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Sensitivity improvement of time overcurrent relays
Electric Power Systems Research 77 (2007) 119–124
Sensitivity improvement of time overcurrent relays Arturo Conde Enr´ıquez ∗ , Ernesto V´azquez Mart´ınez Universidad Aut´onoma de Nuevo Le´on, Facultad de Ingenier´ıa Mec´anica y El´ectrica, Apdo. Postal 114-F, Ciudad Universitaria, CP 66450, San Nicol´as de los Garza, Nuevo Le´on, M´exico Received 1 November 2005; received in revised form 1 February 2006; accepted 2 February 2006 Available online 23 March 2006
Abstract In this paper, we recommend a new adaptive function for time overcurrent relays. The purpose of the adaptation process is to improve the sensitivity of time overcurrent relays, which then operate with a dynamic pickup setting such as a load current. The results obtained from time overcurrent adaptive relays are collated, and the results from analysis of negative sequence relays and conventional time overcurrent relays are also presented. In this article, we describe the control logic structure of an adaptive pickup current and its performance under different operating states. © 2006 Elsevier B.V. All rights reserved. Keywords: Overcurrent relay; Adaptive relay; Sensitivity
1. Introduction The application of time overcurrent relays in electrical networks presents serious limitations in terms of sensitivity. A disproportionate increase in load density, combined with the failure to erect enough sub-transmission and distribution lines to keep pace, leads to a situation in which electrical systems are subjected to more rigorous loadability conditions. The adjustment of an overcurrent relay is mostly compromised because the minimum values of fault current and relay adjustment are comparable, making correct fault detection difficult. A fault under minimum demand conditions represents a lesser current contribution, which is precisely when greater sensitivity of the protection is required. However, the adjustment of the relay’s starting current is accomplished using the maximum values of load current (a few minutes every day) and in critical grid configurations in which a load can trip the relay, such as in power transfer operations (emergency configurations). The adjustment of the pickup current is then established for rare or short-lived scenarios, resulting in greater desensitisation of the protection. More adjustments of the pickup current lead to longer operating times for primary and backup protection. These impairments
∗
Corresponding author. Tel.: +52 81 83294020x5773. E-mail addresses: con [email protected] (A.C. Enr´ıquez), [email protected] (E.V. Mart´ınez). 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.02.004
in sensitivity and backup operating time are typical in overcurrent protection, and cannot be solved by conventional relays. A possible solution to the lack of sensitivity is the negative sequence [1], which responds to unbalanced faults but suffers from impairments of coordination that force an adjustment in its pickup current by the value corresponding to the primary relay, and also requires conventional protection to identify balanced faults. A new adaptive current function for overcurrent relays is proposed. This criterion can be applied to phase time overcurrent relays, and can be applied to both power and industrial systems. The main goal of a time overcurrent adaptive relay is to increase the sensitivity to minimum fault currents in poor load conditions. To accomplish this, the relay should have a dynamic adjustment of the pickup current that reflects the operational state of the electrical grid. This function requires a control logic that distinguishes between a fault condition and an operational change in the network. An adaptive logic has previously been suggested at a substation level in which the relay settings could be modified from a central computer in the substation [2–4]. Updating adjustments of relays through communication channels using online algorithms to coordinate relays has also been proposed [5,6]. In isolated (rural) or highly connected networks in which it is not cost-effective to implement an adequate communication strategy, it is possible to carry out an automatic set-up relay using local current only. In this article, we propose an overcurrent adaptive relay; this relay is effective for such situations and does
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not require communication channels to modify the parameter settings. One of the benefits of the proposed relay is that the pickup current depends on the magnitude of the load current, resulting in greater protection sensitivity when it is most needed. In addition, operating and backup times are reduced. The proposed relay does not require communication channels; the process is handled using the information at the relay location. Finally, the proposed relay is obtained with a minimal change in the relay’s firmware and without an increase in cost. 2. Operating limits of the overcurrent relay The overcurrent protection uses current as the only indicator of fault location. However, the fault current depends on the fault type and the pre-fault steady state. Moreover, the maximum load current can be similar in magnitude to the minimum fault current; this makes it difficult to correctly discriminate between the normal stable state and the fault condition. The limitations of an overcurrent relay are illustrated in the radial grid in Fig. 1(a). Relay B is used as a reference point that provides primary protection of the line itself and backup of the adjacent line. Fig. 1(b) shows a graph of the variation in the short-circuit current as a function of the electrical distance Z to evaluate the sensitivity in the protection. Fig. 1(c) shows the time-electrical distance T = f(Z) used to evaluate the time operation. In Fig. 1(b), curve 1 corresponds to the maximum level of the short-circuit current (three-phase fault at maximum demand), and curve 2 corresponds to the minimum level (twophase fault at minimum demand). The value of the adjustment current Ipickup corresponds to Relay B. One limitation of the overcurrent protection is that its reach (the length of the protection zone) depends on the type of shortcircuit and the system operation. Fig. 1(b) shows that the reach
of Relay B moves between the limits Zmin and Zmax , and can stop providing proper backup to the adjacent line for minimum generation. Because of these factors, the reach of the overcurrent relay changes dynamically, depending on the operational state of the electrical grid; the protection could be lost in minimum operating conditions. This is particularly the case for the phase protection, in which the maximum load current defines the pickup current of the relay. Consequently, the sensitivity limitation of overcurrent relays is poor fault detection in minimum power generation conditions. Another problem in overcurrent protection is the high backup time for minimum fault currents (Fig. 1(c)). This behavior is characteristic of overcurrent relays demonstrated to be appropriate for the protection of electrical systems in which operation above the nominal values is frequent and temporary. This situation is not as convenient when it occurs in backup protection; due to the nature of the overcurrent relay, operating times are long, forcing the system to tolerate non-permissive currents and resulting in thermal and mechanical stress that could be avoided. The load current (high pickup setting) and the divergence of the relay’s time-curve for poor fault currents result in high backup times (see curve 2 for Relay A and backup Relay B in Fig. 1(c)). When both the primary and the backup overcurrent protection have different time-curves, adequate time coordination is difficult. Therefore, the time limitation of overcurrent relays is high backup times for both minimum fault currents and different timecurve devices. 3. Sensitivity improvement The maximum load current is present for only a few minutes per day; for the remaining time, the load current has lower values. Thus, we suggest the pickup current Ipickup of the time overcurrent relay will be a function of the load current Ikr [7]: N
Ipickup =
1 r (Ik )j + I N
(1)
j=1
Fig. 1. Effect of short-circuit current in the reach of time overcurrent relay.
where I represents a safety margin, with a proposed value of 15% of the maximum load current, and N must be selected in such a way that the interval N(t), the integration time used in demand measures, has a duration of between one and several minutes. Eq. (1) ensures that the overcurrent relay has, at any time, the minimum pickup current necessary to avoid incorrect operation due to the load effect. This provides increased sensitivity, because the value of Ipickup is also small due to minimal demand conditions. This adaptability operation can be represented in a time–impedance plane T = F(Z, Ipickup ); this characteristic (contrary to the conventional relay) has a new variable—the pickup current Ipickup , which must be represented in a tri-dimensional form. The characteristic T = F(Z) functions for conventional and adaptive relays have been presented previously [7]. The control logic of the pickup current (Fig. 2) has the task of maintaining constancy of Ipickup during a fault to avoid changes in pickup settings. If the line is de-energised (Ikr < ε), the control
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Fig. 2. Control logic of an adaptive pickup current.
logic assigns a maximum value Ipumax , which can be the setting of a conventional relay (dead line logic). During a complete demand interval, the Ipickup value that was calculated in Eq. (1) is fixed in the relay at the end of the previous interval. The action of low-pass filtering that is inherent in the demand concept simplifies the logic of the adaptive relay. In situations in which a large load is added to a feeder, the relay has a detection fault logic to supervise Ipickup . This logic includes a negative sequence verification and a high-level current detector, both combined in OR logic. The negative sequence was proposed to detect phase-to-phase faults and its setting has been described previously [1]. In low-voltage networks, the negative sequence current of phase-to-phase faults is higher than the maximum unbalanced negative sequence current, leading to the ability to obtain a good setting. A high-level detector is proposed to detect three-phase faults; this uses the same setting as a conventional time overcurrent relay. Therefore, this logic discriminates between a large load and a fault (symmetrical or asymmetrical). 4. Overcurrent relays In this section, results from the three types of phase overcurrent relays are collated: conventional, negative sequence and adaptive. The sensitivity is also analysed. Sensitivity analysis was carried out in the radial power system shown in Fig. 3(a). It is not necessary to consider a more complex configuration of the power system, as use of a complex power system does not lead to unexpected values. Most scenarios have the same effect on the operating current. The minimum
fault current is simulated in Bus 4. The variable load impedances simulate a multi-tap line (Z3 ) and a variable load (Z4 ). For analysis, the phase overcurrent relays are located in Bus 2 (Relay B). Sensitivity is determined by the following relationship: sensitivity =
Iminimum short circuit Ipickup
(2)
The different sensitivities expected for each relay pickup are calculated in different ways. For negative sequence relays and conventional relays, the pickup current method has been described previously [1,8]. For adaptive relays, although the limit of sensitivity is similar to that of the pickup of the primary device, in this sensitivity analysis we assume that adaptive relays are not pickup-limited and therefore the pickup adaptive current is determined by Eq. (1). The minimum short-circuit current is the phase-to-phase fault in Bus 4. The accepted value is 1.5 [8]. Fig. 3(b) shows the relay sensitivities for different load currents (Ikr ) and source contributions (Psource ). The adaptive relay (plane 3), rather than the conventional (plane 1) and negative sequence (plane 2) relays, increases sensitivity for the minimum load. The conventional and negative sequence relays have a pickup setting that is independent of the dynamic load; this is observed in planes 1 and 2 in Fig. 3(b). Adaptive relays have a setting that is dependent on the dynamic load; this characteristic result in plane 3, in which the adaptive relay has greater sensitivity compared with the other two relays in poor load currents. Under high power conditions (high Psource ), the sensitivity of the three relays is higher (larger short-circuit currents), and the effect in the adaptive relay is to increase sensitivity more than in the others relays.
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Fig. 4. Coordination of overcurrent relays with the proposed increase in sensitivity.
The relay’s operating time must be established as a function of the time-curve of the primary device. Thus, the relay’s operating time (dependent on the operating time of the primary protection) is independent of the current (calculated according to Eq. (1) and acting as a fault detector only). Then the adaptive relay uses the pickup current only as a fault detector, the time operation is calculated with the pickup current and with the time-curve of the primary device [7]. Therefore, the adaptive relay emulates the dynamics of the primary device to obtain a fast backup time operation. The relay pickup current is dependent on the load condition, leading to an increase in sensitivity. 5. Test
Fig. 3. Sensitivity curves for phase time overcurrent relays.
It can be observed that adaptive relays are more dependent than the other relays on the power contribution and load current for sensitivity. In multi-terminal lines (Fig. 3(c)), the negative sequence relay has the same or higher sensitivity than the adaptive and conventional relays; negative sequence relays are not affected by the load current, allowing easy current coordination with the high pickup setting for primary protection of multi-tap lines. Under maximum demand conditions, the sensitivity of negative sequence relays is higher than for other relays; however, under minimal demand conditions, the sensitivity of adaptive relays is higher. These conditions may change because of topology dependence or protective schemes, but the results of this study indicate the general trend. In conclusion, conventional relays have poor sensitivity, and negative sequence relays are best used in multi-terminal lines (but only for unbalanced faults). Adaptive relays have a high sensitivity for radial and multi-terminal lines and can be used effectively for three-phase faults and phase-to-phase faults. With the increase in sensitivity (reduction in the pickup current), the operating time of the overcurrent relay is reduced. The effect is illustrated in Fig. 4.
The performance of the logic for the adaptive pickup current was tested in a digital simulation. The test was carried out on the basis of the power system shown in Fig. 3(a). Fig. 5 shows a typical operation of a protective system in a sub-transmission and distribution electric network. The adaptive logic remains in the normal state (load current) until the current is higher than the pickup current (fault current). As the algorithm uses the demand current information, the setting of the pickup current (Ipickup ) is constant in each demand interval. The accumulated value of the integrator (Gk ) or, by analogy, the position of the induction disk in an electromechanical relay at any time, depends on Ikr . During operation, the integration of the function T(Ik ) is positive, increasing the accumulated value in the integrator (distance travelled by the disk towards the tripping position); on the other hand, in the reset zone, the integration is negative and decreases the accumulated value in the integrator (return of the disk towards the actuating position). Fig. 5(b) shows an incomplete operation of adaptive relay due to the operation of the primary device protection; the maximum accumulated value (Gk ) is less that 1. In Fig. 5(c) the evolving fault is presented. The operation logic of the adaptive pickup current is satisfactory. Fig. 6(a) shows a test system that evaluates the performance of the proposed fault detection logic under conditions of severe load unbalance and the unbalanced faults. The simulated operating sequence (PSCAD® ) consists of three stages: unbalance in normal operating conditions, two-phase fault in an adjacent
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Fig. 6. Evaluation of the logic of the proposed fault detection.
Fig. 5. Control logic of an adaptive pickup current using a signal test: (a) selftrip, (b) primary trip, and (c) evolutionary fault.
line, and tripping of the adjacent line (power transfer). During the state of unbalance in the stable state (25% according to Ref. [9]) the algorithm produces no output, tolerating this condition; a value of 80 A [1] was used for this simulation. During the fault, there is an appreciable value for the negative sequence current, allowing effective detection. In the final simulated sequence, tripping line 1 will trigger a power transfer in Relay A. This condition should be tolerated by the relay, allowing the load feed. Adjustment of the symmetrical fault detector was similar to that for a conventional overcurrent relay [8]. During this condition, the output of the proposed fault detector is not present, and thus the performance is satisfactory. The structure of the testing system for the proposed overcurrent relay consists of three operating modules: the real signal acquisition module, which uses the output signal of current transformers connected to an electrical system; the signal file module, which reads an events log; and an internal generation module, which operates internal controls. The tests corresponding to the acquisition module have been reported elsewhere [7].
The tests presented in this article are from the signal file module. While the acquisition module has objectives associated with tests for the design and evaluation of protection algorithms, this module mainly focuses on the analysis of protection operations using actual recorded events. To this end, the record for an actual event was used and the operation of the conventional protection was evaluated and compared with the functional performance of the proposed relay. A fault log was used to test the signal file. This two-phase fault was logged in a 34.5 kV distribution grid. Fig. 7 shows the record of the phase relay event and Table 1 shows information on the relay adjustment and its operating time. For the sake of simplicity, the values shown are relative to the system’s primary relay. Table 1 also shows the operating time of the proposed relay. Although the value of the pre-fault current is 230 A, the conventional relay adjustment is accomplished based on maximum demand (286 A); as the starting current of the protection Table 1 Time overcurrent relay setting Conventional
Adaptive
Ipickup = 430 A
Ipickup = 275 A
Moderate inverse curve [7], dial = 0.2 Time = 0.7067 s or 42.4 cycles (f = 60 Hz)
Time = 0.4322 s or 25.9 cycles (f = 60 Hz)
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Fig. 7. Fault signal recorded in a 34.5 kV distribution network.
increases, so does the operating time. Given the increase in the sensitivity of the adaptive relay, the operating time is less than that for the conventional relay. This is a quantitative example of the benefit of dynamic adjustment of the starting current. Besides increasing fault detections, the relay’s operating time is reduced. 6. Summary The phase-time overcurrent adaptive relay has one main function: to increase the sensitivity for minimum faults under poor load conditions. The adaptive relay proposed in this work modifies the pickup current as a function of the load current. Adaptive relays increase the sensitivity in the most probable operating state. If a large load is added to a feeder, the relay has a detection fault logic to supervise the pickup setting. This logic includes a negative sequence verification and a high-level current detector, both combined in OR logic. This logic discriminates between a large load and a fault (symmetrical or asymmetrical). The increase in sensitivity of the relay results in shorter operating times, with a possible loss of coordination. It is therefore necessary to establish the relay’s operating time as a function of the time-curve of the device being backed up. The adaptive relay will have the same operational dynamic as the primary device, thus guaranteeing coordination. The results show that conventional relays have poor sensitivity. Negative sequence relays are most effective for multiterminal lines, but only for unbalanced faults. Adaptive relays have high sensitivity and are most effectively used for threephase and phase-to-phase faults. The proposed overcurrent relay is obtained with only a small change in the firmware’s relay, without any additional cost.
References [1] A.F. Elneweihi, E.O. Schweitzer III, M.W. Feltis, Negative-sequence overcurrent element application and coordination in distribution protection, in: IEEE Power Engineering Society, PES Summer Meeting, Seattle, WA, July 12–16, 1992. [2] K.R. Shah, E.D. Detjen, A.G. Phadke, Feasibility of adaptive distribution protection system using computer overcurrent relaying concept, IEEE Trans. Ind. Appl. 24 (5) (1988) 792–797. [3] M.S. Sachdev, T.S. Sidhu, B. Chattopadhyay, et al., Design and evaluation of an adaptive protection system for a distribution network, Cigr´e Paper 34-202, Paris, 1995. [4] J. Eisman, G. G´omez, J. Torres, Applied adaptive protection practices based on data transmission between relays, Cigr´e Paper 34-207, Paris, 1995. [5] V. Bapeswara, K. Sankara, Computer-aided coordination of directional relays: determination of break points, IEEE Trans. Power Deliv. 3 (2) (1988) 545–548. [6] A.Y. Abdelaziz, H.E.A. Talaat, A.I. Nosseir, A.A. Hajjar, An adaptive protection scheme for optimal coordination of overcurrent relays, Electr. Power Syst. Res. 61 (1) (2002) 1–9. [7] A. Conde, E. V´azquez, H.J. Altuve, Time overcurrent adaptive relay, Int. J. Electr. Power Energy Syst. 25 (10) (2003) 841–847. [8] IEEE Standard C37.113-1999, Guide for protective relay applications to transmission lines, September 1999. [9] ANSI/IEEE Standard 141-1986, IEEE recommended practice for electric power distribution for industrial plants. Arturo Conde Enr´ıquez received his B.Sc. degree in Mechanical and Electrical Engineering in 1993 from Universidad Veracruzana, Veracruz, M´exico. He received an M.Sc. and Ph.D. in Electrical Engineering in 1996 and 2002, respectively, from the Universidad Aut´onoma de Nuevo Le´on, M´exico. Currently he is a Professor at the same university, and is a member of the National Research System of M´exico. Ernesto V´azquez Mart´ınez received his B.Sc. in Electronic and Communications Engineering in 1988, and his M.Sc. and Ph.D. in Electrical Engineering from the Universidad Aut´onoma de Nuevo Le´on (UANL), M´exico, in 1991 and 1994, respectively. Since 1996, he has worked as Research Professor in Electrical Engineering for the UANL. He is an IEEE member and a member of the National Research System of M´exico.
Sensitivity improvement of time overcurrent relays Arturo Conde Enr´ıquez ∗ , Ernesto V´azquez Mart´ınez Universidad Aut´onoma de Nuevo Le´on, Facultad de Ingenier´ıa Mec´anica y El´ectrica, Apdo. Postal 114-F, Ciudad Universitaria, CP 66450, San Nicol´as de los Garza, Nuevo Le´on, M´exico Received 1 November 2005; received in revised form 1 February 2006; accepted 2 February 2006 Available online 23 March 2006
Abstract In this paper, we recommend a new adaptive function for time overcurrent relays. The purpose of the adaptation process is to improve the sensitivity of time overcurrent relays, which then operate with a dynamic pickup setting such as a load current. The results obtained from time overcurrent adaptive relays are collated, and the results from analysis of negative sequence relays and conventional time overcurrent relays are also presented. In this article, we describe the control logic structure of an adaptive pickup current and its performance under different operating states. © 2006 Elsevier B.V. All rights reserved. Keywords: Overcurrent relay; Adaptive relay; Sensitivity
1. Introduction The application of time overcurrent relays in electrical networks presents serious limitations in terms of sensitivity. A disproportionate increase in load density, combined with the failure to erect enough sub-transmission and distribution lines to keep pace, leads to a situation in which electrical systems are subjected to more rigorous loadability conditions. The adjustment of an overcurrent relay is mostly compromised because the minimum values of fault current and relay adjustment are comparable, making correct fault detection difficult. A fault under minimum demand conditions represents a lesser current contribution, which is precisely when greater sensitivity of the protection is required. However, the adjustment of the relay’s starting current is accomplished using the maximum values of load current (a few minutes every day) and in critical grid configurations in which a load can trip the relay, such as in power transfer operations (emergency configurations). The adjustment of the pickup current is then established for rare or short-lived scenarios, resulting in greater desensitisation of the protection. More adjustments of the pickup current lead to longer operating times for primary and backup protection. These impairments
∗
Corresponding author. Tel.: +52 81 83294020x5773. E-mail addresses: con [email protected] (A.C. Enr´ıquez), [email protected] (E.V. Mart´ınez). 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.02.004
in sensitivity and backup operating time are typical in overcurrent protection, and cannot be solved by conventional relays. A possible solution to the lack of sensitivity is the negative sequence [1], which responds to unbalanced faults but suffers from impairments of coordination that force an adjustment in its pickup current by the value corresponding to the primary relay, and also requires conventional protection to identify balanced faults. A new adaptive current function for overcurrent relays is proposed. This criterion can be applied to phase time overcurrent relays, and can be applied to both power and industrial systems. The main goal of a time overcurrent adaptive relay is to increase the sensitivity to minimum fault currents in poor load conditions. To accomplish this, the relay should have a dynamic adjustment of the pickup current that reflects the operational state of the electrical grid. This function requires a control logic that distinguishes between a fault condition and an operational change in the network. An adaptive logic has previously been suggested at a substation level in which the relay settings could be modified from a central computer in the substation [2–4]. Updating adjustments of relays through communication channels using online algorithms to coordinate relays has also been proposed [5,6]. In isolated (rural) or highly connected networks in which it is not cost-effective to implement an adequate communication strategy, it is possible to carry out an automatic set-up relay using local current only. In this article, we propose an overcurrent adaptive relay; this relay is effective for such situations and does
120
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not require communication channels to modify the parameter settings. One of the benefits of the proposed relay is that the pickup current depends on the magnitude of the load current, resulting in greater protection sensitivity when it is most needed. In addition, operating and backup times are reduced. The proposed relay does not require communication channels; the process is handled using the information at the relay location. Finally, the proposed relay is obtained with a minimal change in the relay’s firmware and without an increase in cost. 2. Operating limits of the overcurrent relay The overcurrent protection uses current as the only indicator of fault location. However, the fault current depends on the fault type and the pre-fault steady state. Moreover, the maximum load current can be similar in magnitude to the minimum fault current; this makes it difficult to correctly discriminate between the normal stable state and the fault condition. The limitations of an overcurrent relay are illustrated in the radial grid in Fig. 1(a). Relay B is used as a reference point that provides primary protection of the line itself and backup of the adjacent line. Fig. 1(b) shows a graph of the variation in the short-circuit current as a function of the electrical distance Z to evaluate the sensitivity in the protection. Fig. 1(c) shows the time-electrical distance T = f(Z) used to evaluate the time operation. In Fig. 1(b), curve 1 corresponds to the maximum level of the short-circuit current (three-phase fault at maximum demand), and curve 2 corresponds to the minimum level (twophase fault at minimum demand). The value of the adjustment current Ipickup corresponds to Relay B. One limitation of the overcurrent protection is that its reach (the length of the protection zone) depends on the type of shortcircuit and the system operation. Fig. 1(b) shows that the reach
of Relay B moves between the limits Zmin and Zmax , and can stop providing proper backup to the adjacent line for minimum generation. Because of these factors, the reach of the overcurrent relay changes dynamically, depending on the operational state of the electrical grid; the protection could be lost in minimum operating conditions. This is particularly the case for the phase protection, in which the maximum load current defines the pickup current of the relay. Consequently, the sensitivity limitation of overcurrent relays is poor fault detection in minimum power generation conditions. Another problem in overcurrent protection is the high backup time for minimum fault currents (Fig. 1(c)). This behavior is characteristic of overcurrent relays demonstrated to be appropriate for the protection of electrical systems in which operation above the nominal values is frequent and temporary. This situation is not as convenient when it occurs in backup protection; due to the nature of the overcurrent relay, operating times are long, forcing the system to tolerate non-permissive currents and resulting in thermal and mechanical stress that could be avoided. The load current (high pickup setting) and the divergence of the relay’s time-curve for poor fault currents result in high backup times (see curve 2 for Relay A and backup Relay B in Fig. 1(c)). When both the primary and the backup overcurrent protection have different time-curves, adequate time coordination is difficult. Therefore, the time limitation of overcurrent relays is high backup times for both minimum fault currents and different timecurve devices. 3. Sensitivity improvement The maximum load current is present for only a few minutes per day; for the remaining time, the load current has lower values. Thus, we suggest the pickup current Ipickup of the time overcurrent relay will be a function of the load current Ikr [7]: N
Ipickup =
1 r (Ik )j + I N
(1)
j=1
Fig. 1. Effect of short-circuit current in the reach of time overcurrent relay.
where I represents a safety margin, with a proposed value of 15% of the maximum load current, and N must be selected in such a way that the interval N(t), the integration time used in demand measures, has a duration of between one and several minutes. Eq. (1) ensures that the overcurrent relay has, at any time, the minimum pickup current necessary to avoid incorrect operation due to the load effect. This provides increased sensitivity, because the value of Ipickup is also small due to minimal demand conditions. This adaptability operation can be represented in a time–impedance plane T = F(Z, Ipickup ); this characteristic (contrary to the conventional relay) has a new variable—the pickup current Ipickup , which must be represented in a tri-dimensional form. The characteristic T = F(Z) functions for conventional and adaptive relays have been presented previously [7]. The control logic of the pickup current (Fig. 2) has the task of maintaining constancy of Ipickup during a fault to avoid changes in pickup settings. If the line is de-energised (Ikr < ε), the control
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Fig. 2. Control logic of an adaptive pickup current.
logic assigns a maximum value Ipumax , which can be the setting of a conventional relay (dead line logic). During a complete demand interval, the Ipickup value that was calculated in Eq. (1) is fixed in the relay at the end of the previous interval. The action of low-pass filtering that is inherent in the demand concept simplifies the logic of the adaptive relay. In situations in which a large load is added to a feeder, the relay has a detection fault logic to supervise Ipickup . This logic includes a negative sequence verification and a high-level current detector, both combined in OR logic. The negative sequence was proposed to detect phase-to-phase faults and its setting has been described previously [1]. In low-voltage networks, the negative sequence current of phase-to-phase faults is higher than the maximum unbalanced negative sequence current, leading to the ability to obtain a good setting. A high-level detector is proposed to detect three-phase faults; this uses the same setting as a conventional time overcurrent relay. Therefore, this logic discriminates between a large load and a fault (symmetrical or asymmetrical). 4. Overcurrent relays In this section, results from the three types of phase overcurrent relays are collated: conventional, negative sequence and adaptive. The sensitivity is also analysed. Sensitivity analysis was carried out in the radial power system shown in Fig. 3(a). It is not necessary to consider a more complex configuration of the power system, as use of a complex power system does not lead to unexpected values. Most scenarios have the same effect on the operating current. The minimum
fault current is simulated in Bus 4. The variable load impedances simulate a multi-tap line (Z3 ) and a variable load (Z4 ). For analysis, the phase overcurrent relays are located in Bus 2 (Relay B). Sensitivity is determined by the following relationship: sensitivity =
Iminimum short circuit Ipickup
(2)
The different sensitivities expected for each relay pickup are calculated in different ways. For negative sequence relays and conventional relays, the pickup current method has been described previously [1,8]. For adaptive relays, although the limit of sensitivity is similar to that of the pickup of the primary device, in this sensitivity analysis we assume that adaptive relays are not pickup-limited and therefore the pickup adaptive current is determined by Eq. (1). The minimum short-circuit current is the phase-to-phase fault in Bus 4. The accepted value is 1.5 [8]. Fig. 3(b) shows the relay sensitivities for different load currents (Ikr ) and source contributions (Psource ). The adaptive relay (plane 3), rather than the conventional (plane 1) and negative sequence (plane 2) relays, increases sensitivity for the minimum load. The conventional and negative sequence relays have a pickup setting that is independent of the dynamic load; this is observed in planes 1 and 2 in Fig. 3(b). Adaptive relays have a setting that is dependent on the dynamic load; this characteristic result in plane 3, in which the adaptive relay has greater sensitivity compared with the other two relays in poor load currents. Under high power conditions (high Psource ), the sensitivity of the three relays is higher (larger short-circuit currents), and the effect in the adaptive relay is to increase sensitivity more than in the others relays.
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Fig. 4. Coordination of overcurrent relays with the proposed increase in sensitivity.
The relay’s operating time must be established as a function of the time-curve of the primary device. Thus, the relay’s operating time (dependent on the operating time of the primary protection) is independent of the current (calculated according to Eq. (1) and acting as a fault detector only). Then the adaptive relay uses the pickup current only as a fault detector, the time operation is calculated with the pickup current and with the time-curve of the primary device [7]. Therefore, the adaptive relay emulates the dynamics of the primary device to obtain a fast backup time operation. The relay pickup current is dependent on the load condition, leading to an increase in sensitivity. 5. Test
Fig. 3. Sensitivity curves for phase time overcurrent relays.
It can be observed that adaptive relays are more dependent than the other relays on the power contribution and load current for sensitivity. In multi-terminal lines (Fig. 3(c)), the negative sequence relay has the same or higher sensitivity than the adaptive and conventional relays; negative sequence relays are not affected by the load current, allowing easy current coordination with the high pickup setting for primary protection of multi-tap lines. Under maximum demand conditions, the sensitivity of negative sequence relays is higher than for other relays; however, under minimal demand conditions, the sensitivity of adaptive relays is higher. These conditions may change because of topology dependence or protective schemes, but the results of this study indicate the general trend. In conclusion, conventional relays have poor sensitivity, and negative sequence relays are best used in multi-terminal lines (but only for unbalanced faults). Adaptive relays have a high sensitivity for radial and multi-terminal lines and can be used effectively for three-phase faults and phase-to-phase faults. With the increase in sensitivity (reduction in the pickup current), the operating time of the overcurrent relay is reduced. The effect is illustrated in Fig. 4.
The performance of the logic for the adaptive pickup current was tested in a digital simulation. The test was carried out on the basis of the power system shown in Fig. 3(a). Fig. 5 shows a typical operation of a protective system in a sub-transmission and distribution electric network. The adaptive logic remains in the normal state (load current) until the current is higher than the pickup current (fault current). As the algorithm uses the demand current information, the setting of the pickup current (Ipickup ) is constant in each demand interval. The accumulated value of the integrator (Gk ) or, by analogy, the position of the induction disk in an electromechanical relay at any time, depends on Ikr . During operation, the integration of the function T(Ik ) is positive, increasing the accumulated value in the integrator (distance travelled by the disk towards the tripping position); on the other hand, in the reset zone, the integration is negative and decreases the accumulated value in the integrator (return of the disk towards the actuating position). Fig. 5(b) shows an incomplete operation of adaptive relay due to the operation of the primary device protection; the maximum accumulated value (Gk ) is less that 1. In Fig. 5(c) the evolving fault is presented. The operation logic of the adaptive pickup current is satisfactory. Fig. 6(a) shows a test system that evaluates the performance of the proposed fault detection logic under conditions of severe load unbalance and the unbalanced faults. The simulated operating sequence (PSCAD® ) consists of three stages: unbalance in normal operating conditions, two-phase fault in an adjacent
A.C. Enr´ıquez, E.V. Mart´ınez / Electric Power Systems Research 77 (2007) 119–124
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Fig. 6. Evaluation of the logic of the proposed fault detection.
Fig. 5. Control logic of an adaptive pickup current using a signal test: (a) selftrip, (b) primary trip, and (c) evolutionary fault.
line, and tripping of the adjacent line (power transfer). During the state of unbalance in the stable state (25% according to Ref. [9]) the algorithm produces no output, tolerating this condition; a value of 80 A [1] was used for this simulation. During the fault, there is an appreciable value for the negative sequence current, allowing effective detection. In the final simulated sequence, tripping line 1 will trigger a power transfer in Relay A. This condition should be tolerated by the relay, allowing the load feed. Adjustment of the symmetrical fault detector was similar to that for a conventional overcurrent relay [8]. During this condition, the output of the proposed fault detector is not present, and thus the performance is satisfactory. The structure of the testing system for the proposed overcurrent relay consists of three operating modules: the real signal acquisition module, which uses the output signal of current transformers connected to an electrical system; the signal file module, which reads an events log; and an internal generation module, which operates internal controls. The tests corresponding to the acquisition module have been reported elsewhere [7].
The tests presented in this article are from the signal file module. While the acquisition module has objectives associated with tests for the design and evaluation of protection algorithms, this module mainly focuses on the analysis of protection operations using actual recorded events. To this end, the record for an actual event was used and the operation of the conventional protection was evaluated and compared with the functional performance of the proposed relay. A fault log was used to test the signal file. This two-phase fault was logged in a 34.5 kV distribution grid. Fig. 7 shows the record of the phase relay event and Table 1 shows information on the relay adjustment and its operating time. For the sake of simplicity, the values shown are relative to the system’s primary relay. Table 1 also shows the operating time of the proposed relay. Although the value of the pre-fault current is 230 A, the conventional relay adjustment is accomplished based on maximum demand (286 A); as the starting current of the protection Table 1 Time overcurrent relay setting Conventional
Adaptive
Ipickup = 430 A
Ipickup = 275 A
Moderate inverse curve [7], dial = 0.2 Time = 0.7067 s or 42.4 cycles (f = 60 Hz)
Time = 0.4322 s or 25.9 cycles (f = 60 Hz)
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A.C. Enr´ıquez, E.V. Mart´ınez / Electric Power Systems Research 77 (2007) 119–124
Fig. 7. Fault signal recorded in a 34.5 kV distribution network.
increases, so does the operating time. Given the increase in the sensitivity of the adaptive relay, the operating time is less than that for the conventional relay. This is a quantitative example of the benefit of dynamic adjustment of the starting current. Besides increasing fault detections, the relay’s operating time is reduced. 6. Summary The phase-time overcurrent adaptive relay has one main function: to increase the sensitivity for minimum faults under poor load conditions. The adaptive relay proposed in this work modifies the pickup current as a function of the load current. Adaptive relays increase the sensitivity in the most probable operating state. If a large load is added to a feeder, the relay has a detection fault logic to supervise the pickup setting. This logic includes a negative sequence verification and a high-level current detector, both combined in OR logic. This logic discriminates between a large load and a fault (symmetrical or asymmetrical). The increase in sensitivity of the relay results in shorter operating times, with a possible loss of coordination. It is therefore necessary to establish the relay’s operating time as a function of the time-curve of the device being backed up. The adaptive relay will have the same operational dynamic as the primary device, thus guaranteeing coordination. The results show that conventional relays have poor sensitivity. Negative sequence relays are most effective for multiterminal lines, but only for unbalanced faults. Adaptive relays have high sensitivity and are most effectively used for threephase and phase-to-phase faults. The proposed overcurrent relay is obtained with only a small change in the firmware’s relay, without any additional cost.
References [1] A.F. Elneweihi, E.O. Schweitzer III, M.W. Feltis, Negative-sequence overcurrent element application and coordination in distribution protection, in: IEEE Power Engineering Society, PES Summer Meeting, Seattle, WA, July 12–16, 1992. [2] K.R. Shah, E.D. Detjen, A.G. Phadke, Feasibility of adaptive distribution protection system using computer overcurrent relaying concept, IEEE Trans. Ind. Appl. 24 (5) (1988) 792–797. [3] M.S. Sachdev, T.S. Sidhu, B. Chattopadhyay, et al., Design and evaluation of an adaptive protection system for a distribution network, Cigr´e Paper 34-202, Paris, 1995. [4] J. Eisman, G. G´omez, J. Torres, Applied adaptive protection practices based on data transmission between relays, Cigr´e Paper 34-207, Paris, 1995. [5] V. Bapeswara, K. Sankara, Computer-aided coordination of directional relays: determination of break points, IEEE Trans. Power Deliv. 3 (2) (1988) 545–548. [6] A.Y. Abdelaziz, H.E.A. Talaat, A.I. Nosseir, A.A. Hajjar, An adaptive protection scheme for optimal coordination of overcurrent relays, Electr. Power Syst. Res. 61 (1) (2002) 1–9. [7] A. Conde, E. V´azquez, H.J. Altuve, Time overcurrent adaptive relay, Int. J. Electr. Power Energy Syst. 25 (10) (2003) 841–847. [8] IEEE Standard C37.113-1999, Guide for protective relay applications to transmission lines, September 1999. [9] ANSI/IEEE Standard 141-1986, IEEE recommended practice for electric power distribution for industrial plants. Arturo Conde Enr´ıquez received his B.Sc. degree in Mechanical and Electrical Engineering in 1993 from Universidad Veracruzana, Veracruz, M´exico. He received an M.Sc. and Ph.D. in Electrical Engineering in 1996 and 2002, respectively, from the Universidad Aut´onoma de Nuevo Le´on, M´exico. Currently he is a Professor at the same university, and is a member of the National Research System of M´exico. Ernesto V´azquez Mart´ınez received his B.Sc. in Electronic and Communications Engineering in 1988, and his M.Sc. and Ph.D. in Electrical Engineering from the Universidad Aut´onoma de Nuevo Le´on (UANL), M´exico, in 1991 and 1994, respectively. Since 1996, he has worked as Research Professor in Electrical Engineering for the UANL. He is an IEEE member and a member of the National Research System of M´exico.