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Coordination of surge protective devices in low voltage AC power installations
The surges protection in low-voltage AC power circuits is commonly realized by Surge Protective Devices (SPDs). These devices divert a large amount of the surge energy to the ground and reduce the surge voltage to the level acceptable for protected device(s). Adequate protection performances are provided by applying multiple-stage protection assuming cascade deployment of SPDs starting from distribution board toward equipment in an installation. Therefore, SPDs coordination, (i.e. selection of SPDs parameters as well as determination of necessary distance between stages) becomes of great importance and the paper deals with this issue. The proper SPDs coordination is significant for both protected equipment and SPDs themselves (their survival during surges). The paper provides an analysis of the SPDs coordination for case of two-stage protection system. The influence of different lengths of cables between protection stages, as well as influence of type and parameters of load on SPDs protection characteristics have been analyzed and discussed with the purpose of determination of requirements for proper surge protection and coordination of applied SPDs.
Electric and modern electronic equipment widely used in industrial and residential objects have become strongly dependent upon the continuous availability and quality of electrical power. Recent studies have shown that industrial and digital business enterprises are having financial losses due to power interruptions and poor power quality.
Power quality or more specifically, a power quality disturbance is generally accepted as any change in the power (voltage, current, or frequency) that interferes with normal operation of electrical equipment and which can result in mal-operation or failure of equipment. Taking the wave shape as a criteria, the IEEE Standard 1159 defines seven categories of power quality disturbances based on the wave shape: transients, interruptions, sag/undervoltage, swell/overvoltage, waveform distortion, voltage fluctuations, frequency variations. Transients are potentially the most damaging type of power disturbance. They fall into two subcategories: impulsive and oscillatory transients. Impulsive transients or surges are sudden high peak events that raise the voltage and/or current levels. The consequences of surges can range from the loss (or corruption) of data to physical damage of equipment.
Duration of surges in low voltage–power installations do not exceed one-half period of the supply voltage waveform. They are random events which appear in any combination of line, neutral, or grounding conductors. The origin of surges occurring in low-voltage ac power circuits are lightning and switching. A third phenomenon that needs to be taken into account is the occurrence of surge voltages resulting from interactions between different systems, such as the power system and a communications system .
Surge Protective Devices (SPDs) are used to provide protection against the surges in low-voltage AC power circuits. SPDs have to fulfill two major tasks: first, to divert a large amount of the surge energy to the ground (which refers to SPD’s energy absorption capability) and second, to clamp the surge voltage to the level below withstand impulse voltage level of protected device(s) (which refers to SPD’s protection voltage).
Numerous studies and research reports have shown that application of only one group of SPDs, usually at the distribution board, (i.e. one-stage protection system) doesn’t provide proper overvoltage protection of equipment . Namely, for equipment with small values of active powers as well as for equipment with capacitive character of load SPDs must to have very low protection voltage in order to provide adequate protection. At the same time they must to have high energy absorption capability. These demands are in contradiction because of the fact that SPDs with relatively high energy absorption capability have higher protection voltages and vice versa.
In order to achieve adequate protection of equipment against surges it is necessary to apply multiple-stage protection which assumes cascade placement of SPD starting from distribution board toward equipment in an installation. This approach should to provide gradually decreasing of the protection level of SPDs in the stages, as well as the appropriate distribution of surge energy between stages. Therefore, SPDs coordination, (i.e. selection of SPDs parameters as well as determination of necessary distance between stages) becomes of great importance and the paper deals with this issue.
Analysis of the SPDs coordination for case of two-stage protection system is given in the paper. The influence of different lengths of cables between protection stages, as well as influence of type and parameters of load on SPDs protection characteristics have been analyzed and discussed with the purpose of determination of requirements for proper surge protection and coordination of applied SPDs. Simulation of surge testing is performed by using ATP/EMTP and MATLAB Simulink software with application of the Combination Wave as one of the representative surges according to relevant standards.
Cascade protection system assumes application of two or more SPDs starting at distribution board toward protected equipment. Protection performances as well as coordination of SPDs can be performed with analysis of two-stage protection scheme given in Fig. 1.
The purpose of the surge testing is to assess response of equipment to the known or unknown surge environment and/or to determine characteristics of SPDs. Therefore, the first decision or assumption that must be made in planning a surge test addresses the nature of the surge environment .
The surges with different characteristics regarding their polarity, duration and wave amplitudes can occur in low-voltage AC power systems . This variety of surges can be reduced to few representative stresses for the purpose of surge testing and determination of protection performances. IEEE and IEC standards define Combination Wave as one of the representative surge waveforms used for surge testing of equipment which impedance is not known in advance and which can change during time. Combination Wave is surge which consists of two waveforms: 1.2/50 µs open circuit voltage waveform and 8/20 µs short circuit current waveform. Selection of amplitudes of these waveforms depends on location categories to which observed circuit belongs. The concept of location category, proposed in IEEE C.62.41.1, rests on the considerations on dispersion and propagation of surge currents and surge voltages. IEEE C62.41.1 recognizes three location categories: A, B and C. Location category A applies to the parts of the installation at some distance from the service entrance. Location category C applies to the external part of the structure, extending some distance into the building. Location category B extends between Location categories C and A.
It will be assumed that circuit in Fig. 1 belongs to location category B. For this category values of open circuit voltage and short circuit current amplitude of the Combination Wave surge are 6 kV and 3 kA, respectively. The model of surge generator delivering Combination Wave with mentioned parameters is given in Fig. 2. Parameters of Combination Wave model generator for location category B are: U = 6.247 kV, C1 = 12.5 µF, L1 = 2.45 µH, L2 = 4 µH, R1 = 5.83 Ω, R2 = 1.41 Ω .
Surge is applied with method of direct coupling between lines to neutral, according to IEEE Std. C62.45. Surge generator is connected directly between neutral conductor and power line. This situation represents un-powered testing, that is direct connection between un-powered equipment under test and the test surge generator.
SPDs used in the model (Fig. 1) are selected from the manufacturer catalogue with following technical parameters: SPD marked as arrester (A) has protection voltage of 1250 V. It is SPD of type 2 according to IEC 61643-11, designed for mounting on distribution board, with maximal discharge current Imax (8/20 µs) of 15kA, which corresponds energy absorption capability of 328J. SPD marked as suppressor (S) is SPD type 3 according to IEC 61643-11, designed for socket mounting, with protection voltage of 800 V, and value of combination wave open circuit voltage of UOC = 6 kV (ISC = 3kA), which corresponds energy absorption capability of 42J.
Equipment under test (EUT) in analyzed circuit is load connected via cable at a socket of single power line. It is assumed that analyzed EUT belongs to the equipment of the overvoltage category I according to IEC 60664-1. This overvoltage category involves equipment with withstands impulse voltage level of 1.5 kV and it is the most rigorous requirement for the protection effect of SPD.
Cables between arrester and suppressor (cable A–S) and between suppressor and EUT (cable S-EUT) are PVC-insulated cables 3 × 2.5 mm2 with electric parameters: R = 0.00561 Ω/m, L = 0.324 µH/m, C = 0.1368 nF/m, G = 0 s/m. It is taken into account that SPD connecting leads (between power and neutral conductors) have length of 0.5 m. These connecting leads cause inductive voltage drop along them, which can have strong influence on the performances of protection system.
Protection performances of the circuit given in Fig. 1 are analyzed by simulations in MATLAB Simulink and ATP/EMTP. Maximal values of voltages across EUT and deposited energy in SPDs are measured and used as criteria for assessment of protection performances and energy coordination of SPDs for cases with different parameters of circuit elements with values which can be found in real household and industrial low-voltage–power systems.
Simulations are performed for three types of EUT load: resistive, inductive and capacitive, and for cases with different lengths of cable A–S and cable S-EUT. For resistive load two values of active power P = 100 W and P = 2000 W, while for inductive and capacitive character of load two values of reactive power Q = 10 VAr and Q = 100 VAr are taken into account.
According to the common practice in two stage protection system the arresters are supposed to be located at the service entrance, while suppressor is located near protected equipment. Therefore, for the analysis of the influence of cable A–S length it is taken that cable S-EUT has length of 1 m. Maximal values of voltage across EUT for different values of cable A–S length in range of 1–100 m are given in Figs. 3, 4 and 5 for resistive, inductive and capacitive character of EUT’s load respectively.
From Figs. 3, 4 and 5 it can be concluded that values of maximal voltages across EUT don’t depend or very little depend on value of EUT’s load power. In case of resistive (Fig. 3) and inductive (Fig. 4) load maximal values of voltages across EUT are close to or lower than suppressor protection voltage (800 V). This is consequence of very short cable S-EUT with length of 1 m and short SPDs connecting leads, which causes attenuation of voltage reflection at the load impedance even in case when load impedance has larger value than characteristic impedance of the cable. However, in the case of capacitive load (Fig. 5) maximal values of voltages across EUT are higher than suppressor protection voltage for cases of lower capacitive power, regardless the fact of short cable S-EUT. The reason for this is voltage oscillations across load impedance with capacitive power. The illustration of these oscillations is given in Fig. 6 for case of capacitive load with power of 10 VAr, cable A–S length of 1m and cable S-EUT length of 1m.
Energies deposited in arrester and suppressors for case of resistive load of EUT (with conditions: active powers of 100 W and 2000 W, different values of cable A–S length in range of 1–100 m and cable S-EUT length of 1 m) are given in the Figs. 7 and 8.
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