facts

A PAPER PRESENTATION ON
FACTS
(DEVICES AND APPLICATIONS)

ABSTRACT

FACTS is an acronym for Fexliable Alternating Current Transmission Systems.
The philosophy of FACTS is to use power electronics controlled devices to control power flow in transmission network, there by allowing transmission line systems to be loaded to its full capacity.
Power electronic controlled devices such as static Var compensators, thyristor controlled series compensators, unified power flow controller. Among this static Var compensators, have been used in transmission network for many years.
These devices are used for dynamic control of voltage, impedance and phase angle of high voltage AC transmission lines.
The benefits of utilizing FACTS devices increase the better utilization, reliability and availability of transmission systems.
It increases the quality of supply systems.
FACTS devices are environmentally friendly. They contain no hazardous materials and produce no waste or pollutants.
The equipment cost depends not only upon the installation rating on protection and control system components such as reactors, capacitors or transformer.
By use of FACTS devices the financial benefits are additional sales, additional wheeling charges, delaying of investments due to increase in transmission capability.
By using FACTS devices, the interconnection of the south Australian, Victoria and New South Wales systems involved transmission at voltages up to 500 kV over distances exceeding 2200 km.
In Brazil there are two independent transmission grids, the North grid and the south grid. These two grids over more than 95% of the electric power transmission in the country.-
Future developments will include the combination of existing devices e.g. combing a STATCOM with a TCS (thyristor switched capacitor) to extend the operational range.
In addition, more sophisticated control systems will improve the operation of FACTS devices.
Improvements in semiconductor technology (e.g. higher current carrying capability, higher blocking voltages) could reduce the costs of FACTS devices and extend their operation ranges.
Finally, developments in superconductor technology open the door to new devices like SCCL (super conducting current limiter) and SMES (Super Conducting Magnetic Energy Storage).

Introduction


The need for more efficient electricity systems management has given rise to innovative technologies in power generation and transmission.
The combined cycle power station is a good example of a new development in power generation and flexible AC transmission systems, FACTS as they are generally known, are new devices that improve transmission systems. Worldwide transmission systems are undergoing continuous changes and restructuring.
They are becoming more heavily loaded and are being operated in ways not originally envisioned. Transmission systems must be flexible to react to more diverse generation and load patterns.
In addition, the economical utilization of transmission system assets is of vital importance to enable utilities in industrialized countries to remain competitive and to survive.
In developing countries, the optimized use of transmission systems investments is also
important to support industry, create employment and utilize efficiently scarce economic resources.
Flexible AC Transmission Systems (FACTS) is a technology that responds to these needs. It significantly alters the way transmission systems are developed and controlled together with improvements in asset utilization, system flexibility and system performance
The use of such devices could reduce the need for additional network and performance of existing facilities
FACTS is an acronym for Fexliable Alternating Current Transmission Systems.
The concept of FACTS as a total network control philosophy was introduced in 1988 by Dr. N. Hungarian from the Electric Power Research Institute (EPRI) in the USA
The significant impact that FACTS devices will make on transmission systems arises from their ability to affect high- speed control. Currently , the main control actions in a power system, such as changing transformer taps, switching current or governing turbine steam pressure, are achieved through the use of mechanical devices, which necessarily impose a limit on the speed at which control action can be made.
FACTS devices are based on solid-state control and so are capable of control actions at far higher speed. The three parameters that control transmission line power flow are line impedance and the magnitude and phase of line end voltages.
Conventional control of these parameters, although adequate during steady-state and slowly changing load conditions, cannot, in general, be achieved quickly enough to handle dynamic system conditions. The use of FASCTS technology will change this situation.

Why FACTS?

Consider the section of power system. If we consider the flow of power from bus bar to bus bar R, there is no way of individually setting the power flow S to R, or via T.
The load sharing is entirely governed by the line impedances- i.e. it is inflexible. If the impedances are dissimilar, one circuit may never realize its full thermal capacity when operating in parallel with the other circuit.
One possible way of controlling the load sharing between circuits is by the use of HVDC schemes. The power can be electronically controlled by adjusting converter firing angles; it is thus possible to load each circuit separately.
However, the use of HVDC schemes is unlikely to be an economic solution to the problem of improving circuit utilization since it requires the installation of costly converter equipment on one circuit and rebuilding of the overhead lines or cables.
The use of FACTS technology is a more attractive option since FACTS devices can be fitted retrospectively to existing AC transmission routes, thus providing an economic solution.


What are FACTS devices?

FACTS devices are used for the dynamic control of voltage, impedance and phase angle of high voltage AC transmission lines. Below the different main types of FACTS devices are described:


Satic Var Compensators (SVC’s):

The most important FACTS devices have been used for a number of years to improve transmission line economics by resolving dynamic voltage problems. The accuracy, availability and fast response enable SVC’s to provide high performance steady state and transient voltage
Control compared with classical shunt compensation. SVC’s are also used to dampen power swings, improve transient stability, and reduce system losses by optimized reactive power control.

Thyristor controlled series compensators (TCSCs):

They are an extension of conventional series capacitors through adding a thyristor-controlled reactor. Placing a controlled reactor in parallel with a series capacitor enables a continuous and rapidly variable series compensation system.
The main benefits of TCSCs are increased energy transfer, dampening of power oscillations, dampening of sub-synchronous resonances, and control of line power flow.
STATCOMs are GTO (gate turn-off type thyristor) based SVC’s. Compared with

Conventional SVC’s (see above) they don’t require large inductive and capacitive components to provide inductive or capacitive reactive power to high voltage transmission systems.
This results in smaller land requirements. An additional advantage is the higher reactive output at low system voltages where a STATCOM can be considered as a current source independent from the system voltage. STATCOMs have been in operation for approximately 5 years.

Unified Power Flow Controller (UPFC):

Connecting a STATCOM, which is a shunt connected device, with a series branch in the transmission line via its DC circuit results in a UPFC.
This device is comparable to a phase shifting transformer but can apply a series

Voltage of the required phase angle instead of a voltage with a fixed phase angle.
The UPFC combines the benefits of a STATCOM and a TCSC.
The section on Worldwide Applications contains Descriptions of typical applications for FACTS

Benefits of utilizing FACTS devices:

The benefits of utilizing FACTS devices in electrical transmission systems can be devices angle
The UPFC combines the benefits of a STATCOM summarized as follows:

Better utilization of existing transmission system assets:


In many countries, increasing the energy transfer capacity and controlling the load flow of transmission lines are of vital importance especially in de-regulated markets, where the locations of generation and the bulk load centers can change rapidly.

Frequently, adding new transmission lines to meet increasing electricity demand is limited by economical and environmental constraints.
FACTS devices help to meet these requirements with the existing transmission systems.

Increased transmission system reliability and availability:

Transmission system reliability and availability is affected by many different factors. FACTS devices cannot prevent faults; they can mitigate the effects of faults and make electricity supply more secure by reducing the number of
line trips. For example, a major load rejection results in an over voltage of the line which can lead to a line trip. SVC’s or STATCOMs counter act the over voltage and avoid line tripping.


Increased dynamic and transient grid stability:

Long transmission lines, interconnected grids, impacts of changing loads and line faults can create instabilities in transmission systems.
These can lead to reduced line power flow, loop Flows or even to line trips. FACTS devices stabilize transmission systems with resulting and TCSC.
Higher energy transfer capability and reduced risk of line trips.

Increased quality of supply for sensitive industries:

Modern industries depend upon high quality electricity supply including constant voltage, and frequency and no supply interruptions. Voltage dips, frequency variations or the loss of supply can lead to interruptions in manufacturing processes with high resulting economic losses.
FACTS devices can help provide the required quality of supply.

Environmental benefits:

FACTS devices are environmentally friendly. They contain no hazardous materials and produce no waste or pollutants.
FACTS help distribute the electrical energy more economically through better utilization of existing installations there by reducing the need for additional transmission lines.

FACTS are a well-proven technology:


The first installations were put into service over 20 years ago.
As of January 2000, the total World wide installed capacity of FACTS devices is more than 40,000 MVAr in several hundred installations.
While FACTS devices are used primarily in the electricity supply industry, they
are also used in computer hardware and steel manufacturing (SVC’s for flicker compensation),
as well as for voltage control in transmission systems for railways and in research centers
(e.g. CERN in Geneva).

Investment costs of FACTS devices:

The investment costs of FACTS devices can be broken down into two categories: (a) the devices’ equipment costs, and (b) the necessary infrastructure costs.

Equipment costs:

Equipment costs depend not only upon the installation rating but also upon special requirements such as:
• redundancy of the control and protection system or even main components such as reactors, capacitors or transformers,
• seismic conditions, • ambient conditions (e.g. temperature, pollution level): and
• Communication with the Substation Control System or the Regional or National Control Center.

Infrastructure Costs:



Infrastructure costs depend on the substation location, where the FACTS device should be installed. These costs include e.g.
• land acquisition, if there is insufficient space in the existing substation,
• modifications in the existing substation, e.g. if new HV switchgear is required, protection, thyristor valves, auxiliaries etc.),
• Construction of a building for the indoor equipment (control, yard civil works (grading, drainage, foundations etc.), and
• connection of the existing communication


What are the financial benefits of FACTS devices?


There are three areas were the financial benefits could be calculated relatively easily.
1. Additional sales due to increased transmission capability.
2. Additional wheeling charges due to increased transmission capability.
3. Avoiding or delaying of investments in new high voltage transmission lines or even new power generation.
There are also indirect benefits of utilizing FACTS devices, which are more difficult to calculate. These include avoidance of industries’ outage costs due to interruption of production processes (e.g. paper industry, textile industry, production of semi conductors / computer chips) or load shedding during peak load times

Maintenance of FACTS devices:



Maintenance of FACTS devices is minimal and similar to that required for shunt capacitors, reactors and transformers. It can be performed by normal substation personnel with no special procedures.
The amount of maintenance ranges from 150 to 250 man-hours per year and depends upon the size of the installation and the local ambient (pollution) conditions
.

Operation of FACTS devices:

FACTS devices are normally operated automatically. They can be located in unmanned substations. Changing of set-points or operation modes can be done locally and remotely (e.g. from a substation control room, a regional control centre, or a national control centre).


Steps for the Identification of FACTS Projects:

1. The first step should always be to conduct a detailed network study to investigate the critical conditions of a grid or grids’ connections. These conditions could include: risks of voltage problems or even voltage collapse, undesired swings or sub-synchronous resonances.
2. For a stable grid, the optimized utilization of the transmission lines – e.g. increasing the energy transfer capability – could be investigated.
3. If there is a potential for improving the transmission system, either through enhanced stability or energy transfer capability, the appropriate FACTS device and its required rating can be determined.
4. Based on this technical information, an economical study can be performed to compare costs of FACTS devices or conventional solutions with the achievable benefits.

Worldwide Applications:

Seven projects are described below, where FACTS devices have proven their benefits over several years.
These descriptions also indicate how the FACTS devices were designed to meet the different requirements of the seven transmission systems.
The construction period for a FACTS device is typically 12 to 18 months from contract signing through commissioning.
Installations with a high degree of complexity, comprehensive approval procedures, and time-consuming equipment tests may have longer construction periods.

The Australian Inter-connector:

The interconnection of the South Australian, Victoria and New South Wales Systems involved transmission at voltages up to 500 kV over distances exceeding 2200 km.
The interconnection is for interchange of 500 MW. Two identical – 100 MVAr (inductive) /+ 150 MVAr (capacitive) SVC’s at Kemps Creek improve transient stability.
Here each SVC consists of two thyristor-switched capacitors and a thyristor-switched reactor that can be switched in combination to provide uniform steps across the full control range.
To ensure reliable operation under all power system conditions, the implementation of the SVC design had to be carefully evaluated prior to installation. The behavior of the SVC was examined at a transient network analyzer under a wide range of system conditions.
The three-state interconnected system and the two SVC’s were successfully put into commercial operation in spring 1990. The two SVC’s are equipped only with thyristor-switched reactors and capacitors with the advantage that no harmonics are generated and therefore no filters are necessary.
The system operates as expected and proved the original concepts. As part of the interconnected system, the compensators at Kemps Creek have been called upon on several occasions to support the system and have done so in an exemplary manner.


SOUTH AFRICA: Increase in Line Capacity with SVC:

The Kwazulu-Natal system of the Eskom Grid, South Africa, serves two major load centers (Durban and Richards Bay) at the extremities of the system. In 1993, the system was loaded close to its voltage stability limit, a situation aggravated by the lack of base load generation capacity in the area.
The 1000 MW Drakensberg pumped storage scheme, by the nature of its duty cycle and location remote from the main load centers, does not provide adequate capacity. The installation of three SVC’s in the major load centers provides superior voltage control performance compared to an additional new line subject to load switching.
A further motivation for choosing SVC’s in this case are their lower capital cost, reduced environmental impact, and the minimization of fault-induced voltage reductions compared to building additional transmission lines. Fault induced voltage reductions cause major disruption of industrial processes, and mainly result from transmission line faults.
The frequency of such reductions is proportional to the total line length exposed to the failure mechanisms (viz. sugar cane fires), resulting in a desire to minimize the total length of transmission lines.
These SVC’s went into commercial operation in 1995.

BRAZIL: North – South Interconnection:

In Brazil there are two independent transmission grids, the North grid and the South grid. These two grids cover more than 95% of the electric power transmission in the country
Detailed studies demonstrated the economic attractiveness of connecting the two grids. Inter alia they compared the attractiveness of building an AC or a HVDC (High Voltage Direct Current) connection of more than 1.000 km long passing through an area with a fast growing economy and also with a high hydropower potential.
As it is technically much easier and more economical to build new connections to an AC line than to an HVDC line it was decided to build a new AC line. The line, which is now in operation since beginning of 1999, is equipped with SC’s (Series Capacitors) and TCSC’s (Controlled Series Capacitors) to reduce the transmission losses and to stabilize the line. Initial studies indicated the potential for low frequency power oscillations between the two grids which TCSC’s can dampen and thereby mitigate the risk of line instability.
In addition, the application of TCSCs can effectively reduce the risk of sub-synchronous resonances (SSRs) caused by the application of SC’s in a line. SSRs in a transmission system are resonance phenomena between the electrical system and the mechanical system of turbine – generator shafts in thermal power stations. Under certain conditions SSRs can damage the shaft of the turbine – generator unit, which results in high repair costs and lost generation during the unit repair time.

USA: More Effective Long-Distance HVDC System:
A major addition to the 500 kV transmission systems between Arizona and California, USA, was installed to increase power transfer.
This addition includes two new series – compensated 500 kV lines and two large SVC’s. These SVC’s are needed to provide system security, safe and secure power transmission, and support the nearby HVDC station of the Los Angeles Department of Water and Power (LADWP).
By installing the SVC’s, the LADWP ensured its capability to supply high quality electric power to its major customers and to minimize the risk of supply interruptions
The control design for these SVC’s, based on detailed analysis, is driven by the unique system requirement of dampening the complex oscillation modes between Arizona and California.
Extensive testing on a real-time simulator was done, including the HVDC system originally delivered by another manufacturer before the controls were delivered on site. Field tests during and after commissioning verified these results.
These SVC’s, ones of the largest installations ever delivered, went into commercial operation early 1996.


INDONESIA: Containerized Design:

Load flow and stability studies of the Indonesian power system identified the need for a SVC with a control range of – 25 MVAr to + 50 MVAr at Jember Substation (Bali).
The SVC provides fast voltage control to allow enhanced power transfer under extreme system contingencies, i.e. loss of a major 150 kV transmission line. Fast implementation of the SVC was required to ensure safe system operation within the shortest time achievable.
To achieve the tight schedule, a unique approach was chosen comprising a SVC design based on containerization to the greatest extent possible to allow prefabrication, pre-installation and pre-commissioning of the SVC system at the manufacturer’s workshop.
This reduced installation and commissioning time on site and is a step


forward for transportable SVC’s that can be easily and economically relocated.
The Jember SVC was put into commercial operation 1995 in only 12months after contract signature.


USA: The Lugo SSR Damper:

The SSR (Sub-synchronous Resonances) damper scheme is a high voltage-thyristor circuit designed to solve a complex problem which in 1970 and 1971 caused damage to the shafts of a turbine-generator connected to the 500 kV transmission network of the Southern California Edison System.
Analysis of the cause of the failure identified the SSR phenomenon. SSR can occur in electrical networks, which utilize high levels of conventional SCs to increase transmission lines power carrying capability by compensating the line series inductance.
The SSR problem occurs when the amount of SC compensation results in an electrical circuit natural frequency that coincides with, and thereby excites, one of the torsion natural frequencies of the turbine-generator shaft.
Dampening is achieved by using anti-parallel thyristor strings to discharge the SCs at controlled times. Network configurations involving Southern California Edison’s Mohave generator were simulated and used to study the worst case SSR problem.
In this case, with a high level of SC (70 percent), the effectiveness of the NGH scheme (comprising outdoor valves at high-voltage potential platforms) was evaluated. This device is in successful commercial operation since the1980’s.

USA: The Kayenta TCSC:

In the Western Area Power Administration (WAPA) system, USA, transmission of low-cost and renewable hydroelectric energy was limited by a major bottleneck in its high-voltage transmission network. To overcome this limitation, WAPA installed a TCSC device at Kayenta Substation, Arizona – the first ever three-phase thyristor-controlled series compensator.
The Kayenta installation, in successful commercial operation since 1992, provides for a power transfer increase of 33 % while maintaining reliable system operation.
The Kayenta ASC has operated successfully under all system conditions, including several transmission line faults.
This installation provides the technology demonstrator for this type of FACTS device, which, in addition to making better use of existing line capacity obviated the need for installing an extra transmission line by the local electrical utility.

Future Developments in FACTS:

Future developments will include the combination of existing devices, e.g. combining a STATCOM with a TSC (thyristor switched capacitor) to extend the operational range. In addition, more sophisticated control systems will improve the operation of FACTS devices.
Improvements in semiconductor technology (e.g. higher current carrying capability, higher blocking voltages) could reduce the costs of FACTS devices and extend their operation ranges. Finally, developments in superconductor technology open the door to new devices like SCCL (Super Conducting Current Limiter) and SMES (Super Conducting Magnetic Energy Storage).

Conclusion:

The use of FACTS technology, as, for sometime, been recognized superior.
In many applications the improved future of FACTS brought many advantages in power transmission. The improvement in the FACTS is increases with the improvement in the relegated technology.
There is a vision for a high voltage transmission system around the world – to generate electrical energy economically and environmentally friendly and provide electrical energy where it’s needed.
FACTS are the key to make this vision live
.





Bibliography:

 Understanding FACTS by -------- NARAIN G.HINGORANI
LASZLO GYUGYI