Creating a Dynamic Load Controller to Mitigate Flickers Caused by Photovoltaic Systems in Cloudy Regions

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"We chose NI devices because of their high reliability, accuracy, and fast response to compensate for the change of PV power. The proposed load controller is a cost-effective approach that saves time because it is fully automated and requires minimal manpower."

- Yun Seng Lim, University Tunku Abdul Rahman

The Challenge:
Compensating for solar irradiance in a photovoltaic (PV) system to reduce fluctuation and flickers in power output to low-voltage distribution networks.

The Solution:
Using NI LabVIEW software and NI hardware to create a dynamic load controller of 200 W power resistors and solid-state relays to mitigate the voltage fluctuation and flickers by rapidly switching the resistors according to PV power output.

Author(s):
Yun Seng Lim - University Tunku Abdul Rahman
Jun Huat Tang - University Tunku Abdul Rahman

We developed an inexpensive dynamic load controller that can rapidly vary the power demand based on changes in the PV power output. The voltage issues and proposed solution are valuable to different parties, including the government, policy makers, utility companies, and PV system owners to ensure growth of PV systems without compromising the quality of the customer electricity supply.

Laboratory Setup

To study how the dynamic load controller can mitigate the flickers caused by the fluctuation in the power output of PV systems, we set up a low-voltage three-phase distribution network (see Figure 1). A PV system and 3 kW load controller connect to the distribution network. Either the utility grid or a 15 kW generator is the supply source to the distribution network. If the utility grid is used as the supply source, the flickers seen by the load controller consists of two portions: the background flickers introduced by the utility grid and the flickers caused by the intermittent power output of the PV system. We use the 15 kW generator to study the flickers purely generated by the PV system.

NI devices such as the NI 9225 voltage module and NI 9227 current module measure the voltage, current, and power at the specified measurement point. In addition, the NI 9403 digital I/O module sends a 5 V signal to turn on or off the solid-state relays that act as switches in the load controller. LabVIEW software controls the load controller and collects the data during a period of time.

Case Study 1

In our first case study, we examined  the severity of flickers introduced by the PV system. We connected a 1.84 kW PV system to Phase A of the distribution network with a 15 kW generator as the supply source to the network. We measured the voltage magnitude at a common point, also known as the point of the common coupling and regularly recorded it using LabVIEW (see Figure 2). The PV power output rapidly fluctuates which causes the voltage magnitude to change frequently throughout the day. We observed three unique characteristics of the PV power outputs from the results:
1. Many high outputs happen within short durations.
2. Some of the high-power outputs drop down suddenly instead of gradually (for example, the high power output of 2.7 kW that happens between 12:00 and 12:22 pm drops immediately to 1.2 kW after 12:22 pm).
3. The magnitude of reduction in the PV power output is significant at about 63 percent.

The voltage profile calculates the short-term flicker index. There are 47 short-term flicker indices as shown in Figure 3. The maximum value of the short-term flicker exceeds 1 percent which is the legal limit.

Control Algorithm of Dynamic Load Controller

We could choose from several options to mitigate voltage fluctuation and flicker issues, including installing static capacitors and, power electronic-based switching devices, or as well as increasing the cable size of the distribution network is one of the mitigation techniques. Super capacitor is claimed to be an effective means for mitigating the voltage fluctuation from the photovoltaic systems. By incorporating the mitigation technique as part of the photovoltaic PV systems, the total cost of the photovoltaic PV systems can be very hhigh. Hence, the photovoltaic systems can be an , making them an unattractive renewable energy source in Malaysia. In order toTo keep the cost of the photovoltaic PV systems as low as possible, the mitigation means must be low,  and yet effective enough to reduce the indices of voltage fluctuation and flickers.

We based the proposed load controller on a number of 200 W power resistors and solid-state relays. We used the NI 9403 measurement system to control the load via solid-state relay. It reduces flicker by rapidly switching the resistors on and off to lower the impacts of sudden increases and decreases in PV power output. The load controller uses real power to change the network voltage because the voltage magnitude predominantly responds to real power changes in the distribution network where the resistance is bigger than the reactants. Furthermore, the amount of real power  required by the load controller is small. Therefore, only a small amount of PV power is dissipated in the load controller. We can use this amount to heat up water for washing purposes. Hence, our proposed load controller is an energy-efficient, cost-effective, time-saving, and technically viable solution for flicker mitigation.
 
The system consists of a load bank with six units of 200 W power resistors with solid-state relays to switch on and off the power resistors. We developed a control algorithm using LabVIEW (see Figure 4). The control algorithm uses the voltage magnitude at the point of the common coupling to determine whether the voltage is within the required tolerance. If not, it activates the load controller or DSM. If the voltage is greater than 243 V, the system sends a command to the solid state of the load bank to switch on a resistor. If it is less than 238 V, it sends a command to the solid state to switch off a resistor.

Case Study 2

We conducted a second case study to investigate the effectiveness of the load controllers in mitigating flickers. In this experiment, we connected a 1.84 kW PV system to the laboratory network with the 15 kW generator the supply source. We measured the voltage magnitude at the point of the common coupling and regularly recorded it when the load controller was in use. We then calculated the short-term flickers (see Figure 5). All the short-term flicker indices are reduced when the load controllers are in use and the maximum flicker is lower than the legal limit. When all the short-term flickers are reduced, the long-term flicker is also reduced to 0.458kW. This experiment shows that the load controller is an effective measure for mitigating voltage fluctuation and flickers.

Highly Reliable NI Hardware and Software Reduce Flicker Impact

We developed a fully automated load controller using NI hardware devices and LabVIEW to reduce the impact of flicker emission. The experiments we performed show that the load controller is an effective means of reducing voltage fluctuation and flicker caused by PV systems. We developed the control algorithm using LabVIEW and the controller consists of several resistors and solid- state relays controlled by the NI 9403 system. The NI 9225 and NI 9227 modules measure the voltage and current at the point of PV connection. The controller can rapidly switch the load on and off with high-performance NI devices to reduce the severity of flickers caused by the PV system. We chose NI devices because of their high reliability, accuracy, and fast response to compensate for the change of PV power. The proposed load controller is a cost-effective approach that saves time because it is fully automated and requires minimal manpower.

Author Information:
Yun Seng Lim
University Tunku Abdul Rahman
Utar Complex, Jalan Genting Kelang, Setapak,
Kuala Lumpur 53300
Malaysia
Tel: 60123459598
yslim@utar.edu.my

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