An Overview of Surge Protection for the Smart Grid IEEE PES/SPDC WG Matt Wakeham, Chair

Transcription

1 An Overview of Surge Protection for the Smart Grid IEEE PES/SPDC WG Matt Wakeham, Chair The document content is of a general nature only and is not intended to address the specific circumstances of any particular individual or entity; nor be necessarily comprehensive, complete, accurate or up to date; nor represent professional or legal advice.

2 2 An Overview of Surge Protection for the Smart Grid IEEE PES/SPDC WG Matt Wakeham, Chair This paper presents a brief overview of surges which occur in ac power, control and communication circuits, and the use of surge protective devices to mitigate these effects within the Smart Grid. An overview of pertinent industry standards is provided. 1.0 INTRODUCTION AND PURPOSE The purpose of this paper is to outline how surge protective devices play an integral part in improving the operability and reliability of the North American Smart Grid and its components. Applications include connection to power equipment, as well as data and signaling circuits for control, monitoring and communication. The selection and application of surge protective devices based on transient exposure levels are discussed. 2.0 SMART GRID 2.1 Smart Grid Overview The IEEE definition of the Smart Grid is the integration of power, communications, and information technologies for an improved electric power infrastructure serving loads while providing for an ongoing evolution of end-use applications. The source for this definition is IEEE Std IEEE Guide for Smart Grid Interoperability of Energy Technology and Information Technology Operation with the Electric Power System (EPS), End-Use Applications, and Loads. Paramount is the two way flow of energy and information. The basic concept of Smart Grid is to provide interoperability of the national electrical delivery system by means of monitoring, information, control, and communication capabilities to the national electrical delivery system to maximize the reliability of the system while reducing the energy consumption. The intent of the Smart Grid is to allow utilities to transmit and distribute electricity throughout the system reliably through the deployment and use of intelligent electronic equipment in the sub-domains associated with the electric power and the public and private communications. This will allow homeowners, businesses and industrial facilities to use electricity more intelligently. The traditional power distribution arrangement is to have central power generation with radial distribution. If there is too much load for the available generation or transmission it will cause the generation to slow down or the transmission system to break apart in an effort to reduce the sag in the transmission lines. Most transmission systems have automatic frequency and undervoltage load shedding plans in place. Modeling the power system determines what are the maximum distributed energy resources that can be absorbed on the transmission facilities. One thing that can occur is that voltage and frequency regulation is going to be degraded any time the grid is put under too much stress. Intelligently interlinking distribution systems in a grid allows for the continued delivery of power during fault or excessive demand situations.

3 3 One of the aspects of the Smart Grid is the ability to monitor, control and coordinate alternative energy resources at various points in the grid. Problems exist when a radial distribution system has to accept two way flow of real and reactive power. To manage this problem there has to be a greater dependence on monitoring, information exchange and control (MIC) (IEEE Std ). It is imperative to understand how to protect all communication system elements from unnecessary upset or damage from lightning, both direct and indirect. It is also necessary to assure that surge protection and/or other technologies are in place to mitigate switching surges anywhere in the electrical delivery system causing upset or damage. Non-traditional sources like energy storage; flywheels, batteries and high energy capacitors and renewables such as; wind photovoltaic, require new standards to define the surge protection requirements. 2.2 Smart Grid and Communications Protection of the communication links from overvoltage disturbances are important in a Smart Grid since operation depends upon a reliable two-way communication between the electric utility and the customer for effective service. This may be as simple as providing information to the customer about the cost of the energy being used called Demand Response (DR) or Demand Side Management (DSM) ; or it can directly control equipment within the facility to manage peak power demand from the grid. Communications typically employ wireless, power line carrier, copper wire or fiber-optic services and in some cases microwave systems or a combination of these systems. The service needs to be interfaced to a network. Generally this interface is implemented by a device called a gateway, which is located where the communications service enters the structure. A prevalent myth is that if this gateway is served by a fiber optic or wireless connection, it is immune from damage by lightning. But damage can still occur, due to ground potential rise.

4 4 Antenna Photovoltaic system CATV Telecom NIU Modem (typical) Set-top box Existing wires [TWP, CAT 5, Coax] Surveillance AC Power Smart power meter (could also be wireless) Telephone Appliance Appliance Appliance Figure 1. A possible configuration of a home network including electronic equipment connected to a gateway via an Ethernet link 2.3 Importance of Surge Protection for Smart Grid Key aspects of Smart Grid point to an increased need for surge protection due to: 1. Addition of electronic based monitoring, analysis, control and communication equipment. Surge protection is required to protect this electronic equipment from damage due to voltage transients. 2. Proliferation of communications, control and monitoring devices and the addition of distributed and alternative power sources, increases the exposure of equipment to lightning and other surges. 3. Increase in residential, commercial and industrial use of energy management systems and distributed generation cause more switching of loads and generation sources within the premise; thereby increasing the need for point-of-use protection inside the premise. 4. The overall geographical reach of the Smart Grid poses an increased exposure to lightning damage or disruption. This risk is proportional to the lightning strike density and capture area of the grid. Manufacturers of electronic equipment and appliances might need to install surge protective devices to mitigate surges. Immunity levels are discussed in IEC Std series and IEEE Std1100.

5 5 Typically, electric utilities require equipment to meet IEEE C , C and C for electronic controls connected to the electric power system; and IEC and In particular, distributed generation systems found in the smart grid often use power inverters and electronic controls that are susceptible to damage from surges, and as such, should have surge protection on both the output (AC) and input (DC) to protect the sensitive internal electronics. In this application both utility equipment and customer equipment might need surge protection. In addition, it is important to assure that the earth conductors of both systems are bonded to the same earthing point. System reliability can be degraded due to the presence of surges, which can be internally or externally generated. This degradation can be prevented by a well-designed protection system. Whether incorporated in a surge panel, a surge strip or a distributed system including a panel, sub-panel and point of use device, surge protection provides valuable down line protection from damage due to surges on the power lines. 3.0 ELECTRICAL DISTURBANCES AND SURGES Temporary electrical events, such as lightning, can be coupled into power and ICT systems as surge voltages, currents or both. These surges can upset or damage electronic equipment within a facility, since they can reach amplitudes of tens of thousands of volts and thousands of amps. 3.1 Internally Generated Disturbances Surge disturbances can originate inside or outside a facility. It is estimated that 60 80% of surges are internally generated within the facility. Common sources of these internal surges are devices that switch power. This can be anything from a simple thermostat switch operating a heating element to a switch-mode power supply found in many electronic devices. Examples of switching surge sources include: Contactor, relay, and breaker operations.: These devices include inductance which stores energy that may be released as an arc across the contacts when switched. The waveform of these surges is often complex with amplitudes that may be several times greater than the nominal system voltage. Switching of capacitor banks: Capacitor banks are commonly used to manage power factor correction within the facility s electrical system. These capacitor banks are switched into and out of operation to regulate the reactive power. This switching action can create ringing, with voltage amplitudes that can be as high as four times the nominal system voltage Switching of inductive devices: Transformers, reactors and motor stored energy, which when switched, is released into the electrical system.

6 6 3.2 Lightning Disturbances Lightning protection systems (LPS) can improve the reliability of the electric power and the communication systems. The design of a LPS is based on statistical information related to the ground flash density (isokeraunic maps) and the location of the structure. The effectiveness of the LPS relies on correct grounding (earthing) and bonding methods. Not all facilities require or need to have the same degree of lightning protection. During a lightning discharge, the passage of the direct lightning current from the lightning receptors (or any other air terminal) at the top of the structure to ground via the LPS down conductors, can induce, harmful overvoltages into the internal wiring system due to electromagnetic coupling,. 3.3 Other line-side disturbances Many of the internally generated surges discussed earlier are similar to those present on the line side of any service entrance equipment of a facility. The difference is generally in the amplitude of the surge. The surges from the line side are typically much higher in amplitude from the surges generated on the load side. Line side surges are generally associated with equipment damage while load side surges are more commonly associated with equipment upset. The surge environment is well discussed in IEEE Std. C IEEE Guide on the Surge Environment in Low-Voltage (1000 V and Less) AC Power Circuits. This includes line-side and load-side surges. IEEE Std. C IEEE Recommended Practice on Characterization of Surges in Low-Voltage (1000 V and Less) AC Power Circuits presents recommendations on the selection of representative surge parameters to be considered in assessing equipment immunity and performance of SPDs. The contents of these two standards address the majority of surge issues that Smart Grid equipment is exposed to. However they do not cover special cases such as electromagnetic pulse (EMP) and geomagnetic disturbances. 4.0 SURGE PROTECTION 4.1 Overview of Surge Protection A Surge Protective Device (SPD) is a device that mitigates the effects of excessive voltage transients. This protection is accomplished by diverting surge current, thereby reducing the level of the voltage transients. These devices are employed to protect electrical and electronic loads or the facility s service equipment. When an overvoltage in excess of the SPD limiting voltage appears on the system, the SPD changes state from being very high impedance and drawing little to no current, to a low impedance state where excess current is diverted and the SPD limits the voltage. If correctly selected, coordinated and installed this SPD will protect the downstream equipment. A second

7 7 important function of an SPD is the ability to equalize or reduce voltage differences that occur across grounded and bonded components of the power and communications systems. Examples of components used in SPDs are: metal oxide varistors (MOV), thermally-protected MOVs, avalanche breakdown diodes (ABD), gas discharge tubes (GDT), passive filters and circuits including combination designs of these components. In addition, communication circuits might use Electronic Current Limiters (ECL) and/or Positive Temperature Coefficient (PTC) devices. Inductors and/or capacitors may also be included to provide filtering. 4.2 Assessment Process to Determine Proper Surge Protection for Different Facilities and Equipment The Institute of Electrical and Electronics Engineers (IEEE) developed the IEEE Recommended Practice on Characterization of Surges in Low-Voltage (1000 V and Less) AC Power Circuits (Std C ) as an electrical transient exposure level/surge severity categorization guideline. Figure 2 shows the concept. These severity levels enable manufacturers to test and users to specify the appropriate protection level based on one of three Categories. The categories include outdoor location, service entrance equipment and inside the facility. Service entrance Service equipment C C/B B B/A A Service equipment Outbuilding XF Service entrance Meter Subpanel Underground service Figure 2. The concept of location categories and transitions as simplification approach.

8 8 Table 1- Classification of location stress levels IEEE Location Stress Level Location Category C High Service equipment Service entrance near utility substation Service entrance on the electric supply system with other large industrial users C/B High-to-Medium Service entrance remotely located from utility power factor correction and electric supply system switching High-lightning area distribution panels feeding roof-top loads IEEE Category B Medium Large distribution panels Non-service entrance distribution switchboards Heavy equipment located near unprotected service entrance Panels feeding variable speed drives Non-service entrance motor control centers utilizing drives, PLCs, soft-start or electronic starters IEEE Category B/A Medium-to-Low Branch panels with heavy sensitive equipment loads Branch panels with combination of "dirty" and sensitive loads Branch panels without upstream protection Busway feeding sensitive loads Bus riser feeding multiple floors with critical or sensitive loads IEEE Category A Low Branch panels with upstream protection Branch panels with primarily sensitive electronic loading Branch panels deep within a facility

9 9 Refer to Table 1 of C for a summary of applicable standards and surge-testing waveforms for Location Categories A, B, and C. 5.0 EXISTING STANDARDS AND RESOURCES 5.1 Existing SPD Related Standards There are a number of industry standards that apply to surge protective devices (SPDs), whether they are connected to the electrical system through a plug-in connection or hard-wired connection to the facility wiring. These include: IEEE Std C (2002): Guide on the Surge Environment in Low-Voltage (1000V and less) AC Power Circuits - This guide provides comprehensive information on surges and the environment in which they occur. It describes the surge voltage, surge current, and temporary overvoltages (TOV) environment in low-voltage (up to 1000V root mean square [RMS]) AC power circuits. It is a reference for the second document, which describes the surge environment. IEEE Std C (2002): Recommended Practice on Characterization of Surges in Low- Voltage (1000 V and less) AC Power Circuits - This guide presents recommendations for selecting surge waveforms and the amplitudes of surge voltages and currents used to evaluate equipment immunity and performance of SPDs. Its recommendations are based on the location within a facility, power line impedance to the surge, total wire length, proximity, and type of electrical loads, wiring quality, and more. IEEE Std C62.45 : Recommended Practice on Surge Testing for Equipment Connected to Low- Voltage (1000V and Less) AC Power Circuits - This guide focuses on surge testing procedures using simplified waveform representations (described in IEEE C ) to obtain reliable measurements and enhance operator safety. This guide provides background information that helps determine whether equipment or a circuit can adequately withstand surges. IEEE Std C : Application of Surge Protectors Used in Low-Voltage (Equal to or Less than 1000 V, rms, or 1200 V, DC) Data, Communications, and Signaling Circuits IEEE Std C : IEEE Guide for the Application of Surge-Protective Devices for Low- Voltage (1000 V or Less) AC Power Circuits National Electrical Code (NEC) and National Fire Protection Association (NFPA) - Developed by the NFPA, the NEC was established to address electrical safety in the workplace. While the code is updated every three years, not all states and municipalities have adopted the same version of the NEC.

10 10 NEC Article 285 Includes requirements for connecting all SPDs rated 1000V or less to the electrical distribution system of a facility. The standard addresses surge protection to help electricians properly install hardwired SPDs. National Fire Protection Association (NFPA) 780 Lightning Protection Code NFPA 780 addresses the protection requirements for ordinary structures, miscellaneous structures, special occupancies, industrial operating environments, etc. It requires that devices suitable for protecting the structure be installed on electric and telephone service entrances, and on radio and television antenna lead-ins. IEC Ed. 2.0 b:2008 Low-voltage surge protective devices - Part 12: Surge protective devices connected to low-voltage power distribution systems Selection and Application principles IEC Ed. 1.0 b:2011, Low-voltage surge protective devices - Part 22: Surge protective devices connected to telecommunications and signaling networks - Selection and application principles ATIS Electric Coordination of Primary and Secondary Surge Protection for Use in Telecommunications Circuits. CEA/CEDIA-CEB29: Recommended Practice for the Installation of Smart Grid Devices. CIGRÉ TB 549 (2013) Lightning Parameters for Engineering Applications ANSI/UL 1449 Standard for Safety - Surge Protective Devices 5.2 SPD Related Standards and guides in development IEEE/PES/SPDC WG is developing an application guide for surge protection of the North American Smart Grid. This will serve to assist the industry at large with a guide for best practices in identifying locations where the application of surge protection needs to be considered and how to determine the specification and selection of these devices.