AC "back to back" switching device in industrial application

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energies

Article

AC “Back to Back” Switching Device in Industrial

Application

†

Roberto Faranda1 , Hossein Hafezi2,* , Kishore Akkala1 and Massimo Lazzaroni3

1 _{Department of Energy, Politecnico di Milano, 20156 Milan, Italy; roberto.faranda@polimi.it (R.F.);} nagavenkata.akkala@polimi.it (K.A.)

2 _{School of Technology and Innovations, Electrical Engineering, University of Vaasa, 65200 Vaasa, Finland} 3 _{Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milan, Italy; massimo.lazzaroni@unimi.it} * Correspondence: hossein.hafezi@uwasa.fi

† _{This paper is an extended version of our paper published in 2018 IEEE International Conference on} Environment and Electrical Engineering and 2018 IEEE Industrial and Commercial Power Systems Europe (EEEIC/I&CPS Europe), Palermo, Italy, 12–15 June 2018; doi:10.1109/EEEIC.2018.8494482.

Received: 13 May 2020; Accepted: 29 June 2020; Published: 9 July 2020 

Abstract: In industrial applications, among several varieties of semiconductor devices available, a silicon-controlled rectifier (SCR) is often used in managing and protecting various systems with different applications. Hence, it is of the utmost importance to design a control system which can operate over a range of electrical loads without any modifications in its hardware and/or software. This paper analyzes and investigates in detail the power circuit effects on conduction delay and SCR functioning. Moreover, two different commonly used driving systems for SCR application have been introduced, discussed, and evaluated. Concerning driving systems, here, three aspects have paramount importance and are consequently taken into consideration, namely the driver system losses, the conduction delay, and in particular, some power quality indices. The conduction delay is a parameter of great importance, as being able to control and reduce it to the minimum allowed by the application can bring significant practical advantages (both in terms of application and economic terms, as better summarized in the article). Theoretical analysis has been performed, followed and verified by simulation studies and, for some cases, laboratory experimental test results are presented which provide credibility to the study.

Keywords: back to back; electromagnetic compatibility (EMC); inductance; industrial automation; measurement; power converters; power quality; silicon-controlled rectifier (SCR)

1. Introduction

In industrial applications, the silicon-controlled rectifier (SCR) (power electronics-based switch and breaker) is a widely-used semi-controlled device due to its controllability and flexibility over diodes (uncontrolled switches) and its reliability and affordable prices with respect to fully-controlled power electronic switches [1–4]. Moreover, in diverse industrial fields, among several varieties of semiconductor devices available, SCRs are a preferred choice in managing and protecting various systems with different applications, especially in the field of industrial control systems [5–7] where it is mandatory to consider the peak value of current and its duration. As examples, in magnetic applications (where the magnets are sensitive to the peak current) and electric protection devices, it is of importance to take into account the peak current value, its shape and its time duration, else it might result in the damage of the system [8] and/or the protected load.

SCR due to its characteristics (controllability, reliability, and affordable price) is the first choice in several other industrial applications as well [4]. Controllability refers to the ability to move the system

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or the device (as it is in this situation) in the proper way as desired and in its entire configuration space using only certain admissible manipulations (as it will be clear in the following). Reliability, instead, is the ability of an item to perform the required function under given conditions for a given time interval. Finally, components should be evaluated also taking into account the cost versus the given functions.

Flexible AC transmission systems (FACTS) is a wide area where SCRs are often used, particularly in Static VAR compensators (SVC) which are utilized in different sectors in electric power system (distribution and transmission lines/substations). Authors in [9] comprehensively reviewed the applications of SVC in different positions in electric railway systems. The evolution of SVC led to the development of static synchronous compensator (STATCOM), where in many topologies SCRs are still being used due to its economic benefits [10].

SCRs have also found their applications in high voltage DC (HVDC) systems, where they act as a converter interface between AC and DC systems and provides smooth operation through phase-control. Other interesting applications, which are under development include a hybrid DC vacuum circuit breaker for medium voltage and is reported in [11]. A self-supplied SCR for medium voltage application for a solid-state starter is proposed in [12].

In addition, Phase-controlled converters in high power AC machine drives is another vast field of application for SCRs and is a topic for many research articles. The recent works on SCR deal with the mitigation effects of common-mode current, as investigated in [13], and harmonic reduction of three-phase controlled converters, as studied in [14].

Furthermore, the wide range of applications of SCRs is not only limited to high voltage or high power. The diode triggered SCR for high temperature, low voltage electrostatic discharge application has been recently introduced and studied [15–17].

Given the wide range of applications of SCRs, under normal operation, a precise control to turn on the SCR (SCR conduction) and with emphasis to close the circuit at the right instant is important. In modern industrial applications, where it is mandatory to obtain high reliability with a mixture of optimal control techniques and diagnostics (particularly techniques related to detect failures), this target can be achieved [18,19]. The study on SCR as a static switch presented in this paper is mainly focused on applications related to power quality, demand response, current management for magnetic load, and in smart grid system applications [20].

Considering that the only way to properly use a SCR is to utilize an electronic control circuit, it is of utmost importance to design an universal SCR driver system which, without any modifications in its hardware and/or software, can operate over a wide variety of applications [21,22], and in turn, it can achieve optimal functionality and high level of system reliability [23].

In order to achieve this goal, it is important to understand the device behaviour and investigate different factor’s (e.g., supply voltage or load characteristics, and driver control circuit characteristics) effects on SCRs ignition. Although the behaviour of the SCR itself is familiar for the scholars and engineers in the field, the effects of different elements on SCR operation have not been studied or reported in any publication. These effects on SCR ignition reported in this paper, are very essential to design an effective control system which can operate over wide range of applications minimizing the SCRs side effects.

This paper fill-in the gaps existing in current literature and investigates in detail the issues in general format and formulate the minimum pulse duration for ignition of an SCR as function of electrical parameters and different driver topologies, in order to lead to a system with better controllability, higher power quality, and to minimize design efforts to allow the industry to reuse a solution in several applications with minor or no changes.

The paper is an extended version of previous work [24] and is organized in the following way. In Section2, a general review of SCR devices and its application is given. In Section3, the problem formulation is explained considering both general and practical aspects. In Section4, we provide

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evidence concerning the driver circuits used, the simulation, and the obtained experimental results are discussed. Finally, the conclusions are summarized in Section5.

2. An Overview on SCR and Its Application

SCR is a semiconductor device widely used as a power switch. Its usage over the years increased due to its characteristics of fast switching current, during switch on and sometimes during switch off, in comparison to the more traditional and historically used electromechanical breaker.

The switching on of the device can be controlled with a gate signal by means of well-defined biasing conditions. Through this, the electrical parameters of the load voltage, current, and average absorbed power by load can be controlled.

The SCR device is typically manufactured by four layers of P and N type semiconductor materials placed one after another in alternating fashion. The operation of the SCR can be explained by a pair of tightly coupled bipolar junction transistors, arranged in such a way to cause a self-latching action as depicted in Figure1. Here, two transistors are shown, namely PNP and an NPN devices.

Energies 2020, 13, x FOR PEER REVIEW 3 of 20

The SCR device is typically manufactured by four layers of P and N type semiconductor materials placed one after another in alternating fashion. The operation of the SCR can be explained by a pair of tightly coupled bipolar junction transistors, arranged in such a way to cause a self-latching action as depicted in Figure 1. Here, two transistors are shown, namely PNP and an NPN devices.

Figure 1. SCR equivalent circuit. The operation of the SCR can be simply explained by a pair of tightly coupled bipolar junction transistors, arranged as in figure in such a way to cause a self-latching action.

When the VGATE is positive with respect to the cathode, the SCR operates in its forward

characteristics. Figure 2 graphically reports SCR’s forward characteristics. In Figure 2, three I-V curves are plotted in order to show gate current (designated as IG) effects on SCR’s forward voltage

(reported on abscissa axis) and the forward current (reported on the ordinate axis). In particular it represents three curves covering IG = 0, relatively low gate current (IG1) and slightly higher gate

current (IG2). It can be noticed that I-V cures follow a similar shape unless the breakover point occurs

sooner with higher IG.

Figure 2. Different kind of SCR Static I-V Characteristics (qualitative representation, not in scale). Legend: VB0 = Forward Breakover Voltage, VBR = Reverse Breakover Voltage, IG = Gate Current, IL =

Latching Current, IH = Holding Current.

In Figure 3, the qualitative variation of Forward Breakdown Voltage (VB0) vs. Gate Current (IG)

is shown. It puts in evidence that the correct way to turn on a SCR, is to apply a fast current pulse with suitable amplitude to the gate terminal when a positive voltage is provided between anode and cathode in order to reduce the breakover voltage of the device.

Figure 1.SCR equivalent circuit. The operation of the SCR can be simply explained by a pair of tightly coupled bipolar junction transistors, arranged as in figure in such a way to cause a self-latching action. When the VGATE is positive with respect to the cathode, the SCR operates in its forward characteristics. Figure2graphically reports SCR’s forward characteristics. In Figure2, three I-V curves are plotted in order to show gate current (designated as IG) effects on SCR’s forward voltage (reported on abscissa axis) and the forward current (reported on the ordinate axis). In particular it represents three curves covering IG= 0, relatively low gate current (IG1) and slightly higher gate current (IG2). It can be noticed that I-V cures follow a similar shape unless the breakover point occurs sooner with higher IG.

The SCR device is typically manufactured by four layers of P and N type semiconductor materials placed one after another in alternating fashion. The operation of the SCR can be explained by a pair of tightly coupled bipolar junction transistors, arranged in such a way to cause a self-latching action as depicted in Figure 1. Here, two transistors are shown, namely PNP and an NPN devices.

Figure 1. SCR equivalent circuit. The operation of the SCR can be simply explained by a pair of tightly coupled bipolar junction transistors, arranged as in figure in such a way to cause a self-latching action.

When the VGATE is positive with respect to the cathode, the SCR operates in its forward

characteristics. Figure 2 graphically reports SCR’s forward characteristics. In Figure 2, three I-V curves are plotted in order to show gate current (designated as IG) effects on SCR’s forward voltage

(reported on abscissa axis) and the forward current (reported on the ordinate axis). In particular it represents three curves covering IG = 0, relatively low gate current (IG1) and slightly higher gate

current (IG2). It can be noticed that I-V cures follow a similar shape unless the breakover point occurs

sooner with higher IG.

Figure 2. Different kind of SCR Static I-V Characteristics (qualitative representation, not in scale). Legend: VB0 = Forward Breakover Voltage, VBR = Reverse Breakover Voltage, IG = Gate Current, IL =

Latching Current, IH = Holding Current.

In Figure 3, the qualitative variation of Forward Breakdown Voltage (VB0) vs. Gate Current (IG)

is shown. It puts in evidence that the correct way to turn on a SCR, is to apply a fast current pulse with suitable amplitude to the gate terminal when a positive voltage is provided between anode and cathode in order to reduce the breakover voltage of the device.

Figure 2. Different kind of SCR Static I-V Characteristics (qualitative representation, not in scale). Legend: VB0= Forward Breakover Voltage, VBR = Reverse Breakover Voltage, IG= Gate Current, IL= Latching Current, IH= Holding Current.

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In Figure3, the qualitative variation of Forward Breakdown Voltage (VB0) vs. Gate Current (IG) is shown. It puts in evidence that the correct way to turn on a SCR, is to apply a fast current pulse with suitable amplitude to the gate terminal when a positive voltage is provided between anode and cathode in order to reduce the breakover voltage of the device.

Figure 3. Qualitative variation of Breakdown Voltage (VB0) vs. Gate Current (IG): the correct way to

turn on a SCR device is to apply a fast current pulse with suitable amplitude on the gate terminal when a positive voltage is applied between anode and cathode to reduce the Breakover Voltage of the device.

Therefore, the gate current is usually made high enough to ensure that the SCR is switched to the ON state at the proper time and it is usually necessary for just an instant because a constant gate current is not required to trigger the SCR and it would only cause more power losses that needs to be dissipated within the device. Therefore, once the device has been switched-on by the gate terminal, it remains latched in the ON state and it does not need a continuous supply of gate current in order to maintain the conduction. This is true only if the anode current has exceeded the latching current (denoted by IL in Figure 2).

As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (denote as IH) and so the voltage across the SCR become zero or

negative.

After the current has reached natural zero point, a finite amount of time delay is required before initiating another switch on procedure and during this period SCR must retain in off state. This minimum time delay is called turn off time (tQ).

To have right SCR gate triggering, following conditions need to be provided:

an appropriate gate to cathode voltage (VGATE) is needed to turn on the device while the SCR is

forward biased;

gate signal shall be removed after the device under test is turned on in order to reduce the losses and avoid higher junction temperature;

while the device is in reverse biased conditions, no VGATE signal should be applied;

when the device is in-off state, negative VGATE should be applied to improve the performance of

the under-test device.

Taking into account the above considerations and noticing that the pulses must occur periodically (considering the network frequency), it is evident that the gate is triggered by means of a pulsed waveform applied across the gate terminals and will be able to work correctly for all the power switch applications and with any load conditions. Therefore, it is very important to know the availability of SCR topologies in the market. Based on the manufacturing technology used, three different topologies of SCR can be realized—they are listed below and are described in more detail in the literature, e.g., in [25]:

 Planar type: Typically used for low current devices because the junctions are obtained by diffusion and come to the same surface on the cathode side. The ratio silicon/ampere is typically high;

 Mesa type: It is used for high current devices but low di/dt ratio. Power SCRs are typically manufactured by this process;

 Press pack type: Widely used in applications where it is necessary to have SCR with center gate and large value of di/dt.

Figure 3.Qualitative variation of Breakdown Voltage (VB0) vs. Gate Current (IG): the correct way to turn on a SCR device is to apply a fast current pulse with suitable amplitude on the gate terminal when a positive voltage is applied between anode and cathode to reduce the Breakover Voltage of the device. Therefore, the gate current is usually made high enough to ensure that the SCR is switched to the ON state at the proper time and it is usually necessary for just an instant because a constant gate current is not required to trigger the SCR and it would only cause more power losses that needs to be dissipated within the device. Therefore, once the device has been switched-on by the gate terminal, it remains latched in the ON state and it does not need a continuous supply of gate current in order to maintain the conduction. This is true only if the anode current has exceeded the latching current (denoted by ILin Figure2).

As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (denote as IH) and so the voltage across the SCR become zero or negative. After the current has reached natural zero point, a finite amount of time delay is required before initiating another switch on procedure and during this period SCR must retain in off state. This minimum time delay is called turn off time (tQ).

negative.

forward biased;

an appropriate gate to cathode voltage (VGATE) is needed to turn on the device while the SCR is forward biased;

negative.

forward biased;

negative.

forward biased;

while the device is in reverse biased conditions, no VGATEsignal should be applied;

negative.

forward biased;

when the device is in-off state, negative VGATEshould be applied to improve the performance of the under-test device.

negative.

forward biased;

Planar type: Typically used for low current devices because the junctions are obtained by diffusion and come to the same surface on the cathode side. The ratio silicon/ampere is typically high;

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Energies 2020, 13, 3539 5 of 20

negative.

forward biased;

Mesa type: It is used for high current devices but low di/dt ratio. Power SCRs are typically manufactured by this process;

negative.

forward biased;

Press pack type: Widely used in applications where it is necessary to have SCR with center gate and large value of di/dt.

Usually, power SCRs use a Mesa process for manufacturing. It is important to note that the device is prevalently used in “back to back” mode in AC circuit configurations, see Figure4where the device is turned off automatically, hence load current is cutoff every zero crossing of the signal. For the DC applications, an ad hoc circuit is necessary for the use of SCR, this is important to force the device into turn off state. However, the SCR application in DC systems is not further developed in this paper as it focuses only on AC applications.

Usually, power SCRs use a Mesa process for manufacturing. It is important to note that the device is prevalently used in “back to back” mode in AC circuit configurations, see Figure 4 where the device is turned off automatically, hence load current is cutoff every zero crossing of the signal. For the DC applications, an ad hoc circuit is necessary for the use of SCR, this is important to force the device into turn off state. However, the SCR application in DC systems is not further developed in this paper as it focuses only on AC applications.

Figure 4. The AC “Back to Back” Switch configuration.

As it can be seen from Table 1, different commercial devices available have different latching currents IL (in Table 1 the holding currents IH are also reported), therefore, it is very important to

evaluate the minimum pulse time duration to be given to an SCR for its ignition. This duration is fundamental to using SCRs as switching devices in different industrial applications, because having knowledge only on latching currents (IL) is not sufficient to assure proper operation of SCR in all

conditions.

Table 1. Examples of SCR characteristics.

Device Manufacturer IF (A) VR (V) IH (A) IL (A)

VSK135 Vishay 135 1600 0.2 0.4 VSK142 Vishay 140 1600 0.2 0.4 VSK162 Vishay 160 1600 0.2 0.4 SKKT132/16 Semikron 220 1600 0.15 0.3 SKKT162/16 Semikron 250 1600 0.15 0.3 SK45KQ16 Semikron 46 1600 0.08 (a)_0.15(a)

SK70KQ16 Semikron 71 1600 0.1 (b)_0.2(b)

TN25 ST 25 1000 0.05 (b)_0.09(b)

TN12 ST 12 1000 0.005 (b)_0.006(b)

PO10 ST 0.8 600 0.2 0.006 (b) (a)_{Max value can be sometimes very high such as 1A for SKKT162/16.}(b)_{Max value.}

Indeed, the minimum value of IL depends on the adopted delay angle control α, load impedance

and on the RMS value of the applied voltage. Considering these points, in most of industrial solutions, the driving pulses length are oversized in time. Hence, the triggering pulse width is much longer than the actual width needed to guarantee the device turning on with a driving signal and to achieve the desired control function. The aim of the paper is to investigate and propose an optimum solution, which can improve overall system efficiency and functionality and can be adopted in wide range of applications.

Here, it is also worth understanding a very important aspect in certain industrial applications. The conduction angle (sometimes also denoted as ignition angle) is very important as it depends on the value of the consequent current’s peak value. In some applications, for example in the case of permanent magnet systems, the state of magnetization of the system depends precisely on the peak value of the magnetizing current. To increase this peak value, with the same system, it is possible to act on the ignition angle, making it small. Therefore, the possibility of having a system that as a whole (hardware and software) is able to turn on the SCR at low conduction angles is certainly an advantage. The advantages are multiple: in terms of ampere-turns, it is therefore possible to either obtain more from the same system or to decrease ampere-turns, obtaining an undeniable advantage

Figure 4.The AC “Back to Back” Switch configuration.

As it can be seen from Table1, different commercial devices available have different latching currents IL(in Table1the holding currents IHare also reported), therefore, it is very important to evaluate the minimum pulse time duration to be given to an SCR for its ignition. This duration is fundamental to using SCRs as switching devices in different industrial applications, because having knowledge only on latching currents (IL) is not sufficient to assure proper operation of SCR in all conditions.

Table 1.Examples of SCR characteristics.

Device Manufacturer IF(A) VR(V) IH(A) IL(A)

VSK135 Vishay 135 1600 0.2 0.4

VSK142 Vishay 140 1600 0.2 0.4

VSK162 Vishay 160 1600 0.2 0.4

SKKT132/16 Semikron 220 1600 0.15 0.3

SKKT162/16 Semikron 250 1600 0.15 0.3

SK45KQ16 Semikron 46 1600 0.08(a) 0.15(a)

SK70KQ16 Semikron 71 1600 0.1(b) 0.2(b)

TN25 ST 25 1000 0.05(b) 0.09(b)

TN12 ST 12 1000 0.005(b) _0.006(b)

PO10 ST 0.8 600 0.2 0.006(b)

(a)_{Max value can be sometimes very high such as 1A for SKKT162/16.}(b)_{Max value.}

Indeed, the minimum value of ILdepends on the adopted delay angle control α, load impedance and on the RMS value of the applied voltage. Considering these points, in most of industrial solutions, the driving pulses length are oversized in time. Hence, the triggering pulse width is much longer than the actual width needed to guarantee the device turning on with a driving signal and to achieve the desired control function. The aim of the paper is to investigate and propose an optimum solution, which can improve overall system efficiency and functionality and can be adopted in wide range of applications.

Here, it is also worth understanding a very important aspect in certain industrial applications. The conduction angle (sometimes also denoted as ignition angle) is very important as it depends on the value of the consequent current’s peak value. In some applications, for example in the case of permanent magnet systems, the state of magnetization of the system depends precisely on the peak

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value of the magnetizing current. To increase this peak value, with the same system, it is possible to act on the ignition angle, making it small. Therefore, the possibility of having a system that as a whole (hardware and software) is able to turn on the SCR at low conduction angles is certainly an advantage. The advantages are multiple: in terms of ampere-turns, it is therefore possible to either obtain more from the same system or to decrease ampere-turns, obtaining an undeniable advantage in economic terms by saving on material and dimensions. All of these are obviously of completely general validity even if, in the aforementioned application, it appears to be of fundamental importance to influence the competitiveness or otherwise of the adopted solution.

The obtained results could be also influenced by the type of application, the type of driver circuit, the adopted control method and, obviously, by the characteristics of the SCR used (and with this the presence and also the importance of the above discussion).

Finally, the importance of the considerations and evaluations (numerical and experimental) that are discussed in the following sections (Sections3and4) are very evident.

3. Problem Formulation: General and Practical Discussions

3.1. General Discusion

The equivalent circuit, in Figure5, shows the typical representation of the SCR application, where two SCRs in back to back configuration are connected to an ohmic-inductive (R-L) load.

in economic terms by saving on material and dimensions. All of these are obviously of completely general validity even if, in the aforementioned application, it appears to be of fundamental importance to influence the competitiveness or otherwise of the adopted solution.

The obtained results could be also influenced by the type of application, the type of driver circuit, the adopted control method and, obviously, by the characteristics of the SCR used (and with this the presence and also the importance of the above discussion).

Finally, the importance of the considerations and evaluations (numerical and experimental) that are discussed in the following sections (Sections 3 and 4) are very evident.

3. Problem Formulation: General and Practical Discussions 3.1. General Discusion

The equivalent circuit, in Figure 5, shows the typical representation of the SCR application, where two SCRs in back to back configuration are connected to an ohmic-inductive (R-L) load.

Figure 5. Equivalent circuit of SCR application for R-L load: two SCRs in back to back configuration are connected to an ohmic-inductive (R-L) load.

The lagging load angle, φ, is the phase displacement between load voltage and current, and its expression is given by:

= (1)

where X is the reactance and R is the resistance of the load. The second Kirchhoff law permits to write the expression illustrate below:

( ) = ∙ ( ) + ( ) ∙ ( )+ ( ) ∙ ( ) (2)

where L is the inductive part of the load. Note that, in many applications, the last term of Equation(2) can be discarded because it represents the dependency of the inductance to the time variation, which it is either very low in magnitude or very slow (different time domain) comparing to electrical current’s dynamic. Ignoring the last term and solving the previous equation, the current expression can be obtained:

( ) = √2 ∙ ∙ ( ∙ − ) − √2 ∙ ∙ ( − ) ∙ ⁄ (3)

where:

 i(t) is the sum of a regimen term (first term) and a transient one (second term), which depends on time constant τ equal to L/R;

 V is the rms voltage value applied by the voltage source, v(t);  Z is the load impedance;

 α is the control phase angle evaluated starting from the voltage zero crossing and it can be managed by the gate driver circuit often triggered through a supervisory microcontroller.

L v(t) R SCR Vgate Vgate LOAD

Figure 5.Equivalent circuit of SCR application for R-L load: two SCRs in back to back configuration are connected to an ohmic-inductive (R-L) load.

The lagging load angle,ϕ, is the phase displacement between load voltage and current, and its expression is given by:

ϕ = arctgX

R (1)

where X is the reactance and R is the resistance of the load. The second Kirchhoff law permits to write the expression illustrate below:

v(t) = R·i(t) +L(t)·di(t) dt +i(t)·

dL(t)

dt (2)

where L is the inductive part of the load. Note that, in many applications, the last term of Equation(2) can be discarded because it represents the dependency of the inductance to the time variation, which it is either very low in magnitude or very slow (different time domain) comparing to electrical current’s dynamic. Ignoring the last term and solving the previous equation, the current expression can be obtained: i(t) = √ 2·V Z·sen(ω·t − ϕ)− √ 2·V Z·sen(α − ϕ)·e −t− αω τ ₍₃₎ where:

negative.

forward biased;

i(t) is the sum of a regimen term (first term) and a transient one (second term), which depends on time constant τ equal to L/R;

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Energies 2020, 13, 3539 7 of 20

negative.

forward biased;

V is the rms voltage value applied by the voltage source, v(t);

negative.

forward biased;

Z is the load impedance;

negative.

forward biased;

α is the control phase angle evaluated starting from the voltage zero crossing and it can be managed by the gate driver circuit often triggered through a supervisory microcontroller. Figure6decomposes the current expression in (3). Here, it is possible to observe both the initial transient component and the regime (steady-state) components of the current. During the first instants, the two components have close values, but considering the negative polarity of the transient component in (3), the results of the Equation (3) are about null at starting point, which can be in the range of the driver pulse duration. In this situation, the SCR ignition can be very difficult, and it is possible not to reach required ILand fail to gain the control of SCR.

Figure 6 decomposes the current expression in (3). Here, it is possible to observe both the initial transient component and the regime (steady-state) components of the current. During the first instants, the two components have close values, but considering the negative polarity of the transient component in (3), the results of the Equation (3) are about null at starting point, which can be in the range of the driver pulse duration. In this situation, the SCR ignition can be very difficult, and it is possible not to reach required IL and fail to gain the control of SCR.

Figure 6. Simulated shape of the current.

Considering this problem, it should be possible to evaluate the necessary time (tα+_{) for the current}

to reach the latching current IL by inversing (3). Given the complexity and nonlinearity of the Equation

(3), it is necessary to simplify it to obtain the desired result. In particular simplified expressions of the elements 𝑒−𝑡−𝛼 𝜔𝜏⁄ _{and 𝑠𝑒𝑛(𝜔 ∙ 𝑡 − 𝜑) as reported in the following expressions (obtained from the}

MacLaurin series expansion).

𝑒−𝑡−𝛼 𝜔𝜏⁄ = 𝑒−𝑥_{= 1 − 𝑥 +}𝑥 2 2!+ 𝑥3 3!… + (−1) 𝑛_∙𝑥 𝑛 𝑛!+ ⋯ 𝑠𝑒𝑛(𝜔 ∙ 𝑡 − 𝜑) = 𝑠𝑒𝑛(𝑥) = 𝑥 −𝑥 3 3!+ 𝑥5 5!… + (−1) 𝑛_∙ 𝑥 2∙𝑛+1 (2 ∙ 𝑛 + 1)!+ ⋯ (4)

It can be seen that when the conduction starts, the element 𝑒−𝑡−𝛼 𝜔

⁄

𝜏 _{can be simplified by the first}

two elements of the MacLaurin series, while the term 𝑠𝑒𝑛(𝜔 ∙ 𝑡 − 𝜑) can be simplified, by the first element of the MacLaurin series, only when the value of it is close to zero.

Considering only the first elements of the MacLaurin series and using a time shift of α (t’ = t- 𝛼 𝜔⁄ ), the following simple expression of the current reported in (5) is obtained.

𝑖(𝑡′) = √2 ∙𝑉 𝑍∙ (𝜔 ∙ 𝑡′ + 𝛼 − 𝜑) − √2 ∙ 𝑉 𝑍∙ 𝑠𝑒𝑛(𝛼 − 𝜑) ∙ (1 − 𝑡′ 𝜏) (5)

Expression (5) provides the possibility to evaluate the time t’, which is the time required to reach a specified current i(t’). Simply replacing i(t) equal to latching current IL it is possible to obtain the

time required by the system to reach IL current and it will result in Expression (6):

𝑡′ = 𝐼𝐿∙ 𝑍

√2 ∙ 𝑉+ 𝑠𝑒𝑛(𝛼 − 𝜑) − 𝛼 + 𝜑 𝜔 +𝑠𝑒𝑛(𝛼 − 𝜑)_𝜏

(6) It is to be noted that expression (6) is applicable only when the conduction starts close to the point where the elements 𝑠𝑒𝑛(𝜔 ∙ 𝑡 − 𝜑) is close to zero. In the case where this element value is far from zero, it is necessary to adopt a different approach to compute the desired time value. In this study, in order to obtain this time value, Expression (3) is simulated in a dedicated simulation software and corresponding time required to reach the latching current (IL) is extracted.

In order to represent the proposed approach, the values of delay angle α is varied between 0 and 10 ms considering a load with R = 10 Ω, L = 55 mH (typical values for the considered application), V = 230 V, f = 50 Hz (standard industrial values for Europe), and IL = 0.4 A (typical value for high current

Figure 6.Simulated shape of the current.

Considering this problem, it should be possible to evaluate the necessary time (tα+) for the current to reach the latching current ILby inversing (3). Given the complexity and nonlinearity of the Equation (3), it is necessary to simplify it to obtain the desired result. In particular simplified expressions of the elements e−t− ατω _{and sen}(ω·t − ϕ)as reported in the following expressions (obtained from the MacLaurin series expansion).

e−t− ατω = e−_x = 1 − x+x_2!2 +x_3!3 . . .+ (−1)n·xn n! +. . . sen(ω·t − ϕ) = sen(x) = x −x_3!3 +x_5!5. . .+ (−1)n· x2·n+1 (2·n+1)!+. . . (4)

It can be seen that when the conduction starts, the element e−t− ατω can be simplified by the first two elements of the MacLaurin series, while the term sen(ω·t − ϕ)can be simplified, by the first element of the MacLaurin series, only when the value of it is close to zero.

Considering only the first elements of the MacLaurin series and using a time shift of α (t0= t-_ωα), the following simple expression of the current reported in (5) is obtained.

i(t0) = √ 2·V Z·(ω·t 0 +α − ϕ)− √ 2·V Z·sen(α − ϕ)· 1 − t0 τ ! (5)

Expression (5) provides the possibility to evaluate the time t0, which is the time required to reach a specified current i(t0). Simply replacing i(t) equal to latching current ILit is possible to obtain the time required by the system to reach ILcurrent and it will result in Expression (6):

t0 =

IL·√Z_2·V+sen(α − ϕ)−α+ϕ ω+sen(α−ϕ)_τ