Flexible AC Transmission Systems | FACTS

Flexible AC Transmission Systems (FACTS) – What and why?

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.

Features of Flexible AC Transmission Systems (FACTS)

• Fast voltage regulation,
• Increased power transfer over long AC lines,
• Damping of active power oscillations, and
• Load flow control in meshed systems,

Thereby significantly improving the stability and performance of existing and future transmission systems.
This means that with Flexible AC Transmission Systems (FACTS), power companies will be able to better utilize their existing transmission networks, substantially increase the availability and reliability of their line networks, and improve both dynamic and transient network stability while ensuring a better quality of supply.

Influence of Reactive Power Flow on Power System Voltage

Reactive Power Compensation in Power Transmission System

Consumer load requires reactive power that varies continuously and increases transmission losses while affecting voltage in the transmission network. To prevent unacceptably high voltage fluctuations or the Reactive power compensation consumer load requires reactive power that varies continuously and increases transmission losses while affecting voltage in the transmission network. To prevent unacceptably high voltage fluctuations or the power failures that can result, this reactive power must be compensated and kept in balance. This function has always been performed by passive elements such as reactors or capacitors, as well as combinations of the two that supply inductive or capacitive reactive power. The more quickly and precisely the reactive power compensation can be accomplished, the more efficiently the various transmission characteristics can be controlled. For this reason, slow mechanically switched components have been almost completely replaced by fast thyristor-switched and thyristor controlled components. Owner failures that can result, this reactive power must be compensated and kept in balance.

Effects of Reactive Power Flow

Reactive power flow has the following effects:

1. Increase in transmission system losses
• Adding to power plant installations
• Adding to operating costs
2. Major influence on system voltage deviation
• Degradation of load performance at under voltage
• Risk of insulation breakdown at over-voltage
3. Limitation of power transfer
4. Steady-state and dynamic stability limits

Parallel and Series

Type	Shor-circuit level	Transmission phase angle	Steady-state voltage	Voltage after load rejection	Application
	nearly unchanged	slightly increased	increased	high	voltage stabilization at heavy load
	nearly unchanged	slightly increased	decreased	low	voltage stabilization at light load
	nearly unchanged	controlled	controlled	limited by control	fast voltage control reactive power control damping of power swings

Fig. Shows today’s most common shunt compensation devices, their influence on the most important transmission parameters, and typical applications.Fig.: The active power/ transmission angle equation illustrates which FACTS components selectively influence which transmission parameters.

Protection and Control of FACTS

To further improve redundancy management, special modules were developed that supplement the standard SIMATIC TDC automation system. Another new module in the instrumentation and control cabinet is responsible for issuing triggering signals to the thyristor valves. Altogether, SIMATIC TDC with its high integration density takes up significantly less space in the plant than the previous technology. Never the less, use of SIMATIC TDC is not limited to new FACTS. With its flexible interface design, it can easily replace existing systems. In this case, the measured values of existing plants are integratedin and further processed by the new control system. Because it requires so little space, the new technology can even be configured in parallel with the existing C and P system in order to integrate the FACTS with as little delay as possible.Human Machine Interface The interface between the operator and the plant
(HMI = Human Machine Inter-face) is the standardized
SIMATIC Win CC visualization system, which further simplifies operation and facilitates the adaptation of graphical user interfaces to the operator’s requirements.

Hardware for Control and Protection

Siemens offers the latest in control and protection for FACTS – the tried and tested SIMATIC TDC (Technology and Drive Control) automation system. SIMATIC TDC is used worldwide in almost every industry and has been proven in both production and process engineering as well as in numerous HVDC and FACTS applications. Operating personnel and project planning engineers work exclusively with a standardized, universal hardware and software platform, enabling them to perform demanding tasks more rapidly. One of the main considerations in developing this automation system was to ensure the highest degree of availability of the FACTS – which is why all control and protection systems as well as the communication links are configured redundantly (if requested by the customer).
The new instrumentation and control technology also permits the use of a high-performance fault recorder operating at a 25 kHz sampling rate. This reduces the period of time between fault recording and the printout of the fault report from several minutes (previously) to 10 seconds (now).

Converter for FACTS

LTT – Light Triggered Thyristors.
Thyristors are a key element in controlling (switching on and off) the passive components in reactive power compensation systems. The system of direct light triggering developed by Siemens activates the thyristors with a pulse of light that lasts for 10 microseconds and has a peak power of 40 mill watts. The device also incorporates over voltage protection, so that it is self-protecting if the forward voltage exceeds the maximum permitted limit. The light pulse is carried by fibre optics at ground potential directly from the valve control to the thyristor gate. Conventional high-voltage thyristor valve technology uses electrically triggered thyristors, which need a pulse with a peak power of several watts. This pulse is generated by complex electronic equipment placed alongside each thyristor. In turn, this electronic equipment, which needs an auxiliary power supply, is activated at ground potential by optical signals from the valve control. Substituting direct light triggering for this electronic equipment reduces the number of electrical and electronic components in the thyristor valve – and, consequently, the possibility of failure – by around 80 percent. This improves reliability and eliminates problems associated with electromagnetic compatibility. The other important fact about the new thyristor technology is that long-term availability of electronic components for replacement purposes over a period of at least 30 years is no longer a problem.
Thyristor valves from Siemens are assembled from 4-inch or 5-inch thyristors, depending on the current- carrying capacity/rated current required. Thyristor technology has been under constant development since the early 1960s. At the present time, thyristors can safely and economically handle blocking voltages of up to 8 kilo volts and rated currents of up to 4,200 amperes.

Power Factor

Electrical Power Factor

In general power is the capacity to do work. In electrical domain, electrical power is the amount of electrical energy that can be transferred to some other form (heat, light etc) per unit time. Mathematically it is the product of voltage drop across the element and currentflowing through it.  Considering first the DC circuits, having only DC voltage sources, the inductors and capacitors behave as short circuit and open circuit respectively in steady state. Hence the entire circuit behaves as resistive circuit and the entire electrical power is dissipated in the form of heat. Here the voltage and current are in same phase and the total electrical power is given by Electrical power = Voltage across the element × Current through the element. Its unit is Watt = Joule/sec.

Now coming to AC circuit, here both inductor and capacitor offer certain amount of impedance given by,The inductor stores electrical energy in the form of magnetic energy and capacitor stores electrical energy in the form of electrostatic energy. Neither of them dissipates it. Further there is a phase shift between voltage and current. Hence when we consider the entire circuit consisting of resistor, inductor and capacitor, there exists some phase difference between the source voltage and current. The cosine of this phase difference is called electrical power factor.

This factor (-1 < cosφ < 1 ) represents the fraction of total power that is used to do the useful work. The other fraction of electrical power is stored in the form of magnetic energy or electrostatic energy in inductor and capacitor respectively. The total power in this case is, Total electrical power = Voltage across the element × current through the element. This is called apparent power and its unit is VA (Volt Amp) and denoted by ‘S’. A fraction of this total electrical power which actually does our useful work is called as active power. It is denoted as ‘P’. P = Active power = Total electrical power.cosφ and its unit is watt. The other fraction of power is called reactive power. This does no useful work, but it is required for the active work to be done. It is denoted by ‘Q’ and mathematically is given by, Q = Reactive power = Total electrical power.sinφ and its unit is VAR (Volt Amp Reactive). This reactive power oscillates between source and load. To help understand this better all these power are represented in the form of triangle.

Mathematically, S² = P² + Q² and electrical power factor is active power / apparent power.

Power Factor Improvement

The term power factor comes into picture in AC circuits only. Mathematically it is cosine of the phase difference between source voltage and current. It refers to the fraction of total power (apparent power) which is utilized to do the useful work called active power.Need for Power Factor Improvement

• Real power is given by P = VIcosφ. To transfer a given amount of power at certain voltage, the electrical current is inversely proportional to cosφ. Hence higher the pf lower will be the current flowing. A small current flow requires less cross sectional area of conductor and thus it saves conductor and money.
• From above relation we saw having poor power factor increases the current flowing in conductor and thus copper loss increases. Further large voltage drop occurs in alternator, electrical transformer and transmission & distribution lines which gives very poor voltage regulation.
• Further the KVA rating of machines is also reduced by having higher power factor as,Hence, the size and cost of machine also reduced. So, electrical power factor should be maintained close to unity.

Methods of Power Factor Improvement

• Capacitors: Improving power factor means reducing the phase difference between voltage and current. Since majority of loads are of inductive nature, they require some amount of reactive power for them to function. This reactive power is provided by the capacitor or bank of capacitors installed parallel to the load. They act as a source of local reactive power and thus less reactive power flows through the line. Basically they reduces the phase difference between the voltage and current.
• Synchronous Condenser: They are 3 phase synchronous motor with no load attached to its shaft. The synchronous motor has the characteristics of operating under any power factor leading, lagging or unity depending upon the excitation. For inductive loads, synchronous condenser is connected towards load side and is overexcited. This makes it behave like a capacitor. It draws the lagging current from the supply or supplies the reactive power.
• Phase Advancer: This is an ac exciter mainly used to improve pf of induction motor. They are mounted on shaft of the motor and is connected in the rotor circuit of the motor. It improves the power factor by providing the exciting ampere turns to produce required flux at slip frequency. Further if ampere turns are increased, it can be made to operate at leading power factor.

Power Factor Calculation

In power factor calculation, we measure the source voltage and current drawn using a voltmeter and ammeter respectively. A wattmeter is used to get the active power. Now, we know P = VIcosφ wattHence, we can get the electrical power factor. Now we can calculate the reactive power Q = VIsinφ VAR This reactive power can now be supplied from the capacitor installed in parallel with load in local. Value of capacitor is calculated as per following formula:IMPORTANT: In power factor improvement, the reactive power requirement by the load does not change. It is just supplied by some device in local, thus reducing the burden on source to provide the required reactive power.