When an inactive transformer is reactivated, the phenomenon of transformer inrush current occurs. This can lead to various problems. The Inrush Limiter T1 suppresses the inrush current by controlling the phase angle at which the circuit breaker closes.
Effective current-limiting
Intended for:
- Limiting inrush currents
- Transformers in utility power take-off stations
- Power take-off facilities in factories
- Substations for wind turbines
- Grid pulse-resistant technology for fusion reactors
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More InformationThe inrush current of a three-phase transformer is caused by:
- Mismatching polarity and magnitude of the initial magnetic flux
- Residual currents in the phase cores before switching on
- Impact of the mains voltage immediately after switching on
The limitation of these misalignments reduces the inrush current.
The key to suppression:
- Accurate determination of polarity and magnitude of the residual current in each phase core when disconnecting via the circuit breaker
Residual current upon shutdown:
- Not identical to the magnetic flux at the moment of switch opening
- Corresponds to the final value of the transient magnetic flux after disconnection
Inrush-Limiter Function:
- Integration of the transformer voltage (VT output) over time for the digital calculation of the residual flux
- Utilization of the transient flux waveforms and their convergence value
On the next power-up:
- Phase control is aligned with the switching moment of the circuit breaker
- Effective suppression of inrush current
Minimal effort, optimal control
The only required input signals are the three-phase transformer voltage on the high-voltage or low-voltage side of the transformer and the transformer voltage on the network side. The only output is a delayed signal that takes into account the optimal connection phase angle.
The E/A interface is extremely user-friendly and requires
no adjustments.
NO RESTRICTIONS
The inrush limiter can be used with any transformer and is not restricted by capacity, voltage, connection type, impedance, or other transformer parameters. The type of circuit breaker and busbar configuration are also irrelevant.
Digital Hub
The central processing unit consists of a circuit board module with a digital processor that calculates the voltage waveform of each phase in real-time when the transformer is switched off and the waveform of the magnetic flux as the integral of the input voltage. This processor then determines the optimal time to turn on the circuit breaker when the transformer is to be switched on again. This enables continuous power without maintenance or regular inspections.
Case Studies
The phenomenon of inrush current is a temporary event that occurs when a stationary transformer is switched back on. The transient effect manifests as large asymmetric, sharp-edged waveforms across the three phases, which are several times greater than the rated current, and a 20% voltage drop that can last from half a second to several seconds. This can lead to various failures on the load side near the transformer in substations used for many different applications. Kodensyas Inrush Limiter effectively suppresses the inrush phenomenon that occurs when operating the circuit breaker for transformers in various applications.
Interconnected substations for distributed power generation systems
Decentralized power generation systems (wind power, solar energy, small hydroelectric plants) are typically connected to low-power transmission and distribution lines (over long distances and with high impedance), where voltage drops associated with inrush current present a serious problem. The inrush limiter plays an important role in these distribution stations.
APPLICATION AREA 1
This is a power substation operated by an electric utility on a remote island. The system’s low voltage potential raised concerns about voltage dips caused by the inrush current of the power-receiving transformer. To prevent this, the inrush current limiter T1 was installed. Figure 3-2 shows measured waveforms of the inrush current and voltage drop, recorded during on-site validation tests, with and without the inrush current limiter.
After repeated testing during power on and off cycles without the inrush limiter, an inrush current of 411 A was observed and a voltage drop of 14.8% was noted. With the inrush current limiter, the inrush current was suppressed to 91 A and the voltage drop was limited to 1.9%.
Substation for electric utilities
The inrush limiter effectively prevents voltage drops in all loads supplied by specialized high-voltage networks and power distribution systems.
APPLICATION AREA 2
A major chemical manufacturer operates an electric furnace and the associated substation for power supply. The substation consists of several rows of current transformers. When a transformer in one of the rows was activated, there was a significant voltage drop in the other rows (which are connected to the primary side of the power grid). The inrush current limiter T1 was installed to prevent this voltage drop and the associated disruptions in production. Figure 3-4 shows measured waveforms of the inrush current and voltage drop with and without the inrush current limiter, recorded during field validation tests.
After repeated opening and closing tests without the inrush limiter, a starting current of 1,514 A and a voltage drop of 7.1% were observed. With the inrush limiter, the starting current was suppressed to 83 A, and the voltage drop was maintained at 0.2%. This manufacturer used a circuit breaker with a resistor for many years to suppress inrush currents in its 66 kV power intake station. (The 66 kV series resistor is used with an additional circuit breaker to limit the inrush current when the switch is closed). The inrush limiter has made these resistors obsolete, so they are now being removed.
Large-scale public facilities and power supply stations for factories
The inrush current limiter prevents failures in power-consuming devices and their control systems that require a stable voltage and current.
APPLICATION AREA 3
In investigations conducted at a substation for a wind power plant, voltage drops of 20% were recorded when the transformer was connected. It was necessary to reduce these to 5%. The inrush limiter achieved less than 3%. (Each of the following countermeasures to achieve 5% or less became unnecessary):
- Installation of a diesel generator
- Custom-designed transformer (change of leakage impedance, etc.)
- Use of a series resistor and circuit breaker
Figure 3-6 shows measured waveforms of the inrush current and voltage drop with and without inrush current limiter, recorded during field validation tests.
After repeated opening and closing tests without the inrush limiter, an inrush current of 839 A was observed along with a voltage drop of 19.5%. With the inrush current limiter, the inrush current was suppressed to 164 A, and the voltage drop was limited to 1.5%.
Applications and Results of Field Validation
APPLICATION AREA 1
Es handelt sich um das Umspannwerk eines Stromversorgungsunternehmens auf einer abgelegenen Insel. Das niedrige Spannungspotenzial des Systems gab Anlass zur Sorge über Spannungseinbrüche aufgrund des Einschaltstroms des Stromabnahmetransformators. Um dies zu verhindern, wurde der Einschaltstrombegrenzer T1 installiert. Abbildung 3-2 zeigt gemessene Wellenformen, die während der Validierungstests vor Ort aufgezeichnet wurden, des Einschaltstroms und des Spannungsabfalls mit und ohne Einschaltstrombegrenzer.
Nach wiederholten Prüfvorgängen beim Einschalten und Ausschalten ohne den Inrush-Limiter zeigte sich ein Einschaltstrom von 411 A und ein Spannungsabfall von 14,8 %. Mit dem Einschaltstrombegrenzer wurde der Einschaltstrom auf 91 A unterdrückt und der Spannungsabfall auf 1,9 % begrenzt.
APPLICATION AREA 2
Ein großer Chemieproduzent betreibt einen Elektro-Ofen und das zugehörige Umspannwerk für die Stromversorgung. Das Umspannwerk besteht aus mehreren Reihen von Stromabnahmetransformatoren. Als ein Transformator in einer der Reihen eingeschaltet wurde, kam es in den anderen Reihen (die mit der Primärseite des Stromnetzes verbunden sind) zu einem erheblichen Spannungsabfall. Der Einschaltstrombegrenzer T1 wurde installiert, um diesen Spannungsabfall und die dadurch verursachten Störungen in der Produktion zu vermeiden. Abbildung 3-4 zeigt gemessene Wellenformen des Einschaltstroms und des Spannungsabfalls mit und ohne Einschaltstrombegrenzer, die während der Feldvalidierungstests aufgezeichnet wurden.
Nach wiederholten Öffnungs- und Schließtests ohne den Inrush-Limiter zeigte sich ein Einschaltstrom von 1.514 A und ein Spannungsabfall von 7,1 %. Mit dem Inrush-Limiter wurde der Einschaltstrom auf 83 A unterdrückt und der Spannungsabfall auf 0,2 % gehalten. Dieser Hersteller verwendete viele Jahre lang einen Leistungsschalter mit einem Widerstand zur Unterdrückung von Einschaltströmen in seiner 66-kV-Stromabnahmestation. (Der 66-kV-Serienwiderstand wird mit einem zusätzlichen Leistungsschalter verwendet, um den Einschaltstrom zu begrenzen, wenn der Schalter geschlossen ist). Der Inrush-Limiter hat diese Widerstände überflüssig gemacht, so dass sie jetzt entfernt werden.
APPLICATION AREA 3
Bei Untersuchungen in einem Umspannwerk für eine Windkraftanlage wurden Spannungsabfälle von 20 % festgestellt, wenn der Transformator angeschlossen war. Es war notwendig, diese auf 5 % zu unterdrücken. Der Inrush-Limiter erreichte weniger als 3%. (Jede der folgenden Gegenmaßnahmen zur Erreichung von 5 % oder weniger wurde überflüssig):
- Einbau eines Dieselgenerators
- Speziell ausgelegter Transformator (Änderung der Streuimpedanz usw.)
- Verwendung eines in Reihe geschalteten Widerstands und Leistungsschalters
Abbildung 3-6 zeigt gemessene Wellenformen des Einschaltstroms und des Spannungsabfalls mit und ohne Einschaltstrombegrenzer, die während der Feldvalidierungstests aufgezeichnet wurden.
Nach wiederholten Öffnungs- und Schließtests ohne den Inrush-Limiter zeigte sich ein Einschaltstrom von 839 A und ein Spannungsabfall von 19,5 %. Mit dem Einschaltstrombegrenzer wurde der Einschaltstrom auf 164 A unterdrückt und der Spannungsabfall auf 1,5 % begrenzt.
How Inrush Current Occurs
When the transformer is disconnected from the power grid by manual operation, the three-phase cores remain magnetized, so that the residual flux φa, φb, φc remains in its original state. The next time the transformer is switched on at phase angle θcl, the initial voltage rise on the utility side adds an initial magnetic flux φa(tcl), φb(tcl), φc(tcl) to the cores. If the residual flux φa, φb, φc and the initial magnetic flux φa(tcl), φb(tcl), φc(tcl) that occurs when the transformer is switched on again have the same polarity and a small scalar difference between the individual phases, the inrush phenomenon can be suppressed. However, if the polarity differs on one of the phases and the magnitude difference is large, the transient flux in the cores exceeds the saturation value and generates an enormous inrush current. Therefore, determining the correct scalar value, including the polarity of the flux remaining in each core after the transformer is turned off, is the key to developing a control algorithm that can suppress the inrush current. Incidentally, the polarity and magnitude of this residual flux φa, φb, φc differ significantly from the flux φa(top0), φb(top0), φc(top0) remaining in the cores when the transformer is switched off at time top0. The reason for this is explained below.
Unloaded state
The diagram below shows a typical phase of the transformer Tr circuit, which is to be energised in the unloaded state (with circuit breaker Br2 switched off). The line from the low-voltage side of the transformer coil features a loop circuit based on floating capacitors and shock absorbers. This means that immediately after circuit breaker Br1 is switched off on the high-voltage side, a short transient voltage, a transient current and a transient magnetic flux occur. The voltages va(t), vb(t), vc(t) and the currents ia(t), ib(t), ic(t) decrease and eventually disappear as the transient process top1 decays. The magnetic flux φa(t), φb(t), φc(t) in the cores (which is the integral of the voltage) undergoes a transient change, allowing the final value at top1 to converge to a constant value φa(top1), φb(top1), φc(top1). This convergence value is the true residual flux in the core. Note that the circuit is balanced across all three phases before and after the transformer is switched off. If the circuit breaker Br1 trips the transformer with an extremely small inrush current, all three phases are tripped at the same time top0 (there is no time delay between the phases as with zero-crossing tripping). This means that the residual flux in the core is balanced across the three phases.
Status after shutdown
The figure above shows the transient phenomenon recorded by the inrush limiter after the transformer was switched off. This installation involves a power transformer (220/110 kVA, 250 MVA, Y-Δ, 60 Hz) in a power company’s substation. The upper diagram shows the transient waveforms of the voltage (measured secondary voltage transformer voltage) and the magnetic flux (generated by calculating the integral of the voltage). The diagram below is a visual representation of the waveforms of voltage and magnetic flux through a rotating phaser. The voltage phaser attenuates during rotation and eventually dissipates as the transient phenomenon at Top1 subsides. The magnetic flux also shrinks during rotation and converges to a residual flux of a specific value, whilst remaining balanced across the three phases at top1. In each phase, the polarity and magnitude of the residual flux have a completely different scalar value from that of the magnetic flux at the moment the circuit breaker is switched off at top0. The inrush current limiter calculates and records the residual fluxes in each phase in a near-balanced state: φa(θop1), φb(θop1), φc(θop1). When the transformer is switched on again, the phase at which the circuit breaker closes is controlled so that the phase angle θcl approximately coincides with the phase θop1 of the residual flux. (Details of this algorithm can be found in our patented method).
Algorithm for suppressing inrush current (patented method)
Numerous field data from various substations demonstrate how effectively the Inrush Limiter suppresses the inrush current phenomenon. See applications for typical uses. Although the fundamental theory behind the Inrush Limiter’s control algorithm is proven in practice, Kodensya has conducted extensive testing using a simulated power transmission system (220 volts) to validate the control method (see below).
Power Transmission Test
During the simulated power transmission test, the transformer was repeatedly switched off at the phase angle θop0. Each time, the voltage waveform (actual measurements) of each phase was recorded during the transient phase and used to calculate and record the waveform of the magnetic flux (generated by integration) and a rotating phasor representing the magnitude and phase angle θop1 of the three-phase residual flux at the end of the transient phase. The peak inrush current (θop1,θcl,I) at the phase angle θcl, at which the circuit breaker was closed to restore power to the transformer, was also recorded. Figure 2-3 shows a curved 3-D surface plot of the measured results (θop1,θcl,I) from repeated opening and closing tests.
Figure 2-4 shows the same test results and the range covered by the maximum and minimum inrush currents as coordinates (θop1, θcl). This figure shows with great accuracy that the inrush current I is extremely low when θop1 and θcl are approximately in phase, and that the inrush current I reaches its maximum value when θop1 and θcl are 180 degrees out of phase. Figure 2-5 shows the waveform of the inrush peak current recorded when the circuit breaker was closed at opposite phases. Figure 2-6 shows how virtually the entire inrush current is eliminated by closing the circuit breaker at the optimal phase angle (in accordance with the inrush current limiter algorithm).
Optimal Algorithm: Adjustment of the Scalar Value to the Phase with the Lowest Residual Flow
Immediately after the transformer is de-energized, the inrush current limiter calculates and detects the actual residual flux in its decayed state at the end of the transient response as an equilateral triangle in no-load operation with phase angle θop1. The circuit breaker is controlled such that the transformer is re-energized when the phase angle θcl is as close as possible to θop1. Figure 2-7 illustrates this control algorithm. The small triangle in the figure indicates the phasor for the residual flux. The phase with the smallest scalar value (phase a in the figure) is selected as the reference phase. The scalar value of the reference phase a in the initial magnetic flux (the large triangle in the figure) corresponds to the scalar value of the residual flux (where ∆Φa = 0) for each cycle with two phase angles (Figures 2-7 and 2-8). In Figure 2-7, the polarities of the residual flux and the initial magnetic flux of phase b match, and ∆Φb is sufficiently small. This also applies to phase c. In this way, the inrush limiter algorithm finds and uses the optimal phase angle at which the circuit breaker is closed. Figure 2-8 shows the other instance where ∆Φa = 0. In this case, an extremely large inrush current would occur because the polarities of the residual flux and the initial magnetic flux for phases b and c are almost exactly opposite, and ∆Φb and ∆Φc are extremely large. The polarities of the fluxes for phases b and c in Figures 2-7 and 2-8 are easily discernible and make it possible to achieve the optimal phase angle shown in Figure 2-7.
Kodensya
Kodensya offers maintenance services for electrical systems in the medium and high voltage range. From these experiences, various devices for controlling circuit breakers and systems in the solar sector have been developed.
