Simulation of Transformer Inrush Current Associated with the Residual Magnetism in the Core
2. PSCAD/EMTDC Study
When energizing a transformer there is a residual magnetism in the core named “remanence”.
The degree of magnetizing inrush current during energizing is a function of the position on the supply voltage wave shape that each phase of the closing circuit breaker actually closes on and the remanence existing in the legs of the transformer core. Remanence is determined by the conditions associated with the de-energized of the transformer that may not be known [2].
It is useful to anticipate the worst conditions that might be expected on any random energization. Usually, the maximum remanence that might exist in any leg of the core is about 80% of the peak flux linkages generated at rated volts. This is determined from the rated r.m.s. voltage Vr of the winding that is being referenced for remanence [2].
A possible way to set a desired remanence with PSCAD/EMTDC in the un-energized transformer is with controlled current sources. The case is run with the circuit breaker open and the current sources in each phase are adjusted to generate the require remanence. The current sources can remain in the circuit at their remanence setting during the run as they do not impact the results [2]. This study presents the results of a study about the worst-case remanence and its impact in the magnitude of inrush current.
The following criteria have been applied when preparing this study:
worst case configurations with/without remanent flux; different models and different parameters for the transformer have been employed for the simulations;
the residual flux can be varied in the rage of ±0.9 p.u. Basically it is determined by adjust the current source in each phase.
A summary of these conditions is presented in Table 1.
Table 1. Transformer models and simulation conditions
TRANSFORMERMODELS SIMULATIONCONDITIONS
With remanent flux Without remanent flux LARGETRANSFORMER*
SMALLTRANSFORMER*
Simultaneous
closing Different
switching times Simultaneous
closing Different switching times
* 1. Three-phase bank (core type) transformer model.
2. Three-phase two-winding transformer model.
The computer models developed for investigating the inrush inrush current consists of a 230kV Wye / 66kV Delta transformer with a 100 MVA capacity, unloaded (secondary open), and a laboratory transformer with the following parameters: 208 V Wye / 208 V Delta and 30kVA capacity. The switching is performed on the Wye side. These parameters were chosen for testing the behavior of both large and small transformers.
Transformer modeling, especially for study the transient phenomena is quite complicated. A general model for using to all frequency and modeling all characteristics of the transformer is not available in PSCAD/EMTDC. In the following the most appropriate models for inrush current study are presented. In PSCAD/EMTDC there are two basic transformer models.
a). The UMEC model has provision for specifying the configuration of the core in single and three phase units. The UMEC transformer model allows for the correct core configuration.
This model allows introducing the saturation curve. Under normal excitation a transformer draws a magnetizing current of between 0.2 and 2% of its rated current. Using this model the description of the process occurring when a three-phase transformer is energized is less simple because the three phases are connected both electrically and magnetically.
b). The second model is the single phase units model. An ideal transformer means zero magnetizing current and no resistive coupling. When a ideal transformer is modeled with the inverted inductance matrix entered into the DATA input file, the magnetizing current and core losses are not represented. Core and winding losses can be neglected because of the little significance to results. In reality, a real transformer operated on no-load will draw some real power from the supply the so-called no-load power or core losses. These losses consist of the core hysteresis losses and eddy current losses. But, losses in the transmission system external to the transformer tend to dominate. The models require that there is leakage reactance and the concept of a fully ideal transformer without leakage reactance is not possible on PSCAD. Due to the double precision calculations of EMTDC, low values of leakage reactance (0.001 to 0.01 per unit) would solve satisfactorily. If the leakage reactance is set to 0.0, the transformer model may actually run, but it may become numerically unstable. The knee point of the saturation curve is usually expressed in percent or per-unit of the operating point defined by rated voltage.
Typical ranges in per-unit are 1.15 to 1.25. Another feature is control over the inrush current decay time. Argument value TD ensures there is a decay time imposed on inrush current such as would occur on energization or recovery from a closed by fault. Inrush current decay realistically if the values 0.5 < TD < 2.0 is used. In our simulations TD = 1.0 is used. In this case the inrush current will fade within one or two seconds. The rate of decay of inrush current is controlled by losses internal and external to the transformer and it is easy to set the rate by this method than in trying to change losses in the model. Minimum „No load loss” parameter is automatically to the value 0.1% if saturation is selected.
In the system under study the maximum transformer inrush current was measured with the circuit presented in Figure 1.
Figure 1
The circuit for measure the maximum transformer inrush current
The oscillograms of the inrush current (phase a), the specified phase flux linkage and the maximum inrush current are presented in Figure 2 for the case without residual flux.
Figure 2
Maximum inrush current without residual flux
The polarity combinations of the residual flux +/-/0 and +/0/- have to be checked for analysing the maximum inrush current under the same switching conditions (Figure 3).
Figure 3
The polarity combinations of the residual flux +/-/0 and +/0/-3. Experimental Results
For a qualitative verification of the simulation models, inrush current simulated with the PSCAD/EMTDC software package are compared to the results obtained in the laboratory of University of Alberta, Edmonton Canada. Nicolet BE256LE Data Acquisition system with Team 256 software was used to record the inrush current data. The experiment was conducted
on a test three-phase transformer with the following parameters: 208 V Wye/208 V Delta and 30 kVA. To study the behavior of inrush current while energizing the transformer the measurements were taken with neutral conductor grounded solidly. All tests were performed without a load on the secondary side. The data obtained are plotted in Figure 4 showing the inrush current at different switching angle.
Figure 4
Inrush current across the 30 kVA transformer for different switching angle (phase a)
The experimental results were analyzed in terms of the statistics of the measured inrush current whose compilation is presented in Table 2. The basic experimental switching case is composed of 36 measurements around the clock.
Table 2 Transformer switching field test results.
V angle 0 20 40 60 80 100 120 140 160 180 200 220 240 250 270 300 320 340 360
Inrush
Current 700 450 400 300 350 405 430 520 600 600 520 340 340 60 120 300 450 550 700
A comparison between simulation results and experimetal results is presented in Figure 5 for the experimental transformer data.
4. Conclusions
The main parameters influencing the magnitude of the inrush current are the magnetic saturation characteristic of the transformer and the magnetic flux remanence in the transformer core at the moment of energization, as was demonstrated through simulations. An analysis of the simulation results shows that the energization of the transformer containing a flux remanence is far more severe than the case of a completely demagnetized transformer. If the closing angle is arbitrary it is possible to hit the condition where the inrush current is very high as presented in Table 2 and Figure 4. The magnitude of the inrush current during energization is a function of the position on the supply voltage waveshape that circuit breaker closes on. A good agreement between the simulation and experimental results were obtained.
Random power transformer energization can create large flux asymmetries and saturation of one or more winding cores of the transformer. These currents can cause false operation of protective relays and fuses, mechanical damage to the transformer windings from magnetic forces, and generally reduce power quality on the system. Further investigation is necessary to detemine some practical methods for reducing transformer inrush current.
5. References
[1] Cipcigan, L., Xu, W., Dinavahi, V., A New Technique to Mitigate Inrush Current Caused by Transformer Energization, IEEE Summer Meeting, Chicago, USA, July, 2002, 0-7803-7519-X102.
[2] *** Introduction to PSCAD/EMTDC V3, Manitoba HVDC Research Centre Inc, 2001.
[3] CIGRE Working Group Task Force 13.07 Controlled Switching of HVAC Circuit Breakers, 1st Part Elektra No. 183, April 1999, pp.43-73.
[4] CIGRE Working Group Task Force 13.07 Controlled Switching of HVAC Circuit Breakers, 2nd Part Elektra No. 185, August 1999, pp.37-57.
[5] Lin, C.E., Cheng, C.L., Huang C.L., Yeh J.C., Investigation of magnetizing inrush current in transformer”, Part I - Numerical simulation, IEEE Transactions on Power Delivery, Vol.8, No.1, January 1993, pp.246-254.
[6] Smith, K., Ran L., Leyman B., Analysis of transformer inrush transients in offshore electrical systems, Generation, Transmission and Distribution, IEE Proceeding, Volume 146, Issue 1, June 1999, pp.89-95.
[7] Braun D., Koeppl G., Azuaje C., Borges F., Inrush currents of a large step-up transformer and means of their reduction, , CIGRE Study Committee, C5.1.13, paper 060.
[8] Rico J., Acha E., The study of inrush current phenomenon using operational matrices, Transactions on Power Delivery Vol.16, No.3, July 2001, pp.231-237.
[9] Adly A., Computation of inrush current forces on transformer windings, Transactions on Power Delivery, Vol.37, No.4, July 2001, pp.2855-2857.
[10] Greenwood, A., Electrical Transients in Power Systems, John Wiley & Sons, 1991.