This article looks at the use of satellite communication technologies by grid operators to optimise monitoring of networks in a bid to address future grid challenges.
The typical large power system grid is a complex engineering machine that consists of millions of components. Its mission is to provide electricity around the clock to a large number of geographically dispersed customers. Continual changes in customer electricity demand necessitate instantaneous changes in electricity production.
Hence the grid is perpetually in a state of flux and conditions are constantly changing every second of the day. The challenge of grid operations in this dynamic environment is to ensure that power system operating conditions stay within safe limits at all times.
The energy management system (EMS) in an electric utility’s control centre that manages the operation of the high voltage transmission grid. The mission of EMS operators is to ensure electricity is available to all customers at all times.
At the EMS, grid conditions are measured every few seconds and unsafe conditions are identified and alarmed. Timely visualisation of grid measurements is essential for successful grid operations. Real-time measurements then need to be intelligently converted into actionable information. Grid operators do not just need to know that a problem exists.
Operators need to fix the problem!
Future grid operation challenges
Today, operating the grid is becoming even more challenging due to many new trends.
• Adverse weather patterns becoming increasingly severe;
• Demand response programmes and customer engagement unpredictably changing load;
• Distributed generation resources with energy outputs that are difficult to predict;
• Electricity market systems resulting in more unpredictable electricity flows;
• Generation company decisions that may be independent of the transmission company decisions;
• Challenges in forecasting load, generation capacities and operating margins; and
• Maximising utilisation of existing grid assets resulting in operation closer to equipment limits.
Fortunately, new technologies and solutions are being developed to address these issues. Two such key solution domains are the focus of this article. They are enhanced visualisation of grid conditions and enhanced automated grid control.
Enhanced visualisation of grid conditions
For many decades, the EMS has monitored grid conditions via supervisory control and data acquisition (SCADA) every two to four seconds. Today a new cadre of grid measurements is rapidly growing worldwide. Called ‘synchrophasor’ measurements, they monitor the grid at a rate of 50 to 60 samples per second using phasor measurement units (PMUs). Time synchronization is achieved with the aid of the timing signals received from the global positioning system (GPS) satellites. The various constituent components of the PMU are depicted in the block diagram illustrated in Fig. 1. This allows for much faster monitoring of power system conditions. The fast PMU data augment the traditional two to four second EMS SCADA data.
Satellite communication technology has undergone tremendous growth over the past few decades. The uniqueness of this technology is that the communication equipment is located and the fact that it is unaffected by natural disasters. Consists of two different segments: the earth segment and the space segment, the technology can be used for the transfer of synchrophasor data when the end equipment is separated by several hundred kilometres. However, the major disadvantage for synchrophasor application is that communication delay is higher compared to any other technology.
In many cases, therefore, communication via powerline communication, fiber-optic, or cellular means is utilised for communication from the PMU to the phasor data concentrator (PDC).
Although PMUs are small in number today compared to SCADA measurements, their numbers are growing at a rapid rate worldwide. The modern EMS for future grid management consists of both SCADA and PMU data.
One challenge for grid operators in this dynamic new environment is to concisely present this voluminous sub-second PMU data for operator decision making. A frequently cited human limitation has been described as Miller’s ‘magical number seven, plus or minus two.’ Miller’s observation was that humans have a limited capacity for the number of items or ‘chunks’ of information that they can maintain in their working memory. As the saying goes, a picture is worth a thousand words. More importantly, to grid operators, the correct picture is priceless!
This means creating a concise picture of what needs immediate attention is immensely more beneficial. Dynamic dashboards are a way to alert operators of current priority concern areas.
Enhanced automated grid control
For decades, the power grid has been operated on an essentially ‘reactive’ paradigm. Current EMS real-time conditions are the basis of determining actions that will ensure grid reliability and security. Cognitive behaviour methods have been utilised to develop operator ‘use-cases’ to document the specific sequence of actions an operator takes in order to accomplish a specific task.
These can be used to develop optimum navigation capabilities (such as the least number of key-strokes to quickly go from receipt of an alert to analysing the ‘correct picture,’ and to implement the appropriate control action.
Today’s wide-area monitoring systems (WAMS) have implemented advanced PMU-based analytics. These include fast monitoring of angular separation, oscillatory stability, voltage stability and islanding detection. These solutions bring additional situational awareness for the grid operator.
Future grid management is moving toward being more proactive by more closely integrating fast synchronised measurements (such as PMUs) with fast thyristor-based controls such as high voltage DC transmission (HVDC) and flexible AC transmission system (FACTS).
Today, a variety of fast-acting controls are available that can respond at a sub-second rate to triggers from PMU analytics that detect sudden grid problems.
These fast control devices are capable of mitigating a variety of problems such as voltage regulation, phase balancing, congestion relief, poorly damped oscillations, voltage instability, angular instability, etc.
The next step is to augment the existing WAMS applications to perform ‘what-if’ predictions and to suggest corrective control actions. Using angular separation monitoring as an example, Line Outage Distribution Factors (LODF) allow computation of the post-contingency angles across a predefined set of contingencies. Then modelbased sensitivity analyses can provide a quantitative ranking of operator actions (such as raise or lower generation) in order to reduce the angular separation and restore them to acceptable limits.
Similarly, the WAMS islanding analytics which quickly detect an islanding condition can be augmented to analyse network topology to identify the specific island boundaries, provide an assessment of the available resources within each island to stabilise the island, and recommend possible options for subsequent resynchronisation of the grid.
Grid operators need a thermostat, not a thermometer
Monitoring is essential, but corrective action is the eventual goal. In other words, we need to transition grid management from just being a thermometer, to becoming a smart thermostat.
Today, such solutions are feasible. For example, a regional utility is working with a vendor to implement a flexible programmable controller platform with a guaranteed response time suitable for automated widearea control solutions. This control platform is designed to be flexible so it can evolve over time to meet future challenges.
Advanced automated grid control will initially start at substations with local information and local controls. In the future, this will transition to a more wide-area implementation, with an underlying ‘think global, act local’ philosophy. SEI
About the authors
Manu Parashar is a senior software manager and has been active in various technical forums in North America such as the North American SynchroPhasor Initiative (NASPI) and IEEE Power Systems Relaying Committee (PSRC).
Jay Giri is an independent consultant with GGM Consulting. He is an affiliate professor at the University of Washington.