Surge Current Characteristics 570 Hybrid SPD White Paper – April 2013 Abstract. This document will analyze the surge current mitigation characteristics of the 570 surge protective device (SPD). Included is a description of the clamping and surge current characteristics of the metal-oxide varistor (MOV) and the silicon avalanche diode (SAD) surge components, as well as, the design philosophy behind the unique combination of the different surge technologies. Introduction. It is well known that the electronic devices utilized to enhance and protect our standard of living are vulnerable to electrical distribution fluctuations and anomalies. These anomalies, like lightning induced and magnetically coupled power line transients, are a source of electrical equipment degradation and/or malfunction. Transients on the electrical distribution network have many characteristics and have been identified by the Institute of Electrical and Electronic Engineers (IEEE) in [1]. Four waveforms associated with lightning induced transients have been identified by the IEEE [1]. The first and second transient waveforms are described by the combination wave, consisting of a specifically identified open circuit voltage and short circuit current, represents transients, which are most likely to occur at the facility’s service entrance and on the electrical distribution network at positions less than twenty meters from the service entrance, see Figure 1 and Figure 2. The third transient waveform, the ring wave, represents transients, which are most likely to occur on a facility’s electrical distribution network at distances greater than twenty meters from the service entrance. The fourth transient waveform is the long wave transient. This is an additional transient that may also occur at the service entrance of a facility. National [2] and international [3], [4] performance and safety standards used to design, develop, test, and specify surge protective devices (SPDs), utilize the lightning induced combination wave transient. The combination transient has unique open circuit voltage and short circuit current characteristics. The open circuit voltage has a rise time of 1.2 s and a time to decay to fifty percent of the peak amplitude of 50 s. The short circuit current has a rise time of 8 s and a time to decay to fifty percent of the peak amplitude of 20 s. The open circuit voltage and short circuit current amplitude ranges vary from specification to specification. Common amplitudes identified by the IEEE at various locations within a facility are shown in Table 1. To mitigate the identified lightning induced transients, all with different amplitudes and energy delivery capabilities, a discussion of the commonly utilized surge suppression components is essential. Surge Suppression Topologies. A variety of topologies and technologies have historically been utilized to provide transient protection. These topologies use devices such as gas discharge tubes (GDTs), metal-oxide varistors (MOVs), silicon avalanche diodes (SADs), and even selenium rectifiers to divert lightning induced transients. Each of these surge suppression components has numerous characteristics that are beneficial in particular applications, but not in others. To limit the effects of adverse characteristics, some manufacturers have combined two or more different surge suppression technologies. Well-known examples of these combinations are MOVs and SADs, MOVs and selenium rectifiers, MOVs and GDTs. Other combinations of the suppression components, such as SADs and GDTs, have been combined, but are not as common. Although the combination of suppression components is prevalent within the transient suppression industry, many manufacturers have not utilized all the potential benefits of these combinations. As an example, many manufacturers utilize MOVs and SADs within their SPDs. The typical topology uses SADs as the primary means of suppression. The MOVs are utilized as a backup protection. In the advent of a large transient being induced on the electrical distribution network, the SADs take the full transient. If the energy capability of the transient is greater than the energy handling capability of the SADs, the SADs fail and are off-line. In this scenario, there has been a failure of the SPD, but the equipment or facility is still protected by the MOVs, which have been provided as back-up protection. Traditionally, manufacturers have designed their SPD systems such that replacement of the SAD suppression components can be accomplished relatively easily, provided the required replacement items are available. Although the concept of back-up protection against transients with extremely large energy delivery is very desirable, the failure of any part of the SPD system is not. The replacement of the SAD suppression components may be easy and quick, but spare parts are required. Furthermore, until maintenance is performed the sensitive equipment deemed important enough to have a SPD system requiring hybrid technology is not fully protected. The 570 hybrid SPD utilizes the full capabilities of combining two different suppression components: SADs and MOVs. To fully understand the workings of the 570, knowledge of the surge suppression components characteristics are required. A description of each device will be presented along with Emerson Network Power’s patent pending technique designed to utilize the beneficial characteristics of each component. Metal-Oxide Varistors (MOVs). The MOV has long been utilized to provide brute force attenuation of lightning induced transients on the AC power grid. The operational characteristics of the MOV can be varied for application on different voltages and transient exposure requirements. By varying the thickness of the component, different operating voltages can be obtained. Additionally, by varying the diameter of the component, different surge current capabilities can be obtained. Descriptively, the MOV is a bipolar, nonlinear voltage controlled device. Activation of the MOV occurs as the applied voltage increases beyond the threshold voltage. Once the threshold voltage is exceeded the MOV changes from a high-impedance state to a low-impedance state. The MOV continues in the low-impedance state, diverting current between opposite polarity, until the applied voltage has decreased to a value below the threshold voltage. The MOV is constructed from numerous boundaries of specifically doped ceramic material. These boundaries create breakdown regions, which can be compared to those of a “back-to-back” zener diode. By manipulating the overall thickness of the device, and hence, the number of boundaries in series, various breakdown voltages can be obtained. Additional manipulation of the overall width of the material results in numerous parallel paths. The number of parallel paths determines the total surge current capability of the component. To determine the clamping characteristics of the MOV [5], EQ1 is utilized 1 I V k EQ1 where V is the clamping voltage of the MOV, I is the current diverted through the MOV, k is an intrinsic characteristic of the MOV, and is the slope which corresponds to the linear surge current region of the MOV. For MOVs, ranges in values from nine to twelve, with a typical value of ten. The value of k ranges from 0.1x10-24 to 10x10-24. A typical linear clamping characteristic for a 130 volt MOV is shown in Figure 3. Specific operational characteristics of the MOV, in relation to other suppression devices, are low-leakage current, no follow-on current, moderate clamping characteristics, high -capacitance, high surge current capability, and low-cost. The response time of the MOV can be described as moderately fast. Silicon Avalanche Diodes (SADs). The SAD has been utilized for decades to provide transient protection to lower-voltage systems, typically located on a printed wiring board (PWB) assembly. The SAD can be configured for numerous voltage and surge current characteristics by adjusting the overall size of the semiconductor p-n junction. The SAD, like the MOV, is also a voltage controlled device. Current is diverted through the device once the threshold voltage has been exceeded. Once the applied voltage has decreased to a level below the threshold voltage, the device returns to a high-impedance state. or high-current regions, the exact clamping voltages of each device can be determined from EQ1 and Figure 3. Once the clamping voltages and the total surge current capability of the SPD has been determined, an effective impedance can be added to force the transition of surge current from the low surge current capability of the SADs to the high surge current capability of the MOVs. This added impedance between the SAD and MOV suppression components is the key to Emerson Network Power’s 570. A schematic representation of the circuit is shown in Figure 4. Mathematically, the impedance that must be added in series to the SAD components to force transient current sharing is determined by VMOV L di Ri VSAD dt EQ2 The SAD is constructed of a semi-conductor p-n junction designed to operate in the reverse breakdown region. In its purest form, the SAD p-n junction is a unipolar device with a specific forward and reverse voltage drop. By applying two SAD p-n junctions in a “back-to-back” configuration, a bipolar device is constructed. where VMOV is the voltage developed across the MOV and the SAD device with the added impedance, L is the added inductance, di/dt is the rate of change of the applied transient current, R is the added resistance, i is the transient current, and VSAD is the clamping voltage of the SAD as described in EQ1. The clamping characteristics of the SAD can also be characterized by equation EQ1. However, the values of and k are different. Values of range from 30 to 40, with a typical value of 37. The value of k ranges from 1x10-89 to 10x10-89. A typical clamping characteristic for a 130 volt, bipolar SAD is shown in Figure 3. Testing of the 570. To demonstrate the surge current transition and survivability of the SAD surge suppression components of the Emerson Network Power 570, testing was performed on a model 570YA16ARCG1S. Testing was accomplished using transient generators manufactured by Ketek Instruments and Emerson Network Power Surge Protection. Specific operational characteristics of the SAD, in relation to other suppression devices, are low-leakage, no follow-on current, tight clamping characteristics, low-capacitance, low surge current capability, and high-cost. The theoretical response time of the SAD is less than most other suppression devices, most notably the MOV. The 570 Topology. As Figure 3 shows, the clamping characteristics of the MOV and SAD suppression components are different. Additionally, the MOV has a higher surge current capability in relationship to an equivalent voltage SAD. When the suppression components are operated in their linear region and not at the low- The Keytek Instruments transient generators were 587 Plus models with the S1/S3 and S7 surge module. Both generators developed the IEEE C62.41-1991 combination wave transient. The S1/S3 module generates an open circuit voltage of 6,000 volts and a short circuit current of 3,000 amperes. The S7 module generates an open circuit voltage of 6,000 volts and a short circuit current of 10,000 amperes. The Emerson Network Power surge generator also developed the IEEE C62.41-1991 combination wave transient. This transient generator develops an open circuit voltage of up to 50,000 volts and a short circuit current of up to 50,000 amperes. The various transient generators were applied to the 570YA16ARCG1S SPD in the line-to-neutral configuration. Transient current was applied in increments up to a total of 29,000 amperes of 8/20 s current. Transient current was measured and recorded at the input to the 570, the input to the MOV module and the input to the SAD module. All data was recorded and is shown in Table 2 and a graphical representation is shown in Figure 5. As can be seen, the surge current has been transitioned from the low surge current capable SAD components to the high surge current MOV components. Conclusion. Combining different types of suppression technologies, such as metal-oxide varistors and silicon avalanche diodes, can only be accomplished by thoroughly understanding the inherent characteristics of the particular devices. Different suppression technologies cannot just be applied together in the same circuit and expected to realize a combined benefit. In the case of the 570 SPD, a series impedance was added to provide low clamping voltages and high surge current capability. The added impedance allows the transition of surge current from the primary, low-clamping voltage, low surge current capability SAD components to the higher-clamping voltage, high surge current capability MOVs. Data shown in Table 2 and Figure 5, demonstrates that the design principles utilized in the 570 SPD truly provide transient transition from the low surge current capability SAD components to the high surge current capability MOVs. This is contrary to the current industry standard of providing primary SAD protection, which must be replaced when subjected to high surge currents. For many years the 500 Series product lines have been the premier SPDs in the market place. They have provided low clamping voltages, high surge current capabilities, and unsurpassed quality and reliability. These traits have continued with the development of the 570 Hybrid SPD. References: 1. Institute of Electrical and Electronic Engineers, IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, Standard C62.41, dated 1991 February. 2. Underwriters Laboratories, Incorporated, Standard for Safety, Transient Voltage Surge Suppressors, Standard 1449, second edition, dated 1996 August. 3. International Electrotechnical Commission, International Standard, Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques, Section 5: Surge Immunity Test , Standard 1000-4-5, dated 1995 February 4. International Electrotechnical Commission, International Standard, Surge Protective Devices Connected to the Low Voltage Power Distribution Systems, Part 1: Performance Requirements and Testing Methods , Standard 61643-1, dated 1998 February. 5. Harris Semiconductor, Transient Voltage Suppression Devices, dated 1995 July Tables: Category Open Circuit Short Circuit Pseudo Impedance Voltage Current () (kVpk) (kApk) C3 20 10 2 C2 10 5 2 C1 6 3 2 B3 6 3 2 B2 4 2 2 B1 2 1 2 Table 1. Lightning Induced Combination Wave Transient Amplitudes and Effective Impedance. Total Input Current (amperes) 342 359 2,620 4,340 6,060 7,788 9,540 10,944 11,944 12,840 13,828 14,836 15,824 16,832 17,808 18,896 19,496 20,440 21,448 22,432 23,280 24,408 24,648 25,768 26,776 27,600 28,928 SAD Module Current (amperes) 342 350 1,162 1,534 1,890 2,256 2,607 2,890 3,089 3,265 3,452 3,660 3,848 4,055 4,231 4,439 4,639 4,783 4,991 5,174 5,383 5,550 5,630 5,951 6,158 6,182 6,390 MOV Module Current (amperes) 0 9 1,500 2,840 4,200 5,560 6,960 8,080 8,880 9,600 10,400 11,200 12,000 12,800 13,600 14,480 14,880 15,680 16,480 17,280 17,920 18,880 19,040 19,840 20,660 21,440 22,560 SAD Module Current (percentage) 100.0 97.5 44.4 35.3 31.2 29.0 27.3 26.4 25.9 25.4 25.0 24.7 24.3 24.1 23.8 23.5 23.8 23.4 23.3 23.1 23.1 22.7 22.8 23.1 23.0 22.4 22.1 MOV Module Current (percentage) 0.0 2.5 55.6 64.7 68.8 71.0 72.7 73.6 74.1 74.6 75.0 75.3 75.7 75.9 76.2 76.5 76.2 76.6 76.7 76.9 76.9 77.3 77.2 76.9 77.0 77.6 77.9 Table 2. Surge Current Distribution Between MOV Modules and SAD Modules of the 570YA16ARCG1S. Figures: C om bination W ave 1.2/50 O pen C ircuit V oltage 6000 Open Circuit Voltage (Volts) 5000 4000 3000 2000 1000 0 0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 T im e (seconds) Figure 1. IEEE C62.41-1991 Combination Wave, 1.2/50 s Open Circuit Voltage at 6000 volts peak. Short Circuit Current (Amperes) C om bination W ave 8/20 S hort C ircuit C urrent 3500 3000 2500 2000 1500 1000 500 0 0 0.00001 0.00002 0.00003 0.00004 0.00005 Tim e (seco n d s) Figure 2. IEEE C62.41-1991 Combination Wave, 8/20 s Short Circuit Current at 3000 Amperes Peak. MOV & SAD Clamping Characteristics Clamping Voltage (Volts) 600.0 500.0 MOV 400.0 300.0 SAD 200.0 100.0 0.0 0 2500 5000 7500 10000 Surge Current (Amperes) Figure 3. Clamping Characteristics of Metal-Oxide Varistors (MOV) and Silicon Avalanche Diodes (SADs). VMOV R1 L1 3 3 1 4 VSAD 4 4 3 R VR2 INPUT VR1 2 5 VR3 5 5 5 Figure 4. Schematic Representation of the Combination of the MOV and the SAD Surge Components in the 570 SPD. Percentage of Surge Current (Percent) 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 MOV MODULE SAD MODULE 0 5000 10000 15000 20000 25000 30000 Surge Current (Amperes) Figure 5. Graphical Representation of the Transient Current Diverted Through the MOV and SAD Modules. 100 Emerson Parkway Binghamton, NY 13905 P (607) 721 8840 P (800) 288 6169 F (607) 722 8713 E contactsurge@emerson.com WP-30023 Rev.3 4/13