5.3 Global Charging Station System Control
5.3.1 Simulation Model and Results
In order to verify the effectiveness of the proposed control system, a simulation model of the whole PET-based EV charging station has been built. Since the PWM effects are of no significant interest compared to the evolution of magnitudes with high time constants, the switching-averaged power converter models have been considered for all conversion stages. For the DHB converters the utilized model, which neglects the AC transformer quantities, is derived in Appendix D. The details for the parameters of the studied system as well as the charging station configuration are given in Table 5.2. A diagram of the considered PET-based ultra-fast EV charging station configuration is also illustrated in Figure 5.19.
Table 5.2: UFEVCS Simulation/Down-Scaled Prototype Parameters
Quantity Value Comment
N 6+2 Submodules/branch (w. redundancy) Nch 4 Charging ports (6 submodules each)
˜
ugk 140 V Grid rms phase voltage Ubat,nki 25.6 V Nominal battery voltage
Qkibat,n 3 Ah Nominal submodule battery capacity C1−4 2.7 mF Submodule/DHB capacitance
Usmki 50±10% V Submodule voltage (star-CHB) Lf 1 mH IAI filter inductance
Uoki 19-31 V DHB output voltage (EV battery) Q1EV,n−3 20 Ah Nominal EV battery capacity np: ns 1 DHB transformer ratio
Lσ 1.5 µH DHB transformer leakage inductance Lo 15 µH DHB filter inductance
δn π/6 rad DHB nominal phase-shift angle
Figure 5.20 shows the simulation results for the different operating modes of the converter architecture utilizing a hypothetical power profile. In Mode I, the converter behaves as an ultra-fast EV charger. It is assumed that the three out of four EV charging ports are used, whereas the fourth is in idle mode. The three EVs arrive at the station with a time difference of one minute and are charged within ten minutes each. The three ports charging the EVs will cause a discharge to the stationary intermediate batteries, whereas the fourth port will be charging the respective storage stage. In Mode II, the DHBs are not operating and the converter structure extracts active power from the grid, in order to recharge the power buffers. Accordingly, in Mode III the charging station provides active power to the grid by discharging the intermediate batteries. Finally in Mode IV, no active power is exchanged with the grid or the EVs and the CHB converter operates with its well known functionality of a STATCOM. At the end of the power profile, all battery SoCs are practically the same, due to the SoC balancing controllers that are in action throughout the whole converter operation.
5.3. Global Charging Station System Control
AC DC
DC DC
AC DC
DC DC
AC DC
DC DC
AC DC
DC DC
AC DC
DC DC
AC DC
DC DC
AC DC
DC DC
AC DC
DC DC
Charging Port 1
Charging Port 2
Charging Port 3
Charging Port 4
Figure 5.19: The considered simulation model for the PET-based ultra-fast EV charging station.
0 5 10 15 20 25
−8 0 8
Active/Reactive Power Flows [kW,kVar]
0 5 10 15 20 25
20 40 60 80
Individual Battery SoCs [%]
0 5 10 15 20 25
−12
−6 0 6 12
Individual Battery Currents [A]
0 5 10 15 20 25
0 20 40 60 80
EV Battery Currents [A]
0 5 10 15 20 25
40 50 60
Submodule Capacitor Voltages [V]
Time [min]
Mode: I II III IV
EV1 Charging EV2 Charging
EV3 Charging
QAC
PAC PEV Pbat
1 2 3 4
P ort
Figure 5.20: Simulation results of the PET-based charging station for a hypothetical power profile, exploiting all operation modes.
5.3. Global Charging Station System Control
−200 0
200 Grid Voltages [V]
300 300.01
−20 0
20 Grid Currents [A]
Time [s] 300.02
(a)
−200 0
200 Grid Voltages [V]
900 900.01
−20 0
20 Grid Currents [A]
Time [s]
900.02 (b)
−200 0
200 Grid Voltages [V]
1200 1200.01
−20 0
20 Grid Currents [A]
Time [s] 1200.02
(c)
−200 0
200 Grid Voltages [V]
1450 1450.01
−20 0
20 Grid Currents [A]
Time [s] 1450.02
(d)
Figure 5.21: Detailed versions of the three-phase grid voltage and current quantities for:
(a) Mode I, (b) Mode II, (c) Mode III and (d) Mode IV corresponding to Figure 5.20.
Figure 5.21 shows some details of the three-phase grid voltage and current quantities for different operation modes. It is clear that the control system is capable of keeping the current symmetry unaffected, even during asymmetric load conditions, e.g., when not all EV charging ports are in use and some intermediate batteries are charging while others are discharging. The active power evolution of the four different charging ports is illustrated in Fig. 5.22. It is clear that the submodule SoC balancing power ∆Psm plays a major role in the asymmetric power distribution between the stationary batteries. For example, if a charging port is not used, the grid power fed to it will be minimal, ensuring a small SoC variation in the respective batteries. On the other hand, the ports that need a larger amount of charging power will absorb more grid power leading to a need for less power coming from the stationary batteries.
The effect of the gain-scheduling controller for the vertical SoC balancing action, which is based in equations (3.38) and (3.42) is depicted in Figure 5.23. It is clear than when the SoC deviation exceeds a predefined value, the rise time of the closed-loop control system increases. This leads to a respective decrease of the control gain ksmp in order to avoid saturation of the control action, which would cause submodule overmodulation in this
−2
−1 0 1 2
Time [min]
Port 1 Active Power [kW]
−2
−1 0 1
2 Port 2 Active Power [kW]
0 5 10 15 20 25
−2
−1 0 1
2 Port 3 Active Power [kW]
0 5 10 15 20 25
−2
−1 0 1
2 Port 4 Active Power [kW]
PEV Pbat ΔPsm
0 5 10 15 20 25 0 5 10 15 20 25
Pgrid
Figure 5.22: Simulation results for the active power distribution within the four charging ports.
0 5 10 15 20 25
8 13 18
Gain of the Vertical SoC Balancing Controller [W/%]
0 5 10 15 20 25
0 6 12
Time [min]
Minimum Rise Times [min]
1 2 34 P ort
psm
k
Figure 5.23: Gain scheduling control behavior of the vertical SoC balancing controller.
5.4. Conclusion case. It is once again mentioned that the gain ksmp has to be the same for all submodules of a branch, in order to ensure that Σ∆Psmki = 0and that the grid current controller is not affected.
5.4 Conclusion
This chapter has presented a novel multiport Power Electronic Transformer-based concept for the realization of multifunctional medium voltage ultra-fast EV charging stations.
Among the numerous isolated converter topologies, the dual half-bridge converter seems to be a reasonable choice. The DHB converter analysis of the converter has revealed that with a change of the primary and secondary converter duty cycle an extension of the soft-switching region in wide input and output voltage variations can take place. The current control has been designed using nonparametric models, overcoming the challenges posed by the high-order and system nonlinearities.
A global system control structure has been described, which is capable of handling all different power flow directions as well as capacitor voltage and battery SoC unbalances.
The operating modes of the system have been presented and simulated, verifying the versatility of the introduced converter structure.
6 Conclusions and Future Research
6.1 Summary and Contributions
Modular Multilevel Converters are undoubtedly expected to play a major role in future large-scale medium- and high-voltage networks. Indeed, features such as a modular design, network friendliness through the injection of currents with very low harmonic contents, redundancy, as well as easy voltage and power scalability, imply evident offered advantages compared to the conventional power conversion concepts. It is no wonder that during the last five years a tremendous academic and industrial interest has been observed in this field. All this comes with a price to pay, i.e., the complexity in terms of physical system behavior, control system design, global hardware and software implementation, as well as arising reliability issues. Even though numerous literature has been appearing, there are many unanswered questions and challenges, as well as constant initiations of novel research directions.