Bidirectional dc-dc converter

A short summary of this paper. Download Download PDF. Translate PDF. To overcome these drawbacks, this paper proposes a new, nonisolated, DC-DC converter for the bidirectional power flow of battery energy storage applications in DC and hybrid microgrids HMGs. The proposed converter uses two back-to-back Boost converters with two battery voltage levels, which eliminates step-down operation to obtain symmetric gains and dynamics in both directions.

In discharge mode, two battery sections are in parallel connection at a voltage level lower than the grid voltage. In charge mode, two battery sections are in series connection at a voltage level higher than the grid voltage.

The results show that the proposed converter has promising performance compared to that of the conventional type. Moreover, the novel converter adds no complexity to the control system and does not incur considerable power loss or capital cost. Keywords: battery energy storage system; bidirectional DC-DC converter; dynamic performance; hybrid microgrid; renewable energy sources 1. Introduction The popularity of DC microgrids and their operation along with AC microgrids is increasing due to the elevated level of DC power renewable generations and ubiquitous DC loads.

HMGs, particularly in islanded operation, are prone to instability and power fluctuations due to the intermittent nature of renewable energy sources RES and the stochastic behavior of the loads.

It is imperative to regulate system oscillations with faster dynamics and reliable controllers. Electronics , 9, ; doi Figure 1 shows Boost converter in step-up mode, with two controllable semiconductor switches. Figure 1 shows the the conventional, nonisolated conventional, nonisolated BDC BDC with with some some drawbacks, drawbacks, as as noted noted below. Figure Conventional bidirectional 1. This asymmetry gain. This asymmetry originates originates from the from the different different circuit circuit structures structures inin both both modes.

Therefore, the modes. Therefore, the converter converter should should work work with with different duty ratios in both modes, resulting in an asymmetric and relatively slow control different duty ratios in both modes, resulting in an asymmetric and relatively slow control response during response during power power flow flow direction direction changeover.

This This issue issue is is addressed addressed to to some some extent extent by by selecting a small enough battery voltage compared to the grid voltage, resulting selecting a small enough battery voltage compared to the grid voltage, resulting in a high voltagein a high voltage difference between difference between two two sides; sides; otherwise, otherwise, the the Buck Buck operation operation would would notnot be be as aseffective.

However, However, this potential this potential difference difference results results in in high high current current peaks peaks inin both both switching switching operations, operations, leading leading to to high current ripples, particularly in heavy load levels.

This issue is usually tackled high current ripples, particularly in heavy load levels. This issue is usually tackled by selecting by selecting a large inductor, thereby incurring a higher capital a large inductor, thereby incurring a higher capital cost.

Other issues Other issues regarding regarding conventional conventional BDC BDC are are reported reported in in the the literature literature [13,14] [13,14] from from different different viewpoints, although this paper does not aim to address them. Previous viewpoints, although this paper does not aim to address them. Previous studies proposed several studies proposed several isolated and isolated and nonisolated nonisolated bidirectional bidirectional topologies topologies to to improve improve dynamic dynamic performance, performance, gain, gain, efficiency, efficiency, and operability and operability ofof BDCs BDCs for for energy energy storage storage and and renewable renewable applications.

Nonisolated,Nonisolated, resonant-type,resonant-type, bidirectional bidirectional converters converters were weretoproposed proposed increase to theincrease overall the overall voltage gainvoltage usinggain usinginductors coupled coupled inductors and clamping and clamping capacitor capacitor circuits operating in zero-voltage-switching ZVS conditions [15,16].

An isolated, three-port, An isolated, three-port, inductor-capacitor-inductor LCL -resonant inductor-capacitor-inductor LCL -resonantconverter converter operating operating in in zero-current-switching zero-current-switching ZCS ZCS condition was also implemented in [17] with the capability of integrating photovoltaic condition was also implemented in [17] with the capability of integrating photovoltaic PV source to PV source to energy storage.

However, However,these these resonant resonant converters converters addadd complexity complexity andtocost and cost bothtopower both power and and control controland stages stages and incur moreincurlossmore due loss due to additional to additional circuit components. In addition, In no addition, controlno control strategies strategies were were presented presented that symmetrically that symmetrically worked forworked both modesfor both modes in bidirectional in bidirectional applications.

A relatively A high voltage relatively high voltage gain gain was was achieved achieved for for step-down step-down operation operation using using aa switched-capacitor switched-capacitor cell augmented to a conventional half-bridge [18].

However, this configuration was cell augmented to a conventional half-bridge [18]. However, this configuration was anan improved improved version of the half-bridge converter, making only a slight improvement only version of the half-bridge converter, making only a slight improvement only in voltage gain at the cost in voltage gain at the of cost of additional additional components components with adesign with a control control designasymmetrically working working asymmetrically for both for both modes.

The modes. The nonisolated nonisolated converter inconverter [13] and its in improved [13] and itshigh-efficiency improved high-efficiency topology intopology in [19]soft-switching [19] provided provided soft-switching operation, operation, employing a ideal transformer and pulse-frequency employing a ideal transformer and pulse-frequency modulation to accommodate modulation to accommodate load load variation variation and reducedandswitching reduced loss switching loss andhigh and to achieve to achieve efficiency.

Electronics , 9, 3 of 21 a single loop Proportional-Integral PI controller that worked for both modes, with applications in residential energy storage systems. Research work in [20] proposed a nonisolated BDC employing two conventional Boost converters to achieve the squared value of voltage gain of that of the conventional BDC.

Although functional improvements in voltage gain, switching operation, and converter efficiency were obtained in the mentioned literature, the problem of bidirectional asymmetry in voltage gain and control scheme persists.

Several noteworthy features of the proposed converter are as follows: i It employs two Boost converters in back-to-back topology and has a minimum additional component compared with other proposed converter configurations.

Since there is no Buck operation and the battery voltage can be raised to higher levels, the converter inductors have less inductance and current capacity due to the reduced battery current. Proposed bidirectional DC-DC converter. Figure 2. Table 1 presents a summarized comparison of the salient features of the proposed BDC in this paper and other BDCs submitted in previously discussed research Main works.

Hybrid microgrid under the study. The HMG 4. Section given in4. Section 4. Electronics , 9, 4 of 21 Table Table1. Comparison Table 1. Comparison conventional, Table and proposed, between other and 1. Table 1. Discharge counted. Body are capacitors are considered. Body considered. Proposed not counted. Figure 3. Principle of Operation 2. Principle of Operation The proposed converter is composed of two Boost converters in a back-to-back configuration.

Therefore, there is only The proposed step-up converter mode in both is composed directions. The proposed configuration has two battery voltage levels in two modes of operation, i.

This is made by employing two battery sections with equal voltage levels, which work in parallel in discharge mode and in series in charge mode. Although two battery levels are used, in practice, battery packs are made up of several cells connected in series and parallel to achieve the desired voltage and current. Hence, this would not be a limitation. S1 and S2 are modulating switches that are separately controlled by the pulse width modulator PWM driver in discharge and charge mode, respectively.

EqualElectronics parameters make , 9, x FOR the base case for two modes. Thus, some adjustments in the power stage and control loops are needed controlled by the pulse width modulator PWM driver in discharge and charge mode, respectively. Thus, someand power switches adjustments diodes in aretheneglected power stage inand thecontrol loops are theoretical analysis for needed to make a symmetrical bidirectional operation.

D1 and D2 are in forward-bias and analysis for simplicity [21—23], but are included in the simulation analysis for accuracy. Discharge mode operation: Figure 4 shows the converter circuit in discharge mode. However, see Figure 4b. However, this should not be the case in low-voltage applications.

Figure 4. Discharge Discharge mode mode circuit. Also, C can be removed since the battery charge level is high enough to changes. Figure 6 shows a simulation keep v1 constant. Electronics , 9, 6 of 21 In mode changeover state, inductors are fully discharged before the current flow direction changes.

Hence, S3 and S4 are switched under zero current conditions. Charge a Current Current flow flow diagram; b equivalent circuit. Figure 5. Charge mode circuit. Inductor currents Figure 6. In2raddition, c. Also, they should bebeimplemented bybydevices deviceswithout withoutbody bodydiodes diodes such as switching loss [24].

Also, Also, theythey should should implemented be implemented by devices without body diodes such as such as insulated-gate insulated-gate bipolar bipolar transistors transistors IGBTs. Converter ConverterTransfer TransferFunctions Functions In Inorder ordertotodesign designthetheconverter convertercontrollers, controllers,aalinearized linearizedmodel modelof ofthe theconverter converterisisderived derivedbased based on onthe thestate-space state-spaceaveraging averagingforforcontinuous continuousconduction conduction mode mode CCM CCM [21,22].

Since Sincethe theconverter converterhas has the the same structure structure but butinequivalent inequivalent parameter parameter values values in both in both modes, modes, one equivalent one equivalent model model is is derived derived that is that is valid forvalid bothfor both as modes, modes, as depicted depicted in Figurein7.

Figure 7. Equivalent Boost converter circuit; e1 and e2 represent input and output voltages, respectively, Figure 7. This mode can moreover be divided into two intervals depending on the conduction on the switch Q1 and diode D2. Interval 1 Q2-on, D2-off ; Q1-off, D2-Off : In this mode Q2 is on and hence can be examined to be short-circuited, hence the lower voltage battery charges the inductor and the inductor current goes on rising till not the gate pulse is separated from the Q2. Also since the diode D1 is reversed biased in this mode and the switch Q1 is off, no current flows into the switch Q1.

Interval 2 Q1-off, D1-off; Q2-off, D2-on : In this mode, Q2 and Q1 both are off and therefore can be considered to be opened circuited.

Now since the current flowing into the inductor cannot change immediately, the polarity of the voltage across it reverses and hence it starts acting in series with the input voltage. Therefore the diode D1 is forward biased and so the inductor current charges the output capacitor C2 to a greater voltage. Therefore the output voltage boosts up. Mode 2 Buck Mode : In this mode switch Q1 and diode D2 begin into conduction depending on the duty cycle whereas the switch Q2 and diode D1 are off all the time.

This mode can moreover be divided into two intervals depending on the conduction on the switch Q2 and diode D1. Again as the inductor current cannot change instantaneously, it gets discharged via the freewheeling diode D2. The voltage across the load is stepped down as correlated to the input voltage. A comparison between the features of the non-isolated bidirectional topologies have been explained below:. During step-up mode, in the buck-boost bidirectional converter the RMS value of the current in the inductor and the power switches is greater by an amount equivalent to the output current as compared to the buck-boost cascade bidirectional converter.

Hence in the bidirectional buck-boost converter the power switches, inductor, and the capacitor work under more thermal and electrical stresses as compared to the buck-boost cascade converter following in greater power loss and also creating the saturation of the inductor core. Also as the stress on the MOSFET and the diode is higher, the buck-boost bidirectional converter needs power devices with larger device ratings.

Higher RMS currents also result in greater conduction losses and thus reducing the overall efficiency of the buck-boost bidirectional converter. Protection The block contains an integral protection diode for each switching device. Diode with no dynamics The Diode block Prioritize model fidelity by precisely specifying reverse-mode charge dynamics. Diode with charge dynamics The dynamic model of the Diode block.

Connect the vector signal to the G port. Variables Use the Variables settings to specify the priority and initial target values for the block variables before simulation.

Conserving expand all G — Switching device gate control electrical vector. Data Types: double. Parameters expand all Switching Devices These tables show how the visibility of Switching Devices parameters depends on the converter model and switching devices that you select. Switching device type for the nonisolated converter model. Forward voltage — Voltage 0. On-state resistance — Resistance 0. For the different switching device types, the On-state resistance is taken as: GTO — Rate of change of voltage versus current above the forward voltage Ideal semiconductor switch — Anode-cathode resistance when the device is on IGBT — Collector-emitter resistance when the device is on Thyristor — Anode-cathode resistance when the device is on Averaged switch — Anode-cathode resistance when the device is on.

Drain-source on resistance — Resistance 0. Threshold voltage — Voltage threshold 6 V default scalar. Gate trigger voltage, Vgt — Voltage threshold 1 V default scalar. Holding current — Current threshold 1 A default scalar.

Forward voltage HV — Voltage 0. Drain-source on resistance HV — Resistance 0. On-state resistance HV — Resistance 0. For the different switching device types, the On-state resistance HV is taken as: GTO — Rate of change of voltage versus current above the forward voltage Ideal semiconductor switch — Anode-cathode resistance when the device is on IGBT — Collector-emitter resistance when the device is on Thyristor — Anode-cathode resistance when the device is on Averaged switch — Anode-cathode resistance when the device is on.

Threshold voltage HV — Voltage threshold 6 V default scalar. Holding current HV — Current threshold 1 A default scalar. Forward voltage LV — Voltage 0. Drain-source on resistance LV — Resistance 0. On-state resistance LV — Resistance 0. For the different switching device types, the On-state resistance LV is taken as: GTO — Rate of change of voltage versus current above the forward voltage Ideal semiconductor switch — Anode-cathode resistance when the device is on IGBT — Collector-emitter resistance when the device is on Thyristor — Anode-cathode resistance when the device is on Averaged switch — Anode-cathode resistance when the device is on.

Threshold voltage LV — Voltage threshold 6 V default scalar. Holding current LV — Current threshold 1 A default scalar. Protection Diode The visibility of Protection Diode parameters depends on how you configure the protection diode Model dynamics and Reverse recovery time parameterization parameters. Protection Diode Parameter Dependencies Parameters and Options Model dynamics Diode with no dynamics Diode with charge dynamics Forward voltage Forward voltage On resistance On resistance Off conductance Off conductance Junction capacitance Peak reverse current, iRM Initial forward current when measuring iRM Rate of change of current when measuring iRM Reverse recovery time parameterization Specify stretch factor Specify reverse recovery time directly Specify reverse recovery charge Reverse recovery time stretch factor Reverse recovery time, trr Reverse recovery charge, Qrr.

Model dynamics — Diode model Diode with no dynamics default Diode with charge dynamics. Diode type. The options are: Diode with no dynamics — Select this option to prioritize simulation speed using the Diode block. Note If you select Averaged Switch for the Switching Device parameter in the Switching Device setting, this parameter is not visible and Diode with no dynamics is automatically selected. On resistance — Resistance 0.

Conductance of the reverse-biased diode. Junction capacitance — Capacitance 50 nF default scalar. Diode junction capacitance. Peak reverse current, iRM — Current A default scalar less than 0. Peak reverse current measured by an external test circuit. Initial forward current when measuring iRM — Current A default scalar greater than 0. Rate of change of current when measuring peak reverse current.

Reverse recovery time parameterization — Recovery-time model Specify stretch factor default Specify reverse recovery time directly Specify reverse recovery charge. Reverse recovery time stretch factor — Stretch factor 3 default scalar greater than 1. Reverse recovery time, trr — Time 15 us default scalar. Transformer The Transformer parameters are only visible when Block choice is set to Isolated converter.

Transformer inductance L1 — Inductance 10 H default positive scalar. Self-inductance of the first winding of the transformer. Dependencies This parameter is only visible when Block choice is set to Isolated converter. Transformer inductance L2 — Inductance 0. Self-inductance of the second winding of the transformer. Transformer coefficient of coupling — Coupling coefficient 0.

Defines the mutual inductance of the transformer. Inductor series resistance — Inductor series resistance 0 Ohm default zero or positive scalar. Series resistance of the inductor. Capacitance, C1 — Capacitance 1e-7 F default positive scalar. Capacitance of the first DC terminal. Capacitance, C2 — Capacitance 1e-7 F default positive scalar. Capacitance of the second DC terminal. C1 effective series resistance — Resistance 1e-6 Ohm default zero or positive scalar. Series resistance of capacitor C1.

C2 effective series resistance — Resistance 1e-6 Ohm default zero or positive scalar. Series resistance of capacitor C2. Snubber — Snubber model None default RC snubber.

Snubber for each switching device. Dependencies See the Snubbers Parameter Dependencies table. Snubber resistance — Resistance 0. Resistance of the snubbers. Snubber capacitance — Capacitance 1e-7 F default scalar. Capacitance of the snubbers.