Read Microsoft Word - CIRED inrush final.doc text version

CIRED

18th International Conference on Electricity Distribution

Turin, 6-9 June 2005

PRACTICAL ASPECTS OF TRANSFORMER INRUSH CURRENT CALCULATION USING TWO SOFTWARE PACKAGES

J.G. SLOOTWEG*, C.P.J. JANSEN** *Essent Netwerk B.V., P.O. Box 856, 5201 AW, 's-Hertogenbosch **KEMA T&D Consulting, P.O. Box 9035, 6800 ET, Arnhem The Netherlands [email protected], [email protected] SUMMARY In the paper, the simulation of the energizing of transformers and more specifically the calculation of inrush currents is dealt with. First, the physical origin of inrush currents is discussed. Then, two different transformers, namely a 20 MVA 10.6/66 kV and a 160 MVA 66/220 kV transformer, are each modelled in two different software packages, namely ATP and DIgSILENT Power Factory. The models are described elaborately and the values of all relevant quantities are given. The models are used to simulate the energizing of the studied transformers from a generator and to investigate the impact of various quantities, such as remanent flux and switching instant. Finally, the simulation results are compared with measurements and it is observed that a satisfactory level of correspondence has been achieved. INTRODUCTION The working principle of a transformer is based on the presence of a magnetic flux, that couples the transformer windings. The transformer's iron core, that carries the flux and is hence referred to as the magnetic circuit, is dimensioned for flux densities occurring during normal operation. However, when the transformer is connected to the grid after standstill, it must be magnetized, or energized, and flux densities much higher than the values during normal operation may occur. These currents are referred to as inrush currents. In practice, inrush currents may pose problems for several reasons [1, 2]: overvoltages, it is important to be able to simulate the switchon of a transformer accurately. This is, however, not very straightforward, because it requires more advanced simulation approaches than used in routine network analysis and because more elaborate data than the usual name plate data of the transformer must be available. The investigation of transformer energizing and the calculation of inrush currents form the subject of this paper. It is organized as follows. First, the physical origin of inrush currents is shortly commented upon. Then, the topology and characteristics of the investigated system and its modelling in ATP and DIgSILENT Power Factory are described. Finally, simulation results are presented and compared with measurements. ORIGIN OF INRUSH CURRENTS The working principle of a transformer is governed by Faraday's and Ampère's laws [3-5]. Faraday's law states that induced voltage is equal to the derivative of flux linkage, or, stated differently, that the coupled flux equals the integral of the applied voltage. Ampère's law relates the flux linked by a coil to the current flowing through it. During normal operation, the transformer current and voltage, as well as the flux in the iron core, vary sinusoidally. When the transformer is not loaded, i.e. one of the terminals is open, the phase shift between voltage on the one hand and flux and current on the other, is approximately 90 degrees. Assume now that the transformer is idle and a voltage is applied. When the presence of remanent magnetism is neglected and the connection happens at the instant at which the flux would equal zero during normal operation, i.e. at the voltage's maximum, this does not give rise to any exceptional current or flux. The flux is the integral of the applied voltage and is exactly equal to what it would be if this had not been the first period of the applied voltage. However, when the transformer is connected at the moment that the voltage equals zero, the flux will reach twice its normal height when integrating the voltage. Had the transformer already been connected, the integration would have started from the negative maximum of the flux and end at the positive maximum of the flux. However, because this is the first period of the voltage, the integration starts at zero flux and the flux reaches twice the maximum value occurring during normal operation. Because these high flux densities, the iron core is driven into saturation. Therefore, the transformer starts to exhibit nonlinear behaviour. As a result, the current is no longer

· ·

Protection schemes may act upon the amplitude and/or the shape of the transformer inrush current, mistakenly assuming that the observed currents are caused by some fault. When a transformer is energized from a weak grid or a single generator, the voltage drop caused by the large inrush currents may be such that undervoltage protection schemes, if present, are triggered and again disconnect the transformer that is just connected.

·

Due to their non sinusoidal shape and the presence of higher harmonics, inrush currents can lead to resonance overvoltages caused by the transformer non-linear impedance and the capacitance of the line or cable connected to it. In order to be able to prevent the occurrence of unintended protection system actions as well as damage due to resonance

CIRED2005 Session No 1

CIRED

18th International Conference on Electricity Distribution

Turin, 6-9 June 2005

approximately proportional to the flux. Instead, much more current is needed to increase the flux and the current can easily reach much more than two times its maximum. This large current is referred to as the inrush current. As stated earlier, inrush currents can lead to undesirable effects, such as the triggering of protection devices and resonant overvoltages. The presence of remanent magnetism can increase this effect, leading to even larger inrush currents. MODELLING System description The system studied in this paper consists of transformers and generators in island operation, as depicted in figure 1. The generators have a rating of 34 MVA and 50 MVA at a voltage of 10.6 kV. The first generator is connected to a 66 kV busbar through a 26.5 MVA 10.6/66 kV transformers. It is driven by a gas turbine. The second generator is connected to the same 66 kV busbar through a 50 MVA transformer. To investigate the inrush phenomenon and to validate the developed models, firstly a second 26.5 MVA 10.6/66 kV transformer and then a 160 MVA 66/220 kV transformer is connected to this 66 kV busbar. In the first case, only one generator is in operation, whereas in the second case, both are in operation. The relevant parameters of the generator are given in table 1 and of the transformers in table 2. The parameters not given in the generator documentation have been estimated and are marked with *. The one line diagrams of the systems are depicted in figure 1.

Table 1. Generator parameters Value Quantity Generator Generator 1 2 Snom 34.083 50 MVA MVA Unom 10.5 kV 10.5 kV Xl Rl Xd Xd' Xd'' Xq 0.10 p.u.* 1.481 mp.u. 2.007 p.u. 0.194 p.u. 0.131 p.u. 1.8 p.u.* 0.10 p.u.* 1.317 mp.u. 1.910 p.u. 0.226 p.u. 0.151 p.u. 1.72 p.u.* Quantity Value Generator Generator 1 2 0.5 p.u.* 0.5 p.u.* 0.131 p.u.* 0.075 p.u. 6,54 s 0,04 s* 0,50 s* 0,15 s* 0.151 p.u. 0.085 p.u. 5.49 s 0.018 s 0.04 s* 0,15 s*

34.0 MVA 10.5 kV

~ G

20.0 MVA 10.6/66.0 kV

20.0 MVA 66.0/10.6 kV

Transformer 1 10.5 kV

50 MVA 10.5 kV

~ G

Transformer 2 66.0 kV 10.5 kV

50 MVA 10.5/70.5 kV

Transformer 3 10.5 kV

160 MVA 66.0/220.0 kV

Transformer 4 34.0 MVA 10.5 kV

~ G

20.0 MVA 10.6/66.0 kV

220.0 kV

Transformer 1 10.5 kV 66.0 kV

Figure 1. Investigated systems

ATP Model The ATP model was developed using ATPDraw. The generators were modelled as synchronous generators using the model SM59 for which all relevant parameters are given in table 1. The transformers were modelled using the BCTRAN dialog box. For those transformers connected directly to the generators, which were assumed to be already energized at the start of the simulation, all nonlinearities were neglected. In case of the transformer to be energized, the non-linear behaviour was modelled using the pseudo-nonlinear hysteretic inductor component Type 96 [6]. The -I characteristic of the nonlinear inductor proved critical for matching the simulation results with the measurements. However, it was unfortunately not available for the investigated transformers. Therefore, it was calculated using the supporting routine HYSDAT that comes with ATP. The values for and I to be inserted in HYSDAT were derived using the "View+" function of the BCTRAN dialog box. It was assumed that the positive saturation point, to be inserted in HYSDAT, lied at a voltage of 1.18 p.u., and the corresponding no-load current was assumed to equal 10 times that at a voltage of 1.1 p.u. The assumption of a factor 10 increase in the current was based on measurements taken from one of the transformers, of which no-load measurements up to a voltage of 1.18 p.u. were available. These were used to extrapolate the value for the other transformers of which only measurements up to a voltage of 1.1 p.u. were available. Power Factory Model In Power Factory the generators were modelled using the standard synchronous machine model. The parameters for the generators are given in table 1. The transformer model in Power Factory used for the inrush simulations, is the two winding transformer model shown in figure 2 [7]. The model contains the leakage reactances at the low and high voltage

Xq' Xq'' X0 Td0' Td0'' Tq0' Tq0''

Table 2. Transformer parameters Quantity Transformer 1, 2 Snom 20 MVA UHV 66 kV ULV 10.6 kV Uk 14.7 % PCu 87.6 kW P0, 100% 10.3 kW I0, 100% 0.77 mp.u.

Value Transformer 3 50 MVA 70.5 kV 10.5 kV 11.0% 116 kW 31.3 kW 2.3 mp.u.

Transformer 4 160 MVA 220 kV 66 kV 17.8 % 459 kW 71.4 kW 1.04 mp.u.

CIRED2005 Session No 1

CIRED

18th International Conference on Electricity Distribution

Turin, 6-9 June 2005

side together with the winding resistance at low and high voltage side. The model also shows the magnetizing reactance Xm and the ion losses resistance RFe.

Rprim Xprim Rsec Xsec

Uprim XM RFe n1 : n2

Figure 2. Transformer model in Power Factory

Usec

To model the effect of inrush currents, the magnetizing reactance must be modelled as a function of the magnetizing current. For that, the magnetic flux is given as a function of the magnetizing current in figure 3.

knee

saturated

unsaturated

As can be seen from the simulations, the shape and amplitude of the inrush currents depend strongly on the instant of connection for both transformers. The cause of this is the physical mechanism that causes the occurrence of the inrush current, which makes them dependent on voltage at connection instant as well as on remanent magnetism, as described earlier. Further investigations with ATP on the impact of the presence of remanent magnetism (which was created by first starting up the simulation with the transformer connected and then disconnecting and again reconnecting it) have shown that when this was included, the simulation results varied even more widely [8]. However, in the rest of this paper, remanent magnetism is not included in the ATP simulations in order to reduce the number of quantities to be varied. The simulations with Power Factory also showed that the inrush current is dependent on the instant of connection and on the assumed amount of remanent magnetism. In the Power Factory model, the instant of connection and the amount of remanent magnetism were tuned together with the value of the slope of the saturated flux (figure 3) in order to get the best possible agreement between the simulation results and the measurements described in the next section. The tuning showed that the amount of remanent magnetism and the value of the saturated flux were up to a certain extent interchangeable.

150 [A] 100 50 0 -50 -100 -150 0.01 200 [A] 150 100 50 0 -50 -100 -150 -200 0.015

iM

Figure 3. Flux saturation model in Power Factory

The unsaturated part of the curve is determined by the simulation software from the no-load characteristics of the transformer. The saturated part and also the knee flux, the normal operating point of the transformer, is given by the -I characteristic of the transformer. As already mentioned, this characteristic was not known for the transformers. For the knee flux a typical value 1.04 p.u. is used. For the ratio between the gradient of the saturated and unsaturated curve, a typical value of 500 is used. This value is tuned to get a better match between simulated and measured curve of the inrush current during the simulations. To model the remanent magnetism in Power Factory, the value of the transformer magnetizing flux is set at a predefined value (between 0 and 0.7 p.u.) at the start of a simulation. SIMULATION RESULTS AND MEASUREMENTS Simulation results Now, simulation results obtained with the models described earlier are presented. Figures 4 and 5 show simulation results obtained with ATP. Figures 6 and 7 show simulation results obtained with Power Factory. In figures 4 and 6, the phase currents for the switching of the 26.5 MVA transformer (transformer 2) for two different instants are depicted. In figures 5 and 7, the phase currents for the switching of the 160 MVA transformer (transformer 4) for two different instants are depicted. CIRED2005 Session No 1

0.03

0.05

0.07

c:X0055A-X0051A

0.09

(f ile SW_20_2.pl4; x-v ar t) c:X0055C-X0051C

c:X0055B-X0051B

[s] 0.11

0.035

0.055

0.075

c:X0055A-X0051A

0.095

(f ile SW_20_2.pl4; x-v ar t) c:X0055C-X0051C

c:X0055B-X0051B

[s] 0.115

Figure 4. Simulation of the connection of transformer 2 with ATP

CIRED

2000 [A] 1500 1000 500 0 -500 -1000 -1500 -2000 0.008 1500 [A] 1000 500 0 -500 -1000 -1500 0.008

18th International Conference on Electricity Distribution

2000.00

Turin, 6-9 June 2005

1000.00

0.00

-1000.00

-2000.00

0.00

25.00

50.00

75.00

[ms]

100.

0.028

0.048

0.068

c:X0024C-X0001C

0.088

(f ile SW_160_2.pl4; x-v ar t) c:X0024B-X0001B

c:X0024A-X0001A

[s] 0.108

1500.00 1000.00 500.00 0.00 -500.00 -1000.00 -1500.00

38-1T261: Phase Current A/LV-Side in A 38-1T261: Phase Current B/LV-Side in A 38-1T261: Phase Current C/LV-Side in A

0.028

0.048

0.068

c:X0024C-X0001C

0.088

(f ile SW_160_2.pl4; x-v ar t) c:X0024B-X0001B

c:X0024A-X0001A

[s] 0.108

0.00

25.00

50.00

75.00

[ms]

100.

Figure 5. Simulation of the connection of transformer 4 with ATP

150.00 100.00 50.00 0.00 -50.00 -100.00 -150.00

38-1T261: Phase Current A/LV-Side in A 38-1T261: Phase Current B/LV-Side in A 38-1T261: Phase Current C/LV-Side in A

Figure 7. Simulation of the connection of transformer 4 with Power Factory

Comparison of simulations and measurements In figure 8 and 9, two measurements of the connection to the 66 kV busbar are shown for transformer 2 and transformer 4. When these figures are compared to figures 4 and 5 and to figures 6 and 7 respectively, it can be concluded that for transformer 2 both the amplitude and shape of the measured inrush currents are quite similar to the simulation results obtained with ATP and Power Factory. For transformer 4, the observed discrepancies both with respect to shape as well as to amplitude are larger, particularly in ATP, but to a lesser extent also in Power Factory. It proved not feasible to get a better match. Explanations for this observation could be the that the magnetizing characteristic of the iron differs more from the assumed characteristic than in case of transformer 2 or that the effect of the simplifications in the transformers model (e.g. neglecting the third winding of transformer 4, which was not present in transformer 2) negatively affect the similarity between simulation results and measurements. Further, in some cases the phases had to be exchanged to get a better correspondence. This is, however, not seen as a major weakness of the model, because in practice it does not matter very much which phase exhibits which behaviour, as long as the behaviour of each of the phases in the measurements is reflected in the simulation results as well.

0.00

25.00

50.00

75.00

[ms]

100.

38-2T02: Phase Current A/HV-Side in A 38-2T02: Phase Current B/HV-Side in A 38-2T02: Phase Current C/HV-Side in A

200.00

100.00

0.00

-100.00

-200.00

0.00

25.00

50.00

75.00

[ms]

100.

38-2T02: Phase Current A/HV-Side in A 38-2T02: Phase Current B/HV-Side in A 38-2T02: Phase Current C/HV-Side in A

Figure 6. Simulation of the connection of transformer 2 with Power Factory

CIRED2005 Session No 1

CIRED

18th International Conference on Electricity Distribution

Turin, 6-9 June 2005

Figure 8. Measurements of the connection of transformer 2

how inrush currents can be simulated using two state of the art power systems simulation software packages. Particular attention was paid to the modelling of the transformer including magnetic saturation: it was indicated that in most cases not all necessary data is available, so that either measurements must be available to tune the model or assumptions must be made. In the latter case, the modelling approach presented in the paper could be used advantageously for formulating these assumptions. Simulation results obtained with the developed models were presented and it was shown that the shape and the amplitude of the inrush currents strongly depend on the connection instant, as well as on the assumptions with respect to the amount of remanent magnetism. Finally, the simulation results were compared with measurements. It was concluded that the degree of correspondence was satisfactory, although differences remain, which vary from case to case. Factors explaining these could be discrepancies between the shape of the modelled and the real magnetizing characteristic and the impact of remanent magnetism. Overall, the conclusion is that the modelling of inrush currents is a complex task, because of a lack of appropriate data and due to the strong nonlinearity and the stochastic nature of the phenomenon. In order to cope with the latter, always a number of simulations should be run to investigate the extremes in the obtained results. Further, it can be concluded that investigations of inrush currents purely based on simulations should be handled with care, because significant errors can easily occur due to the great sensitivity of the results for changes in the input parameters. ACKNOWLEDGEMENT Delesto B.V. is acknowledged for carrying out the measurements, providing the equipment data and financially supporting the work reported in the paper. REFERENCES [1] M.M. Adibi, 1992, R.W. Alexander, B. Avramovic, "Overvoltage control during restoration", IEEE Transactions on Power Systems, v. 7, n. 4, pp. 1464-1470. [2] C.P. Cheng, S. Chen, "Simulation of resonance overvoltage during energization of high voltage power network", International Conference on Power System Transients, New Orleans. [3] M.J. Heathcote, 1998, The J&P Transformer Book, Reed Educational and Professional Publishing, Oxford. [4] J.J. Winders, 2004, Power Transformers, Marcel Dekker, New York. [5] http://www.allaboutcircuits.com/vol_2/chpt_9/12.html [6] Can/Am EMTP User Group, 1995, Rule Book-Alternative Transient Program, Oregon. [7] DIgSILENT, 2004, Power Factory Version 13; User Manual, Gomaringen. [8] M. Rioual, C. Sicre, 2001, "Energization of a no-load transformer for power restoration purposes: Impact of the sensitivity to parameters", International Conference on Power System Transients, Rio de Janeiro.

Figure 9. Measurements of the connection of transformer 4

CONCLUSIONS In this paper, a practical approach towards the simulation of transformer inrush currents was discussed. First, the physical origin of inrush currents was described. Then it was discussed CIRED2005 Session No 1

Information

Microsoft Word - CIRED inrush final.doc

5 pages

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

51514


You might also be interested in

BETA
RTAA_SB.book
US_Buch_Power_Academy.indb
Protective Relay Application Guide