Introduction

These widespread outages have demonstrated the vulnerability of the interconnected power systems worldwide when operated outside intended design limits.  The exposure of the power system to wide area collapse have increased in recent years as the power systems have been pushed to their operating limits - often resulting in violation of operating policies and planning standards.

One of the U.S. Department of Energy (DOE) and the Canadian Natural Resources (NRCAN)'s top priorities is modernizing North America’s electricity infrastructure.  This effort focuses, amongst others, on the application of technology to enhance the reliability and efficiency of the entire energy delivery system. 

Electric reliability and efficiency are affected by four segments of the electricity value chain: generation, transmission, distribution, and end-use.  Satisfactory system performance requires investments in all these segments of the system.  Increasing supply without improving transmission and distribution infrastructure, for example, may actually lead to more serious reliability issues. The Transmission Reliability Program is developing advanced technologies, including information technology, software programs, and reliability/ analysis tools, to support grid reliability and efficient markets during this critical transition.

The National Transmission Grid Study has stated that without dramatic improvements and upgrades over the next decade our nation's transmission system will fall short of the reliability standards our economy requires, and will result in higher costs to consumers.

The Transmission program's mission specifically is to develop technologies and policy options that will contribute to maintaining and enhancing the reliability of the nation's electric power delivery system during the transition to competitive power markets.

There are often many issues to address in terms of reliable system operation, however the primary issue is typically the heavily loaded transmission system (with subsequent high reactive power losses/requirements).  This overloading is often at the root of system instability problems. The understanding of the reactive margin requirements is not lost on legislators and regulatory bodies who have expressed their concerns about potential blackout scenarios. Reactive power flow analysis, including mitigation of voltage instability, should become an integral part of planning and operating studies and have been mandated in the recent industry recommendations for prevention and mitigation of future blackout scenarios.

The Union for the Coordination of Electricity Transmission (UCTE) report lists 14 observations for the September 28, 2003 outage, that are very similar to those identified for the outages in North America and in Europe.

It should be noted that the issues faced, in many cases, are not easily overcome.  Even when new generation is justified, transmission owners are faced with challenges such as market forces, permit availability, sighting opportunities, and strict environmental constraints as opposed to system studies. Under these scenarios, load centers often end up connected far from generation resources and through heavily loaded weak transmission systems.  Subsequently, deregulation and the high cost of building new transmission infrastructures have placed the transmission owners under increasing pressure to maximize asset utilization.  Transmission operators note that they have credible contingency situations that can result in voltage collapse or system instability challenges imposed by insufficient levels of reactive compensation.  The potential risk of voltage instability, especially during contingent conditions has been evident without the continued dynamic reactive support.

Solution Space

As mentioned above, one of the issues to address is lack of reactive power sources.  In the case of the 2003 North East outage in North America for example, the Electric Reliability Council (NERC) Planning Standard specifies that: “Proper control of reactive power supply and provision of adequate reactive power supply reserve on the transmission system are required to maintain stability limits and reliable system performance.  Entities should plan for coordinated use of voltage control equipment to maintain transmission voltages at sufficient levels to ensure system stability with operating range of electrical equipment.” 

Dynamic VAR support is often needed to maintain the desired operating voltage levels and mitigate voltage instability from unscheduled generation and transmission contingencies during high load conditions.  As such, one piece of the solution space is addition of VAR sources on the system. 

In addition to reactive compensation, power flow regulation devices such as series capacitors, Thyristor Controlled Series Capacitors (TCSC), and DC lines can be installed on a system.  In the total stability solution space, these technologies may be required, however they tend to have long lead times and are capital intensive.

Advancements in the real time monitoring of power system parameters and availability of secure high-speed telecommunication networks now provide opportunities for implementing wide area protection and control schemes generically known as Special Protection Schemes.  NERC defines SPS as:  “an automatic protection system (also known as a Remedial Action Scheme - RAS) designed to detect abnormal or predetermined system conditions, and take corrective actions other than and/or in addition to the isolation of faulted components to maintain system reliability. Such action may include changes in demand, generation (MW and Mvar), or system configuration to maintain system stability, acceptable voltage, or power flows.