Is Your Surge Protective Device

Capable of Protecting the Smart Grid

and Microgrids?

December 2013

The US electrical grid is being transformed with an infusion of modern technology and communication systems. The goal is to create intelligent distributed electrical generation and transmission systems that will increase electricity availability, reliability, and quality (the Smart Grid and Microgrids). The Smart Grid, Microgrids, and other distributed electrical generation systems will increase switching surges that have the potential to damage the electrical infrastructure, electronic systems and critical processes. Surge protective devices (SPDs) that incorporate coordinated surge components (selenium suppressors and metal-oxide varistors (MOVs)) can protect equipment and processes from switching surges. An SPD that incorporates selenium suppressors and MOVs, and has been protecting equipment and processes from switching surges is the Current Technology Select Series SPD.

The US elecTrical Grid

The US electrical grid is capable of generating 1,000,000 MW of electricity through more than 9,200 generators consisting of more than 300,000 miles of transmission lines, with a power availability rate of 99.97 percent. However, outages still occur [1]. Studies commissioned by the US Department of Energy (DOE) show that [1]:

• A one-hour outage in 2000 caused the Chicago Board of Trade to delay $20 million in trades,• Every minute that Sun Microsystems is without power costs the company $1 million in economic

losses,• The Northeast blackout of 2003 resulted in $6 billion in economic loss to the region,• Electrical power outages and interruptions cost Americans at least $150 billion each year, which

translates to approximately $500 for each citizen.

The Energy Independence and Security Act (EISA) of 2007 requires that advancements in electrical, telecommunications, and information technologies be integrated into the electrical system [1]. It is estimated that $1.5 trillion will be needed through 2027 to enhance the infrastructure, with additional revenue needed for equipment and process improvements [1]. EISA will decrease dependency on foreign energy sources, reduce the emission of global warming components, create a more reliable source of electricity, and allow consumers more choices in how and when they utilize electrical energy.

To address the requirements for modernization, the problems with reliability, additional electricity demand, and to update the existing US electrical grid, the government, energy producers and transmission companies are pushing the Smart Grid concept, while unique electricity consumers are pushing the Microgrid concept [2].

Is Your Surge Protective Device Capable of Protecting the Smart Grid and Microgrids?

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The SmarT Grid

The Smart Grid will integrate information systems and communication technology to provide information on the demand of electricity, which will result in increased efficiency, reliability, and sustainability in the production and distribution of electricity [1]. Smart Grid systems are becoming more prevalent as methods to ensure electrical system reliability, increase energy efficiency, deploy energy sources that have lower carbon foot-prints, and integrate advanced technology [1]. The technologies and equipment being deployed will include advanced power generation, transmission, information and communications systems.

The Smart Grid will consist of existing fossil fuel generator sources plus the addition of “green” or renewable sources of electrical generation. Electric generation will be accomplished by utilities, consumers, and industry. Electricity generated by industry and consumers will be primarily used for individual consumption, but excess electrical energy will be placed back onto the electrical grid. In addition to the changes in the electrical grid infrastructure, communications between all aspects of the electrical grid will be incorporated. Communications will assist consumers in determining the most efficient times to consume electrical energy, and will allow system operators to coordinate consumption with generation, maintenance, and servicing of the electrical grid components (Figure 1).

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The micrOGrid

Microgrids are defined as small, independent power systems that can function either in conjunction with or autonomously from the traditional electrical grid, achieving increased reliability through distributed generation and increased efficiency through reduced transmission distances between generation and consumption locations [3]. Instead of using local or distributed generation (DG) of electrical power to provide electricity for local loads and then selling the remaining electricity to the utility for consumption by remote customers, Microgrids generate electricity and utilize the electricity locally; there is no selling of the electricity to the utility for utilization by remote customers [3]. Microgrids are typically set-up by individual consumers, e.g. small businesses, business campuses, etc. Microgrids can also be configured to include systems that provide sufficient electrical power to multiple consumers, e.g. office complexes, shopping malls, or municipalities.

Equipment to generate electricity for a Microgrid includes wind turbines, photovoltaic solar arrays, as well as gas turbines, small-scale hydro-electric generators, and fossil fuel generators [3]. These devices are similar to those used in the Smart Grid by utilities, but are typically smaller in their generating capacity. Communication equipment in Microgrids are also limited to the needs of the Microgrid owner.

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NOTe 1definition of TraNSieNT [6]: A sub-cycle disturbance in the AC waveform that is evident by a sharp, brief discontinuity of the waveform. May be of either polarity and may be additive to, or subtractive from, the nominal waveform.Transients are commonly represented by IEEE Category A, B, and C waveforms [5, 6, 7].

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TraNSieNTS aNd SWiTchiNG NeTWOrkS

Overvoltage transients can occur as a result of natural phenomena (e.g. lightning, static build-up, etc.) or during any type of switching operation (e.g. energizing or de-energizing Power Factor Correction Capacitors (PFCCs), fault initiation, or fault interruption within the electrical system, switching between electrical generating sources, etc.) [4, 5].

Overvoltage transients that occur when switching electrical equipment are referred to as “switching surges”, (which are different than “lightning induced transients”).

Switching surges have the following characteristics:

1. The magnitude of switching surges have a theoretical maximum of 2 times the source voltage [5, 6]

2. The frequency of the switching surge depends on system characteristics but is in the range of a few hundred Hertz to a few kilo-Hertz [6]

3. The switching current and voltage are typically out of phase [7]

The frequency (f) of the switching surges can be determined by EQ1, where L is the inductance in the system, and C is the capacitance in the system. The current associated with switching transients is determined by the capacitance value of the PFCC through i=vωC, where i is current, v is voltage, C is the capacitive value of the PFCC, and ω is 2πf, where f is the fundamental frequency (60 Hz) [8].

To ensure that the electrical grid functions efficiently, utility companies deploy PFCCs to minimize the effects of inductive losses from motors, compressors, transformers and other inductive devices. PFCCs are typically energized in the morning and de-energized in the evening to account for changing loads of industrial facilities that are connected to the electrical system. Energizing or de-energizing PFCCs causes switching surges to occur (Figure 2) [6, 7, 8]. While PFCCs are located on the transmission side of the electrical grid, switching surges migrate throughout the entire electrical grid. Switching surges can be imposed on parallel or downstream circuits and affect electrical systems, processes and electronic equipment.

In a 208Y/120V, three-phase system, the peak line-to-line (L-L) voltage is 294V (VRMS

x [2]1/2) and the peak line-to-neutral (L-N) and line-to-ground (L-G) voltage would be 170V. Switching surges in the 208Y/120V, three-phase system can have peak voltages between 294V and 588V (L-L), and between 170V and 339V (L-N, L-G).

1 2π√LCf =

EQ1

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VUlNeraBiliTieS OF PrOceSSeS aNd elecTrical eqUiPmeNT

Electric Power Research Institute (EPRI) estimates that approximately 16 percent of the equipment and processes connected to the electrical grid require digital-quality power with less tolerance to voltage fluctuation and variance in frequency [1]. Most digital equipment, including Variable Frequency Drives (VFDs), Uninterruptible Power Supplies (UPSs), DC power supplies, Programmable Logic Controllers (PLCs), etc., rely on Switch Mode Power Supplies (SMPSs) to convert AC electrical power to AC or DC electrical power for use in industrial, telecommunication, medical, or commercial processes. The Information Technology Industry Council (ITIC) curve (formerly the CBEMA curve) is used as guidance for electronic devices (Figure 3).

FiGUre 3iTic cUrVe [9]

NOTe 2Let-through voltages of an MOV SPD are determined by the MOV threshold voltage PLUS the added inductance and resistance associated with the SPD construction and installation.SPDs constructed with MOV only components may have threshold voltages that exceed the protection needs of power semiconductors.

Most industrial, telecommunications, medical, and some commercial equipment are powered from three-phase power systems, e.g. 208Y/120V, 480Y/277V, etc. The SMPSs in equipment of this type utilizes 300V, 600V or 1200V semiconductors (e.g. diodes, silicon controlled rectifiers [SCRs], insulated gate bipolar transistors [IGBTs], etc.) to convert electrical power from AC to DC and then to AC or DC as needed for the application. Sometimes, MOVs are provided in electronic equipment to provide protection from impulse type transients; transients coupled to the electrical system from direct or indirect lightning strikes.

Systems operating at 208Y/120V would generally incorporate semiconductors that have a typical Peak Inverse Voltage (PIV) of 300V; any voltage greater than 300V would cause a reverse breakdown in the p-n junction to occur and damage the power semiconductor. Commonly used MOVs on

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208Y/120V systems have Maximum Continuous Operating Voltages (MCOV) of 300V (L-L) and 150V (L-N, L-G). The threshold or turn-on voltage for MOVs ranges from 432V to 528V (L-L) and 216V to 264V (L-N, L-G), which does not include the additional voltage drop associated with the construction (circuit board traces, bus-bars) and installation (conductor lengths) of the SPD [10]. SPDs constructed with MOVs only may have threshold protection voltages that exceed the PIV of the power semiconductors, leaving the power semiconductors unprotected and vulnerable to switching surges.

miTiGaTiNG SWiTchiNG SUrGeS

Selenium rectifiers have been used in electronic equipment since the 1940s [11]. Selenium rectifiers were originally designed for use in power conversion equipment as a replacement for vacuum tubes for converting AC power to DC power [11]. In power conversion equipment, selenium rectifiers are intended to be used in the forward bias region, which requires that the voltage drop be minimized to allow maximum power transfer to the load. While selenium rectifiers were a significant improvement over vacuum tubes in power conversion equipment, they were eventually replaced by germanium and silicon based rectifiers.

Selenium suppressors (which are different in construction and operation than selenium rectifiers) have been used in SPDs for more than 30 years as a means to attenuate overvoltage transients. Selenium suppressors for use in SPDs are non-linear components designed to operate in the reverse-bias region of the voltage-current (V-I) curve. Characteristics of the selenium suppressor include a predictable stand-off voltage, self-healing properties and large power dissipation capabilities.

Construction of selenium suppressors starts with a conductive base (e.g. aluminum) that is cleaned and plated with Nickel using a vacuum deposition process [11, 12]. Once cleaned and prepared, the base is coated with a Selenium layer and then annealed [11, 12]. After the annealing process, a second layer of selenium is applied and annealed [11, 12]. After a complete annealing of the selenium to the base, a protective layer is applied to the device [11, 12]. This process creates selenium suppressors that have a sharp and distinct transition region (knee) on the V-I curve resulting in a stand-off voltage of approximately 70V; stand-off voltages can vary depending on the manufacturers processes [12]. The final step in the manufacturing process is to create selenium suppressor devices that have power and voltage handling requirements as determined by the designer. This is accomplished by the series or paralleling selenium suppressors and the attachment of various heat sinks.

Testing of selenium suppressors used in SPDs has been conducted by Thomas & Betts Power Solutions to assess the ability of selenium suppressors to attenuate and withstand temporary overvoltage conditions with limited currents of 100A. Testing has shown that SPDs that contain selenium suppressors constructed of four-inch by four-inch (4" x 4") cells are capable of attenuating temporary overvoltage conditions for up to 20 seconds without failure [13]. When SPDs are constructed using larger selenium cells (7.25" x 6"), the selenium suppressor is capable of attenuating temporary over voltage for up to 35 seconds without failure [13].

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Selenium suppressors used in SPDs are ideal for repetitive transients, limited current temporary overvoltage conditions and switching surges because of their self-healing properties and their ability to be combined with aluminum heat-sinks (Figure 4). Selenium suppressors can dissipate up to 80W of power continuously [14]. For attenuation of switching surges that are less than one cycle (16.7 milliseconds for 60 Hz power), selenium suppressors can dissipate large amounts of power, with currents up to 300A [14].

FiGUre 4SeleNiUm SUPPreSSOrS iNSTalled iN a hYBrid

SUrGe PrOTecTiVe deVice (SPd)

Because of their power handling capabilities, selenium suppressors can be sized with a lower MCOV than MOVs without degradation. Selenium suppressors can have an MCOV of 225V (L-L) and 130V (L-N, L-G) for a 208Y/120V system. This equates to typical selenium suppressor threshold voltages of 318V (L-L), and 184V (L-N, L-G) [15]. When combined with the aluminum heat-sinks, the selenium suppressors allow for a lower let-through voltage than MOV components, which provides improved protection to electronic equipment and power semiconductors from switching surges PLUS other induced overvoltage transients.

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cONclUSiON

The US electrical grid is undergoing significant change. Electrical infrastructure, equipment and processes are subjected to overvoltage transients as a result of natural phenomena and switching operations. The Smart Grid and Microgrid systems are becoming more prevalent as methods to ensure electrical system reliability, increase energy efficiency, deploy energy sources that have lower carbon foot-prints, and deploy advanced technology. These changes will increase the occurrences of switching surges on the electrical grid as systems are turned ON and OFF and ON again. The greater number of switching surges on the electrical power system increases the chances of damage to electrical systems, processes, and electronic equipment. Switching surges have amplitudes that approach twice the operating voltage and frequencies that range from a few Hertz to a few kilo-Hertz [4, 6].

MOVs and passive filters have been used successfully to protect equipment and processes from static or lightning induced transients. However, because of the components or circuit limitations, protection capabilities or physical size, MOVs and passive filters alone may not be suitable to protect the power supplies that are the front-end (input) of modern industrial, telecommunications, medical, or commercial equipment from switching surges. The surge component that has shown the ability to divert low-frequency transients from switching operations (switching surges) of electrical equipment, and the interaction between multiple sources are selenium suppressors. The SPD that utilizes Selenium Suppressors and demonstrates the ability to attenuate lower-amplitude switching surges is the Current Technology Select Series SPD.

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reFereNceS

1. Litos Strategic Communication (2009). The Smart Grid: An Introduction. Prepared for the US Department of Energy under contract DE-AC26-04NT41817, Subtask 560.01.04.

2. Kamenetz, A. Why the Microgrid Could be the Answer to our Energy Crisis. Available [on-line] at http://www.fastcompany.com/1297936/why-microgrid-could-be-answer-our-energy-crisis. Retrieved 2012 May 11.

3. Mark, B.A., Weaver, W.W. (2009). Smart Grids and Micro-grids: What are they really? Available [on-line] at http://www.ece.mtu.edu/faculty/bamork/EE5223/MIPSYCON09.SmartGrid.pdf. Retrieved 2013 June 09

4. Russelectric. Eliminating Power Transients When Switching Large Inductive Loads. Available [on-line] at http://www.russelectric.com/wp-content/uploads/Tech-Brief-Dual-Operator-Trnsfr-Swtch.pdf. Retrieved 2013 May 13.

5. Institute of Electrical and Electronic Engineers (2002). IEEE Guide on the Surge Environment in Low-Voltage (1000 V and Less) AC Power Circuits. IEEE C62.41.1TM. Piscataway, NJ, USA

6. Institute of Electrical and Electronic Engineers (2002). IEEE Recommended Practice on Characterization of Surges in Low-Voltage (1000 V or Less) AC Power Circuits. IEEE C62.41.2TM. Piscataway, NJ USA

7. Institute of Electrical and Electronic Engineers (2006). IEEE Recommended Practice for Powering and Grounding Electronic Equipment. IEEE-STD-1100 (IEEE Emerald BookTM). Piscataway, NJ USA.

8. Greenwood, A. (1991). Electrical Transients in Power Systems, second edition. John Wiley and Sons, Inc., NY NY USA

9. ITIC Curve – Power Acceptability Curve for Information Technology Equipment. Available [on-line] at http://www.powerqualityworld.com/2011/04/itic-power-acceptability-curve.html Retrieved 2013 May 23.

10. Littlefuse. Varistor Products TPMOV 25S Series. Available [on-line] at http://www.littelfuse.com/~/media/Electronics_Technical/Datasheets/Varistors/Littelfuse_Varistors_TMOV25S_Datasheet.pdf Retrieved 2013 June 29.

11. Gramels, J. (1953). Selenium Rectifiers – Factors in Their Application. Available [on-line] at http://www3.alcatel-lucent.com/bstj/vol32-1953/articles/bstj32-6-1469.pdf, Retrieved 2013 July 29

12. C.S. Smith (1958). Selenium Rectifiers and the Method Making the Same. US Patent 2,819,433.13. Thomas & Betts (2013). Swell Testing on Mersen 320TPMOVSL with Selenium Cells. Test

Number 4478. Thomas & Betts Power Solutions, Richmond, VA USA.14. Maharaj, R.K. Selenium Suppressors Outperform MOV Cousins. Available [on-line] at: http://

www.deantechnology.com. Retrieved 2013 May 21.15. Dean Technology (2013). Product Catalog. Available [on-line] at: http://www.deantechnology.

com. Retrieved 2013 October 20.

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© 2013, Thomas & Betts Power Solutions, LLC. • Thomas & Betts Power Solutions, LLC., is a wholly owned subsidiary of Thomas & Betts Corporation. Current Technology® is a registered trademark of Thomas & Betts International, Inc. • Specifications are subject to change without notice. • Visit our website for latest revisions.

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