FACTS and HVDC

SIMILARITY BETWEEN FACTS AND HVDC

While some of the relevant technology i.e., Static VAR Compensation is already in wide use, the FACTS concept has brought to the table a tremendous potential for thyristor based controllers which will surely revolutionize the power system. The technology offers the utilities the ability to:

1. Control power flows on their transmission routes;
2. Allow secure loading of transmission lines to their full thermal capacity.

FACTS technology, while allowing use of transmission to its thermal capacity, does not do away with the need for additional transmission lines or the upgrading of existing lines where thermal limits have been reached or when evaluation of losses added to the cost of FACTS technology shows that new lines or upgrading of existing lines is the most optimum answer.

Often, ac transmission systems are thought of as being “inflexible”. Power flow in ac networks simply follows Ohm’s law and ordinarily cannot be made to flow along specific desired paths. As a result, ac networks suffer from parallel-path, or “loop” flows. The power flows from source to load in inverse proportion to the relative impedances of the transmission paths. Low  impedance paths take the largest fraction of flow, but all lines in the interconnection are a part of the flow path. Thus, utilities not involved in an interchange power transaction can be affected.

A fundamental notion behind FACTS is that it is possible to continuously vary the apparent impedance of specific transmission lines so as to force power to flow along a “contract path”. This is a brand-new concept for many system planners. As illustrated in Figure 1, with precise control of the impedance of transmission lines using FACTS devices, it is possible to maintain constant power flow along a desired path in the presence of continuous changes of load levels in the external ac network, and to react in a planned way to contingencies. Just as in HVDC applications, FACTS controls could be designed to enhance the behavior of the uncontrolled systems.

Fig. 1: FACTS can direct flow along a desired path

The flexible system owes its tighter transmission control to its ability to manage the interrelated parameters that constrain today’s systems ,including series impedance, shunt impedance, phase angle, and the occurrence of oscillations at various frequencies below the rated frequency. By adding to in this way, the controllers enable a transmission line to function nearer its thermal rating. For example, a 500-kV line may have a loading limit of 1000-2000 MW for safe operation, but a thermal limit of 3000 MW.

It is often not possible both to overcome these constraints and maintain the required system reliability by conventional mechanical means alone, such as tap changers, phase shifters, and switched capacitors and reactors (inductors). Granted, mechanical controllers are on the whole  less expensive, but they increasingly need to be supplemented by rapidly responding power electronics controllers.

The new technology is not a single, high-power electronic controller, but rather a collection of controllers, which can be applied individually or collectively in a specific power system to control the five interrelated functions already mentioned. The thyristor is their basic element, just as the transistor is the basic element for a whole variety of microelectronic circuit. Because all controllers for the flexible transmission system are applications of similar technology, their use will eventually benefit from volume production and further development of high-power electronics.

Electric power networks integrate generation and load centers within each utility system and through interconnections among neighboring systems, share power with vast regional grids. The purpose of this is to take advantage of the diversity of loads, changes in peak demand due to weather and time differences, the availability of different generation reserves in various geographic regions, power sharing arrangements among utilities , shifts in fuel prices, regulatory changes, and other discrepancies.

 

Applying Flexibility to the Electric Power System

The power industry term FACTS (Flexible AC Transmission Systems) covers a number of technologies that enhance the security, capacity and flexibility of power transmission systems. FACTS solutions enable power grid owners to increase existing transmission network capacity while maintaining or improving the operating margins necessary for grid stability. As a result, more power can reach consumers with a minimum impact on the environment, after substantially shorter project implementation times, and at lower investment costs – all compared to the alternative of building new transmission lines or power generation facilities.

The two main reasons for incorporating FACTS devices in electric power systems are:
· Raising dynamic stability limits

· Provide better power flow control

Benefits of FACTS

Rapidly Implemented Installations: FACTS projects are installed at existing substations and avoid the taking of public or private lands. They can be completed in less than 12 to 18 months—a substantially shorter timeframe than the process required for constructing new transmission lines.

Increased System Capacity: FACTS provide increased capacity on the existing electrical transmission system infrastructure by allowing maximum operational efficiency of existing transmission lines and other equipment.

Enhanced System Reliability: FACTS strengthen the operational integrity of transmission networks, allowing greater voltage stability and power flow control, which leads to enhanced system reliability and security.

Improved System Controllability: FACTS allow improved system controllability by building “intelligence” into the transmission network via the ability to instantaneously respond to system disturbances and gridlock constraints and to enable redirection of power flows.

Seamless System Interconnections: FACTS, in the form of BTB dc-link configurations, can establish “seamless” interconnections within and between regional and local networks, allowing controlled power transfer and an increase in grid stability.

 

–ANAND MANDAL

E-mail: anandkalyaneee@gmail.com