introduction to Electric Power Quality

Even a few years back, the main concern of consumers of electricity was the reliability of supply. Here we define reliability as the continuity of electric supply. Even though the power generation in most advanced countries is fairly reliable, the distribution is not always so. The transmission systems compound the problem further as they are exposed to the vagaries of Mother Nature. It is however not only reliability that the consumers want these days, quality too is very important to them. For example, a consumer that is connected to the same bus that supplies a large motor load may have to face a severe dip in his supply voltage every time the motor load is switched on. 

In some extreme cases, he may even have to bear with blackouts. This may be quite unacceptable to most consumers. There are also very sensitive loads such as hospitals (life support, operation theater, and patient database system), processing plants (semiconductor, food, rayon and fabrics). Air traffic control, financial institutions and numerous other data processing and service providers that require clean and uninterrupted power. In several processes such as semiconductor manufacturing or food processing plants, a batch of product can be ruined by a voltage dip of very short duration. Such customers are very wary of such dips since each such interruptions cost them a substantial amount of money. 

Even short dips are sufficient to cause contractors on motor drives to drop out. Stoppage in a portion of a process can destroy the conditions for quality control of the product and requires restarting of production. Thus in this changed scenario in which the customers increasingly demand quality power, the term power quality (PQ) attains increased significance. Transmission lines are exposed to the forces of nature. Furthermore, each transmission line has its load-ability limit that is often determined by either stability considerations or by thermal limits. 

Even though the power quality problem is a distribution side problem, transmission lines often have an impact on the quality of power supplied. It is however to be noted that while most problems associated with transmission systems arise due to the forces of nature or due to the interconnection of power systems, individual customers are responsible for a more substantial fraction of the problems of power distribution systems.