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OVERCURRENT PROTECTION AND VOLTAGE SAG COORDINATION IN SYSTEMS WITH DISTRIBUTED GENERATION Juan Carlos Gomez Rio Cuarto National University Rio Cuarto, Cordoba ARGENTINA M. M. Morcos Kansas State University Manhattan, KS 66506 USA Abstract- Overcurrent protection and voltage sag coordination studies are based on the upstream impedance of the point of common coupling (PCC) or system impedance (Z1). Voltage sag magnitude is obtained taking into account the voltage drop on Z1. Sag duration depends on the operating time of the overcurrent protective device. The normal coordination procedure is obtained by comparing the sensitive equipment (SE) dropout curve with the timevoltage characteristic (TVC) of the protective device. Under normal operating system conditions Z1 is approximately constant, but in the case of distributed generation this impedance can change from a low value to a high value when the non-utility generators are the only supply. The new scenario implies that the protective device TVC is changed into a zone which modifies previous methodologies and increases the SE dropout susceptibility. Index terms- Distributed generation, power quality, voltage sags, overcurrent protection. Each of these power generators has its own advantages and disadvantages, such as easy installation based on its small size, fast start, voltage distortion generator, and uncertain power availability. In spite of the distribution– generation promotion carried out by utilities, they have been expressing concerns related with potential distribution system problems. I. INTRODUCTION One of the important issues that the new scenario will introduce is overcurrent protection, where there will be changes in system behavior and flow of power under short-circuit conditions. Normally there is no concern with the DR when its power is less than 10 % of the minimum load demand in the feeder [1]. It has been predicted that by the year 2010 approximately 20 % of the new generation will be distributed generation (DG) [1]. There are several types of power generators that can be used as DG which, in order of importance, are: Wind turbines Fuel cells Photovoltaic cells Small and micro turbines Internal combustion engines. Currently an extensive task is being carried out by the Standards Coordination Committee 21 (IEEE SCC 21), where there are numerous activities by the working group in the new IEEE Standard P1547 which will provide guidelines for interconnecting distributed generation with the power system [2]. In general, distributed resources are defined as sources of electrical power that are not directly connected to a bulk power transmission system. They include both generators and energy storage technologies, with power rating of 10 MW or less [2]. The most immediate consequence is the need for verification of the protective device breaking capacity, which might not be enough due to the increase of the available short-circuit power. Short-circuit current value and transient behavior of generator that provides power through inverters - such as photovoltaic cells, fuel cells, wind turbines, and induction generators - are completely different from the synchronous generator response. Induction generators directly connected to the supply will show a special behavior when a shortcircuit takes place. Due to the fact that this type of generator gets its excitation from the mains, it is not able to maintain the short-circuit currents for a relatively long time. The situation when the main supply is intentionally or unintentionally disconnected from the power system having at least one DG, which continues operating with this single source, is called Islanded Mode Operation. An aspect that will be studied is the coordination between overcurrent protection and the voltage sag ride-through capability of sensitive equipment (SE). Voltage sag is almost universally considered as a non-permanent voltage reduction with values between 10 % and 90 % of the rated voltage. The ability of sensitive equipment to withstand voltage sags without dropout is called ride-through capability. Many typical ridethrough capability curves have been proposed as guidelines. The present work is confined to the lower part only of the voltage sag ride-through capability curves adopting the Computer Business Equipment Manufacturing Association (CBEMA) curve as guideline. generally separated from the distorted or toohigh current path. In this case it is the shortcircuit main flow path as can be seen in Fig. 1. Adapted protective device TCC is a curve transformed into time-voltage characteristic (TVC), that represents the voltage sag which the protective device allows to be applied to the SE under study. Voltage sag applied to SE is directly the voltage at the PCC or the supply voltage minus the voltage drop across Z1 due to short-circuit current [3]. B. Circuit with Distributed Resources Figure 1 shows the new scheme of the system with utility and distributed generation where DG may be representing more than one single device. When the islanding circuit breaker (ICB) is closed the source impedance is approximately the parallel combination of the utility (Area EPS) and DG impedances. When ICB is open the source impedance jumps to a larger value. In case of a fault (Fig. 1), short-circuit fault current will be greatly attenuated and the operating time of the protective device will be increased subsequently. The coordination study needs to be done only for the two branches on the DG side of the ICB, with maximum and minimum rated currents of the protective device. II. COORDINATION BETWEEN OVERCURRENT PROTECTION AND SENSITIVE EQUIPMENT VOLTAGE IMMUNITY PCC DG ZDR SE A. Classical Study The coordination study between the overcurrent protection device (PD) and the SE voltage immunity located in parallel branches or feeders, is done in graphic form, comparing the adapted time-current characteristic (TCC) of the protective device with the previously cited CBEMA curve. Basically, the PCC will be defined as the point of the circuit where the SE current is PD ICB EPS Z1 Fig. 1 System with utility and distributed resource C. Transformation of Protecting Device TimeCurrent Characteristic into Time–Voltage Characteristic Considering the utility and the DG have nearly the same Voltage Regulation value, the voltage sag magnitude at the SE connection point (or PCC) is given by, Vs = VEPS – (Z1 // ZDR) x Isc (1) Where, Vs = voltage sag value VEPS = electric power system voltage Z1 = utility impedance ZDR = distributed resource impedance Isc = short-circuit fault current The product Z x Isc in (1) can be identified. During the protective device TVC calculation, once the rated current has been adopted, each selected time value will correspond to one value of the voltage sag magnitude. Isc can be expressed as function of the PD rated current, thus the fault current will be a number of times the rated current. Normally fuses are built in a homogeneous way, with the same structure and with characteristic curves approximately parallel, thus increasing the number of fuse elements will increase the rated current. Hence, the Isc value expressed as multiple of the fuse rated current has a multiplier that increases with the decrease in the fuse rated current and vice versa. A short-circuit current of 1,000 A will trigger the operation of fuse of 100 A rated current in nearly 1.4 cycles and the same current will melt the 200 A rated fuse in approximately 20 cycles. In conclusion, the product Z x Isc can be considered as function of the source impedance and PD rated current. Thus, a reduction in the short-circuit power (islanding mode) is equivalent to an increase in the rated current of the protective device and vice versa. The previous analysis, although was applied to fuses, is applicable also to circuit breakers and to any other type of low-voltage protective device. The effect of protective devices rated-current can be seen in Fig. 2, where the TVCs of the two fuses mentioned above where plotted. To clarify the procedure, the methodology for obtaining a point on each curve is as follows. Considering an application with a load rated current of 1000 A and a source impedance of 4 % (source impedance voltage drop - due to rated current is of the same value as voltage regulation), with two feeders coming out from the bus-bar. They are protected by fuses with rated currents of 100 A and 200 A, respectively. An arbitrary time of 10 cycles (200 ms) was selected for the explanation, for which the fuses will need currents (to melt) of 600 A and 1200 A, respectively. The fuse rated currents expressed in pu of the circuit rated current result 0.1 and 0.2, respectively, then the base current for 10 cycles is 6000 A. Voltage sags which the fuses allow to be applied to SE connected to the parallel feeders are: For the 100 A fuse; Vs (%) = 100 – (0.04 * 0.1 * 6000) = 97.6 % For the 200 A fuse, Vs (%) = 100 – (0.04 * 0.2 * 6000) = 95.2 %. The same procedure allows the calculation of the complete fuse TVC curves. D. Effect of Protective Device Rated Current on Coordination The protecting device rated current is crucial for the coordination between protecting device and SE immunity curve. The reason is due to the fact that, as the rating current increases the curves go nearer, and when they intersect there are points of mis-coordination. E. Transformation of the Protective Device TVC in a Time–Voltage Zone If during parallel operation of utility and DG, due to any cause, the IB opens, the circuit suffers a source impedance increase, modifying the situation. Fig. 2 was drawn for a similar situation as before, but changing the source impedance from 0.04 to 0.06 pu and maintaining similar fuse rated currents. It can be seen that the protection given by the 100 A fuse is still satisfactory, but the curve representing the 200 A fuse now intersects with the immunity curve of the sensitive equipment, which might signify SE dropout. 100 Voltage (%) 80 by the curve locations when the supply is the utility plus DG and the new curve location having DG as only supply. In the present case, it is the zone enclosed by the positions of the 200 A-fuse curves for both source impedances. III. CONCLUSIONS Sensitive equipment protection against voltage sags can be given for overcurrent protective devices, provided that careful coordination has been carried out. The readily available protective device TVC moves now into a zone which shall be up and left to the SE immunity curve if effective coordination is desired. The area is bordered by the two locations of the maximum protective device TVCs, and will be wider as the difference between utility and DR impedances increases. 60 IV. REFERENCES 40 20 0 0.01 0.1 CBEMA 1 10 100 Time (cycles) Fuse 100 A 1000 10000 Fuse 200 A Fig. 2 Fuse time-voltage characteristics and CBEMA curve for DG alone It may be deduced that the protective device TVC has been transformed into a zone bordered [1] A.M. Borbely and J.F. Kreide, Distributed Generation, The Power Paradigm for the New Millennium, CRC Press, Boca Raton, FL, 2001. [2] IEEE Draft Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard P1547, 2001. [3] J.C. Gómez and M.M. Morcos, “Voltage sag mitigation using overcurrent protection devices,” Electric Power Components and Systems, Vol. 29, pp. 71-81, 2001.