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3 Resistibility to Surges of Smart Grid Equipment Connected to either DC or 120/240 V Single Phase AC, and Metallic Communication Line(s) 4 1. Overview 5 1.1 Scope 1 2 6 7 8 9 10 11 12 13 This Standard applies equipment which is connected to one or more metallic conductive communication line(s) and either a DC power source, or a 120/240 V single phase AC power service with the neutral grounded at the service entrance. It specifies the test procedures and resistibility requirements under which the communications ports of the equipment shall continue to demonstrate basic functionality, when subjected to overvoltages and overcurrents on either the power lines or the communications line(s). Overvoltages or overcurrents covered by this Standard include surges due to lightning on or near the power lines or telecommunications line(s). This standard covers the case where two or more services connected to the equipment have ground connections which may be separated by a significant impedance. 14 1.2 Purpose 15 16 17 18 19 20 21 22 23 24 Most standards for the resistibility of equipment to electrical surges assume that a zero [or very low] impedance exists among all the grounds in the equipment, or among the connections to separate earth grounds. For equipment installed in the Smart Grid (or indeed, anywhere), the impedance of the ground connections may be significant. The purpose of this standard is to provide tests and performance criteria for the resistibility to lightning strikes of equipment connected to two or more services having at least one ground connection separated from the others by a significant impedance. 25 1.3 Contents and context 26 27 28 29 30 31 32 33 34 This standard is divided into 6 clauses. Clause 1 provides the scope and purpose of this standard. Clause 2 lists references to other standards that are needed or useful in applying this standard. Clause 3 provides definitions that are either not found in other standards, or have been modified for use with this standard. Clause 4 provides a description of the surge environment for equipment with multiple services, at least two of which have grounds separated by a significant impedance, and the rationale for the tests for survivability in this environment. Clause 5 provides test procedures for testing resistibility of equipment to lightning surges when the equipment has separate grounds in the same local area. Clause 6 provides test procedures for testing resistibility of equipment to lightning surges when the equipment has a local and a remote ground. 35 2. References 36 37 38 39 40 [note: The clause numbers for the references will be removed in the final draft. They are here to help the editing process] Note: Examples of equipment with services having separate grounds include a Smart Grid power meter which is connected to the AC power at one side of a building and a communications service at the opposite side; and a roof-mounted photovoltaic system with a communications link to the Smart Grid. 2.1 IEEE Std C62.41.2™, IEEE Recommended Practice on Characterization of Surges in Low-Voltage (1000 V and Less) AC Power Circuits 1 2 3 2.2 K. Phipps and F. Martzloff, Application Guidelines: Electromagnetic compatibility of Computer 4 5 2.3 Telcordia GR-1089-CORE, Issue 5, Electromagnetic Compatibility and Electrical Safety - Generic 6 2.4 UL1449, 3rd Edition, Surge Protective Devices 7 3. Definitions Networks, Internet Equipment, Medical Equipment, and Home Entertainment systems, EPRI Palo Alto, CA: 2001, Report No.1005934 Criteria for Network Telecommunications Equipment. 8 9 10 3.1 Local ground potential difference (LGPD) 11 12 3.2 Port 13 14 15 16 3.3 Service 17 18 19 20 21 22 3.4 Significant impedance 23 4. Surge environment and rationale for tests 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 For information technology equipment surges at either the power port alone, or the communications port alone, have been the subject of many EMC standards (e.g. IEEE C62.41.2™ [2.1] and UL 1449 [2.4] for AC power; Telcordia GR-1089-CORE [2.3], ITU-T K.44 [B5] and UL 60950 [B7] for telecommunications lines). This standard takes note that Smart Grid equipment needs to be tested to one or more of these EMC standards. This standard is meant to supplement this EMC testing, and will address the case where a lightning surge current flowing through the impedance of an earth ground or ground connection can develop enough voltage to result in damage to the associated equipment. No present standards address this issue. The difference in potential between two separate ground points located in the same area, due to a lightning strike. The place where a service enters an equipment A connection to either a power or a communications source Note: Examples of a service are AC power, telephone, CATV, and surveillance systems An impedance which is large enough that lightning current flowing through it can develop a voltage sufficient to cause damage to an associated equipment, or shock hazard Note: Due to the variability of lightning strikes, a numerical value can’t be assigned to this impedance. Generally an impedance of one ohm or greater can be significant As an illustration of this situation, consider a possible home network as shown in Figure 1. The important thing to notice is that much of the equipment connected to the home network is connected more than one service, e.g. AC power, wired communications lines, roof-top antennas. All ports of the equipment may be grounded at the same point, but frequently this is not the case. If the ground points are different, a significant impedance can exist between them. The high current from a lightning surge flowing through this impedance can develop enough voltage to damage the equipment Two cases need to be considered (and they may occur simultaneously): Case 1: Equipment with two or more ports having a metallic interface, and whose grounding points are connected together with a wire that may be 10 meters long Case 2: The signal lines that terminate at an equipment in a remote site. Photovoltaic system Antenna Surveillance CATV Set-top box NIU Telecom Existing wires [TWP, CAT5, Coax] Power line Smart power meter Appliance Appliance 1 2 3 4 Figure 1. A possible home network 5 4.1 Case 1: Same local area but separate grounds 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Appliance Typically the two ports with separate grounds are the AC power and a communications line. This case has been discussed by Martzloff and Samotyj [B2], and by Cohen et al [B1], and is illustrated in Figure 2 (where the communications line is shown as a CATV connection). The equipment is referenced on the AC side to point B (via the branch circuit neutral and ground), but on the signal side, to point A (via the coax sheath). A lightning strike can develop a large potential difference between point A and point B, either by the resulting current flowing through a bond wire [as illustrated in Fig. 2]; or if the bond wire is not present, by a current flowing through resistive earth. For example, with a 3000 A surge (10% of a moderately strong lightning pulse) having a 3 μs rise time, and a 30 foot (~9 meter) long ground connection between A and B, the voltage developed in wire A–B is ~10,000 V. This is enough voltage difference to flash over most ordinary insulating barriers in the equipment, with equipment damage likely to result. If this potential is developed by a current flowing through resistive earth, the voltage difference is called ground potential rise, abbreviated as GPR. Service Entrance TV Set-top Box AC Power A CATV High potential difference Ground Block AC Branch Circuit COAX Sheath Ground Bond B 1 2 3 Figure 2. Equipment [a TV set-top box, in this example] with two ground reference points connected with a relatively long wire, shown here as the COAX Sheath Ground Bond. 4 4.2 Case 2: The signal lines terminate at a remote site. 5 6 7 8 9 10 11 12 13 This case has been discussed by Melton [B3], and is illustrated in Figure 3. Here the issue is that a lightning strike at site A will drive a large current IL toward a remote ground. This current flowing through the ground impedance Zg creates a GPR, which will raise site A to a high potential with respect to the remote ground at site B. If installations at site A and site B are connected by a conductor [e.g. a telephone line or coax] a high-energy surge can be propagated down the line from A to B. This surge may cause damage to equipment connected to either end of the line. Conductor Site A Site B IL Zg 14 15 16 17 GPR Figure 3. Lightning striking site A drives a current IL through the impedance of the ground, Zg, creating a ground potential rise GPR. 1 4.3 General considerations 2 3 4 The tests described in this standard are meant to be applied in addition to tests that should be done for compliance to standards such as Telcordia GR-1089-CORE [2.3], ITU-T K.44 [B5], and UL 60950 [B7], UL 1449 [2.4] , and IEC 62368 [B4]. 5 5. Tests for ports with separate local grounds 6 5.1 Rationale 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 The wire connecting the two grounds, as shown in figure 2, can have significant inductance. For fast-rising lightning pulses, the inductance of the wire produces large voltage drops [the inductance of the wire is the determining factor rather than the resistance of the wire]. The inductance is a property mainly of the length of the wire, and to a lesser extent, to the diameter of the wire. The inductance of a typical ground wire is approximately 1 µHy/m. The inductive voltage drop is L × di/dt, so it is proportional to the current rate of rise (A/μs). Since lightning currents rise very fast (typically less than 3 μs), the current rate of rise is very large, resulting in a high voltage. 29 5.2 Purpose 30 31 32 33 34 The high voltage that a lightning surge can apply to an equipment with multiple local ground connections can cause flashover and resulting equipment damage. The purpose of these tests is to verify that after applying a lightning surge as described in clause 5.3 and its subclauses, the equipment operates normally after the test. 35 5.3 Test Setup and Procedure 36 37 38 39 There are 2 general cases to test: One for the existence of the bond [if any] between the various ports with metallic interfaces; and one for the robustness of a selected port to surges on the other ports. The test for the former is a modification of the test illustrated in Figure 3-3 of the EPRI report [2.2]. 40 Bond impedance 41 5.3.1.1 Introduction 42 43 44 Bonding of the grounds within an equipment to establish a common ground will eliminate the possibility of damage due to GPR or LGPD. This test verifies that the individual services of the equipment have a robust common ground connection by applying the surge generator (Hi) output to the power line ground or The separation of two or more grounds by a significant impedance raises two issues. If the grounds of all the services entering the equipment do not have a common low impedance bond in the equipment, then a high voltage due to LGPD can occur in the equipment, with likely damage resulting. If the grounds of all the services entering the equipment have a common low impedance bond in the equipment, then no voltage due to LGPD will occur in the equipment [although the equipment itself may be at a high potential relative to a nearby ground]. Clause 5.3.1 addresses this issue. The second issue is that if two or more ports of the equipment do not have common low impedance grounds, then a potential at one port, whether due to an LGPD or to a GPR, can cause insulation flashover and resulting equipment damage. Even if all ports have a common low-impedance bond, there is a possibility that a surge on one port can couple enough voltage or current into another port to damage the equipment. Clause 5.3.2 and 5.3.3 test for these cases. 1 2 3 4 5 6 7 8 9 10 neutral and connecting the generator return (Lo) to a selected service port ground. A 10 µH air-core inductance having a 6 kV insulation withstand rating is connected across the terminals of the surge generator, to simulate the inductance of an assumed worst-case of a 10 meter length of wire connecting the ground of the power service to the ground of the selected service port. If the equipment has a common ground between the power port and the selected service port, very little voltage will be developed across the inductor, and no damage to the equipment is expected. If the equipment does not have a common ground between the power port and the selected service port, a high voltage will be developed across the inductor, potentially damaging the equipment. If the common ground is not robust, circuit board traces may be damaged. 11 5.3.1.2 Test procedure 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Refer to Figure 4. For this test the 6 kV combination wave generator specified in UL 1449 [2.4] is used. 34 5.3.1.3 Verification values 35 36 37 38 39 After the test sequence in clause 5.3.1.2 has been completed, verify that the EUT operates normally, and that all bonds between the grounds are intact. 1) Connect a 10 µH air-core inductance having a 6 kV insulation withstand rating across the terminals of the surge generator [see Figure 4]. 2) Select a service port 3) Connect the (Hi) output terminal of the surge-generator-plus-inductor [see step 1] to the neutral terminal of the power feed. 4) Connect the surge-generator-plus-inductor return (Lo) to the selected service port B, C, or S terminal [see figure 4] 5) Power the generator and set the voltage to 6 kV 6) Surge the circuit as connected in both positive and negative polarities. 7) Power down the generator 8) Connect the surge-generator-plus-inductor return (Lo) to another untested service port B, C, or S terminal. 9) Power the generator and set the voltage to 6 kV 10) Surge the circuit as connected in both positive and negative polarities. 11) Power down the generator 12) Repeat tasks 8, 9, 10 and 11 until all available B, C, or S connections have been tested. 13) If the power feed has a ground terminal, move the (Hi) output terminal of the surgegenerator-plus-inductor to the ground terminal of the power feed. 14) Repeat tasks 2 through 12. Generic Equipment Communications terminals A = Signal B = Ground (if present) C = Common S = Screen or shield L,N,G Power in Power terminals L = Line N = Neutral G = Ground Combination Wave Generator S A C Assymetrical Cable input A1 Symmetrical Cable Pair input 10 μH Lo 1 2 3 4 5 Coaxial Cable input A Hi Power Circuits B A2 Figure 4. Generic equipment with AC power and various communications inputs. Test set-up to verify a common service ground connection 6 Robustness of communications ports to power line surges 7 5.3.1.4 Introduction 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 This test checks for two things: The presence of a protector on the AC or DC power service; and for collateral damage that a surge on the power service can cause to communications ports in the equipment. If a protector is not present, the applied surge can cause significant damage to the equipment. Even if a protector is present, the applied surge may cause damage to other circuits due to magnetic coupling between the power circuits and circuit affected. This coupling can be enhanced by the presence of an impedance (possibly unintended, but a result of the installation) between the neutral or ground of the power system and the ground of the affected communications port. 24 5.3.1.5 Test procedure 25 26 A combination wave generator having a 6 kV open-circuit voltage and a 3 kA short-circuit current is used for this test As pointed out in clause 5.3.1.1, a significant impedance may exist between the AC or DC power ground and one or more communications port grounds. The assumed worst-case is that the grounds between the power service and the communications service are connected by a 10 meter length of wire, resulting in a 10 µH inductance between the two grounds. This test verifies that a surge on the AC or DC power port of the equipment does not cause damage to the power circuits, or to any communications port of the equipment, when the grounds of the AC or DC power port and the communications port are connected by a 10 µH inductance. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 1) Select a communications port 2) Short the signal line(s) of the port to its ground. 3) Leave all other signal lines open 4) Connect the ground of the selected port to the AC or DC power ground with a 10 µH air-core inductance having a 6 kV insulation withstand rating. 5) Connect the Hi side of the combination wave generator to the Line terminal. 6) Skip steps 7 – 10, if the power source is DC 7) Connect the generator return (Lo) to the Neutral terminal 8) Power the generator and set the voltage to 6 kV 9) Surge the circuit as connected, in both positive and negative polarities. 10) Power down the generator 11) Move the generator return (Lo) to the AC or DC power Ground terminal 12) Power the generator and set the voltage to 6 kV 13) Surge the circuit as connected, in both positive and negative polarities. 14) Power down the generator 15) Open the connection to ground of the signal line(s) at the selected port. 16) Connect the Hi side of the combination wave generator to the Line terminal. 17) Skip steps 18 – 21, if the power source is DC 18) Connect the generator return (Lo) to the Neutral terminal 19) Power the generator and set the voltage to 6 kV 20) Surge the circuit as connected, in both positive and negative polarities. 21) Power down the generator 22) Move the generator return (Lo) to the AC or DC power Ground terminal 23) Power the generator and set the voltage to 6 kV 24) Surge the circuit as connected, in both positive and negative polarities. 25) Power down the generator 26) Select another port, and repeat steps 2 - 23 27) Continue selecting ports, until all ports have been tested. 30 31 5.3.1.6 Verification values 32 33 After the test sequence in clause 5.3.2.2 has been completed, verify that the EUT operates normally. 34 Robustness of one communications port to surges on another communications port 35 5.3.1.7 Introduction 36 37 38 39 If an equipment has two or more communications ports, a lightning strike on one of the communications ports could cause damage on another communications port, if a significant impedance exists between their grounds. The damage could be due to GPR or LGPD, as was discussed in clause 5.3.1.1. The assumed worst-case is that the grounds between the two communications services are connected by a 10 meter 1 2 3 4 length of wire, resulting in a 10 µH inductance between the two grounds. This test verifies that a surge on one of the equipment communications ports does not cause damage to any other port of the equipment, when the grounds of the two services are connected by a 10 µH inductance. . 5 5.3.1.8 Test procedure 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 This test applies only to equipment having two or more communications ports. A combination wave generator having a 6 kV open-circuit voltage and a 3 kA short-circuit current is used for this test 24 5.3.1.9 Verification values 25 26 After the test sequence in clause 5.3.3.2 has been completed, verify that the EUT operates normally. 27 6. The signal lines that terminate at a remote site 28 6.1 Rationale 29 30 31 32 Figure 5 shows two separate sites connected together by one or more communications lines. These could be phone lines, coax lines, or any wired connection. If lightning strikes at site A, it will cause a GPR there. 1) Select a communications port to be surged 2) Short all other ports to their respective grounds. 3) Connect the ground of each of the non-surged ports to the ground of the surged port with a 10 µH air-core inductance having a 6 kV insulation withstand rating. 4) Connect the Hi side of the surge generator to the signal line of the port being surged 5) Connect the generator return (Lo) to the ground of the port being surged 6) Power the generator and set the voltage to 6 kV 7) Surge the circuit as connected, in both positive and negative polarities. 8) Power down the generator 9) Open all ports except one being surged. 10) Surge the circuit as connected, in both positive and negative polarities. 11) Power down the generator 12) Select another port, and repeat steps 1 - 10 13) Continue selecting ports, until all ports have been tested. Communications line Equipment Site B B A Ze Equipment Site A Equipment ground GPR Zg 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Figure 5. Effect of a lightning strike on equipment with a communications line connected to a remote ground Because the wires have a low resistance, the high potential at A will appear at B, and fire the protectors there. As a result, the lightning current will flow through the equipment impedance Ze, through the protector at B, and will return back through the ground impedance Zg. If Ze is small [e.g. if Ze is the impedance of a secondary protector, such as a thyristor], then the voltage drop across Z e caused by the flow of lightning current may not be sufficient to fire the protector at A. In that case the equipment at site A may not be protected, and the high lightning current can damage or disable the equipment. If Ze is large enough that the voltage drop across it caused by the flow of lightning current is sufficient to fire the protector at A (coordinated protection – see T1-338-2004 [B6]), then the lightning current will be diverted around the equipment at site A, and the equipment at site A will generally not be damaged (although the equipment at site B could be). The surge applied should approximate the largest surge that the equipment would experience in service. To coordinate with GR-1089-CORE 5th Edition, Clause 4.7, a 10/250 surge generator capable of a 4 kV, 500 A surge is used for this test 20 6.2 Purpose 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 The high currents that can be driven down a wire by a GPR can disable or cause damage to the equipment to which the wire is connected. The purpose of this test is to verify that the equipment operates normally after the test. 1 6.3 Test Setup and Procedure 2 3 All signal lines shorted together Equipment Under Test Ground R=3Ω 4 5 6 7 8 9 10 11 12 10/250 4 kV, 400 A Surge Generator Figure 6. Test setup for testing robustness of equipment with a remote ground reference to a local GPR Refer to Figure 6. Use a 10/250 surge generator capable of a 4 kV, 500 A surge 1) Select a communications port to be tested 2) Short all signal lines of the selected port together; leave all other signal lines open. 13 14 3) Connect one side of a 6 kV impulse rated, 3 resistor to the (shorted) communications line(s) of the selected port 15 16 17 18 19 20 21 22 4) Connect the other side of the 3 resistor to the Lo side of the combination wave generator. 5) Connect the Hi side of the combination wave generator to the ground of the EUT 6) Power the generator and set the voltage to 6 kV 7) Surge the circuit as connected, in both positive and negative polarities. 8) Power down the generator 9) Select another communications port, and repeat steps 1 – 8 10) Continue selecting communications ports, until all ports have been tested. 23 6.4 Expected Results and Pass Criteria 24 25 26 27 The equipment shall operate normally after the test, and shall not become a fire or fragmentation hazard. 1 2 3 4 5 6 Annex A Bibliography 7 8 [B2] F. D. Martzloff and M. Samotyj, “An Important Link in Whole-House Protection: Surge Reference equalizers”. Proceedings, 1993 EMC Zurich Symposium 9 [B3] B. Melton, “Metallic Wire Line Service for Cell Sites”. ATIS PEG General Meeting -2007 [B1] R. L. Cohen, D. Dorr, J. Funke, C. Jensen and S. F. Waterer, How To Protect Your House and Its Contents From Lightning: IEEE Guide for Surge Protection of Equipment Connected to AC Power and Communication Circuits. 2005, ISBN: 0-7381-4634-X 10 11 [B4] IEC 62368-1, Audio/video, information and communication technology equipment - Part 1: Safety requirements 12 13 [B5] ITU-T K.44, Resistibility tests for telecommunication equipment exposed to overvoltages and overcurrents – Basic Recommendation 14 15 [B6] T1-338-2004, Electrical Coordination of Primary and Secondary Surge Protection for use in Telecommunications Circuits 16 [B7] UL 60950, 3rd Edition, Safety of Information Technology Equipment