Power to be efficient

The idea behind open power markets is that the consumer should be able to purchase power from the cheapest, most efficient or least polluting source. Reality, however, is not quite there yet. Insufficient capacity on the network often requires efficient plants to be run at reduced capacity forcing the customer to buy power from less efficient sources closer by. The solution lies in a combination of new transmission corridors and better and more efficient use of existing ones through the adoption of new technologies. ABB Review takes a tour. Power to be efficient Transmission and distribution technologies are the key to increased energy efficiency Enrique Santacana, Tammy Zucco, Xiaoming Feng, Jiuping Pan, Mirrasoul Mousavi, Le Tang The electrical energy generated by power plants is delivered to the end users hundreds to thousands of miles away via a network of intercon- nected transmission and distribution wires 1 2 3 . Key components on this network include transmission towers, conductors/cables, transform- ers, circuit breakers, capacitors/reac- tors, HVDC/FACTS devices, and moni- toring, protection, and control devic- es. In general, the network that deliv- ers energy over long distances from power plants to substations near population centers and operates at high voltages is called the bulk power transmission network. The distribution system delivers energy from the sub- station to end users over shorter dis- tances, is less interconnected and operates at lower voltages. The trans- mission and distribution (T&D) system is designed to ensure reliable, secure, and economic operation of energy delivery, subject to load demand and system constraints. The blackouts of recent years provide testimony to the lack of sufficient reli- ability and optimization capability in T&D systems on all continents. A T&D system can be designed to provide three levels of services 4 : The first level of service provides the minimum level of connectivity and energy transfer capability under nor- mal operation conditions. This is the most basic service. If this service fails to live up to its requirements, eco- nomic development of the areas served is compromised. The second level of service allows for secure and reliable service to consumers in the event of plausible component failures. It requires redun- dant paths between power plants and consumers and thus a higher level of redundance in T&D capability. The third level of services enables the optimization of geographically distrib- uted and diverse energy resources to achieve maximum social and econom- 14 ABB Review 2/2007 Energy efficient grids 15ABB Review 2/2007 under-investment in network expan- sion and modernization, current T&D infrastructure in the United States of- ten forestalls such measures 5 . Transmission congestion issues in the United States Transmission congestion occurs when flows of elec- tricity across a line or piece of equipment must be curtailed to keep the flow levels under the limits re- quired, either by physical capacity or by system opera- tional security restrictions. Power purchasers always look for the least expensive source of energy available to transmit across the grid to the load centers. When a transmission constraint limits the amount of energy that can be transferred safely to a load center from the most desirable source, the grid operator must find an alternative and more expensive (or less efficient) source of generation to meet the system demand. An industry survey in 2003 examined the six operating ISOs1) in the United States including New England, New York, PJM2), Midwest, Texas, and California [1]. This survey found that the total congestion costs experienced by the six ISOs for the four-year period from 1999 to 2002 totaled approximately $4.8 billion. Public data available from the RTO- administered3) energy markets have North America 14-Aug-03 London 28-Aug-03 Denmark/Sweden 23-Sep-03 Italy 28-Sep-03 Greece 12-Jul-04 Australia 14-Mar-05 Moscow 25-May-05 European blackout 4-Nov-06 Victoria, Australia 17-Jan-07 South Africa 18-Jan-07 Colombia 26-Apr-07 Factbox Significant power outages in recent years ic welfare. This can include optimizing the utilization of the various power plants to reduce the greenhouse gases that may contribute to global warming, and maximizing the overall economic efficiency of meeting the energy demand through market-based energy transactions. Such optimiza- tions are simply not possible without sufficient T&D capa- bilities beyond the level re- quired by the second level of service. Unfortunately, most T&D systems in the world today achieve only the second level and partially the third level service. The blackouts of recent years Factbox provide ample testimony to the lack of sufficient reliability and opti- mization capability in T&D systems on all continents. As is illustrated in the following sec- tion, a well built T&D system also af- fects the level of energy efficiency in power delivery. Inadequate T&D hinders energy efficiency: An example from North America Sufficient transmission and distribu- tion capacities are an essential prereq- uisite for the efficient operation of electric power systems through the optimization of generation resources and loss minimization in the energy delivery system. Due to significant Energy efficient grids Power to be efficient 2 Power plant locations in the United States (source: US Department of Energy) Gas Coal Hydroelectric Oil Nuclear 3 Transmission grid in the United States (source: US Department of Energy) Eastern Interconnection Texas Interconnection Western Interconnection 230,000 volts 345,000 volts 500,000 volts 765,000 volts High-voltage direct current Footnotes 1) ISO: Independent System Operator 2) PJM: Pennsylvania New Jersey Maryland Intercon- nection 3) RTO: Regional Transmission Organization 1 Transmission and Distribution systems connect the power plants to the end users (source: www.howstuffworks.com) Power substation High voltage transmission lines Transmission substation Power plant Transformer Transformer drum Power poles 16 ABB Review 2/2007 shown increased congestion costs over time. A more recent study has indicated that, based on the reported congestion costs for New York ISO and PJM from 2001 to 2005, the total congestion costs are nearly $ 1 billion per year in New York and more than $ 2 billion per year in PJM [2]. Trans- mission congestion also calls for fre- quent transmission load relief actions 6 . When demands are very high and local generation is limited, grid opera- tors may have to curtail service to consumers in some areas to protect the reliability of the grid. HVDC transmission is more efficient for long distance bulk power transfer when using overhead lines. HVDC systems can carry 2–5 times the capacity of an AC line of similar voltage. Electricity losses in T&D systems Transporting power from the genera- tion source to the load always in- volves some losses. These losses add to the total electrical load and so require additional generation, hence wasted resources. Overall, the losses in transmission and distribution sys- tems account for 6 to 7.5 percent of the total electric energy produced [3]. Typical losses are about 3.5 percent in the transmission system and about 4.5 percent in the distribution system. Losses vary greatly in terms of net- work configuration, generator loca- tions and outputs, and customer locations and demands. In particular, losses during heavy loading periods or on heavily loaded lines are often much higher than un- der average or light loading conditions. This is because a quadratic relationship be- tween losses and line flows can be assumed for most devices of power delivery systems. The annual mone- tary impact of T&D losses is estimated at over $ 21 billion (based on the average national retail price of elec- tricity and the total T&D losses in 2005 [3]). In recent years, T&D losses in the United States have been marked by an increasing trend, mainly due to in- creased power transactions and ineffi- cient T&D system operations 8 . Technologies to improve efficiency in transmission and distribution systems Technology options for improving efficiency of transmission and distri- bution systems may be classified into the following three categories: technologies for expanding trans- mission capacity to enable optimal deployment and use of generation resources technologies for optimizing trans- mission and distribution system design and operations to reduce overall energy losses new industry standards for energy efficiency power apparatus Expanding transmission capacity to enable optimal deployment and use of generation resources There are three major technology op- tions that permit transmission capacity to be augmented: building new lines – AC or DC, upgrading existing lines, and utilizing existing lines closer to the thermal limits. Constructing new lines There are two technological options for new lines: high voltage AC (HVAC) and high voltage DC (HVDC). Ther- mal constraints typically limit trans- mission capacities of HVAC lines to 400 MW for 230 kV, 1100 MW for 345 kV, 2300 MW for 500 kV and about 7000 MW for 765 kV. However, in ad- dition to these thermal constraints, the capability of AC transmission systems is also limited by voltage constraints, stability constraints and system oper- ating constraints. As such, the power handing capability of long HVAC transmission lines is usually lower than these values. HVDC HVDC transmission is more efficient for long distance bulk power transfer (eg over 600–1000 km) when using overhead lines 9 . HVDC systems can carry 2–5 times the capacity of an AC line of similar voltage 7 . The environ- mental impact of HVDC is more favor- able than AC lines because less land is needed for the right-of-way4). HVDC transmission has been widely used to interconnect AC systems in situations where AC ties would not be feasible on account of system stability prob- lems or different nominal frequencies of the two systems. In addition, HVDC transmission is also used for underwa- ter cables longer than 50 km where HVAC transmission is impractical be- cause of the high capacitances of the cable (otherwise intermediate compensation stations would be required). A recent devel- opment in HVDC transmission utilizes a compact voltage source converter with IGBT5) technology permitting an im- proved quality of supply in AC power networks. The technology uses small and 5 Transmission investment is falling behind electricity demand growth (source: EEI) Transmission Investment Electricity Retail Sales 1975 1978 1981 1984 1987 1990 1993 1996 1999 m ill io n $ 20 01 /y ea r $7.000 $6.000 $5.000 $4.000 $3.000 $2.000 $1.000 $0 m ill io n ki lo w at t- ho ur s 4.000 3.500 3.000 2.500 2.000 1.500 1.000 500 0 +67 billion kWh/year -$103 million/year Energy efficient grids Power to be efficient 4 The three levels of services provided by transmission and distribution systems B es t B et te r B as ic T & D capability Current status Desired status Meeting basic connectivity need Meeting system security need Meeting optimization & energy efficienty need Enabling capability Footnotes 4) See also “Light and invisible, Under- ground transmission with HVDC Light”, Dag Ravemark, Bo Normark, ABB Review 4/2005 pp 25–29. 5) IGBT: Integrated Gate Bipolar Transistor (a power electronic switching device). 17ABB Review 2/2007 low profile converter stations and un- derground cable transmission – reduc- ing environmental impact. This tech- nology, which is called HVDC LightTM, opens up new possibilities for im- proving the quality of supply in AC power networks with rapid and inde- pendent control of active and reactive power, emergency power support and black start possibility. Efficiency of HVDC Losses in an HVDC system include line losses and losses in the AC to DC converters. The losses in the converter terminals are approximately 1.0 to 1.5 percent of the transmitted power. This is low compared to the line loss- es, which are a function of conductor resistance and current. Since no reac- tive power is transmitted in DC lines, line losses are lower for DC than for AC. In practically all cases, the total HVDC transmission losses are lower than the AC losses for the same power transfer 7 . Obstacles to new lines One significant barrier to line con- struction, whether AC or DC, is the cost allocation controversy. Lines fre- quently cross regions in which the lo- cal benefits are questionable. Should these costs be socialized or should they be allocated directly to the bene- factors only? This remains an area of disagreement in politics and society. Even if a line has financial support, the issue of permitting and siting can become a long, arduous process that many utilities struggle with for years. By the time permission is finally granted, the requirements may have changed and additional studies be needed. Upgrading existing lines There are three ways to upgrade the capacity of existing lines: raise the voltage, increase the size and/or num- ber of conductors per phase, or use high-temperature conductor materials. Increasing a line’s voltage reduces the current required to move the same power. An upgrade from, 230 kV to the next voltage level of 345 kV for example, increases a line’s capacity from about 400 MW to 1100 MW. A high-temperature conductor is capable of transmitting two to three times more current than conventional power lines of the same diameter without increasing structure loads. Reconductoring Since a conductors’ resistance is ap- proximately inversely proportional to its cross-section, increasing this cross section or adding parallel conductors increases the line’s current-carrying capacity. For instance, A 230 kV line can be increased from 400 MW to 1100 MW by adding new, larger and bundled conductors. Recent technological development in the area of high-temperature conduc- tors provides an effective way of miti- gating thermally constrained bottle- necks for short- and medium-length lines. A high-temperature conductor is capable of transmitting two to three times more current than conventional power lines (ie, aluminum-steel rein- forced conductors – ACSR) of the same diameter without increasing structure loads. For both of the above options (raising voltage or reconductoring), the same right-of-way is used and new land use is normally not required. However, because of the increased weight of the new conductors or increased insu- lation requirements, the towers may need to be strengthened or rebuilt. The major substation equipment, such as transformers and circuit breakers may also need to be changed. New lines or upgrades? The issue of constructing new lines versus upgrading existing corridors is 6 Transmission loading relief (TLR) incidents are on the increase (source: NERC) nu m be r of lo gs year Total number of TLRs per year 3000 2500 2000 1500 1000 500 0 1997 1998 1999 2000 2001 2002 2003 2004 2005 Energy efficient grids Power to be efficient 8 Transmission and distribution losses in the USA, 2001–2005 (source: EIA) T&D losses in US, 2001–2005 300 250 200 150 100 50 0 2001 2002 2003 2004 2005 8 6 4 2 0 T&D losses (billion kWh) T&D losses (% of total use) 7 HVDC lines have lower transmission losses over long distances than HVAC lines 150 transmission distance (km) terminals HVDC ±400kV 1620 mm3 1200 mm3 AC 2x400kV Losses (MW) 100 50 500 1000 18 ABB Review 2/2007 certainly not determined by technical questions alone. As mentioned earlier, the process of obtaining permission to build a line takes many years in the U.S., and there is no guarantee of suc- cess. However, the DOE6) recently is- sued two draft designations for na- tional interest electricity transmission corridors as part of the implementa- tion of the EPACT 20057). This is in- tended to simplify the permit-granting process in order to speed up the con- struction of large lines in the most critically congested areas. Make full use of transmission capacity In many cases, transmission lines are operated well below their thermal loading capacity due to voltage con- straints, stability constraints, or system operating constraints. Several technolo- gies are available and are being ap- plied to improve the utilization of the transmission capacity. The phase-angle regulator (PAR) is the device most of- ten used to remove thermal constraints associated with “parallel path flow” or “loop flow” problems. Series capacitor compensation is another commonly used technology for increasing transfer capability of long-distance HVAC trans- mission lines. A family of devices based on power electronics technolo- gy, often referred to as FACTS devices (Flexible AC Transmission System)8) can be used to enable better utilization of lines and cables and other associat- ed equipment such as transformers 10 . The simplest of these devices are the thyristor controlled capacitor and reac- tor banks (SVC) that have been widely used to provide quick reactive power compensation at critical locations in the transmission grid. Another com- monly used device is thyristor-con- trolled series capacitors (TCSC) that can provide reactive compensation as well as damping of power system os- cillations. More sophisticated use of power electronics is employed in what is called static synchronous compensa- tors (STATCOM). This device can ab- sorb and deliver reactive power to the system based on the variations of the system voltage fluctuations. The most sophisticated of these devices is the Unified Power Flow Controller (UPFC). The UPFC can regulate both real and reactive power in a line, allowing for rapid voltage support and power flow control. It is estimated that FACTS devices can boost the transmission capacity of lines now limited by volt- age or stability considerations by as much as 20 to 40 percent. Potential benefits of building and operating unconstrained transmission grids Reduce the prices for electricity The operation of unconstrained trans- mission grids provides cost effective generators access to the load and so increases the efficiency of the electric power market. The operation of an unconstrained transmission grid has the potential advantage that it may permit full use of the regional load shape diversity that may result from weather condition differences and time zone differences. As a result, effi- cient generation resources can be dis- patched at full capacity for more hours, permitting the usage of less economic resources to be reduced. Losses vary greatly in terms of network con figuration, generator locations and outputs, and customer locations and demands. Improve system reliability Unconstrained transmission grids will potentially improve overall system re- liability. At the given level of capacity Energy efficient grids Power to be efficient 11 Distribution transformers account for a considerable part of total transmission and distribution losses. New materials help reduce these losses 9 An HVDC station: HVDC is seeing increasing use in bulk transmission over long distances and other applications 10 FACTS equipmen