Flexible AC Transmission System – Need, Definition & Types

Why is a Flexible AC Transmission System Needed?

In a conventional AC transmission system, the ability to transfer AC power is limited by several factors like thermal limits, transient stability limit, voltage limit, short circuit current limit, etc. These limits define the maximum electric power which can be efficiently transmitted through the transmission line without causing any damage to the electrical equipment and the transmission lines. This is normally achieved by bringing changes in the power system layout. However, this is not feasible and another way of achieving maximum power transfer capability without any changes in the power system layout.  Also with the introduction of variable impedance devices like capacitors and inductors, the whole of the energy or power from the source is not transferred to the load, but a part is stored in these devices as reactive power and returned to the source.  Thus the actual amount of power transferred to the load or the active power is always less than the apparent power or the net power. For ideal transmission, the active power should be equal to the apparent power. In other words, the power factor (the ratio of active power to apparent power) should be unity. This is where the role of a Flexible AC transmission System comes.

Before going to details about FACTS, let us brief about the power factor.

What is Power Factor?

The power factor is defined as it is the ratio of active power to the apparent power in the circuit.

Whatever the power factor is, on the other hand, the generating power should place machines to delivering a specific voltage and current. The generators must have the ability to withstand the appraised voltage and current of the power produced. The power factor (PF) value is between 0.0 and 1.0.

If the power factor is zero, the current flow is entirely reactive and power stored in the load returns to on every cycle. When the power factor is 1, all the current supplied by the source is devoured by the load. Generally, the Power factor is expressed as leading or lagging of the voltage.

Unity Power Factor Test Circuit

The circuit with power supply is 230v and a choke is all connected in series. Capacitors are required to be connected in parallel through SCR switches to improve the power factor. While the by-pass switch is off, the choke acts as an inductor and the same current will flow in both the 10R/10W resistors. A CT is used as the primary side of which is connected to the common point of the resistors. The other point of the CT goes to one of the common points of a DPDT S1 switch. While the DPDT switch is moved to left then the voltage drop proportional to the current is sensed by it to develop increased voltage. The voltage drop is proportional to the lagging current. Thus the primary voltage from the CT provides lagging current.

If used microcontroller-based control circuit then receives zero current references and compares with the zero voltage reference for calculating the power factor based on their time difference. So depending on the time difference required no. of SCR switches are switched on, thereby switching additional capacitors till the power factor is near unity.

Thus depending on the switch position, one can sense the lagging current or the compensated current and the display provides accordingly the time delay between voltages, current with power factor display.

What is Flexible AC Transmission System (FACTS)?

A Flexible AC transmission System refers to the system consisting of power electronic devices along with power system devices to enhance the controllability and stability of the transmission system and increase the power transfer capabilities. With the invention of thyristor switch, opened the door for the development of power electronics devices known as Flexible AC transmission systems (FACTS) controllers.  The FACT system is used to provide the controllability of the high voltage side of the network by incorporating power electronic devices to introduce inductive or capacitive power in the network.

4 Types of FACTS Controllers

• Series Controllers: Series Controllers consist of capacitors or reactors which introduce voltage in series with the line. They are variable impedance devices. Their major task is to reduce the inductivity of the transmission line. They supply or consume variable reactive power. Examples of series controllers are SSSC, TCSC, TSSC, etc.
• Shunt Controllers:  Shunt controllers consist of variable impedance devices like capacitors or reactors which introduce current in series with the line. Their major task is to reduce the capacitive of the transmission line. The injected current is in phase with the line voltage. Examples of shunt controllers are STATCOM, TSR, TSC, SVC.
• Shunt-Series Controllers:  These controllers introduce current in series using the series controllers and voltage in shunt using the shunt controllers. An example is UPFC.
• Series-Series Controllers: These controllers consist of a combination of series controllers with each controller providing series compensation and also the transfer real power along the line. An example is IPFC.

2 Types of Series Controllers

• Thyristor controlled series capacitor (TCSC): Thyristor controlled series capacitor (TCSC) uses silicon controlled rectifiers to manage a capacitor bank connected in series with a line. This allows utility for transferring more power on a specified line. It generally consists of the thyristors in series with an inductor and connected across a capacitor. It can work in the blocking mode where the thyristor is not triggered and current passes through the capacitor only. It can work in the bypass mode where the current is bypassed to the thyristor and the whole system behaves as a shunt impedance network.

• Static Series Synchronous Compensators: SSSC is simply a series version of STATCOM. These are not used in commercial applications as independent controllers. They consist of the synchronous voltage source in series with the line such that it introduces a compensating voltage in series with the line. They can increase or decrease the voltage drop across the line.

2 Parallel Controllers

• Static Variable Compensators: Static variable compensator is the most primitive and first generation of FACTS controller. This compensator consists of a fast thyristor switch controlling a reactor and/or shunt capacitive bank to provide dynamic shunt compensation. They generally consist of shunt connected variable impedance devices whose output can be adjusted using power electronic switches, to introduce capacitive or inductive reactance in the line. It can be placed in the middle of the line to increase the maximum power transfer capability and can also be placed at the end of the line to compensate for variations due to load.

3 Types of SVC are

1. TSR(Thyristor Switched Reactor):  It consists of a shunt connected inductor whose impedance is controlled in a gradual manner using a Thyristor switch. The Thyristor is fired at angles of 90 and 180 degrees only.
2. TSC(Thyristor Switched Capacitor): It consists of a shunt connected capacitor whose impedance is controlled in a stepwise manner using a Thyristor. The manner of control using the SCR is the same as that of TSR.
3. TCR(Thyristor Controlled Reactor): It consists of a shunt connected inductor whose impedance is controlled by the firing angle delay method of SCR wherein the firing of Thyristor is controlled causing a variation in the current through the inductor.

• STATCOM(Static Synchronous Compensator): It consists of a voltage source that can be a DC energy source or a capacitor or an inductor whose output can be controlled using a Thyristor It is used to absorb or generate reactive power.

A  Series-Shunt Controller- Unified Power Flow Controller:

They are a combination of STATCOM and SSSC such that both are combined using a common dc source and provide both active and reactive series line compensation. It controls all the parameters of the AC power transmission.

Steady-State Voltage Control using SVC for Flexible AC Transmission Systems

To generate zero-crossing voltage pulses we need digitalized voltage and current signals. The voltage signal from the mains is taken and is converted into pulsating DC by bridge rectifier and is given to a comparator that generates the digital voltage signal. Similarly, the current signal is converted into the voltage signal by taking the voltage drop of the load current across a resistor.  This AC signal will be again converted into the digital signal as the voltage signal. Then this digitalized voltage and current signals are sent to the microcontroller. The microcontroller will calculate the time difference between the zero-crossing points of voltage and current, whose ratio directly proportional to the power factor and determines the range in which the power is. In the same manner, using Thyristor switched reactor (TSR) also zero-cross voltage pulses can be generated for voltage stability improvement.

Flexible AC Transmission System by SVC

The above circuit can be used to improve the power factor of transmission lines using SVC. It uses thyristor switched capacitors (TSC) based on shunt compensation duly controlled from a programmed microcontroller. This is useful to improve the power factor. If the inductive load is connected, the power factor is lagging because of load current lagging. To compensate for this, a shunt capacitor is connected, which draws current leading the source voltage. Then the improvement in power factor will be made. The time lag between zero voltage and zero current pulses is duly generated by operational amplifiers in comparator mode which are fed to the 8051 series of microcontrollers.

Using the FACTS controller the reactive power can be controlled. Sub synchronous resonance (SSR) is a phenomenon that can be associated with series compensation under certain adverse conditions. SSR elimination can be done using FACTS controllers. The benefits of the FACTS devices are many like a financial benefit, increased quality of supply, increased stability, etc.

A Problem with Flexible AC Transmission System and a way to solve it

For a flexible transmission of AC power, solid-state devices are often incorporated in the circuits which are used for power factor improvement and to raise the limits of the AC transmission system. However, a major disadvantage is that these devices are nonlinear and induce harmonics in the output signal of the system.

To remove the harmonics created due to the inclusion of power electronic devices in the AC transmission system, it is required to use active filters which can be current source power filters or a voltage source power filter.  The former involves making the AC sinusoidal. The technique is to either directly control current or control the output voltage of the filter capacitor. This is the Voltage regulation or Indirect Current control method.  The active power filters inject a current which is equal in magnitude but opposite in phase to the harmonic current which is drawn by the load, such that these two currents cancel out each other and the source current is completely sinusoidal. The Active power filters incorporate power electronic devices to produce harmonic current components that cancel out the harmonic current components of the output signal due to the nonlinear loads. Generally, the Active power filters consist of a combination of an insulated gate bipolar transistor and a diode powered by a DC bus capacitor. The active filter is controlled using an Indirect current control method.  IGBT or Insulated Gate Bipolar Transistor is a voltage controlled bipolar active device which incorporates the features of both BJT and MOSFET. For the AC transmission system, a shunt active filter can eliminate the harmonics, improve the power factor and balance the loads.

Transformer Power Management

Problem Statement:

1. Chronic high voltage is most often attributable to excessive correction for voltage drop on the utility transmission and distribution system. Voltage drop on electrical conductors is a common situation anywhere. But, in locations with low electrical load density, such as suburban and rural areas, long conductor runs magnifies the problem.

2. Impedance causes the voltage to decrease along the length of a conductor as the current flow increases to meet the demand. To correct a voltage drops, the utility employs on-load tap changing voltage regulators (OLTCs) and line drop compensating voltage regulators (LDCs) to boost (raise) or buck (lower) the voltage.

3. Customers nearest to an OLTC or LDC can experience over voltage as the utility tries to overcome conductor voltage drop for those customers at the far end of the line.

4. In many locations, the impact of load-driven voltage drop is seen as daily fluctuations that result in voltage levels being the highest at the time of the lowest load demand.

5. Due to time-varying loads and propagation nonlinearity causes great disturbances will enter into the system which will also enter into the consumer lines leads to the whole system unhealthy.

6. A less typical cause of high voltage problems is caused by local transformers that have been set to boost voltage to offset reduced voltage levels. This most often occurs at facilities with heavy loads at the end of distribution lines. When the heavy loads are operating, a normal voltage level is maintained but when the loads are shut off, the voltage levels shoot up.

7. During strange events, the transformer is burned out due to the overload and short circuit in their winding. Also, the oil temperature is increased due to the increase in the level of current flowing through their internal windings. This results in an unexpected rise in voltage, current or temperature in the distribution transformer.

8. Electrical devices are designed to operate at a certain standard voltage for the product to achieve specified levels of performance, efficiency, safety, and reliability. Operating an electrical device above the specified voltage level range can lead to problems such as malfunction, shut down, overheating, premature failure, etc. For example, a printed circuit board can be expected to have a shorter life when operated above its rated voltage for long periods.

Solution:

1. The design of Microcontroller based system is to monitor the voltage fluctuations on the input /output side of the transformer and acquire real-time data.
2. Development of automatic transformer tap changing using servo/stepper motors.
3. The system should raise the alarm during threshold voltage levels or emergency.
4. The system should be reliable rugged.
5. The system can be fitted on outdoor transformers.
6. The design of continuous monitoring of the oil temperature of distribution transformers will compare by the rated values and the corresponding action will take care.
7.  Use of devices like Automatic voltage stabilizes (AVR’s), Power system stabilizers, FACTS, etc in the power system network.

Technical Feasibility:

Microcontroller Based Data Logger System (MDLS):

MDLS requires no additional hardware and allows the selection of the amount of data and the time intervals between them. The collected data can easily be exported to a PC via a serial port. MDLS is very compact because it employs a few integrated circuits. An MDLS design which is selected should meet the following requirements

1. It should be easily programmable.
2. The user must be able to choose measurement rates.
3. It should backup data when sys power is momentarily disrupted or removed entirely.
4. It should be able to export data to a PC via a serial port.
5. It should be simple and inexpensive.

I hope you have understood the concept of flexible AC transmission from the above article. If you have any queries on this concept or the electrical and electronic projects leave the comments section below.

Photo Credit

• Flexible cir by mathworks