Power Electronic Interface

The bidirectional converter has been optional for many applications in power system, such as EVs, and connection of storage system and renewable energy to the microgrid network. It is used to improve the system performance, reduce the cost, and enhance efficiency.

The power flow from the network to the storage systems (batteries or capacitors) during the charging period. Storage systems should boost the power to the grid during fluctuation or instability period. Therefore, the bidirectional converter has been used to transfer power between sources.

Most of the bidirectional converters are classified into voltage source or current source converter. It depends on the energy storage system as shown in Figure 2-11. Bi-directional energy flow is achieved by a unidirectional semiconductor switch such as MOSFET or IGBT connected in parallel with a diode.

Bidirectional converters are also classified into non-isolated and isolated converters depending on the construction to employ for different applications.

Bidirectional Dc-Dc

converter

+

-

+

-

Forward power flow

V1>V2

Backward power flow

V1<V2

V1

V2

I1

I2

Figure 2-11: Bi-directional power flow

2.10.1 Non-isolated converter:

This converter is constructed without a transformer. Usually, a buck-boost type is used. The buck converter is used to step the voltage down. Meanwhile, the boost converter is used to step the voltage up. The transformerless converter is considered to improve efficiency, cost, weight, and size for the high-power applications. Single phase or multi-phase is presented by the literature according to the power density application. The advantages of the multi-phase converter that is implemented by the device are low current stress, minimising inductance and capacitance on both sides of converter for acceptable voltage ripple, and increasing efficiency [192]–[200].

2.10.2 Isolated converter

Many specifications have been achieved according to the power supply industrial applications such as isolation between supply and demand, high efficiency with low total harmonics, and

high power density to reduce the size and weight of the equipment. However, these devices have many complexities to achieve a design feature and prevent electromagnetic interference [201], [202, Ch. 14].

Isolated bi-directional DC-DC converter, which has been used through the last years by researchers, has a structure as shown in Figure 2-12. The converter works on both sides; the incoming DC voltage is converted to AC waveform then it is transferred to the secondary side through the transformer. The AC waveform is reconverted to the DC waveform again at a different step, which is the output of the converter [203], [204]. Even though the transformer can isolate two voltage sources, it works as impedance matching between them; nevertheless, it adds additional cost and losses.

+

-

+

-

Primary

converter

Secondary

converter

Isolation

Transformer

Figure 2-12: Bi-directional topology

Researchers after DeDoncker et.al [205], who patented first dual active bridge (DAB) converter, have identified different topologies for the converter depending on the converter rating, such as the isolated converters of clamped Bi-directional flyback converter; current fed push-pull converter, and bridge converter (Half and full bridge) [206]–[211].

2.10.3 Fly back converter:

A researcher suggested connecting two converters back to back to make them work in the bidirectional mode [212]. MOS1 and MOS2 modulation achieve the bidirectional power flow. The flyback converter that has a simple topology with fewer components without any output filter supplies a different voltage level by multi-winding transformer [213]. However, the high peak current due to the discontinuous current through this circuit reduces the efficiency. The converter works without any filter. Therefore, the whole energy is stored in the huge transformer magnetisation winding and increases the switching rating for this circuit. Very low leakage inductance design requires transferring energy effectively through transformer [201], [214]. This converter is used for low power applications (less than 100 W) with the high output voltage. The converter is mostly used in discontinuous mode [202, Sec. 14.2.2]

2.10.4 Push-pull converter:

The converter could be bidirectional by connecting a secondary converter to the secondary side of the transformer, which is called hybrid converter. This converter has different primary and secondary voltages [215].

The voltage of primary winding swings between +Vs to –Vs during the complementary switching, the primary voltage is transferred to the output through the secondary transformer and diodes [201], [202, Sec. 14.2.4].

The input current to the transformer, which has grown as a result of the non-ideal modulation signal, could be regulated by the inductor to prevent converter failure due to transformer core saturation as a result of overvoltage event. This saturation is called staircase [201], [216, Sec. 2.2]. However, double input dc voltage should be blocked during each switching cycle. The high blocking voltage makes this converter too expensive. This converter is used in different low voltage applications (less than 400 V) and power levels such as power factor correction, inverter battery charger, supercapacitor connection, fuel cell connected to batteries and hybrid EV [217], [218].

2.10.5 Bridge converter

This converter is classified according to the number of legs to the half bridge (one leg) or full bridge converter (two legs for single phase bridge and three legs for three phase bridge). Each leg operates in complementary switch signal with small delay time to prevent working at the same time. Half-bridge converter is popular for the Isolated DC-DC converter.

The main operation of this converter is to convert DC voltage to AC voltage using the first part (primary side of the transformer) and then to the DC voltage using another part (secondary side of the transformer). This configuration makes the converter work in bidirectional mode at the symmetry construction. Each part can work as a bidirectional power flow from AC to DC or vice versa [219], [220]. The current flow between these two parts can be limited by the inductor, which is connected to them.

Half-bridge converter has less switching than full bridge converter. However, it needs a bulky capacitor to manage a large ripple current amount, which is produced on the AC side. This capacitor can also be used to reduce the oscillation in the power flow. This converter is used for low power application (less than 2 kW) with low voltage level (less than 400V) [202, Sec. 14.2.5,14.2.6], [205], [216, Sec. 3.2]. The full bridge converter is more popular than half bridge converter because the current flow is distributed through more than one leg. Two legs and three legs converters have 180 and 120 phase shift between legs that help to reduce AC component for the power flow through the transformer.

The more the legs, the smaller DC capacitor used, and the less transformed core material is used because the summation of flux is reduced due to the offset between legs. However, the switch modulation complexity increases and the thermal problem appears with the legs increase [221], [222]. Full bridge converter has more flexibility in power rating than others. Thus, the voltage level reaches up to 1KV with high power rating up to 50KW [223].

The converter topology selection depends on the power rating. Fly back, and the push-pull converters are used for rated power up to 100 W and 2 KW respectively. Meanwhile, bridge converter is more appropriate for high power rating. Three phase full bridge converter has more complex construction and switching control than the two-phase full bridge converter. In addition, high cost and heat management are obtained due to the less transformer core.

2.10.6 Multi-Level inverter topology

The multi-level inverter is one of the solutions for high power and high voltage conversion system including a hybrid electric vehicle. The most attractive benefits of the multilevel inverter are generating an output voltage and drawing input current with low distortion in addition to operating with lower switching frequency [224]. There are many existing topologies for multi-level converter; the optimum solution has chosen the topology with less harmonic distortion, and less electromagnetic interface. A modified Cascade H-Bridge Converter has been chosen for this study, due to using a low number of components with separate DC resources. One of them is the battery that connects to the three-leg inverter (one leg for each phase) while the other is a supercapacitor that is connected in series to the battery through H-bridge cascade converter, as shown in Figure 2-13. The modified H-bridge cascade converter works with DC resources separately to be easily accommodated. This topology uses single battery bank and separated supercapacitors that connect in series with the battery for each phase. The supercapacitors work as an active filter.

Figure 2-13: Three-phase H-bridge cascade multi-level convert

V_SCA V_SCB V_SCC V_bat Ia (t ) Ic (t ) Ib (t )

S1_u S2_u S3_u

S3_l S2_l S1_l A1_u A1_l B1_u A2_u A2_l B1_l B2_l

B2_u C1_u C2_u

C2_l C1_l Van(t) n Vcn(t) Vbn(t)

2.10.7 Other topologies

Power factor correction converters or improved power quality converters are other topologies, which could be used to enhance power quality for a system in both sides; line and load. In addition, it provides a reduction of harmonic distortion, regulating dc voltage, and achieves higher efficiency. Even though, the sudden change of load or source voltage fluctuation occurs. These topologies could be classified according to the power transfer to the following:

1- AC-DC-DC using voltage source inverter, and a half or full wave bridge DC-DC converter.

2- AC-AC-DC using cyclo-converter or matrix converter, and a half or full wave DC-DC converter [219], [220], [225].