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1 Sizing Current Transformers Rating To Enhance Digital Relay Operations Using Advanced Saturation Voltage Model *J.O. Aibangbee 1 and S.O. Onohaebi 2 *Department of Electrical &Computer Engineering, Bells University of Technology, Ota, Nigeria Department of Electrical / Electronic Engineering, University of Benin, Nigeria ABSTRACT Current transformers for transmission lines are rated to avoid saturation during the asymmetrical portion of the maximum fault current. The selection of current transformers (CTs) rating to maximize microprocessor-based (MPB) relay operations was investigated. This paper examines the effects of saturation on protection relays, and developed an advanced saturation voltage (VS) algorithm for the selection of CT rating for Microprocessorbased relays applications in transmission lines with high short circuit levels. Results show that Instantaneous MPB relay operation using 1200:5A ratio can be guaranteed with Voltage burden of 525, within 1-cycle trip time as compared to Voltage burden of 20 times rated voltage at standard burden to avoid saturation specified by IEEE standard for EMR. A criterion for selecting minimum CT rating for MPB overcurrent relay operations in transmission lines to avoid saturation using the developed voltage saturation (VS) algorithm is presented. It allows protection engineers to determine the limits of high current applications in terms of the fault current magnitude, the X/R ratio, and relay burden. The criteria address the limitation of CT ratings used in line protections. Small cores, long leads, high burdens, high currents, and dc offset lead to saturated CTs. Improper selection of CTs results in CT s saturation during fault conditions and this causes relay mal-operation or fail-to-trip. Keywords: Microprocessor-based relays, Relay burden, EMR, Current Transformers, Relay burden, Saturation Voltage (VS), dc offset, Fault current. 1. Introduction Most of the generating stations in Nigeria are retrofitted with Microprocessor-based (digital) relays and protection equipment with the rest of the installation such as Current transformers (CTs) and control cables (lead) remaining unchanged. In the past, due to the high burden of electromechanical relays (EMRs), it was the rated power of the CTs and potential transformers (PT) that was the crucial parameter. In recent time, it is the transient performance of instrument transformers that has become the over-riding influence within the digital relays, measuring and controlling devices. It is utmost important therefore to evaluate the dimensioning of CTs while planning such up-grade. The existing CT sizes and lead sizes of EMRs may not be adequate for the latest protective relays which might demand a different CT performance [1, 2, 4]. CTs are the basic interconnection between the power system networks and protective relays. CTs step the primary current down to a nominal secondary level for use by protective relays, meters, and other monitoring devices. CTs perform reasonably in most operating scenarios, faithfully reproducing primary current as a secondary current, with little distortion or error CTs for transmission lines are rated to avoid saturation during the asymmetrical portion of the maximum fault current. Saturation is avoided by selecting the voltage rating larger than the maximum fault burden voltage with the (X/R+1) factors applied according to [3]. This is possible in line protection applications since large load currents result in the use of high ratio CTs, and fault currents are typically limited from 5 to 7, and rarely exceed 10 times the CT primary current rating. In this paper, the relationship between the flux density and the time integral of the voltage is examined. Using this as a basis, the relationship between fault current, CT burdens, and system X/R ratio is established and this ultimately determines the useful range of any CT. II. Current Transformers Burden and Accuracy Class The burden of current transformers is the property of the circuit connected to the secondary winding, which determines the active and reactive power at the secondary terminals [3]. The external load applied to the secondary of a CT is called the burden The total burden is the combination of internal CT burden and the external burden connected to the CT terminals. In selecting CT, the process is to minimize the total burden that consists of the internal resistance of the CT winding itself, the resistance of the leads from the CT to the relay including the return path, and the burden of the connected relays. The accuracy of CT s is defined as the extent to which the current in the secondary circuit reproduces the current in the primary circuit as stated by the marked ratio. In many applications the use of the ANSI/IEEE accuracy class designation is adequate to assure satisfactory relay performance. The accuracy class can be obtained by calculation or by test, followed by the minimum secondary terminal voltage that transformer will produce at 20 times rated secondary current with one of the standard burdens without exceeding the accuracy class limit of 10%. The secondary voltage rating is the voltage the CT will deliver to a standard burden at 20 time s rated secondary current without exceeding 10% ratio correction. The standard burden values ISSN: Page 298

2 for relaying are 1.0, 2.0, 4.0, and 8.0 ohms all with an impedance angle of 60 0 [4,5,6]. Consequently, at 20 times the rated current of 5 amperes the standard voltage ratings are 100, 200, 400, and 800 volts respectively. Table 1 presents standard current transformers burdens with 5A secondary windings. Table 1: Standard burdens for current transformers with 5A Secondary [6] Burden Desig- nation Resis- tance (Ω) Induct- ance (mh) Imped- ance (Ω) Volt- Amp (VA) Pf Secon- dary voltage C C C C , Figure 1 shows the shaded volt-time area produced by asymmetrical fault current. Where I f is the magnitude of the fault current in the secondary; Z b is the burden impedance, and L/R is the time constant of the primary fault current. The sine wave and the exponential components of the wave are shown dashed for comparison. The plot shows the change of burden voltage with the time. The volt-time area of the asymmetrical fault is increases compared to the normal sine wave and hence will affect the performance of the CT and the relay [8]. III. System X/R Ratio on CT s Saturation CT performance is affected significantly by the D.C component of the ac current. When a current change occurs in the primary ac system, one or more of the three-phase currents may obtain some D.C offset. This results from the necessity to satisfy two conflicting requirements that may occur [7]. In a highly inductive network, the current wave must be near maximum when the voltage is at or near zero, and The actual current at the time of the change, which is determined by the prior networks conditions. During asymmetrical faults, the fault current can be represents by two parts, namely the d.c and ac components as follows: The total fault current can be rewritten as: Figure 1: Burden Voltage for Asymmetrical Fault current Using the asymmetrical burden voltage equation (3) and substituting in equation (5) will result in the following equation: The burden voltage can be expressed as follows: In equation (6), the limit of the integral of the exponential term is the X/R ratio of the primary circuit. Since the limit integral of the cosine term is unity, we can write the equation as follows: The burden voltage V is related to the core turn (N) of the CT and the rate of change of the core flux by the induction equation. Integrating equation (4) show that the flux density in the core is represented by the area under the voltage waveform. In other words, for a given secondary fault current, more burden voltage from the CT will be required and the core density is proportional to the time interval of this voltage. Therefore the flux linkages in the core are given by the interval of equation (5) where the flux is expressed as flux density B times the core cross sectional area A Equation (7) expresses the CT-rating voltage in terms of the physical parameters of the CT, namely the saturated flux density Bs, system turns ratio N, the core cross sectional areas A; and angular frequency w. A different form of Equation (7) can be derived by recognizing that the rating voltage is 20 times the voltage across the standard burden at rated current. If we then express the fault current I f in per unit of the rated current and the burden Z b in per unit of the standard burden, equation (8) becomes the simple criterion to avoid saturation. 20 (X/R + 1).* I F * Z b (8) Where VS = 20 ISSN: Page 299

3 I F = the maximum fault current in per unit of CT rating, undistorted CT secondary current to perform phasor Z b = the CT burden in per unit of standard burden and measurement in the presents of the dc offset. X/R = the system X/R ratio, that is the primary system Using class 1200:5 CT ratio with standard burden of reactance and resistance up to the point of fault. 4Ω, the calculated total burden is express as: However, most times, these rules result in impractically large CTs, which are not economically acceptable. CT cables of m full circuit run of #10 AWG (1.0 Ω/1000 feet) = 0.40 A. CT Selection Procedures CT winding of 240 turns at Ω/turn = 0.60 The algorithms for the selection of current transformers in a given transmission line relay application is outline as follows (IEEE standard C ). (1) Determine the maximum fault current I f in primary amperes (2) Determine the corresponding primary circuit X/R ratio. (3) Select the CT voltage rating based on the load requirements. (4) Determine the total burden in per unit of the CT standard burden. (5) Divide the maximum primary fault current I f by the per unit current to determine the CT nominal current rating. Select the nearest standard rating greater than the calculated value. (6) Calculate the saturation voltage VS. (7) Finally, compare VS to the current transformer rating. If the voltage rating is greater than or equal to the calculated secondary voltage, the current transformer is acceptable for the application. IV. Methodology Description Advanced Saturation Voltage Model Microprocessor-based (MPB) relays have a relatively small burden of 0.2 VA (0.008 Ω). The secondary burden is the impedance of the CT windings and cables (leads) full circuit runs; and it is purely resistive. In this paper, the CT selection criteria are based on the degree of saturation voltage produced by the 120KA fault in the 1200:5 CT primary rating. Evaluation of the effects of asymmetrical faults with DC offset on operation of instantaneous microprocessor-based relay is considered. In order to avoid DC saturation, the saturation voltage equation VS is as expressed in equation 9. VS (9) Where I f = the maximum faults current in per unit of CT rating, Z b is the CT burden in per unit of standard burden, VS is the CT saturation voltage in per unit, X/R is the X/R ratio of the primary fault current. A. CTs Selection for line Protection Modern Microprocessor-based relays clear faults in few cycles to preserve stability, accurately identify fault type for single-pole reclosing applications, and determine an accurate fault location. To accomplish this, line relays require Total burden = 1.00 Ω The burden is primarily due to the CT winding and external cables to the relay. Equation (10) determines the maximum burden in per unit of the standard burden: Z b = (10) With an X/R ratio of 20, the saturation voltage can be calculated using equation (9) as follows: Vs [20+1]* 100 * 0.25 = 525 Consequently, CTs used with the microprocessor relays should meet the criteria in equation 11; 525 (11) Calculated maximum load current of the protected line is A, so select CT of 1200:5A, being the nearest CT standard rating greater than the calculated value. Also, the calculated Voltage Saturation of CT 1200:5A rating from the enhanced model is 302 Volts. This is less than the 400 Volts rating of CT 1200:5A ratio, hence the selected CT rating of 1200:5A is thus adequate and suitable for protection application. For evaluation purposes, microprocessor overcurrent relay with a 240A instantaneous setting and X/R ratio of 20 (worst case) is considered. The equation for maximum fault current in terms of primary CT and ANSI voltage rating, burden in ohms, and X/R ratio is as express in equation 12, I max = * (12) The acceptable calculated saturated voltage VS is 525 volts, substituting equation (12) above, the maximum fault current for secure operation can be calculated as follows: I max = B. Simulation: Evaluation of CT Secondary Burden: * 1200 = 120,000 A for MPBRs In the simulation conducted at BELLS University of Technology, Power Laboratory, different secondary burdens of 1.0, 3.0, 5.0, and 10Ω were injected at different times into the SEL-551 MPB instantaneous overcurrent relay by the secondary test equipment ISA T2000, using a CT of 1200:5A ratio. Starting from a burden of 1.0 Ω; the burden is ISSN: Page 300

4 primary due to CT winding itself, and external leads (a wire size of #10 AWG) of a total length of 400ft (121.92m) from CTs to the relay including the return path. With total burden of 1.0 ohm, secondary current waveform results was obtained from ATP simulations base on transmission network parameters as shows in Figure 2; it indicates an undistorted waveform throughout the duration of the simulation. When secondary burden of 3.0 ohm was injected into the secondary circuit, the CT core saturates for at least four (4) cycles after the fault occurs. Subsequently increasing the burden to 5.0 ohm, and then 10 ohms respectively, the waveform distortion became more present (visible) during the entire simulation. The severe CT saturation (distortions) caused by the heavy fault current (115 KA) with maximum offset and different values of secondary burden (3, 5, and 10 ohms) is shown in Figure 3; these caused RMS current to be smaller than in fact it was, i.e. the secondary current is substantially reduced under severe saturation of main CTs. This proves that relative small burden can influence CT accuracy if the fault current is not correctly anticipated. Conclusively, proper selection of a CT is required for a better protection scheme operation. Figure 2: Non-saturated Waveform secondary burden (1.0 ohm). Since MPB instantaneous relays operates within one cycle (1) and that the CTs operation was accurate from one to four cycles of its operation before saturation start to occurs, this proved that MPB instantaneous relays operation is not affected by CT saturation. Thus the trip time of MPB relay is unimpaired by CT saturation due to DC offset with extremely high fault currents. Figure 3: Severe CT Saturation (Distorted Waveforms) due to varying burden (3, 5, and10ω). V. Case Study and Application A case study is considered to show the area of application of the work. A transmission line under protection with MPB overcurrent relay installed at the source of the busbar to protect against over load and short circuits is rated 600 MVA, three phase, 330 KV, 50Hz, line reactance (X) and resistance (R) of Ω/km and Ω/km (i.e, X/R of 11.4 ratios). A CT rating of 1200:5A ratio is selected based on the full load current of A. Short circuit faults occurred at 5km away from source, with maximum fault current calculated to be A. The CT performance data is shown in table 2. Table 2: CT Performance Data Description Current transformer in used Instantaneous element pickup setting (1200:5) Values Class 1200:5 240A secondary X/R ratio ( / ) 11.4 (85º) Maximum fault current at 5 km A Using and a total burden of 1.0 ohms, the minimum CT rating can be determine in terms of maximum fault current, X/R ratio, ANSI rating, and burden as in equation (13), CT Rating =.. 13 The parameters presented are the maximum fault current of 120 KA, X/R = 20 (worst case) and a total burden of 1.0 Ω. CT rating = = 1200 Table 3 lists the maximum fault current versus X/R ratio for which saturation is avoided. Using ANSI Class of CT ratio s of 2000:5, 1500: 5, 1200: 5, or 1000: 5, with varying burdens at various inception angles is presented. ISSN: Page 301

5 Table 3: Maximum current (Amps) to avoid saturation using Microprocessorbased Relays Line Angle X/R Ratio 2000:5 1.5 Ω 1500:5 1.2 Ω Amperes 1200:5 1.0 Ω 1000:5 0.8 Ω , , , , The CT primary current accuracy describes the highest fault current magnitude at which the CT will fulfill the specified accuracy. Beyond this level the secondary current of the CT will be distorted and it might have severe effects to the performance of the protective relay. Compliance with saturation voltage equation depends on the X/R ratio and the magnitude of the maximum fault current. Either by calculation or measuring the burden and the X/R ratio, the saturation voltage equation can be applied to calculate the maximum fault current for a given CT ratio. Thus, relay operations with 1200:5 CT can be selected and guaranteed with a saturation voltage of 525 for the relay trip operations as show in table 3. b) Transient Behavior of CT during Faults. The total secondary burden of the installation is Ω primarily due to the CT winding and external CT control wire of (400ft) m full circuit runs of #10 AWG to the relay. From equation (11), saturation voltage VS = 525 (X/R+I) * I f * Z b Fault current at 5 km away from source I f(5) = = = A Substitute these values into equation (11) to determine the limit of saturation voltage. 525 (11.4+1) * * = 302 Volts Result shows that the instantaneous operations of MPB relay using 1200:5A CT can be guaranteed with 302 times CT rating with X/R ratio of Figure 4 show the response of the relay with A short circuit fault currents for the case presented. The thick line indicates the root mean square current (I RMS) while the dash waveform is the secondary currents. It confirmed that the MPB relay with 240A instantaneous setting will respond to the fault in this case with no more than three cycles. Figure 4: Case Study Test Result with X/R =11.4 Applying the developed algorithm, instantaneous microprocessor-based relays will operate properly with 1200:5A CT ratio and high short circuit levels. CT performance will be satisfactory as the CT secondary maximum symmetrical fault current multiplied by the total secondary burden is less than the ANSI-class voltage rating of the CT. The criteria allow protection engineers to determine the limits of high current applications in terms of the fault current magnitudes, the X/R ratio and relay burden. The developed algorithm has addressed the limitation of CT ratings used in line protection where fault current exceed 525 times the CT primary rating. As shown in equation (9), a smaller size CT rating and accuracy class with adequate capability can be employ for line protection. However, the CT ratings that avoid saturation for asymmetrical current are impractical in applications near a generator bus-bar where X/R ratio and the fault current are both extremely high. A limit to the criterion is shown in table 3 where the permissible maximum fault current for a given CT rating is severely decreasing with increasing X/R ratio. The limit occurs where high X/R ratio and high fault current are experienced near the bus-bar. VI. Conclusion The effect of CT saturation on protective relays has been investigated. The relation between the ANSI voltage rating, CT burden, maximum fault current, and system X/R ratio defines the threshold of CT saturation. The derived equation provides the CT rating criteria for line protection in new installations and can identify the threshold of CT saturation in older installations. CT rating is selected based on short circuit level available, secondary burden connected, X/R ratio and required relay trip time. Consequently, the need for higher CT ratio with higher class could be eliminated. ISSN: Page 302

6 CT of smaller size rating can be used for MPBR as [4] Aibangbee J.O. Ph.D. Thesis, Current Transformer saturation impacts on Microprocessor-Based Protective Relays Application in Nigeria against the heavy and bulky type CT rating currently Electricity Transmission Network, university of Benin, Nigeria, used for electromechanical relays. The criterion used CT selection procedure for line [5] ABB Inc, Instrument Transformers Technical Information and protection which determined the CT ratio using the Application Guide maximum fault in amperes, the system X/R ratio and [6] U.S. Department of the Interior Bureau of Reclamation Denver, the ct burden expressed in per unit of a CT rated Colorado: Facilities Instructions, Operation, Maintenance, and Field standard burden. Testing Procedures for Protective Relays and associated circuits Vol.3- The instantaneous operations of microprocessor 8, May based relay can be guarantee with the advanced [7] ABB Inc Instrument Transformers Application Guides SE saturation voltage Algorithm developed. LUDVIKA, Sweden, REFERENCES [1] G. Benmouyal, and S. E. Zocholl Current Transformer Concepts Proceedings of the 46 th Annual Georgia Tech Protective Relay Conference, Atlanta, GA, April 29, [2] S. E. Zocholl, Armando Guzmán, and Daqing Hou, Transformer Modeling as Applied to Differential Relaying, Proceedings of the 22nd Annual Western Protective Relay Conference,Spokane, WA. October 24-26, [3] IEEE C , the Guide for the Application of Current Transformers Used for Protective Relaying Purposes. [8] Blackburn J. L. and Thomas J. Domin Protective Relaying Principles and Applications third edition CRC Press, 2007 [9] G. Benmouyal, E. O. Schweitzer III and Jeff Roberts, Distance Relay Element Design, Proceedings of the 19th Annual Western Protective Relay Conference, Spokane, WA, [10] IEEE Standard C57.13, 2008.Standard requirements for Instrument Transformers ISSN: Page 303