- Email: [email protected]
Instrument current transducers with Rogowski coils in protective relaying applications
Electrical Power and Energy Systems 73 (2015) 107–113
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Instrument current transducers with Rogowski coils in protective relaying applications Denis B. Solovev ⇑, Alexander S. Shadrin Far Eastern Federal University (FEFU), Engineering School, City of Vladivostok, Russian Federation
a r t i c l e
i n f o
Article history: Received 23 April 2014 Accepted 22 April 2015
Keywords: Rogowski coils Relay protection Electromechanical protection system Negative sequence Current transducers Power supply
a b s t r a c t The purpose of this work is informing the scientific community and vendors of a certain type of instrument transducers for protection and control applications in industrial electric installations. Such transducers can increase the accuracy and decrease the size of the measuring system for all service conditions at any industrial facilities. The work contains circuit diagrams of negative sequence current transducers offered by the author and obtaining primary currents from Rogowski coils. The statistical data based on the field tests of the discussed equipment is also presented in the paper. The algorithm for calculation of parameter values for negative sequence current transducers is discussed. The paper may be of interest for investigators and engineers engaged in research, design and commissioning of protection and control equipment, current instrument and measurement devices used in industrial applications, and also for undergraduate and postgraduate students in electrical engineering. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction
Material and methods
Presently microprocessor-based protective relaying is becoming more and more widespread and continues to force out conventional relaying equipment. It is confirmed by the fact that for the last 10 years microprocessor-based relaying has been the mandatory subject for all students in the pertaining fields. The transition to digital relaying is inevitable [1]. It is known that such large international relay vendors as ABB, General Electric, Siemens, Alstom may soon stop the production of electromechanical relays. This fact is mainly explained by the argument that it is much more profitable to manufacture complex microprocessor-based relays than their electromechanical or solid-state counterparts [1–3]. These sources, however, claim that microprocessor-based relays are not perfect and may have some disadvantages. The source [1,3,4] suggests that in order to use microprocessor-based relays ‘‘more advanced instrument current transducers must be introduced for measurements’’. The integrator filter is unnecessary in this case. An integrator filter can be implemented as a virtual device as part of the microprocessor program.
The number of failures of industrial equipment and mechanisms is significantly rising at most of facilities in the developing countries. About 50% of such failures are attributed to faults in electrical parts of machines and mechanisms, motors, and drive control systems. Every year about 35% of general purpose industrial and domestic electric motors fail. The number of electric motor failures reaches even 60% in larger industries with harsh operation environments (such as mining and construction). The fact that about 50% of all industrial electrical failures in Russia are attributed to open-phase conditions of electric drives is of particular interest with this respect. The motors of industrial machines and mechanisms are equipped with protections against overloads, short circuits, single-phase faults and sometimes are not able to respond to asymmetrical or other abnormal conditions. At the same time the design of control and protection systems is of great importance in the power industry today. Large investments and appearance of new products have become characteristic of this area. The microprocessor technology has become very important in the production of protective relays. Among the famous vendors of protection and control equipment are such European companies as ALSTOM, ABB and SIEMENS. The cost of these digital protections is rather large, but it is justified by high technical characteristics and advanced functionality of the devices.
⇑ Corresponding author. Tel.: +7 9502904396. E-mail address: [email protected] (D.B. Solovev). http://dx.doi.org/10.1016/j.ijepes.2015.04.011 0142-0615/Ó 2015 Elsevier Ltd. All rights reserved.
108
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
The modern digital devices are integrated into the information infrastructure which includes protective relaying, measurements, regulation and control of the electric utility. Within the automated process control system such devices can be viewed as terminal data collection equipment. The main obstacles for the wide use of such protection are the disadvantages of primary current transducers. In 90% of cases such transducers are conventional current transformers with large size, weight and high costs. The mode of a current transformer is close to short circuit when the secondary voltage is significantly less than the e.m.f. The secondary winding inductive reactance is tens or hundreds times greater than the total resistance of the winding and transformer burden. Therefore in steady-state conditions for cases when there is no dc component present, the secondary current is almost proportional to e.m.f. integral, i.e. it is proportional to the measured primary current. The nominal secondary current is 1. . .5 A; the nominal capacity of a current transformer is no greater than tens of watts. However this large difference between e.m.f. and output voltage results in large weight of a current transformer. For instance, at voltage of 700 kV the weight of the transformer is about a ton or sometimes greater. This large weight is the main disadvantage of current transformers, but it is not the only disadvantage. The other disadvantage is failed accuracy of measurement in some conditions. This disadvantage is especially evident when the dc component is large due to ferromagnetic core saturation. Such errors may result in false operations of protections using current measurements, particularly bus differential protections. That is why the main purpose of the research carried out at Far Eastern Federal University (Vladivostok, Russia) has been the design and investigation of instrument transducers for industrial protective relaying and control applications that could provide the required accuracy and decrease the size and weight of the measurement equipment. The accuracy of current measurement can be increased significantly if Rogowski coils are used as primary instrument transducers instead of conventional current transformers. Rogowski coils however measure the rate of change of current, not the current itself. Operation of Rogowski coils in conditions close to no-load results in much smaller weights of such transducers as compared to conventional current transformers. The best accuracy is shown by Rogowski coils without magnetic core. There is no saturation during measurements in such coils; large currents can be measured with no need to increase the transducer size; rate of change of current up to 40 kA/ls can be measured; no electrical connection with the primary circuit is required for such measurements. Therefore the characteristics of protections which require current measurements can be increased significantly due to higher accuracy of primary current transducers applied in the protection system. Above all, Rogowski coils can be considered as alternative to conventional current transformers for applications in harsh operation environments mentioned previously. It must be also said that a wider use of Rogowski coils instead of conventional current transformers is often quite difficult due to problems pertaining to design and calculation of symmetrical component filters in such new applications. The symmetrical component filters for three-conductor industrial systems have been designed at Far Eastern Federal University (Vladivostok, Russia) in order to use Rogowski coils for open-phase condition protections. The suggested application of instrument transducers obtaining primary currents from Rogowski coils for three-conductor lines is shown in Fig. 1. The sources of voltage for both instrument negative sequence current transducers are e.m.f. Ea and Eb of differentiating inductance transducers Rogowski coil 1 and Rogowski coil 2. The coils are inductively connected with current-carrying conductors in
phases A and B. (Another combination of the phases can be chosen provided that the following requirement is met: the positive sequence component of the phase current associated with Rogowski coil 1 must lead the same component of current associated with Rogowski coil 2 by 120°.) The Rogowski coils in the negative sequence current transducer with 5-element filter (see Fig. 1a) have equal impedances and mutual inductances M with their respective conductors. The relations between current phasors Ia (Ib ) and the respective e.m.f. phasors in steady-state condition for sinusoidal currents are as follows: Ea ¼ jx M Ia and Eb ¼ jx M Ib , where j – imaginary 1, and x – angular frequency of currents. The resistor-capacitance circuit of the negative sequence voltage filter comprises two capacitors C1 and C2 with reactances (jX1) and (jX2), and three resistors R1, R2 and R3. This filter differs from the traditional 4-element negative sequence voltage filter in the way that it has a fifth element, i.e. the 3rd resistor. This additional element is necessary to compensate the effect of the inducpffiffiffi tive reactance jXk of Rogowski coil 2. If the relation R1 ¼ X 2 ¼ 3 R2 pffiffiffi and conditions X 1 ¼ R2 þ X k and R3 ¼ 3 X k are met, the output voltage of the negative sequence voltage filter U ab1 equals zero in steady-state condition when currents Ia , Ib and Ic make up the positive sequence of current. The following three parameters are necessary to evaluate the relationship between Rogowski coil impedance and the load resistance: m ¼ X k =R2 , mr ¼ Rk =X k and mng ¼ R2 =Rng , where Rng – resistance of filter load (resistance of the protection relay). With the use of these parameters and all the conditions given above the reactances and resistances in the negative sequence current transducer can be expressed in per unit values. The resistance Rng is taken as base value. For instrument current transducer with 2-element negative sequence voltage filter (see Fig. 1b) the mutual inductance between Rogowski coil in phase A and the respective conductor is 2 times the mutual inductance between Rogowski coil in phase B and conductor B. To simplify the manufacturing of Rogowski coils their parameters shall be unified as much as possible. Therefore, in case with Rogowski coils without magnetic core the same coil sizes should be used (one coil per phase), but the number of turns of the coil for phase A should be increased by a factor 2. Given that the inductance of the toroidal coil is proportional to the squared number of turns and that its resistance is proportional to the number of turns, the inductive reactance X kB and the resistance RkB of Rogowski coil 2 are less than those of Rogowski coil 1 (X kA and RkA ). This difference in values is respectively 4 and 2 times as given pffiffiffi . pffiffiffi by the following equations: X kA ¼ 3 m R, X kB ¼ 3 m R 4, pffiffiffi . pffiffiffi 3 m mr R 2, where m ¼ X kA = RkA ¼ 3 m mr R, RkB ¼ pffiffiffi 3 R :, mr ¼ RkA =X kA . The effect of XkA can be compensated by the capacitor with reacpffiffiffi tance of X ¼ ð1 þ mÞ X 0 , where X 0 ¼ 3 R – reactance of capacitor when reactance of the coil is negligibly small. Thus, the effect of the Rogowski coil inductive reactance requires decreasing of the capacitance in the negative sequence voltage filter by a factor of ð1 þ mÞ. The relation between the resistance R and the load resistance is the following: R ¼ Rng mng . Protective relaying equipment used at ore mining and processing plants can be rather diversified. The power supply systems at such plants can be both of 3-conductor or 4-conductor type. To raise the efficiency of inductor motor protective relays a special filter for three-phase 4-conductor systems has been designed. This filter contains 4 Rogowski coils of similar design: one in each of the phases A and C and two in phase B. The diagram of this instrument transducer is shown in Fig. 2.
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
109
Fig. 1. Symmetrical component filters with two Rogowski coils for three-conductor lines: (a) with 5-element filter, (b) with 2-element filter.
Theory/calculation It has been stressed by many sources [5–8] that the quality of the signal entering the microprocessor-based protection device must be rather high. There are specific requirements concerning noise level and frequency range of the signal. In most advanced protections the signal is not directly applied to ADC from the primary transformers, but rather from filters or, ideally, from instrument current transducers. When instrument current transducers are used the negative effect from electromagnetic fields can be significantly decreased and the operation of protections will be more stable. The operation of a protective relay usually results in operation of the associated switching equipment and disconnection of the protected circuit. The operating principles or microprocessor and electromechanical relays are sometimes different, but both can employ the same primary transducers (e.g., instrument negative sequence current transducers). Therefore, the input circuits of microprocessor-based protections can be connected in parallel with the electromechanical relays. The example of such combined protection system is shown in Fig. 3. The design of negative sequence current transducer described above can also be used for such applications. Fig. 2. Symmetrical component filter with four Rogowski coils for 4-conductor lines.
The differences of e.m.f. (A, B and C, B) are applied to the input of the 5-element negative sequence voltage filter. This helps to eliminate the negative sequence component of the measured currents. The following relations between the elements are to be observed: .pffiffiffi pffiffiffi pffiffiffi X 1 ¼ R1 3 þ X k ; X 2 ¼ 3 R2 ; R3 ¼ 3 X k ; m ¼ X k =R2 ; X k ¼ 2 X k0 – where X k0 is inductive reactance (it is comparable with the resistances and reactances of all the remaining elements of the filter circuit). The above relations between the elements of the instrument transducer provide separation of the correct sequence in the four-conductor line.
Fig. 3. Combined microprocessor-based and electromechanical protection system.
110
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
The effectiveness of calculation algorithm for elements of negative sequence current transducers is very important when it comes to analysis and, especially, optimization of systems with combined operation of electromechanical and microprocessor-based relays. The above relations between the parameters of the elements used in current transducers connected to Rogowski coils not only allow to calculate reactances, resistances and capacitances of the filter, but can also be used to quickly determine the parameters of Rogowski coils. This is particularly useful when the variable parameters during analysis and optimization are the non-changeable parameters of relays connected to transducers (such as relay resistances). If the relay resistance is known, it is possible to determine all parameters of the negative sequence voltage filter and its output signal (which will cause both the electromechanical and microprocessor-based relay to operate). It was already mentioned earlier that Far Eastern Federal University has conducted a research aimed at design of Rogowski coils for different operation environments (with different voltage ranges). The analytical and experimental data obtained during the research can be used to make certain suggestions concerning the design of Rogowski coils to be used as primary current transducers in negative sequence circuits. For voltages up to 1000 V, Rogowski coils with magnetic core and air gap should be used. In such applications the coils can be of minimum size and weight, the maximum dimension being no greater than 5 cm. The standard single-phase transformer cores (for transformer capacity up to 20 VA and voltages up to 400 V) can be used. The new primary winding of thinner wire with greater number of turns is used. The bus which carries the measured current goes through the transformer core. The air gap and the number of turns in the winding determine the mutual inductance M between the winding and the bus, as well as the inductance L of Rogowski coil winding. The relation between the values is as follows: the decrease of the gap and the increase in cross-section of the core and number of turns result in the increase of M and L. The calculation of such Rogowski coil is described in source [9]. The Rogowski coils for protection of 400 V induction motors (e.g., boring machines in open mines) should be designed as transformer reactors with magnetic core having air gaps. In this case Rogowski coils can have very small physical dimensions. The calculation of such coils is not much different from the traditional calculation used for ordinary a.c. reactors. Let us assume that 6. . .35 kV lines and loads are used at the mine in question. In this case Rogowski coils should be of another design: no magnetic core, the coil is placed around the bushing of the respective conductor in which the current is measured. The power being equal, the consumer current is inversely proportional to the voltage. Therefore when the voltage changes from 400 V to 6 kV the consumer current will decrease by a factor of 15. The mutual inductance between the Rogowski coil and the conductor must in this case increase by a factor of 15. Some new designs of Rogowski coils can be used for such applications. They have printed-type winding located on a flat ring. The inner diameter of the ring is larger than the outer diameter of the bushing. The main disadvantage of this design is small cross-section of the winding. The e.m.f. in such cases (tens or hundreds of mV) is insufficient for direct connection (without amplifiers) of Rogowski coils to negative sequence voltage filter which is connected to electromechanical relay. In order to make possible operation of electromechanical protections or combined type protections, it would be better to avoid the use of amplifiers and to design Rogowski coils that allow for direct connection to negative sequence voltage filter. At voltages up to 6 kV it is better to use Rogowski coils placed on bushings of the equipment (circuit breakers or voltage transformers). The coils are comprised of short solenoid sections placed
on a flexible hollow cylindrical core. The sections are divided by gaps that are shorter than the sections. The ends of the core are united on the bushing during the coil installation process. The sections have a two-layer winding. This type of Rogowski coil is described in the source [10]. All negative sequence current transducer designs are linear systems where the positive and negative sequences of three-phase currents are independent. Therefore in the general case when the three-phase current has both of the sequences the output voltage of the transducer is proportional to the negative sequence current. For microprocessor modules used in the protective relaying systems it is necessary to carefully choose the parameters of negative sequence current transducers basing on traditional recommendations relying on theory of electric circuits and protective relaying. Therefore choosing the parameters of components included into the negative sequence current transducers represents an important optimization problem. This problem must be solved step by step (depending on the characteristics of relays connected to the measurement circuit). In case of parallel connection of electromechanical and microprocessor-based relays to negative sequence current transducers (see Fig. 3) the total resistance of the relays is given by Rtot ¼ ðRMP REM Þ=ðRMP þ REM Þ, where RMP and REM – resistances of microprocessor and electromechanical relays respectively. The calculation of permissible loads in current circuits of protective relaying equipment is one of the stages in design of protection and control systems for utilities. Such sources as [11] provide a number of methods for calculation of parameters of current transducers for relaying needs. However, if Rogowski coils are used as primary transducers, such calculation methods do not always allow determining the true relationships between the parameters of negative sequence voltage filter components and coil parameters. This becomes even more difficult when such parameters for Rogowski coil based negative sequence current transducers are to be calculated for distribution networks at open mines where transient currents can be very large due to increased capacity of machines and equipment in such environments. For open mines it is also true that loads in current circuits of protective relaying equipment become larger due to longer wiring and larger number of protection devices. The effectiveness of calculation algorithm for elements of instrument negative sequence current transducers is very important when it comes to analysis and, especially, optimization of combined operation of electromechanical and microprocessor-based relays. The obtained relations between parameters of elements in current transducers connected to Rogowski coils not only allow to calculate reactances, resistances and capacitances in the negative sequence voltage filter, but can also be used to quickly determine the parameters of Rogowski coils. This is particularly useful when the variable parameters during analysis and optimization are the non-changeable parameters of relays connected to transducers (such as relay resistances). If the relay resistance is known, it is possible to determine all parameters of the negative sequence voltage filter and its output signal (which will cause both the electromechanical and microprocessor-based relays to operate) can be predicted in advance. The following consecutive steps can be used for choosing the parameters of instrument current transducers: 1. The main input parameter for calculation is the output resistance of voltage filter, i.e. equivalent resistance of microprocessor and electromechanical relay inputs connected in parallel. Therefore, at first it is necessary to determine the input resistance Rng (from nameplate data) of electromechanical or microprocessor-based relay connected to instrument current transducer as load of negative sequence voltage filter (if the
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
relays are connected in parallel the total resistance must be determined using the formula given earlier). 2. The relay operating settings must be calculated or selected using recommendations provided by the relay manufacturer [11]. 2.1. If the instrument negative sequence current transducer with 5-element voltage filter is used, the relay time delay of approximately 0.02 s must be introduced [11]. Microprocessor-based relays are very sensitive to overvoltages on relay input (relays may be damaged if the overvoltage is too high). Therefore for such applications it must be considered that the voltage produced by current transducers at open phase conditions depends on currents of the protected induction motor, and such currents can be 8 times the nominal current. 2.2. If the current transducer with 2-element voltage filter is used, the relay time delay, as in the previous case, must be approximately 0.02 s. The maximum output voltages on current transducer output at open phase conditions are 30% higher than those for the 5-element filter design. This fact must also be considered if a microprocessor-based relay it used. 3. Resistances must be calculated which can be further used to determine the parameters of the voltage filter: 3.1. For current transducer with 5-element voltage filter the resistance R2 is given by: R2 ¼ 0:5Rng . 3.2. For current transducer with 2-element voltage filter the resistance R is given by: R ¼ Rng . 3.3 The resistors must be selected basing on the preliminary values of R2 and R calculated at steps 3.1 and 3.2. The nominal values of the resistors must be larger than the obtained values (closest in the standard series). The power rating of the resistors is calculated from the resistance values and output voltage of the instrument negative sequence current transducer. The selected power rating must be no less than the power rating values obtained in calculation. 4. The preliminary value of parameter m must be selected. The parameters of negative sequence voltage filter depend on Rogowski coil impedances while the mutual inductance between the coils and the conductors is determined by the parameters of the filter. So the further calculation of instrument negative sequence current transducer is iterative. At first the approximate value of parameter m should be defined. During the further iteration this value will be corrected. For instrument negative sequence current transducers in 6. . .35 kV networks the value m = 0 can be selected. In other cases this parameter must be defined on the basis of relationships given earlier in this paper. 5. The size, number of turns, resistance and reactance of Rogowski coils must be defined (as described in [9] for voltages up to 1000 V; as described in [12] for voltages over 35 kV; as described in [13,14] for voltages 6–35 kV). 5.1. The calculated value of negative sequence current transducer output voltage Urat and phase current Irat at open phase condition further allow to determine the required value of mutual inductance between Rogowski coil A with the respective conductor
MA ¼
U rat e Irat x
ð1Þ
111
where per unit e.m.f. value e for 5-element negative sequence voltage filter is given by: e¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi 2 pffiffiffi 1 2 3 þ 6 m mng 3 mng þ 6 þ 6 mng þ 3 3 mng at mng ¼ 0:5, 6 and
the same value for 2-element filter is given by: ffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi pffiffiffi 2 at mng ¼ 1 e ¼ 3 ð3mng ðm 1ÞÞ2 þ ð4 3 þ 3 3mng Þ (these values of parameter mng must be chosen for the calculation of the respective instrument negative sequence current transducer to ensure the best operation condition of the transducer. This empirical data was obtained during the investigations carried out at Far Eastern Federal University (Vladivostok, Russia)). In both of these cases the parameter m equals 0 (for Rogowski coils at voltage of 6. . .35 kV). 1 6
5.2. For instrument negative sequence current transducer with 5-element voltage filter the identical Rogowski coils are selected for both phases. For instrument negative sequence current transducer with 2-element voltage filter the mutual inductance of Rogowski coil for phase B is less by a factor of 2 than the value given by formula (1). 5.3. Then the calculation of Rogowski coil windings is to be done: choosing the wire type, diameter of the window, centerline radius and tube diameter of the toroid, as well as number of turns in the windings (for Rogowski coils rated for voltages over 6 kV). For Rogowski coils with voltages up to 1000 V the cross-section, the frame and the air gap of the core are to be calculated and the number of turns and size of the coil are to be determined. This step in the calculation must ensure such parameters that will provide the lowest possible values of both the weight of the coil and the parameter m. Resistance Rk and inductive reactance X k are to be calculated for the Rogowski coil of phase A. In 6. . .35 kV systems this calculation step may be omitted. 5.4. The value of m must be corrected using the following formulas: m ¼ X k =R2 for 5-element negative sequence voltage filter and m ¼ X k =R for 2-element negative sequence voltage filter. 6. The elements of negative sequence voltage filter must be further calculated: 6.1. For 5-element voltage filter design the reactances and resistances of the remaining filter elements are to be calculated: pffiffiffi pffiffiffi X 1 ¼ ð1 þ mÞ R2 , X 2 ¼ R1 ¼ 3 R2 , R3 ¼ m R2 3. 6.2. For 2-element voltage filter design the reactance of the pffiffiffi capacitor is to be calculated: X ¼ ð1 þ mÞ 3 R. 6.3. The obtained X, X1, X2, R1 and R3 values must be used to select capacitors and resistors (their nominal values must be closest to the obtained values). These nominal values and the output voltage of the instrument negative sequence current transducer must be further used to define the power rating of resistors and the capacitor voltage rating. The capacitors whose capacitance value is close to the defined value and voltage rating is larger than the defined value must be selected. The adjustable resistors with power ratings of no less than the defined ratings must be selected. The parameters of elements in the instrument negative sequence current transducers for the actual resistances of microprocessor-based or electromechanical relays (and in combined designs) can be calculated using software written for PC.
112
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
The calculation method described above is realized in the calculation application «IPTOP» written in Pascal. The block diagram of the algorithm is shown in Fig. 4. The algorithm combines the following steps: (1) Entering data into unit 1 (resistance Rng which includes the resistances of electromechanical and/or microprocessor-based relays; variant of instrument negative sequence current transducer used; approximate value of m); (2) Defining type of instrument negative sequence current transducer (with 2-element or 5-element voltage filter); (3) Entering voltage filter load (resistances of electromechanical and/or microprocessor-based relays); (4) Calculating the total resistance of electromechanical and/or microprocessor-based relays; (5) Calculating ratings of resistors (R – for current transducer with 2-element voltage filter and R2 – for current transducer with 5-element voltage filter); (6) Calculating the power rating of these resistors; (7) Calculation of mutual inductance between the Rogowski coil in phase A and the respective conductor for open-phase condition;
Table 1 Number of electric equipment failures due to open-phase conditions at ore processing plants «Novoshakhtinskoye» and «Luchegorskoye». Type of protection
Distribution boards with protections obtaining primary data from new filters (38 loads with induction motor drives) Distribution boards with protections obtaining primary data from filters with conventional current transformers (39 loads) Distribution boards with no protection against open-phase condition (35 loads)
Years 2010
2011
2
3
2012 2
2013 3
7
8
5
6
17
25
20
18
(8) Calculating the mutual inductances for Rogowski coils in phase A and phase B; (9) Calculating the parameters of Rogowski coil windings; (10) Correction of parameter m. If for new value of m the per unit e.m.f. value e is much different from the earlier obtained value (more than 5%), the elements must be recalculated; (11) Comparing the obtained value e with the setting; (12) Calculating values of capacitors and resistors in voltage filter; (13) End of program and output of calculation results. The current transducer parameter calculation algorithm (Fig. 4) can be simplified: if it is known in advance which of the voltage filter variants will be used (2-element or 5-element), step 2 in the algorithm is not necessary. The algorithm written in Pascal can be also used to calculate the parameters for combined electromechanical and microprocessor relaying applications. Results The tests of the transducer prototypes designed in 2010–2013 have been carried out at ore processing plants «Novoshakhtinsko ye» and «Luchegorskoye» (Primorsky Krai, Russia). Each distribution board controls a similar number of similar types of loads involved in similar technological processes. The results of tests are given in Table 1. These results show that the use of symmetrical component filters obtaining primary data from Rogowski coils can decrease the yearly average number of equipment outages due to open-phase conditions by 50%. Discussion It must be mentioned that failure to operate in protections in the first group was basically due to personnel errors. It is known that problems may arise as a result of traditions and poor knowledge of up-to-date protective relaying among the personnel; additional training is always required to introduce new types of equipment in a power utility. Failure to operate in protections in the second group was mostly due to distortion of signals obtained from primary transducers which can cause failures or may even result in damage of protection relays during operation. Conclusions
Fig. 4. Block diagram (algorithm) of calculating parameters for instrument negative sequence current transducer with 2- and 5-element voltage filter.
It can be concluded that new symmetrical component filters obtaining primary data from Rogowski coils can provide a high degree of selectivity. They can be directly connected to analog-to-digital converters and have smaller total weights. They are also more reliable and provide better accuracy in comparison with filters used with conventional current transformers.
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
References [1] Kojovic L. Rogowski coils suit relay protection and measurement. IEEE Comput Appl Power 1997;10(3):47–52. [2] Guihua Weng, He Jiang. Measurement of medium and high-voltage system bus current based on Rogowski coil. Electrician Electr 2009;2:43–7. [3] Kojovic L, Bishop MT. Field experience with differential protection of power transformers based on Rogowski coil current sensors. Actual trends in development of power system protection and automation 7–10 September 2009, Moscow, Russia. [4] Ramboz JD. Machinable Rogowski coil, design, and calibration. IEEE Instrum Meas 1996;45(2):511–5. [5] Marinescu A. A calibration laboratory for Rogowski Coil used in energy systems and power electronics. In: IEEE optimization of electrical and electronic equipment (OPTIM), 2010 12th international conference on 20–22 May 2010, conference location: Basov; May 2010. p. 913–9. [6] Ramoz JD, Destefan DE, Stant RS. The verification of Rogowski coil linearity from 200 A to greater than 100 kA using ratio methods. In: Proc of IMTC, vol. 1; 2002. p. 687–92. [7] Kojovic L, Colopy C, Arden D. Volt/var control for smart grid solutions. CIRED 2011, Frankfurt, Germany; June 9–11, 2011.
113
[8] Shafiq Muhammad, Amjad Hussain G, Kütt Lauri, Lehtonen Matti. Effect of geometrical parameters on high frequency performance of Rogowski coil for partial discharge measurements. Measurement 2014;49:126–37. [9] Zhang Yu, Liu Jinliang, Bai Guoqiang, Feng Jiahuai. Analysis of damping resistor’s effects on pulse response of self-integrating Rogowski coil with magnetic core. Measurement 2012;45(5):1277–85. [10] Draxler K, Styblikova R. Determination of Rogowski coil constant. In: IEEE applied electronics, 2006. AE 2006. International conference on 6–7, September 2006; September 2006. p. 241–3. [11] Winders JJ. Power transformers: principles and applications. New York, Basel: Marcel Dekker, Inc., 2002. p. 286. ISBN: 0-8247-0766-4. [12] Draxler K, Styblikova R, Hlavacek J, Prochazka R. Calibration of Rogowski coils with an integrator at high currents. IEEE Trans Instrum Meas 2011;60(7):2434–9. [13] Mariscotti A. A Rogowski winding with high voltage immunity. IEEE; January 2009. p. 1129–34. [14] Muzamir IS. Partial discharge location technique for covered-conductor overhead distribution lines. PhD thesis published in 2013 by School of Electrical Engineering and The department of Electrical Engineering, Aalto University.
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Instrument current transducers with Rogowski coils in protective relaying applications Denis B. Solovev ⇑, Alexander S. Shadrin Far Eastern Federal University (FEFU), Engineering School, City of Vladivostok, Russian Federation
a r t i c l e
i n f o
Article history: Received 23 April 2014 Accepted 22 April 2015
Keywords: Rogowski coils Relay protection Electromechanical protection system Negative sequence Current transducers Power supply
a b s t r a c t The purpose of this work is informing the scientific community and vendors of a certain type of instrument transducers for protection and control applications in industrial electric installations. Such transducers can increase the accuracy and decrease the size of the measuring system for all service conditions at any industrial facilities. The work contains circuit diagrams of negative sequence current transducers offered by the author and obtaining primary currents from Rogowski coils. The statistical data based on the field tests of the discussed equipment is also presented in the paper. The algorithm for calculation of parameter values for negative sequence current transducers is discussed. The paper may be of interest for investigators and engineers engaged in research, design and commissioning of protection and control equipment, current instrument and measurement devices used in industrial applications, and also for undergraduate and postgraduate students in electrical engineering. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction
Material and methods
Presently microprocessor-based protective relaying is becoming more and more widespread and continues to force out conventional relaying equipment. It is confirmed by the fact that for the last 10 years microprocessor-based relaying has been the mandatory subject for all students in the pertaining fields. The transition to digital relaying is inevitable [1]. It is known that such large international relay vendors as ABB, General Electric, Siemens, Alstom may soon stop the production of electromechanical relays. This fact is mainly explained by the argument that it is much more profitable to manufacture complex microprocessor-based relays than their electromechanical or solid-state counterparts [1–3]. These sources, however, claim that microprocessor-based relays are not perfect and may have some disadvantages. The source [1,3,4] suggests that in order to use microprocessor-based relays ‘‘more advanced instrument current transducers must be introduced for measurements’’. The integrator filter is unnecessary in this case. An integrator filter can be implemented as a virtual device as part of the microprocessor program.
The number of failures of industrial equipment and mechanisms is significantly rising at most of facilities in the developing countries. About 50% of such failures are attributed to faults in electrical parts of machines and mechanisms, motors, and drive control systems. Every year about 35% of general purpose industrial and domestic electric motors fail. The number of electric motor failures reaches even 60% in larger industries with harsh operation environments (such as mining and construction). The fact that about 50% of all industrial electrical failures in Russia are attributed to open-phase conditions of electric drives is of particular interest with this respect. The motors of industrial machines and mechanisms are equipped with protections against overloads, short circuits, single-phase faults and sometimes are not able to respond to asymmetrical or other abnormal conditions. At the same time the design of control and protection systems is of great importance in the power industry today. Large investments and appearance of new products have become characteristic of this area. The microprocessor technology has become very important in the production of protective relays. Among the famous vendors of protection and control equipment are such European companies as ALSTOM, ABB and SIEMENS. The cost of these digital protections is rather large, but it is justified by high technical characteristics and advanced functionality of the devices.
⇑ Corresponding author. Tel.: +7 9502904396. E-mail address: [email protected] (D.B. Solovev). http://dx.doi.org/10.1016/j.ijepes.2015.04.011 0142-0615/Ó 2015 Elsevier Ltd. All rights reserved.
108
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
The modern digital devices are integrated into the information infrastructure which includes protective relaying, measurements, regulation and control of the electric utility. Within the automated process control system such devices can be viewed as terminal data collection equipment. The main obstacles for the wide use of such protection are the disadvantages of primary current transducers. In 90% of cases such transducers are conventional current transformers with large size, weight and high costs. The mode of a current transformer is close to short circuit when the secondary voltage is significantly less than the e.m.f. The secondary winding inductive reactance is tens or hundreds times greater than the total resistance of the winding and transformer burden. Therefore in steady-state conditions for cases when there is no dc component present, the secondary current is almost proportional to e.m.f. integral, i.e. it is proportional to the measured primary current. The nominal secondary current is 1. . .5 A; the nominal capacity of a current transformer is no greater than tens of watts. However this large difference between e.m.f. and output voltage results in large weight of a current transformer. For instance, at voltage of 700 kV the weight of the transformer is about a ton or sometimes greater. This large weight is the main disadvantage of current transformers, but it is not the only disadvantage. The other disadvantage is failed accuracy of measurement in some conditions. This disadvantage is especially evident when the dc component is large due to ferromagnetic core saturation. Such errors may result in false operations of protections using current measurements, particularly bus differential protections. That is why the main purpose of the research carried out at Far Eastern Federal University (Vladivostok, Russia) has been the design and investigation of instrument transducers for industrial protective relaying and control applications that could provide the required accuracy and decrease the size and weight of the measurement equipment. The accuracy of current measurement can be increased significantly if Rogowski coils are used as primary instrument transducers instead of conventional current transformers. Rogowski coils however measure the rate of change of current, not the current itself. Operation of Rogowski coils in conditions close to no-load results in much smaller weights of such transducers as compared to conventional current transformers. The best accuracy is shown by Rogowski coils without magnetic core. There is no saturation during measurements in such coils; large currents can be measured with no need to increase the transducer size; rate of change of current up to 40 kA/ls can be measured; no electrical connection with the primary circuit is required for such measurements. Therefore the characteristics of protections which require current measurements can be increased significantly due to higher accuracy of primary current transducers applied in the protection system. Above all, Rogowski coils can be considered as alternative to conventional current transformers for applications in harsh operation environments mentioned previously. It must be also said that a wider use of Rogowski coils instead of conventional current transformers is often quite difficult due to problems pertaining to design and calculation of symmetrical component filters in such new applications. The symmetrical component filters for three-conductor industrial systems have been designed at Far Eastern Federal University (Vladivostok, Russia) in order to use Rogowski coils for open-phase condition protections. The suggested application of instrument transducers obtaining primary currents from Rogowski coils for three-conductor lines is shown in Fig. 1. The sources of voltage for both instrument negative sequence current transducers are e.m.f. Ea and Eb of differentiating inductance transducers Rogowski coil 1 and Rogowski coil 2. The coils are inductively connected with current-carrying conductors in
phases A and B. (Another combination of the phases can be chosen provided that the following requirement is met: the positive sequence component of the phase current associated with Rogowski coil 1 must lead the same component of current associated with Rogowski coil 2 by 120°.) The Rogowski coils in the negative sequence current transducer with 5-element filter (see Fig. 1a) have equal impedances and mutual inductances M with their respective conductors. The relations between current phasors Ia (Ib ) and the respective e.m.f. phasors in steady-state condition for sinusoidal currents are as follows: Ea ¼ jx M Ia and Eb ¼ jx M Ib , where j – imaginary 1, and x – angular frequency of currents. The resistor-capacitance circuit of the negative sequence voltage filter comprises two capacitors C1 and C2 with reactances (jX1) and (jX2), and three resistors R1, R2 and R3. This filter differs from the traditional 4-element negative sequence voltage filter in the way that it has a fifth element, i.e. the 3rd resistor. This additional element is necessary to compensate the effect of the inducpffiffiffi tive reactance jXk of Rogowski coil 2. If the relation R1 ¼ X 2 ¼ 3 R2 pffiffiffi and conditions X 1 ¼ R2 þ X k and R3 ¼ 3 X k are met, the output voltage of the negative sequence voltage filter U ab1 equals zero in steady-state condition when currents Ia , Ib and Ic make up the positive sequence of current. The following three parameters are necessary to evaluate the relationship between Rogowski coil impedance and the load resistance: m ¼ X k =R2 , mr ¼ Rk =X k and mng ¼ R2 =Rng , where Rng – resistance of filter load (resistance of the protection relay). With the use of these parameters and all the conditions given above the reactances and resistances in the negative sequence current transducer can be expressed in per unit values. The resistance Rng is taken as base value. For instrument current transducer with 2-element negative sequence voltage filter (see Fig. 1b) the mutual inductance between Rogowski coil in phase A and the respective conductor is 2 times the mutual inductance between Rogowski coil in phase B and conductor B. To simplify the manufacturing of Rogowski coils their parameters shall be unified as much as possible. Therefore, in case with Rogowski coils without magnetic core the same coil sizes should be used (one coil per phase), but the number of turns of the coil for phase A should be increased by a factor 2. Given that the inductance of the toroidal coil is proportional to the squared number of turns and that its resistance is proportional to the number of turns, the inductive reactance X kB and the resistance RkB of Rogowski coil 2 are less than those of Rogowski coil 1 (X kA and RkA ). This difference in values is respectively 4 and 2 times as given pffiffiffi . pffiffiffi by the following equations: X kA ¼ 3 m R, X kB ¼ 3 m R 4, pffiffiffi . pffiffiffi 3 m mr R 2, where m ¼ X kA = RkA ¼ 3 m mr R, RkB ¼ pffiffiffi 3 R :, mr ¼ RkA =X kA . The effect of XkA can be compensated by the capacitor with reacpffiffiffi tance of X ¼ ð1 þ mÞ X 0 , where X 0 ¼ 3 R – reactance of capacitor when reactance of the coil is negligibly small. Thus, the effect of the Rogowski coil inductive reactance requires decreasing of the capacitance in the negative sequence voltage filter by a factor of ð1 þ mÞ. The relation between the resistance R and the load resistance is the following: R ¼ Rng mng . Protective relaying equipment used at ore mining and processing plants can be rather diversified. The power supply systems at such plants can be both of 3-conductor or 4-conductor type. To raise the efficiency of inductor motor protective relays a special filter for three-phase 4-conductor systems has been designed. This filter contains 4 Rogowski coils of similar design: one in each of the phases A and C and two in phase B. The diagram of this instrument transducer is shown in Fig. 2.
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
109
Fig. 1. Symmetrical component filters with two Rogowski coils for three-conductor lines: (a) with 5-element filter, (b) with 2-element filter.
Theory/calculation It has been stressed by many sources [5–8] that the quality of the signal entering the microprocessor-based protection device must be rather high. There are specific requirements concerning noise level and frequency range of the signal. In most advanced protections the signal is not directly applied to ADC from the primary transformers, but rather from filters or, ideally, from instrument current transducers. When instrument current transducers are used the negative effect from electromagnetic fields can be significantly decreased and the operation of protections will be more stable. The operation of a protective relay usually results in operation of the associated switching equipment and disconnection of the protected circuit. The operating principles or microprocessor and electromechanical relays are sometimes different, but both can employ the same primary transducers (e.g., instrument negative sequence current transducers). Therefore, the input circuits of microprocessor-based protections can be connected in parallel with the electromechanical relays. The example of such combined protection system is shown in Fig. 3. The design of negative sequence current transducer described above can also be used for such applications. Fig. 2. Symmetrical component filter with four Rogowski coils for 4-conductor lines.
The differences of e.m.f. (A, B and C, B) are applied to the input of the 5-element negative sequence voltage filter. This helps to eliminate the negative sequence component of the measured currents. The following relations between the elements are to be observed: .pffiffiffi pffiffiffi pffiffiffi X 1 ¼ R1 3 þ X k ; X 2 ¼ 3 R2 ; R3 ¼ 3 X k ; m ¼ X k =R2 ; X k ¼ 2 X k0 – where X k0 is inductive reactance (it is comparable with the resistances and reactances of all the remaining elements of the filter circuit). The above relations between the elements of the instrument transducer provide separation of the correct sequence in the four-conductor line.
Fig. 3. Combined microprocessor-based and electromechanical protection system.
110
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
The effectiveness of calculation algorithm for elements of negative sequence current transducers is very important when it comes to analysis and, especially, optimization of systems with combined operation of electromechanical and microprocessor-based relays. The above relations between the parameters of the elements used in current transducers connected to Rogowski coils not only allow to calculate reactances, resistances and capacitances of the filter, but can also be used to quickly determine the parameters of Rogowski coils. This is particularly useful when the variable parameters during analysis and optimization are the non-changeable parameters of relays connected to transducers (such as relay resistances). If the relay resistance is known, it is possible to determine all parameters of the negative sequence voltage filter and its output signal (which will cause both the electromechanical and microprocessor-based relay to operate). It was already mentioned earlier that Far Eastern Federal University has conducted a research aimed at design of Rogowski coils for different operation environments (with different voltage ranges). The analytical and experimental data obtained during the research can be used to make certain suggestions concerning the design of Rogowski coils to be used as primary current transducers in negative sequence circuits. For voltages up to 1000 V, Rogowski coils with magnetic core and air gap should be used. In such applications the coils can be of minimum size and weight, the maximum dimension being no greater than 5 cm. The standard single-phase transformer cores (for transformer capacity up to 20 VA and voltages up to 400 V) can be used. The new primary winding of thinner wire with greater number of turns is used. The bus which carries the measured current goes through the transformer core. The air gap and the number of turns in the winding determine the mutual inductance M between the winding and the bus, as well as the inductance L of Rogowski coil winding. The relation between the values is as follows: the decrease of the gap and the increase in cross-section of the core and number of turns result in the increase of M and L. The calculation of such Rogowski coil is described in source [9]. The Rogowski coils for protection of 400 V induction motors (e.g., boring machines in open mines) should be designed as transformer reactors with magnetic core having air gaps. In this case Rogowski coils can have very small physical dimensions. The calculation of such coils is not much different from the traditional calculation used for ordinary a.c. reactors. Let us assume that 6. . .35 kV lines and loads are used at the mine in question. In this case Rogowski coils should be of another design: no magnetic core, the coil is placed around the bushing of the respective conductor in which the current is measured. The power being equal, the consumer current is inversely proportional to the voltage. Therefore when the voltage changes from 400 V to 6 kV the consumer current will decrease by a factor of 15. The mutual inductance between the Rogowski coil and the conductor must in this case increase by a factor of 15. Some new designs of Rogowski coils can be used for such applications. They have printed-type winding located on a flat ring. The inner diameter of the ring is larger than the outer diameter of the bushing. The main disadvantage of this design is small cross-section of the winding. The e.m.f. in such cases (tens or hundreds of mV) is insufficient for direct connection (without amplifiers) of Rogowski coils to negative sequence voltage filter which is connected to electromechanical relay. In order to make possible operation of electromechanical protections or combined type protections, it would be better to avoid the use of amplifiers and to design Rogowski coils that allow for direct connection to negative sequence voltage filter. At voltages up to 6 kV it is better to use Rogowski coils placed on bushings of the equipment (circuit breakers or voltage transformers). The coils are comprised of short solenoid sections placed
on a flexible hollow cylindrical core. The sections are divided by gaps that are shorter than the sections. The ends of the core are united on the bushing during the coil installation process. The sections have a two-layer winding. This type of Rogowski coil is described in the source [10]. All negative sequence current transducer designs are linear systems where the positive and negative sequences of three-phase currents are independent. Therefore in the general case when the three-phase current has both of the sequences the output voltage of the transducer is proportional to the negative sequence current. For microprocessor modules used in the protective relaying systems it is necessary to carefully choose the parameters of negative sequence current transducers basing on traditional recommendations relying on theory of electric circuits and protective relaying. Therefore choosing the parameters of components included into the negative sequence current transducers represents an important optimization problem. This problem must be solved step by step (depending on the characteristics of relays connected to the measurement circuit). In case of parallel connection of electromechanical and microprocessor-based relays to negative sequence current transducers (see Fig. 3) the total resistance of the relays is given by Rtot ¼ ðRMP REM Þ=ðRMP þ REM Þ, where RMP and REM – resistances of microprocessor and electromechanical relays respectively. The calculation of permissible loads in current circuits of protective relaying equipment is one of the stages in design of protection and control systems for utilities. Such sources as [11] provide a number of methods for calculation of parameters of current transducers for relaying needs. However, if Rogowski coils are used as primary transducers, such calculation methods do not always allow determining the true relationships between the parameters of negative sequence voltage filter components and coil parameters. This becomes even more difficult when such parameters for Rogowski coil based negative sequence current transducers are to be calculated for distribution networks at open mines where transient currents can be very large due to increased capacity of machines and equipment in such environments. For open mines it is also true that loads in current circuits of protective relaying equipment become larger due to longer wiring and larger number of protection devices. The effectiveness of calculation algorithm for elements of instrument negative sequence current transducers is very important when it comes to analysis and, especially, optimization of combined operation of electromechanical and microprocessor-based relays. The obtained relations between parameters of elements in current transducers connected to Rogowski coils not only allow to calculate reactances, resistances and capacitances in the negative sequence voltage filter, but can also be used to quickly determine the parameters of Rogowski coils. This is particularly useful when the variable parameters during analysis and optimization are the non-changeable parameters of relays connected to transducers (such as relay resistances). If the relay resistance is known, it is possible to determine all parameters of the negative sequence voltage filter and its output signal (which will cause both the electromechanical and microprocessor-based relays to operate) can be predicted in advance. The following consecutive steps can be used for choosing the parameters of instrument current transducers: 1. The main input parameter for calculation is the output resistance of voltage filter, i.e. equivalent resistance of microprocessor and electromechanical relay inputs connected in parallel. Therefore, at first it is necessary to determine the input resistance Rng (from nameplate data) of electromechanical or microprocessor-based relay connected to instrument current transducer as load of negative sequence voltage filter (if the
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
relays are connected in parallel the total resistance must be determined using the formula given earlier). 2. The relay operating settings must be calculated or selected using recommendations provided by the relay manufacturer [11]. 2.1. If the instrument negative sequence current transducer with 5-element voltage filter is used, the relay time delay of approximately 0.02 s must be introduced [11]. Microprocessor-based relays are very sensitive to overvoltages on relay input (relays may be damaged if the overvoltage is too high). Therefore for such applications it must be considered that the voltage produced by current transducers at open phase conditions depends on currents of the protected induction motor, and such currents can be 8 times the nominal current. 2.2. If the current transducer with 2-element voltage filter is used, the relay time delay, as in the previous case, must be approximately 0.02 s. The maximum output voltages on current transducer output at open phase conditions are 30% higher than those for the 5-element filter design. This fact must also be considered if a microprocessor-based relay it used. 3. Resistances must be calculated which can be further used to determine the parameters of the voltage filter: 3.1. For current transducer with 5-element voltage filter the resistance R2 is given by: R2 ¼ 0:5Rng . 3.2. For current transducer with 2-element voltage filter the resistance R is given by: R ¼ Rng . 3.3 The resistors must be selected basing on the preliminary values of R2 and R calculated at steps 3.1 and 3.2. The nominal values of the resistors must be larger than the obtained values (closest in the standard series). The power rating of the resistors is calculated from the resistance values and output voltage of the instrument negative sequence current transducer. The selected power rating must be no less than the power rating values obtained in calculation. 4. The preliminary value of parameter m must be selected. The parameters of negative sequence voltage filter depend on Rogowski coil impedances while the mutual inductance between the coils and the conductors is determined by the parameters of the filter. So the further calculation of instrument negative sequence current transducer is iterative. At first the approximate value of parameter m should be defined. During the further iteration this value will be corrected. For instrument negative sequence current transducers in 6. . .35 kV networks the value m = 0 can be selected. In other cases this parameter must be defined on the basis of relationships given earlier in this paper. 5. The size, number of turns, resistance and reactance of Rogowski coils must be defined (as described in [9] for voltages up to 1000 V; as described in [12] for voltages over 35 kV; as described in [13,14] for voltages 6–35 kV). 5.1. The calculated value of negative sequence current transducer output voltage Urat and phase current Irat at open phase condition further allow to determine the required value of mutual inductance between Rogowski coil A with the respective conductor
MA ¼
U rat e Irat x
ð1Þ
111
where per unit e.m.f. value e for 5-element negative sequence voltage filter is given by: e¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi 2 pffiffiffi 1 2 3 þ 6 m mng 3 mng þ 6 þ 6 mng þ 3 3 mng at mng ¼ 0:5, 6 and
the same value for 2-element filter is given by: ffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi pffiffiffi 2 at mng ¼ 1 e ¼ 3 ð3mng ðm 1ÞÞ2 þ ð4 3 þ 3 3mng Þ (these values of parameter mng must be chosen for the calculation of the respective instrument negative sequence current transducer to ensure the best operation condition of the transducer. This empirical data was obtained during the investigations carried out at Far Eastern Federal University (Vladivostok, Russia)). In both of these cases the parameter m equals 0 (for Rogowski coils at voltage of 6. . .35 kV). 1 6
5.2. For instrument negative sequence current transducer with 5-element voltage filter the identical Rogowski coils are selected for both phases. For instrument negative sequence current transducer with 2-element voltage filter the mutual inductance of Rogowski coil for phase B is less by a factor of 2 than the value given by formula (1). 5.3. Then the calculation of Rogowski coil windings is to be done: choosing the wire type, diameter of the window, centerline radius and tube diameter of the toroid, as well as number of turns in the windings (for Rogowski coils rated for voltages over 6 kV). For Rogowski coils with voltages up to 1000 V the cross-section, the frame and the air gap of the core are to be calculated and the number of turns and size of the coil are to be determined. This step in the calculation must ensure such parameters that will provide the lowest possible values of both the weight of the coil and the parameter m. Resistance Rk and inductive reactance X k are to be calculated for the Rogowski coil of phase A. In 6. . .35 kV systems this calculation step may be omitted. 5.4. The value of m must be corrected using the following formulas: m ¼ X k =R2 for 5-element negative sequence voltage filter and m ¼ X k =R for 2-element negative sequence voltage filter. 6. The elements of negative sequence voltage filter must be further calculated: 6.1. For 5-element voltage filter design the reactances and resistances of the remaining filter elements are to be calculated: pffiffiffi pffiffiffi X 1 ¼ ð1 þ mÞ R2 , X 2 ¼ R1 ¼ 3 R2 , R3 ¼ m R2 3. 6.2. For 2-element voltage filter design the reactance of the pffiffiffi capacitor is to be calculated: X ¼ ð1 þ mÞ 3 R. 6.3. The obtained X, X1, X2, R1 and R3 values must be used to select capacitors and resistors (their nominal values must be closest to the obtained values). These nominal values and the output voltage of the instrument negative sequence current transducer must be further used to define the power rating of resistors and the capacitor voltage rating. The capacitors whose capacitance value is close to the defined value and voltage rating is larger than the defined value must be selected. The adjustable resistors with power ratings of no less than the defined ratings must be selected. The parameters of elements in the instrument negative sequence current transducers for the actual resistances of microprocessor-based or electromechanical relays (and in combined designs) can be calculated using software written for PC.
112
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
The calculation method described above is realized in the calculation application «IPTOP» written in Pascal. The block diagram of the algorithm is shown in Fig. 4. The algorithm combines the following steps: (1) Entering data into unit 1 (resistance Rng which includes the resistances of electromechanical and/or microprocessor-based relays; variant of instrument negative sequence current transducer used; approximate value of m); (2) Defining type of instrument negative sequence current transducer (with 2-element or 5-element voltage filter); (3) Entering voltage filter load (resistances of electromechanical and/or microprocessor-based relays); (4) Calculating the total resistance of electromechanical and/or microprocessor-based relays; (5) Calculating ratings of resistors (R – for current transducer with 2-element voltage filter and R2 – for current transducer with 5-element voltage filter); (6) Calculating the power rating of these resistors; (7) Calculation of mutual inductance between the Rogowski coil in phase A and the respective conductor for open-phase condition;
Table 1 Number of electric equipment failures due to open-phase conditions at ore processing plants «Novoshakhtinskoye» and «Luchegorskoye». Type of protection
Distribution boards with protections obtaining primary data from new filters (38 loads with induction motor drives) Distribution boards with protections obtaining primary data from filters with conventional current transformers (39 loads) Distribution boards with no protection against open-phase condition (35 loads)
Years 2010
2011
2
3
2012 2
2013 3
7
8
5
6
17
25
20
18
(8) Calculating the mutual inductances for Rogowski coils in phase A and phase B; (9) Calculating the parameters of Rogowski coil windings; (10) Correction of parameter m. If for new value of m the per unit e.m.f. value e is much different from the earlier obtained value (more than 5%), the elements must be recalculated; (11) Comparing the obtained value e with the setting; (12) Calculating values of capacitors and resistors in voltage filter; (13) End of program and output of calculation results. The current transducer parameter calculation algorithm (Fig. 4) can be simplified: if it is known in advance which of the voltage filter variants will be used (2-element or 5-element), step 2 in the algorithm is not necessary. The algorithm written in Pascal can be also used to calculate the parameters for combined electromechanical and microprocessor relaying applications. Results The tests of the transducer prototypes designed in 2010–2013 have been carried out at ore processing plants «Novoshakhtinsko ye» and «Luchegorskoye» (Primorsky Krai, Russia). Each distribution board controls a similar number of similar types of loads involved in similar technological processes. The results of tests are given in Table 1. These results show that the use of symmetrical component filters obtaining primary data from Rogowski coils can decrease the yearly average number of equipment outages due to open-phase conditions by 50%. Discussion It must be mentioned that failure to operate in protections in the first group was basically due to personnel errors. It is known that problems may arise as a result of traditions and poor knowledge of up-to-date protective relaying among the personnel; additional training is always required to introduce new types of equipment in a power utility. Failure to operate in protections in the second group was mostly due to distortion of signals obtained from primary transducers which can cause failures or may even result in damage of protection relays during operation. Conclusions
Fig. 4. Block diagram (algorithm) of calculating parameters for instrument negative sequence current transducer with 2- and 5-element voltage filter.
It can be concluded that new symmetrical component filters obtaining primary data from Rogowski coils can provide a high degree of selectivity. They can be directly connected to analog-to-digital converters and have smaller total weights. They are also more reliable and provide better accuracy in comparison with filters used with conventional current transformers.
D.B. Solovev, A.S. Shadrin / Electrical Power and Energy Systems 73 (2015) 107–113
References [1] Kojovic L. Rogowski coils suit relay protection and measurement. IEEE Comput Appl Power 1997;10(3):47–52. [2] Guihua Weng, He Jiang. Measurement of medium and high-voltage system bus current based on Rogowski coil. Electrician Electr 2009;2:43–7. [3] Kojovic L, Bishop MT. Field experience with differential protection of power transformers based on Rogowski coil current sensors. Actual trends in development of power system protection and automation 7–10 September 2009, Moscow, Russia. [4] Ramboz JD. Machinable Rogowski coil, design, and calibration. IEEE Instrum Meas 1996;45(2):511–5. [5] Marinescu A. A calibration laboratory for Rogowski Coil used in energy systems and power electronics. In: IEEE optimization of electrical and electronic equipment (OPTIM), 2010 12th international conference on 20–22 May 2010, conference location: Basov; May 2010. p. 913–9. [6] Ramoz JD, Destefan DE, Stant RS. The verification of Rogowski coil linearity from 200 A to greater than 100 kA using ratio methods. In: Proc of IMTC, vol. 1; 2002. p. 687–92. [7] Kojovic L, Colopy C, Arden D. Volt/var control for smart grid solutions. CIRED 2011, Frankfurt, Germany; June 9–11, 2011.
113
[8] Shafiq Muhammad, Amjad Hussain G, Kütt Lauri, Lehtonen Matti. Effect of geometrical parameters on high frequency performance of Rogowski coil for partial discharge measurements. Measurement 2014;49:126–37. [9] Zhang Yu, Liu Jinliang, Bai Guoqiang, Feng Jiahuai. Analysis of damping resistor’s effects on pulse response of self-integrating Rogowski coil with magnetic core. Measurement 2012;45(5):1277–85. [10] Draxler K, Styblikova R. Determination of Rogowski coil constant. In: IEEE applied electronics, 2006. AE 2006. International conference on 6–7, September 2006; September 2006. p. 241–3. [11] Winders JJ. Power transformers: principles and applications. New York, Basel: Marcel Dekker, Inc., 2002. p. 286. ISBN: 0-8247-0766-4. [12] Draxler K, Styblikova R, Hlavacek J, Prochazka R. Calibration of Rogowski coils with an integrator at high currents. IEEE Trans Instrum Meas 2011;60(7):2434–9. [13] Mariscotti A. A Rogowski winding with high voltage immunity. IEEE; January 2009. p. 1129–34. [14] Muzamir IS. Partial discharge location technique for covered-conductor overhead distribution lines. PhD thesis published in 2013 by School of Electrical Engineering and The department of Electrical Engineering, Aalto University.