The US power grid is getting old with parts of it dating back over a century. In fact, 70 percent of transmission lines and power transformers are over 25 years old, and power plants average over 30 years. Consequently, its reliability has been declining for decades. According to the Department of Energy, the “second half of the 1990s saw 41 percent more outages affecting 50,000 or more consumers than in the first half of the decade.” Interruptions cost Americans $150 billion, or around $500 per person annually.
In response, the industry is spending billions of dollars to replace and upgrade aging infrastructure, but the winds of change are blowing.
For one, the nature of the relationship between utilities and consumers is changing. The rise of alternate energy sources such as solar and wind power has turned many consumers into “prosumers” who can both purchase energy and sell it back to the utility depending on instantaneous changes in usage patterns. The intermittent nature of sun and wind is driving the development of more efficient energy storage technology and the means to switch it into and out of the grid when needed.
The Dawn of the Smart Grid
To make the most efficient use of resources, accommodate new sources of power, and respond to new trends, the last decade has seen the increasing integration of digital information and control technology into the power grid, leading to its gradual transformation into the Smart Grid.
For many residential consumers, the appearance of smart meters is the most noticeable change, but that’s only the tip of the iceberg. The Smart Grid uses bidirectional digital communication and control at every point in the system to deliver power from suppliers to homes and industrial locations. A formal definition of what it includes first appeared in the Energy Independence and Security Act of 2007.
Smart Grid devices currently in place or being installed include synchrophasors in substations and on transmission lines, smart sensors on transmission and distribution lines, distributed generation systems, and smart meters at the customer interface and within the power grid.
The power delivery system includes several main components:
Bulk Power Generation – primary sources of power such as nuclear, hydro-electric, steam, gas turbine, solar, or wind generators, etc.
Transmission – included in substations at generating plants, transmission substations, and in the conductors and cables between towers.
Distribution – transfers power from the transmission system to consumers of a geographical region and includes substations to reduce voltage levels and provide isolation and protection.
Distributed Energy Resources and Generation – secondary sources of electric power: small-scale generators (micro-turbines, fuel cells, etc.); renewable sources; stored energy from batteries or other systems.
Control Centers – provide real-time control of operations including dynamic load management, fault management, outage management, maintenance scheduling, etc.
Meters And Infrastructure – precise recording of electricity usage. Smart meters, already being installed, allow bidirectional communication with the utility.
For the Smart Grid to be fully operational, it must integrate many electronic systems into a grid that was never designed to accommodate current generation electronic technology such as RF and optical communications, and high-speed low-voltage digital and analog circuitry.
The integration of such a broad spectrum of technologies is raising concerns about EMC performance, especially since in many cases electronic devices are replacing mechanical devices that were immune to EM fields or conducted disturbances. Substations, in particular, have a harsh electromagnetic environment, and the failure of Smart Grid devices in substations may have far-reaching consequences to the electric power grid.
What are some of the potential EMC issues that designers must take into account?
• Conducted noise from sources such as power line harmonics, lightning, power system switching surges, or fast transients. For example, circuit breakers, disconnect switches, and their control systems are all powered by a station battery (in most cases 125 V dc). When inductances, such as the dc operating coils of auxiliary relays, are de-energized, a very fast rising dc transient is created on the dc supply circuit. Since these are often slowly opening contacts, there are repeated flashovers of the contact until it opens wide enough to create a high enough dielectric strength to stop the current flow.
• Radiated noise, or signals from AM, FM, and TV broadcast transmitters, communications radios, and wireless devices. For example, maintenance personnel commonly use low-power portable transceivers to communicate in generating stations, substations, and remotely. There have been multiple reports of electronic protective relays tripping as a result of two-way radio use.
• High ESD levels, which were not high on the priority list forty years ago. Many substation control rooms have vinyl asbestos insulated flooring. Insulated shoes combined with low humidity can result in high electrostatic discharges that can spell doom for semiconductors.
• Addition of New Power Sources – As solar, wind, and other power sources are included in the grid, designers need to consider any EMC implications, especially since many alternative energy products are not designed with EMC in mind. A 2014 European study, for example, found that only 9 percent of PV inverters tested complied with the EU’s EMC Directive. Further, the Fraunhofer Institute in Germany found that one meter under test measured less than 82 percent of the true output value when connected to a PV system. Over the years, the average wind power generator output has increased to 3,000 kW, giving rise to dv/dt rates of 6 kV/µs or higher.
• Multiple Communication Standards – In contrast to other applications with demanding EMC requirements (automotive, for example), the Smart Grid does not have a small number of standards that are accepted by the majority of players. Figure 2 illustrates the issue.
• Integration of New Equipment – The Smart Grid will require a significant increase in high-voltage, high-current power devices in numerous locations. For example, synchrophasors measurement units (PMUs) are installed in substations to provide real-time power measurement and wide-area control. And wireless powerline sensors, located on transmission lines, measure conductor temperature, line sag, wind movement, electricity consumption, and other parameters.
There will also be upgrades to older devices to make them compatible with the new technology. Flexible Alternating Current Transmission System (FACTS) devices, for example, have been used since the 1970s to provide more effective line flow control, loss minimization, and voltage control. A new generation of FACTS devices will be needed with faster switching speeds and wireless capabilities, increasing the frequency of radiated and conducted noise.
Since the electrical grid is global in scope, designers must also consider high-power events such as geomagnetic storms, EM interference from portable transmitters, and lightning strikes. Scientists at the DOE even worry about EM pulses associated with high-altitude nuclear detonation (HEMP).
There is a large number of regulatory standards that cover various aspects of EMC in the power generating industry. Since the Smart Grid is so diverse, standards from multiple agencies may be applicable.
The IEC’s International Special Committee on Radio Interference (CISPR) issues standards covering the control of emissions at frequencies above 9 kHz and has provided a guidance document on EMC performance of equipment connected to the Smart Grid. Some of the applicable CISPR standards are:
• CISPR 11 (emissions from industrial, scientific, and medical devices).
• CISPR 32 (emissions from information technology and multimedia equipment, as well as receivers). This is identified in the IEC Smart Grid Standardization Roadmap.
• CISPR 24 for immunity of information technology equipment (ITE) in Smart Grid control and appliances/devices.
• CISPR 12/25 for vehicles, providing test methods for emission measurements and taking into account the impact of electrical vehicles and distributed charging stations.
• CISPR 16, which includes basic RF measurement methods and test instrumentation specifications.
Other interested agencies include the ITU-R and FCC for wireless communications, and TC77, another IEC committee covering EMC matters including frequencies below 9 kHz, ESD, and high-power events such as EMP.
In the U.S., the National Institute of Standards and Technology (NIST) has a Smart Grid Interoperability Panel (SGIP) with the mission of accelerating the implementation of interoperable Smart Grid devices and systems.
EMC issues related to Smart Grid adoption are many and varied. The procedures needed to solve them bring to mind the old maxim about the best way to eat an elephant: one bite at a time.
Multiple groups worldwide are working to accomplish the task in just that way.