Conny Wahlberg, ABB Power Technologies, Västerås, Sweden
Saudi Arabia is facing the challenge of meeting robust growth in power demand and has plans for huge investments in both generation and transmission. Making better use of existing transmission capacity is one way of increasing available capacity.
In July 2003, ABB won a turnkey contract worth $90 million to install Flexible AC Transmission Systems (FACTS) technology in Saudi Arabia. The installation will improve the transmission capability of a 380 kV power transmission corridor by 80 per cent. This will help prevent power shortages in the Riyadh area during summer peaks and eliminate the immediate need to build costly new transmission lines or power generation facilities to the rapidly growing capital.
Figure 1. FACTS with series compensation in operation at Dafang, China
Compensating for demand
To meet an expected 4.5 per cent annual increase in demand for electric power, Saudi Arabia plans to add some 50 000 MW of installed power generating capacity to the existing base over the next 20 years.
This ambitious strategy signals a need for vast future investments in power transmission. The Saudi Electric Commission (SEC) has already begun the process of extending and securing optimal and safe transmission facilities.
Under its contract, ABB will design, manufacture, install and commission four series compensation units in the existing transmission corridor between the Eastern and Central regions of the Saudi Electricity Commission (SEC) grid. ABB’s system is the first series compensation technology to be used in Saudi Arabia. Series compensation is part of FACTS technology and enables grid owners to increase capacity while maintaining or improving grid stability. The technology ensures stable voltage levels and optimizes transmission capacity under controlled angular stability conditions.
ABB will also expand a 230 kV sub station, install a 300 km long, high speed fibre optic telecommunication link, and build an auxiliary power feeder and associated equipment.
In particular, ABB has been contracted for deliveries of FACTS devices of shunt type, the so called SVC, to the eastern, central and southern region and of series type to compensate the 400 kV lines connecting Riyadh to the eastern region.
SEC has established series compensation with a total rating of some 2200 MVAr. Compared to the alternative investments, the compensation would be substantially less costly. The process to allow for the first installation of series compensation ever in Saudi Arabia, included an absolute request that the supplier should have the necessary competence, proven by relevant references, to carry out the project on a turnkey basis.
Environmental concerns have to be considered when energy projects are planned. Unlike when building new transmission lines and power plants, series compensation has minimal environmental impact.
Constructing new overhead transmission lines takes several years. A FACTS installation such as the one in Saudi Arabia requires no or limited access to new land, and will be in service in less than 18 months. In addition to providing increased capacity and improved stability to the system, FACTS also has the flexibility for future upgrades.
Figure 2. FACTS equipment at the McLeese site in western Canada. Similar equipment will be supplied to Saudi Arabia
Commercially available FACTS technologies can be divided into two main branches: dynamic shunt compensation and series compensation. The former is connected in shunt with the power system, and the latter in series. Both technologies have been applied in several hundred installations around the world.
Dynamic shunt compensation: Dynamic shunt compensation can automatically support the voltage level in a specific area of the power system. The voltage level is an immediate image of the reactive power balance – a too high voltage means a surplus of reactive power and vice versa. A dynamic shunt compensator automatically and instantaneously adjusts the reactive power output smoothly compared to the reference voltage level, thus maintaining the voltage stability.
An event such as lightning striking a line section causes rapid changes in the power system and it is very important that the faulty section is disconnected. The remaining, healthy part of the power system needs the ability to instantaneously overcome the event in order to remain in service. The ability for the healthy part of the power system to rapidly recover from an event is transient stability. A dynamic shunt compensator improves transient stability by quickly detecting and automatically adjusting its output in response to system events.
There are currently two commercially available non-rotating dynamic shunt compensation technologies on the market: the static var compensator (SVC) and the static compensator (STATCOM).
An SVC comprises reactors and capacitors, controlled by thyristor valves. It operates by measuring the actual voltage, and automatically generating or consuming reactive power to the system through its capacitors and reactors, hence automatically providing voltage and transient stability. This technology originates from the mid 1970s, and there are more than 800 installations worldwide.
A STATCOM is based on voltage source converter (VSC) technology, meaning that the valve is built with power electronics with turn-off capabilities. A comparison with an SVC yields that the capacitors and reactors are replaced with intelligent switching of semiconductors. VSC technology utilizing power transistors (IGBTs) operates at a frequency in the kHz range, giving possibilities to implement advanced algorithms in the control system. By connecting DC capacitors on one side of the converter, the STATCOM is able to vary its output with respect to magnitude, frequency and phase angle. This means that the way the converter is operated; the STATCOM is automatically giving the requested output to provide voltage and transient stability. This technology originates from the mid 1990s, and there are approximately 20 installations worldwide.
Series compensation: Series compensation increases the transmission capacity and improves the stability of a power system. A long and loaded overhead transmission line is, in brief, characterized as an inductive reactance, meaning that the transmission line itself consumes reactive power as it transmits the active power. Once again, this means that the transmission system is not operated in the optimum way. By adding series compensation technology to the transmission system, the transmission capacity is drastically increased, as the capacitors will produce (capacitive) reactive power. Furthermore, it is a self-regulating phenomenon; as more current is transmitted, the power system will consume more reactive power and the capacitors will also automatically produce more reactive power. As a result, the transmission line is utilized more effectively, and more active power can reach consumers on the existing infrastructure. Series compensation supports the voltage, as long lines otherwise see a decaying voltage profile along the line.
Specifically, the transient stability of a series compensated transmission line is dramatically improved. The angle between the voltages in the sending and receiving ends is a measure of transient (or angular) stability. With series compensation, the power flow can be held constant and angle can be decreased, increasing the transient stability margins. Alternatively, power flow can be increased if the angle between sending and receiving end voltages is held constant.
Since the introduction of metal oxide varistor (MOV) over-voltage protection in the beginning of the 1980s, the series capacitor will automatically and instantaneously support the remaining and healthy part of the transmission system after the faulty part has been disconnected. Series compensation technology has been utilized since the 1950s, and there are currently some 500 installations worldwide.
One alternative to building new lines is re-conductoring existing transmission corridors with lines able to carry larger currents. However, re-conductoring does not reduce the system demand for reactive power. In fact, re-conductoring will generally increase reactive power consumed by the system, as reactive power used by the system is proportional to the square of line current. However, re-conductoring in combination with series compensation can provide a superior means of increasing power flow over existing rights-of-way.
Since the mid 1990s, controllable series compensation technology is available on the market, i.e. thyristor controlled series compensation (TCSC). This technology opens up new applications.
One example is that effective interconnections of power systems can be accomplished by means of AC (previously only feasible with HVDC technology). When strong isolated power systems are interconnected, power oscillations may appear between the power systems; for instance, upon the disconnection of generation. By applying TCSC technology on the interconnecting line(s), and controlling it to effectively mitigate these power oscillations, the power exchange between the systems is maintained safely.
Figure 3. Breamar SVC 1, Australia. An SVC automatically provides voltage and transient stability
Progress on the Saudi contract is going according to schedule. Civil construction is underway. Local contractor GreenTop (SA) is carrying out civil construction (control buildings and foundations). NCC (SA) will be performing fibre installation on existing overhead lines, 33 kV distribution lines as well as 80 km of roadworks.
ABB (SA) is in the process of manufacturing switchgear and transformers for the auxiliary supplies.
The in-service date of the FACTS system is scheduled for May 2005.