The rapid growth of renewable power generation delivers savings in terms of carbon dioxide emissions and climate change but not cost. Wind and solar power are typically generated far from where power is used and power transmission systems have capacity limits.
The UK has installed wind power generating capacity equal to 30 coal power generating plants. These plants are located offshore, largely in Scotland where winds are often the highest. But the biggest power demand is hundreds of miles away in Southern England. While the power transmission lines connecting Scotland and England typically operate below their thermal limit—the maximum amount of power before heating up, sagging and possibly hitting an obstruction—getting the power from where it is generated to where it is needed is not easy.
Electrical power is delivered using alternating current, with power flow constantly alternating directions. System operators controlling the network have to keep all of the online power generators operating in lockstep so all change direction at the same time. One technical concern is that transmission lines shift the phase – the position on the waveform cycle — of the electricity they are transporting. If the phase difference or angle between two generators at any point on the grid becomes too great, one or both may suffer serious damage. Generators are set up with pole trip detection systems designed to instantly take them off-line when phase differences approach dangerous values. The demand being met by that generator will then instantly be dumped onto other generators. In the worst case scenario, one generator after another trips, causing a power blackout that takes hours or even days to bring the grid back online.
System operators monitoring transmission lines have to make quick decisions. The risk is managed by a combination of offline study and experience that is used to determine whether or not lines are able to handle low-cost sources of power such as wind when it becomes available. The phase angles will always vary but as long as it is an amount that approaches the critical value for any generator that is currently online, there is nothing to fear. Today’s monitoring systems are not capable of measuring the phase shifts that are occurring throughout the network and transmitting that information hundreds of miles to a national control center. So, operators don’t know if the phase shift at various points in the grid is approaching dangerous values. They rely upon operating rules that are determined by studies performed in a simulation environment.
Without live information on the state of the grid, operating rules have to be very conservative. “Getting stability wrong can have catastrophic consequences,” says Peter Haigh, Senior Power Systems Engineer for National Grid, a major supplier of electricity and gas in the UK and Northeastern US and operator of the UK’s electric power transmission system. “We play it safe by adding appropriate margins of safety to the operating rules. Occasionally, this can mean that renewable power sources in Scotland must be taken off-line and replaced by fossil fuel plants in England to avoid the possibility of overstressing transmission lines.”
National Grid is applying the Internet of Things to address this challenge. It is installing 110 grid monitoring sensors at substations and has built 26 portable units. Each sensor is synchronized with a Global Positioning System (GPS) time source so they can make extremely accurate phase angle measurements. The monitors also measure a wide range of other values like voltage waveform quality which is becoming increasingly important because renewable resources can lower the quality of the voltage waveform by introducing harmonic distortion. These distortions can be managed with the help of the measurements from these devices. Each monitoring unit is equipped with National Instrument’s CompactRIO controller which provides control, data logging, processing and network communications. The controllers connect to a central server via National Grid’s secure virtual local area network (VLAN).
The database is currently available to engineers and managers in the company who use it to gain a better understanding of the condition and performance of the power transmission network and to make better decisions such as prioritizing capital investment. In the near future, the phase angle and other key measurements at any point in the grid will be integrated into the control panel’s information system.. This will enable system operators in national control centers to make power dispatch decisions based on actual real-time conditions rather than just relying on the operating rules.
“By knowing the phase angle of voltages at critical points in the transmission network our operators can understand where the actual stability limit is and operate closer to it,” says Haigh. “In practice, this means that in some instances they will be able to take fossil fuel plants offline and replace them with renewable resources that operate at a lower cost and with fewer emissions. As our network evolves, real-time monitoring will enable our system operators to use the lowest cost resources available to keep energy prices low and reduce our carbon footprint.”