Scarcity of fossil fuel resources has motivated the researchers to develop renewable energy based power projects. Instead of using a single or independent renewable energy source, it is preferable to use the combination of such energy sources in a distributed way to compensate the power fluctuations of the system and this leads to the concept of hybrid micro-grid energy. Voltage stability is an important parameter for the secure operation of the hybrid-micro grid, and IEEE 1547 Standard defines the limit of the voltage for the successful operation of the micro-grid. Although Vanadium Redox Batteries (VRBs) can help the system to stabilize the voltage when voltage sag occurs when a heavy load is suddenly connected to the system, this stabilization process takes some time. This paper discusses the application of super capacitors to the hybrid micro-grid system, as a higher energy density element, to help the system quickly recover its transient voltage.
The centralized power generation system has been used for decades and usually the system utilizes fossil fuel as a resource, but unfortunately, the use of fossil fuel in the power generation poses a threat to human life and more specifically to those people who are situated near the power generation plants. Therefore, due to environmental issues i.e. emission of CO2 and other hazardous gases, the centralized power generation plants are located remotely [
Nowadays, the demand of electricity is also increasing and to overcome the shortage of electricity, development of new centralized systems is not a wise decision. Therefore, instead of increasing the quantity of energy there is a need to improve the quality of energy by employing renewable energy sources i.e. wind and solar energy. Since renewable energy technologies unlike fossil fuel based power generation systems are “eco-friendly”, locating these “eco-friendly” power generation systems in remote areas is not required. It not only helps in the reduction of the transmission line losses, but also decreases the risk of large blackouts. Hence, this idea leads to the concept of Distributed Generation (DG) using micro-grids because the large scale applications of the DG are possible by employing micro-grid. Micro-grid is developed by grouping a cluster of loads and parallel DG units in local distribution network.
It must be noted that the micro-grid is connected to the grid via Point of Common Coupling (PCC), which, in case of a fault, will disconnect from the grid, and then the micro-grid will operate in the islanded mode. So, micro-grid must be able to operate in grid-connected as well as in islanded mode [
In order to keep the micro-grid in the grid-connected mode, certain constraints must be satisfied such as maximum voltage sag and swell, and the limits for maximum and minimum voltages are defined in ANSI C84.1- 2006 [
This paper presents the practical study of a micro-grid model under the influence of load change, and this is due to the fact that the transient voltage of the system is largely affected by the increase of load. The voltage sag due to the increase of load will increase the amplitude of current, which consequently can cause damage to the power electronic converter. This paper proposes to employ super capacitors in a DG hybrid micro-grid system. Therefore, the inclusion of super capacitors to the micro-grid system becomes necessary.
Today, independent PV based renewable energy system is being employed but due to changing intensity of sun, the power of the PV system is not constant.
Therefore, in order to avoid fluctuations, instead of using an independent PV system, a hybrid system, integrating wind energy system (WES) and energy storage (ESS) elements with the PV system, is usually used. Similarly, the power of an independent WES will adversely affect the power system. Hence, a hybrid system formed due to the integration of ESS i.e. battery energy storage system (BESS) and flywheel energy storage system (FESS) with renewable energy technologies i.e. PV and wind energy is best possible solution to avoid power fluctuation problems in a power system. A hybrid micro-grid system which is shown in
The micro-grid hybrid system shown in Fig. 1 includes three PV Energy Systems (PVES) with two of them adding about 400 kW to the system i.e. PV1 and PV2 while another PV panel system i.e. PV3 contributing 100 kW to the system. The mentioned system also includes a wind energy system, with a capacity of 2.5 MW, connected to the 400 V AC channel via step-down transformer.
As discussed before, in order to secure the system from power surges from WECS and power fluctuations from PV system, two different technologies of EES, each with a capacity of 800 kWh, have also been integrated via bi-directional power electronic converters. One of the EES being used in the system employs Vanadium Redox Battery Energy Storage System (VRBESS). VRB’s are usually used in the system to due to its capability of providing high energy density, long cycle life of 10,000 cycles (which is 3 times more than that of lithium ion battery), increased operating temperature range and a long time scale of minutes to hours [
The mentioned hybrid micro-grid system also includes two units of 65kW Natural Gas Micro Turbine. C65
Energy Sources/Energy Storage Elements/Load | |||||||
---|---|---|---|---|---|---|---|
Number of Units | Specifications Per Unit | Total Power | |||||
PV Energy System (PVES) | 3 | PV1 = 200 kW | 200 kW × 2 +100 kW | 500 kW | |||
PV2 = 200 kW | |||||||
PV3 = 100 kW | |||||||
Wind Energy System (WES) | 2 | WT1 = 1.5 kW | 2.5 MW | ||||
WT2 = 1.5 kW | |||||||
Micro Turbine Energy System (MTES) | 2 | MT1 = 65 kW | 65 kW × 2 | 130 kW | |||
MT2 = 65 kW | |||||||
Vanadium Redox Battery Energy Storage System (VRBESS) | 1 | 200 kW for a duration of 4 hours | 800 kWh | ||||
Lithium Ion Battery Energy Storage System (LIBESS) | 1 | 200 kW for a duration of 4 hours | 800 kWh | ||||
Super Capacitor Energy Storage System (SCESS) | 1 | 200 kW for a duration of 10 seconds | 0.55 kWh | ||||
Three Phase Induction Motor (TPIM) | 4 | IM1 = 75 kW | 75 kW �� 4 | 300 kW | |||
IM2 = 75 kW | |||||||
IM3 = 75 kW | |||||||
IM4 = 75 kW |
ICHP combined heat and power (CHP) natural gas based micro turbine has a capability of providing 100A in grid-connected as well as in islanded mode operation of micro-grid whereas, operational voltage range is between 400VAC and 480VAC. One of the advantages is that C65 ICHP has ultra low NOx emissions.
The purpose of this research is to experimentally check the effect of super capacitor units on micro-grid under different load conditions. However, there are different control schemes that have been developed for grid-con- nected mode and islanded mode of the micro-grid system which has been presented in [
The Super Capacitor or Ultra-Capacitor has very higher value of energy density in comparison to the normal capacitors. Unlike a conventional capacitor, the super capacitor doesn’t use the dielectric material and it uses “plates”.
The equivalent model of the super capacitor is shown in
The super capacitor is connected to the grid via bi-directional power electronic converter that can be controlled using a decoupled d-q axis control algorithm [
AC filter improves the quality of the current and decreases THD value and then power is fed to the step-up transformer that step ups the voltage from 150VAC to the required grid voltage of 400VAC. The parameters for experiment, transformer and SCESS’s converter are given in
Parameters for Experiment | Parameters for Converter System | Parameters for Grid Side Transformer | Parameters for Super Capacitor | ||
---|---|---|---|---|---|
Altitude (<2000 m) | Rated Voltage (150 V ± 10% Fluctuation) | Type of Transformer (Y-connection) | Rated Capacitance (Approx. 30F) | ||
Protection Type (IP22) | Active Power Capacity (200 kW) | Capacity in Apparent Power (500 kVA) | Operating Temperature Range (−40˚C - +60˚C) | ||
Humidity (10% - 100%) | Rated Frequency (50 Hz ± 5%) | System Working Time (10 s) | |||
Pressure (77,500 Pa - 106,500 Pa) | Switching Frequency (Approx. 2.7 kHz - 3.0 kHz) | ||||
Storage Temperature Range (−40˚C - +60˚C) | Approx. Power Factor (0.9) | ||||
Operating Temperature Range (−30˚C - +50˚C) | |||||
The transient response of the grid is experimentally studied with and without the super capacitor. In order to check the effect of super capacitor on the grid, an inductive load of about 12 KVA was added to the initial resistive load at t = 80 ms.
It can be noted from
In order to experimentally check the response of the system under the influence of super capacitor, the load is increased to about 5 times the previous value i.e. 60 KVA and the transient response of the voltages are shown in
need 90 milliseconds to recover the voltage and it also shows that it has successfully recovered the voltage sag within a short time of one to two cycles and super capacitor has helped the system to quickly stabilize the response.
The starting current for an induction motor has usually a very high value and therefore, the effect of a sudden start of an induction motor on system with and without super capacitor is studied.
in about 180 ms but for the system with super capacitor, the induction motor starts-up at t = 0.33 s approx., the voltage sets stabilized in less than 70ms. Similarly, the amplitude of three phase currents increase when the induction motor is connected and current takes less time to stabilized for the system with super capacitor as shown in
The concept of hybrid micro-grid technology is emerging because of its stable and eco-friendly behavior. Transient analysis of the system is very important in this regard for its successful and stable operation. The parameters such as upper and lower limits of the system’s voltages must be taken into consideration, and a careful observation is required to check the transient response of the system under different loads because the hybrid micro-grid must develop a robust system that can work in grid-connected as well as islanded mode. The transient response of the voltage is disturbed for some cycles when a particular load change occurs in the micro-grid system. Although vanadium redox batteries (VRBESS) have higher energy density than lithium-ion batteries (LIBESS), they are not fast enough to stabilize the response. Therefore, in addition to VRBESS and LIBESS, an introduction of a super capacitor (SCESS), as very high energy-density energy storage equipment, will help the system to recover and stabilize three phase voltages rapidly. Hence, using a super capacitor in a hybrid micro-grid system is a requirement of today’s micro-grid technology.
We would like to thank Goldwind Science & Technology Co. Ltd. Beijing, China for providing us an opportunity to enhance our research potential by co-operating with us and helping us to use their equipment for experimental verification of stabilizing voltage using super capacitors. Mr. Jamshed Khan is associated with Goldwind Science & Technology Co. Ltd. Beijing, China.