1. Power Flow Interactive Sharing between Two DC Nanogrids Photovoltaic Local Branch Dynamic Systems at Island Operation
Maged F. Bauomy1 , Haytham Gamal2, Adel A. Shaltout3, Mostafa Noah 4, Masayoshi Yamamoto 5
1SHAKER Consultancy, Doha, Qatar, 2Modern Academy, Cairo, Egypt, 3Cairo University, Giza, Egypt, 4Shimane University, Japan, 5Nagoya University, Japan
Speaker : Eng. / Mostafa Noah , Shimane University, Japan
EPE 2017, ECCE Europe, September 2017, Warsaw, Poland
Abstract—Nowadays DC nanogrid topologies are going spread with the deployment of distributed generation. Therefore, the investigation of interactive sharing between DC nanogrid branches is mandatory to realize a continuous operation of the photovoltaic considering: maximum power tracking, Off-maximum power point tracking, load sharing, and battery charge sharing. This paper discusses the power flow interaction among two DC nanogrids in island mode. To demonstrate that, a detailed dynamic branch modeling systems is proposed. The MPPT and Off-MPPT control are tested on this nanogrid dynamic model; the MPPT is working continuously unless the two BSS are fully charged which is known by the reference indicator state of charge (SOC) value. If the BSS reaches SOC 100 %, the Off-MPPT control will be operated to stop extraction of more power. The objective of interactive sharing is proposed by a control approach for the bidirectional converter utilized in charging and discharging of BSS. This control approach with the support of simple nanogrid controller transfers power via transmission line to performs load sharing and BSS charging of the nanogrid. The power transfer between the two buses will be performed by rising the sending bus voltage above the receiving bus.
Introduction
Two NG-LBS Description & Power Sharing Interaction Three Scenarios Explanation
Indeed, there is a new trend for distributed generation types progressing up with various topologies named nanogrid. In this regard, the DC nanogrids interconnection in island mode in residential field distributed generation by applying photovoltaic renewable source will be under investigation. This paper proposed an interactive systems depend on a detailed dynamic modeling for nanogrid local branch system (NG-LBS) for photovoltaic-battery coupled with DC boost converter and adding dynamic buck–boost bidirectional converter, this is done to manage the BSS charging and discharging operations. This paper focuses on interactive sharing among NG-LBS inside two residential homes. The control schemes for BSS will perform a vital task for dual action for charging and discharging for same BSS and for power flow sharing with the other NG-LBS, where the reference signals provided from nanogrid home controller based on an off-line calculations. The effectiveness of the proposed control schemes are demonstrated with a standalone mode and island mode. The Off-MPPT control is presented here, if the BSS is fully charged.
The PV array for both NG-LBS is (5x10) modules of EPV array at different G are 1,387 W at 400 W/m2, 2,844 W at 800 W/m2, and 3,549 W at 1000 W/m2.
The BSS system consists of two parallel branches Nb,par each one is 24 cell in series Nb,ser (2 x 24 cell).
H1 SOCH1=0.95% is almost fully charged, H2 SOCH2=0.3% is almost at last discharge state.
Load of H1 is always 15 ohm which means 960 W.
Load of H2 is 15 ohm and changed two scenarios (2 and 3) to be 5 ohm demand average power 2,880 W.
The (T.C.) is 0.2 ohm for copper cable 10 mm2, where the connecting length is 100 meters distance.
Fig. 3: Photovoltaic Voltage Vpv
Table I: Three Scenarios of Power Flow
Scenario Cases
NG-LBS-H1
NG-LBS-H2
PV power
Ppv,1
BSS power
Pbat,1
Load Power
PL,1
Scenario (1)
2,844 W
1,830 W (Chg.)
961.5 W
1,387 W
395 W
(Chg.)
960 W
Scenario (2)
627 W (Chg.)
1034 W
-537 W
(Disc.)
3,030 W
Scenario (3)
3,549 W
620 W (Chg.)
1,010 W
295 W
(Chg. By H1)
2,881 W
Two Nanogrids Local Branch Dynamic Systems Architecture
Fig. 4: Photovoltaic Power Home-1 Ppv,1
Fig. 5: Load Bus Voltage Home-1 VL,1
Fig. 6: Load Bus Power Home-1 PL,1
Fig. 1: Overall Architecture of the two nanogrids local branch dynamic system (NG-LBS)
Main Parts of Nanogrid:
Three main parts; PV generator PV-HX with DC-DC boost converter, battery storage system BSS-HX and load Bus, where X denotes home number. The two homes have a circuit breakers C.B-HX at (STDC).
Two Nanogrid System Analogy:
One nanogrid may be considered as a supply side and the other nanogrid home system as a demand side which depends on power flow path.
Fig. 7: Photovoltaic Power Home-2 Ppv,2
Fig. 8: Load Bus Voltage Home-2 VL,2
Fig. 9: Load Bus Power Home-2 PL,2
Fig. 11: Battery Power Home-2
Pbat,2
Fig. 10: Battery Power Home-1
Pbat,1
Modeling Components of Nanogrid Dynamic Local Branch Systems
Simulations &Results of Two NG-LBS Scenario (4) at Off-MPPT
For H(1), the maximum power is 2,844 W which exceed the connected load 960 W. The operation takes 1.75 hour for the home (1) till fully charged from the initial SOCH1.
For H(2), the maximum power is 1,387 W which exceed the connected load 960 W. The operation takes 2.5 hour for home (2) till fully charged from the initial SOCH2.
The indicator for charging value foe battery storage system is the SOC. In this regard, figures 14 and 17 are self-explanatory for the initial SOC till reach full charge value (1) which is equivalent to (100%).
Fig. 2: Typical nanogrid PV local branch dynamic system type components and control strategies
The nanogrid electric circuit is shown in Fig. 2 using the following components:
Enhanced Photovoltaic Diode Model for Shell SP-70 Type
The PV module can be simply represent by the Enhanced PV diode model
DC-DC Boost Converter dynamical Model
Switch On-Mode:
Switch On-mode and Switch Off-Mode
DC-DC Buck-Boost Converter dynamical Model
Buck mode for charging and Boost mode for discharging.
Lead Acid Battery Dynamical Model
Using Capacity, estimation of average current, extracted charge, state of charge , depth of charge and electrolyte temperature.
Fig. 12: Photovoltaic Power H-1 Ppv,1
Fig. 13: Battery Power H-1 Pbat,1
Fig. 14: Battery State of Charge H-1 SOCbat,1
Proposed Controllers
8. Conclusions
Fig. 15: Battery Power H-2 Ppv,2
Fig. 16: Battery Power H-2 Pbat,2
Fig. 17: Battery State of Charge H-2 SOCbat,2
PV Generator: MPP Controller
The MPPT controller presented is the polynomial computational method PCM.
PV Generator: Off - MPPT Controller
The Off-MPP controller for PV generator is required to stop the maximum power whenever there is no capacity in battery storage system.
Battery Storage System: Bidirectional Controller
This bidirectional DC-DC converter controller has three embedded controllers as follows; buck operation for charging mode, boost operation for discharging mode and switching selection mode of operation fed by switching signal K4 from nanogrid home controller.
Nanogrid LBS: Off-line Controller
This is done to avoid complexity of calculation with change of losses values of dynamical bidirectional converter at each reference value and change of bus voltage operation for each case.
Conclusion
This paper presents two NG-LBS based on a derived detailed dynamic models to study the system outputs performance.
The MPP is continuously tracked on both sides via PCM controller.
The Off-MPPT is operated when both BSS are fully charged to avoid overcharging.
In island mode, one nanogrid resembles a supply side sending bus and the other nanogrid resembles demand side receiving bus.
The bidirectional controller plays an important role in power flow interaction within nanogrids for load sharing and battery charge sharing.
Control schemes of bidirectional buck boost converter had been approached.
The nanogrid home controller fed the bidirectional controller by the following reference values; charging, discharging currents and bus voltage.
The validity of the power flow interaction is tested through three designed scenarios for load sharing, then for battery charge sharing which has a good financial benefit in future by decrease BSS sizing.
Fig. 3: Typical nanogrid PV local branch dynamic system type components and control strategies