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Optimal gang-operated switching for transformer inrush current reduction
Abstract
Controlled switching technology with independent-pole-operated circuit breakers is an effective way to eliminate transient transformer inrush currents. This technique cannot usually be applied at lower voltage levels where three-pole-operated circuit breakers are more frequent. In this paper, the optimal instant for a simultaneous closing is obtained as a solution of a min-max problem. The proposed strategy has been evaluated in a test system using EMTP/ATP and has presented highly satisfactory results, even when actual characteristics of circuit breakers are taken into account.
... The magnitude of the inrush current depends on the switch point (turn-ON) of the AC input voltage waveform. If the turn-on occurs when the AC voltage wave is at its peak value, there will be no inrush current drawn by the transformer (Cano-Gonz alez et al., 2016). The magnitude of the current in this case will be at a normal value equal to no load value. ...
... When the transformer is energized at zero crossing of the applied input voltage and prospective flux is the opposite of the polarity as residual flux, then the flux value exceeds the normal maximum (Cano-Gonz alez et al., 2016). So, the phase cross between the AC input source side and the load side should be measured first, and then the ARM controller will delay the trigger (switch ON or OFF) signal for 5 msec, which equals half a cycle of AC voltage waveform. ...
... The optimal closing instant is the instant at which residual flux and prospective flux match for the phase with the lowest absolute value of the residual flux, and the polarities of the residual flux and the prospective flux coincide in the other two phases. This strategy has been described in [12] and is illustrated in Fig. 5. ...
... Since the use of independent-pole-operated circuit-breakers represents an additional cost, three-pole-operated circuit-breakers are usually employed at voltage levels below about 145 kV [11]. In these cases, optimal gang-operated switching [12] (strategy S2) has proved to be the most highly recommended strategy, due to the inrush current mitigation obtained and its straightforward application. ...
Transformer inrush current can be efficiently mitigated by controlled switching. For energization of three-phase transformers, several strategies have been developed that depend on whether the residual flux can be measured and whether independent-pole-operated or three-pole-operated circuit-breakers are used. In this paper, a comparative analysis of these strategies has been carried out. Residual flux uncertainty and dispersion of the characteristics of the circuit-breaker can significantly affect the performance of the controlled switching. These issues have been taken into account in the analysis of each strategy since they can influence the performance in different ways. The study has resulted in the selection of the most appropriate strategy for each case.
... CS technique has also been typically approached in the literature with the assumption that the CB phases can be controlled independently. Although such CBs are available and used with dedicated relays [12], there remains a high number of implemented three-pole (3-PL) CBs, especially in medium and low voltage networks [13]. Since many IBRs are integrated in distribution networks and capable of participating in blackstart, industrial efforts are aiming to develop and test solutions Strathclyde, Glasgow, UK, along with Khaled Ahmed and Agusti Egea-Alvarez (email: Khaled.ahmed@strath.ac.uk; agusti.egea@strath.ac.uk). ...
The use of inverters-based resources (IBRs) is rising rapidly in power networks due to increased renewable energy penetration. This requires revisiting of classical network operation standards. For instance, high transformer energization inrush current has been studied extensively under the classical network paradigm. Whereas this paper investigates transformers' energization techniques in the context of inverters dominated grids, where inverters with limited short-circuit current are expected to utilize their grid-forming capabilities for black-start. Common transformer energization techniques such as controlled switching and soft energization are first analyzed with a new perspective aiming to assess their feasibility when used with grid-forming inverters and existing network assets. Parameters influencing soft energization voltage ramp-up time (
) are investigated, and a new
estimation framework for transformer energization from IBRs is introduced. Due to the variability of available point-on-wave circuit breakers (CBs) in distribution networks, controlled energization using single-pole and three-pole CBs is investigated for various configurations and their application limits are identified. A comprehensive case study is then presented using a test network with multiple transformers to benchmark the performance and requirements of each technique when the network is energized from an IBR, followed by a set of practical recommendations.
... For example, electrical protection is a Mini Circuit Breaker (MCB) that protects from overload and short circuit conditions which results in rising conductor temperatures and damage to electrical equipment. The working principle of the MCB is to cut off the electric current if the value of the current through it exceeds the nominal limit [18][19][20]. ...
The amount of electricity used is followed by the magnitude of the risk of disasters caused by power disruptions. In general, PLN as the manager of electricity distribution has provided standard protection for users, but it has not provided good protection from disasters due to disturbances, because in general the protection is specific to one type of disturbance, and the cause of the disturbance cannot be known through protection devices provided. Therefore we need a smarter electrical protection device by applying fuzzy logic to the protection device. Fuzzy on this protection device aims as a decision support system to show the condition of the electricity network based on the parameters of voltage, current, and temperature of the cable. The results of fuzzy logic in the form of a hazard level with a range of 1-10 which is divided into 3 status states, namely safe, alert, and hazardous, which in danger conditions the protective device will cut off the electricity so as not to damage electronic devices. Besides that, from the results of fuzzification, the parameters measured can be known to cause interference.
Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated.
The paper documents a new transformer model in ATPDraw v4.2 called XFMR. This model handles 3-phase transformers with two or three windings. Autotransformers and all Wye and Delta couplings are supported. The model includes an inverse inductance matrix for the leakage description, optional frequency dependent winding resistance, capacitive coupling, and a topologically correct core model with individual saturation and losses in legs and yokes. At the moment 3-and 5-legged stacked transformer cores are handled. The user can base the transformer model on three sources of data: Design (specify geometry and material parameters of the core and windings), Test report (similar to BCTRAN except for the core model), and Typical (typical values based on the voltage and power ratings).
Transformer inrush current is harmful to the transformer's windings and may bring about inadvertent tripping of the protective relays. In order to reduce the magnitude of inrush current, a bidirectional impedance-type inrush current limiter (BIT-ICL) is proposed in this paper. When a power transformer is energized, the BIT-ICL immediately and automatically inserts into the circuit to suppress the inrush current without any control or detection circuit. After completing transformer inrush suppression, the BIT-ICL is bypassed automatically, and the transformer appears to connect directly to the voltage source. Thus, when the transformer is loaded, there is almost no distortion in the steady-state load voltage and current waveforms due to the voltage-compensation effect in the limiter. The feasibility and performance of the proposed BIT-ICL are verified by a low-voltage single-phase power transformer under different residual flux and load conditions. Moreover, as long as the power loss that results from the related components in the BIT-ICL can be greatly reduced, this limiter can be applied to high-voltage and large-capacity distribution transformers.
a b s t r a c t The paper documents a new transformer model in ATPDraw called XFMR. This model handles three-phase transformers with two or three windings. Autotransformers and all Wye and Delta couplings are supported. The model includes an inverse inductance matrix for the leakage description, optional fre-quency dependent winding resistance, capacitive coupling, and a topologically correct core model (3-and 5-legged) with individual saturation and losses in legs and yokes. Three different sources of parameters are supported; typical values, standard test reports, and design information. The hybrid model XFMR is compared to the UMEC model in PSCAD showing good agreement at rated, stationary operation, but con-siderable differences in transient situations. Both models need further benchmarking and development to reproduce all switching transient behaviors properly.
This paper presents a new methodology for distinguish between inrush currents and internal faults based on the differential current gradient. The scheme is based on calculating the differential current gradient vector angles in phases A-B-C at all points of the data window. Using statistical calculations, the inrush current is then identified, because its gradient vector behavior will be different in the case of a short circuit occurrence. The method effectiveness has been verified in several computational simulation cases using EMTP/ATP and MATLAB®, analyzing situations of internal faults and inrush currents, including cases of sympathetic inrush in a power transformer, presenting highly satisfactory results.
Rapid changes and developments are being witnessed in the transformer design technologies. The phenomenal growth of power systems has put tremendous responsibilities on the industry to supply reliable and cost-effective transformers. The advent of high-temperature superconductor (HTS) materials has increased interest in research and development of superconducting transformers with major projects being carried out worldwide. The major challenges in the design and development of HTS transformers are the modeling of short-circuit and inrush currents the transformer can withstand. Even though HTS technology is claimed to be more efficient, reliable, and eco-friendly, use of HTS transformers must be appropriately verified through the proper modeling of power system network.
Transformer inrush currents have always been a concern in a power industry. Inrush currents generated by unloaded power transformer often reduce power quality on the system. Over the last decades, methods have been proposed to remove transformer inrush currents. To improve this situation, this paper proposes an active inrush current compensator that is capable of reducing the inrush current effectively during start up mode. The method uses a voltage source PWM converter is connected in series to the transformer that produce a dynamic resistor in series with transformer and remove inrush current. This method was tested by PSCAD/EMTDC simulation. Simulate shows that proposed method removes inrush current completely. This strategy is easier to implement because it has simple control method and requires no information of the transformer parameters, power on angle circuit breaker and measurement of residual flux and so on.
It was found that a neutral resistor together with `simultaneous' switching didn't have any effect on either the magnitudes or the time constant of inrush currents. The pre-insertion resistors were recommended as the most effective means of controlling inrush currents. Through simulations, it was found that the neutral resistor had little effect on reducing the inrush current peak or even the rate of decay as compared to the cases without a neutral resistor. The use of neutral impedances was concluded to be ineffective compared to the use of pre-insertion resistors. This finding was explained by the low neutral current value as compared to that of high phase currents during inrush. The inrush currents could be mitigated by using a neutral resistor when sequential switching is implemented. From the sequential energizing scheme performance, the neutral resistor size plays the significant role in the scheme effectiveness. Through simulation, it was found that a few ohms neutral grounding resistor can effectively achieve inrush currents reduction. If the neutral resistor is directly selected to minimize the peak of the actual inrush current, a much lower resistor value could be found. This paper presents an analytical method to select optimal neutral grounding resistor for mitigation of inrush current. In this method nonlinearity and core loss of the transformer has been modeled and derived analytical equations.
The inrush current occurs during transformer energization due to flux saturation in the core. Controlled switching connects transformer to power grid in proper phase angle derived from core remanent fluxes. The DC forced magnetization prior to transformer energization is one of simultaneous closing strategy methods. Active intervention to the transformer remanent flux was realized as a DC power supply with current splitter and automated disconnection circuit. Inrush currents were measured in dependency on switch on angle. Resulting current maps determine the range of switch on angles in which the current does not exceed the limits. This angle range restricts the choice of synchronous switch closing time scatter. This paper shows the results of a research on possible closing time scatter dependency both on forced pre-magnetizing current and on transformer working flux for star, grounded star and delta winding connections.
When an unloaded transformer is being energized, a large inrush current may flow depending upon the residual flux in the core of the transformer and the closing phase of the circuit breaker. A large inrush current may cause voltage fluctuations in the transmission system. One method of suppressing the inrush current is to perform controlled switching of the circuit breaker. In this paper, a new method of controlled switching of a 3-phase transformer in an isolated neutral system was examined. A method employing 3-phase circuit breakers with one operating mechanism and also single phase circuit breakers was used to devise a method of control switching for suppressing inrush current when the transformer is energized. In experiment using a 3 kV transformer, actual inrush currents were greatly suppressed.
Energization of unloaded transformers results in magnetizing inrush current (IR) very often with high amplitude. These currents have many unfavorable effects, including operation failure of transformer differential protection, deterioration of the insulation and mechanical support structure of windings and reduced power quality of the system. Without controlled switching the energization may occur at any time on the voltage wave producing high inrush current peak when the transformer core is driven into saturation. The control strategy presented in this paper has been elaborated to eliminate the inrush currents of 132/15 kV, 155 MVA Y n /∆ generator step-up transformers switched very often in two quick start gas-turbine power station. Existing control methods proposed by former papers could not be applied here, because the transformer breakers are mechanically staggered common drive types (3-pole operated breaker with single spring drive and fixed time delay between the operating poles). The paper proposes a new control method to minimize the residual flux by controlled transformer de-energization combined with the traditional point-on-wave controlled energization. The new concept has been tested by several field tests and the elaborated new point-on-wave controller has been put into service in two new substations in Hungary.
When a transformer is energized, a transient current, known as magnetizing inrush current, generally flows for a short period of time until normal flux conditions are established. Under most practical system conditions, this current transient is of little consequence. However, in very rare cases a combination of circumstances may be obtained which results in this inrush being of such consequence as to impair momentarily the proper operation of the system. Because of the numerous faetors bearing upon this general problem, an investigation has been made to determine the effects of transformer inrush currents under a wide variety of system conditions. It is the purpose of this paper to discuss the mechanism by which inrush currents are produced, the results of tests and calculations, and studies made with the miniature-system analyzer. Factors that determine the significance, of inrush current from the standpoint of system operation and methods for reducing the inrush current or mitigating its effects are also discussed.
Controlled switching taking into account the residual flux can - in theory - completely eliminate transient transformer inrush currents. Real factors such as closing time scatter of the circuit breaker and uncertainty in the residual flux measurement significantly reduce its performance in the field. To quantify the acceptable tolerances of these factors if no inrush currents have to occur, a systematic inrush current study was carried out in the laboratory. Instantaneous switches and ideal residual flux measurement in combination with a controller unit are used to emulate the scatter respectively the uncertainty by presetting deterministic deviations. The analyses were carried out for ldquorapid closing strategyrdquo and ldquodelayed closing strategyrdquo and showed that the acceptable tolerances for both strategies can be evaluated independent of the controlled switching strategy with the inrush current studies of the first phase that is energized.
The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation.
The topological structure and basic approaches for parameter estimation for a new hybrid transformer model are presented in Part I of this two-paper set. Part II deals with the model benchmarking and also discusses additional methods for parameter estimation based on laboratory measurements. The simulation results confirm the validity of the model for the low- and medium-frequency range
Transformers are essential elements, which facilitate the transmission of electric power at high voltages over long distances and transformer energization is a common occurrence. The energization can lead to excessive transient inrush current, especially when the transformer core has remnant flux that adds to the flux build-up after switching. Inrush current sags the system voltage, thereby affecting the power quality of the network in proximity of the transformer. The extent to which power quality is degraded depends on short circuit MVA at the source bus, and the magnitude and decay time constant of the transient current. Present day thyristor-controlled machinery used in mining, pulp and paper industries and semiconductor manufacturing require high quality power. Some of these industries even pay a premium price for high quality power. It is, therefore, necessary to first assess the impact of energizing a large transformer on power quality and then develop a technique to limit this impact. This paper presents results from field tests and simulations using Electromagnetic Transients Program a 138 kV 315 MVA transformer energization in the BC Hydro system. The transformer is situated close to a pulp and paper mill with voltage sensitive loads. Methods to assess and to limit the voltage sags during energization are discussed.
This paper presents a new, simple and low cost method to reduce inrush currents caused by transformer energization. The method uses a grounding resistor connected at a transformer neutral point. By energizing each phase of the transformer in sequence, the neutral resistor behaves as a series-inserted resistor and thereby significantly reduces the energization inrush currents. The proposed method has been tested by computer simulation and laboratory experiments. Both results show that the method has a performance similar to that of the resistor pre-insertion scheme. The proposed method is much less expensive, however, since there is only one resistor involved and the resistor carries only a small neutral current in steady-state.
From failure experience on power transformers, it was very often
suspected that inrush currents, occurring when energizing unloaded
transformers, were reason for damage. In this paper, it was investigated
how mechanical forces within the transformer coils build up under inrush
compared to those occurring at short circuit. Two-dimensional and
three-dimensional computer modeling for a real 268 MVA, 525/17.75 kV
three-legged step up transformer was employed. The results show that
inrush current peaks of 70% of the rated short circuit current cause
local forces in the same order of magnitude as those at short circuit.
The resulting force summed up over the high voltage coil is even three
times higher. Although inrush currents normally are smaller, the forces
can have similar amplitudes as those at short circuit however with
longer exposure time. Therefore, care has to be taken to avoid such high
inrush currents. Today controlled switching offers an elegant and
practical solution
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