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level.

           

t

t k k

k k

k ref

 

  SoC -SoC 1 DR

DR R

D  (6.14)

All of the distributed storage units in the feeder operate according to the control strategy described above, based on the local information at the PCC, such as the PV output, the local demand, and the PCC voltage.

Although this thesis proposes a storage control strategy based on local parameters only, a coordinated control strategy of multiple storage devices in the feeder may also be possible if a communication system become available as envisaged in the future smart grid. In such a coordinated operation, the storage devices at different locations would be able to exchange information on the available storage capacity and the surplus power at different PCCs using the communication systems, to make a better use of the total storage capacity available in the feeder to store an optimum amount of total excess PV generation in the feeder.

The present thesis mainly discusses the mitigation of reverse power flow caused by active power injection from rooftop solar PV in LV distribution feeders, resulting in the avoidance of a voltage-rise, although the system does not intentionally control the voltage. As there is no voltage control set-point in the proposed strategy, it is different than an on load tap-changer or voltage regulator located in medium voltage system where a feed-back arrangement is used to control the voltage at the load centre. For the same reason, it will not interfere with the operation of the on load tap-changer or the voltage regulator. The only effect of this controller is to make the on-load tap changer and the voltage regulator to have less switching operation and hence less stress. With a communication system, the battery storage system can be coordinated with the on-load tap changer as presented in [9].

shown in Fig. 6.11. One of the LV feeders is shown in detail in a dashed circular shape.

The LV feeder data used for analysis are presented in Table 6-I.

Fig. 6.11. A practical distribution system in Australia.

Typical electric appliances used in residential households were used to model the aggregate loads at the different phases of the test feeder. The PV system sizes at the residential households were limited within 4 kW range and operated at unity power factor. The selection of a proper sizing methodology for energy storage is a complex task and may require optimization process, as discussed in [19]. In this research, a lead-acid energy storage device rated at 250 Ah, 24 V, was integrated at each of the rooftop PV system which is a typical size for residential rooftop PV applications in Australia for a PV rating of 2-4 kW [20]. The standard charging/discharging curves of a 12-V lead-acid battery available in [13] were used to model a 24-V lead-lead-acid battery using the method described in section 6.3.1 and the obtained charging/discharging curves are shown in Fig. 6.12. The specifications of the charging/discharging control strategy are given in Table 6-II. The charging/discharging parameters CRSat, DRSat, SCR, and SDR are calculated using (6.9)-(6.10).

TABLE6-I

DATA OF A TYPICAL AUSTRALIAN LOW VOLTAGE FEEDER

Feeder Length (metre) 350

Pole to Pole Distance (metre) 30-40

Conductor 7/3.00 AAC

MV/LV Transformer Size 160 kVA

PV Size (kW) 2-4 kW

PV Module Manufacturer Kyocera

Inverter Manufacturer SMA Sunnyboy

Battery Storage Model Lead-acid Trojan L-16W Battery Storage Voltage (V) 212

Battery Storage Capacity (Ah) 250

TABLE6-II

SPECIFICATIONS OF CHARGING/DISCHARGING CONTROL STRATEGY

Parameter Value

CRSat, DRSat C/6.56, C/3.34

SCR, SDR (per minute) 0.1619, 0.6243 Charging and Discharging Profile Triangular

ToS1, ToS2 70%, 70%

SoCmax, DoDmax 100%, 40%

 for CR, DR compensation 100%

Fig. 6.12. Charge/discharge curves of a 24-V lead acid battery modeled for different charging/discharging rates.

The network analysis is performed using a three-phase four-wire power-flow tool

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State of Charge, SoC [%]

Battery Voltage [V]

Charge C/5 Discharge C/5 Charge C/10 Discharge C/10 Charge C/20 Discharge C/20

[21] developed by the authors, based on the current mismatch variant of the Newton-Raphson power flow algorithm [22]. The proposed storage control strategy is tested with steady-state simulations of the test network at each instant of time under consideration.

To test the developed methodology in a realistic PV generation environment, a solar PV output profile captured on the 9th of February 2011, in the Renewable Energy Integration Facility, CSIRO Energy Technology, Australia, is used. The PV output, the load demand, the energy storage power and the net active power injection into the grid from 8:00 hours to 24:00 hours for the PCC corresponding to phase a of node 10 of the test LV feeder is shown in Fig. 6.13(a). The PV output was distorted by the sudden changes in the sun irradiance level. It is assumed that the SoC of the storage device is at DoDmax at the start of the simulation.

The storage starts to charge at about 9:00 hours by detecting a reverse power flow larger than a set threshold of 0.1 kW at the PCC. Using a triangular charging profile, the charging rate increases at the SCR specified in Table 6-II until it attains ToS1, which is 70% of the total battery capacity. The SoC profile is shown in Fig. 6.13(b). Due to the triangular charging profile, the second threshold ToS2 is also 70%, and therefore, the charging rate starts to decrease from this point at the same SCR until the charging rate becomes zero. Due to the power consumed by the storage in the charging operation, the active power injection into the grid without storage (dotted blue line) in Fig. 6.13(a) is higher than the active power injection with storage (solid blue line).

Within the charging period, several fluctuations in PV output appear, as marked using the dotted circular shapes in Fig. 6.13(a). The proposed control strategy detects the sudden changes in PV output and places the storage in a short-term discharge mode, as observed by the negative spikes in the battery power shown in Fig. 6.13(a). This counteracts the sudden fluctuations in the PV power. As a result, the fluctuations in surplus power injection at the PCC are also reduced, as observed by comparing the solid and dotted blue lines in Fig. 6.13(a). The drops in the SoC levels in the zoomed box in Fig. 6.13(b) indicate the short-term discharges. It is also observed in Fig. 6.13(b) that the SoC level without the charging rate adjustment does not reach the SoCmax level.

Fig. 6.13. (a) Load demand, PV output, storage power and active power injection into grid; (b) SoC profile; (c) Voltage profile without storage; (d) Voltage profile with storage.

The discharge operation starts at 19:00 hours by detecting the drop of PCC voltage below a threshold level. Using a triangular discharge profile, the discharge rate increases at the SDR defined in Table 6-II until the SoC drops to 70% of the SoC and then decreases at the same SDR until DoDmax is reached. The discharge power of storage and the SoC during the discharge operation is shown in Fig. 6.13(a) and 13(b), respectively. A sudden increase in the PCC load (due to an event such as a motor start) is observed in Fig. 6.13(a) at about 20:00 hours. This is partly mitigated by putting the

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storage in a short-term high discharge mode, as observed in Fig. 6.13(a) by a momentary increase in storage discharge power.

The PCC voltage profile with and without storage is shown in Fig. 6.13(c) and 13(d), respectively. The voltage-rise during the peak PV generation, the voltage fluctuations during the sudden reductions in PV output, the voltage drop during the evening peak and a voltage dip due to a sudden increase in the load are observed in Fig. 6.13(c). Fig.

6.13(d) shows that the voltage-rise is reduced by the charging operation; the voltage fluctuations are reduced by the short-term discharging operation; the voltage profile is improved during evening peak by the discharging operation; and the voltage dip is partly mitigated by the short-term high discharge operation.

A comparison of the reverse power flow at the secondary side of LV substation connected at bus 68 in Fig. 6.11 with and without distributed storage devices is shown in Fig. 6.14. It is observed that the reverse power flow with storage devices is reduced by 44% compared to the case without storage devices.

Fig. 6.14. Reverse power flow mitigation with storage.

The reduction in reverse power flow is limited due to the size of the installed storage devices. With a sufficient storage capacity, it is possible to totally eliminate the reverse power. The only limitation is the cost of the energy storage system. A trade-off analysis between the cost of the energy storage and the impacts of the reverse power flow and voltage-rise in the system has to be performed to find the optimum size of the energy storage system. Further, it is important to note that the reverse flow is a complex function of time, load demand and sun insolation. The result in Fig. 6.14 is only for a

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Power Flow at LV Substation [kW]

particular day, a particular load demand and the given sun insolation profile. The size of the energy storage system (250 Ah, 24 V) has been found to be adequate for the size of PV modules chosen in terms of the acceptable Australian voltage limits.

A case is studied in Fig. 6.15 where the PV generation drops down to such a low level for a certain period that no surplus power is available at the PCC for the storage to be charged during that time. The PV output profile is obtained from the same source (Renewable Energy Integration Facility at CSIRO), and the data was captured on the 8th February, 2011.

Fig. 6.15. (a) Load demand, PV output, storage power and active power injection into grid; (b) SoC profile; (c) Voltage profile without storage; (d) Voltage profile with storage.

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As shown in Fig. 6.15(a), the PV output drops below the load demand in the period from 10:00 hours to about 11:30 hours. At this time, no reverse power is available at the PCC and the storage device is not charged. The PV output again exceeds the load demand from about 11:45 hours and the storage device consumes the surplus power available at the PCC. The storage charging follows a triangular profile. The charging rate starts to decrease once 70% SoC level is reached at about 13:30 hours, as shown in Fig. 6.15(b). It is important to observe that due to multiple short-term discharge operations from 14:00 hours to 16:30 hours, and the unavailability of surplus PV power from 16:30 hours, the desired SoCmax level is not reached at the end of the charging period. The SoC level in this scenario reaches to about 88.5%. To ensure that the available stored charge is used wisely to serve the evening peak load, the discharge parameters are re-calculated using (6.9)-(6.11). In this case, the discharge rate starts to decrease when the SoC drops to 65.2% (not to 70% that would be the case for full SoC level) at 21:00 hours as shown in Fig. 6.15(b). The mitigation of voltage-rise, the reduction of voltage fluctuations, and voltage support during the evening peak demand are shown in Fig. 6.15(c) and 15(d).

The usefulness of the proposed charging strategy is shown in Fig. 6.16 for the same PV output profile in terms of the utilization of battery capacity and the mitigation of voltage-rise. Fig. 6.16(a) shows that the storage power consumption is higher with the charging rate adjustment than that without the adjustment, because according to (6.13), the charging rate is adjusted to be higher than the normal charging rate to recover the charge not stored during the time of unavailability of surplus PV power. Also, the storage power consumption is higher with the proposed strategy than that with the constant charging strategy during peak PV generation period.

Fig. 6.16(b) shows that at the end of the charging period, 28% higher utilization of the storage capacity is achieved by the proposed adjustment strategy when compared to the constant charging strategy (220Ah instead of 150Ah). However, if the adjustment is not performed to account for unstable weather condition, the utilization of the battery capacity would not be significantly higher than constant charging rate, as shown in Fig.

6.16(b). Fig. 6.16(c) shows that the proposed strategy can mitigate voltage-rise better than that using the constant charging rate, particularly during peak PV period in the midday.

Fig. 6.16. Usefulness of the proposed charging strategy. (a) effect on power consumed by storage in charging operation; (b) utilization of battery capacity (c) voltage rise mitigation performance.

The methodology proposed in this thesis is generic in nature and can be adapted for any integrated PV and storage system connected to the PCC. It can be used by the distribution network operators as a general guideline for the charging/discharging of storage devices integrated at the LV customer premises with rooftop PV. It can also be applied for operation of the energy storage devices in autonomous distribution systems and microgrids. Further, a graphical user interface can be developed to allow network operators to input the relevant information such as the intended length of charging/discharging period and the shape of the charging/discharging profile, the PV irradiance data, the PV rating, the PV inverter characteristics, the energy storage data, such as rating, charging/discharging characteristics, SoCmax, DoDmax. It is envisaged that with the availability of smart meters and monitoring devices, especially with the introduction of Advanced Metering Infrastructure [23], the low voltage distribution networks will be automated using a two-way communication with the utility system.

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Through this arrangement, the AMI interface can communicate with network control center and the required parameters for storage controller can be made available. In such an environment, the distributed storage devices will be able to mitigate the PV impacts using the continuously updated control parameters. A manual can be developed and used to inform the operators on how to input the correct data to the graphical user interface to obtain a generic charging/discharging strategy for an integrated PV and storage system under consideration.