SUPERCAPACITORS ENERGY STORAGE SYSTEM FOR POWER QUALITY IMPROVEMENT: AN OVERVIEW

INTRODUCTION The reason behind overview of supercapacitors energy storage system is that supercapacitors are less weighty than that of battery of the same energy storage capacity, A fast access to the stored energy, Charging very fast than battery, Charge/discharge cycle is 106 time, storage capacity independent of number of charging/discharging cycles, energy density for Supercapacitor is 10 to 100 times larger than of traditional capacitors(typical 20-70 MJ/m3), capacities up to 5F/cm2, life cycles 25-30 years, high efficiency (95%), power density 10 times greater than batteries, Charge and discharge time very less, rated capacitance value ranging from 0.043-2700F, nominal voltage ranging from 2.3 to 400V, rated current ranging from 3-600 A, operating temperature ranging -40°C to 85°C, Maintenance free, very low leakage current, also non polar.

Sustained Interruptions Power Quality Issues Voltage sags Voltage flicker Sustained Interruptions Voltage regulation Harmonics

Existing Storage Systems ENERGY STORAGE SYSTEMS Existing Storage Systems Pumped Hydro Storage of Compressed Air Electrochemical Storage Hydrogen Storage Heat Storage Mechanical Storage

Oncoming energy storage systems Superconducting Magnetic Energy Storage (SMES)

Redox Flow Batteries Super Capacitor Energy Storage (SCES)

Available Supercapacitors in market

Supercapacitors cost trend

Models of Supercapacitors The RC series model of Supercapacitor

The two branches model of Bonert and Zubieta for a Supercapacitor RC model of Supercapacitor

Supercapacitors model reduced to the mainline (generator convention)

Measurement Methods of Supercapacitor Parameters Constant Current Charge Method General schema used for constant current charge method

The work stages according to this method are: a constant current charge is applied at the supercapacitors terminals; during the charge process, the voltage across supercapacitors terminals is measured; data is processed to obtain the experimental response: curves Isc(t), Usc(t); Model parameters are extracted from the experimental response Usc (t); are identified: the internal resistance and the equivalent capacity of the pack.

Capacitance Time-Domain Conversion Method The work stages according to this method are: a constant current charge is applied; a potential difference is created, preselected as function of supercapacitor’s working voltage; measurement of time period needed to create the voltage variation; Data processing and computing in order to obtain the supercapacitor capacity.

The measurement process involves the following steps: a) at Uc = V1 , the output of comparator U1 is ‘1’ and the output Q of flip flop U3 switches to ‘high’ state (logical ‘1’); b) at Uc = V2, the output of comparator U2 is ‘1’ and the output Q of flip-flop U3 switches to ‘low’ value(logical ‘0’). At the output Q of circuit U3 a time pulse τ will be obtained, that is directly proportional to the capacitance Cx, according to relation (3); c) turning off the switch S1 (stopping the charging process); d) turning on the switch S2 (discharging the capacitor); e) watching the value Uc < V1 , after which the measuring process can be restarted; f) Determining the time value τ, that will give the size of capacitance Cx.

Supercapacitors Parameters Determination: Case Study Using Constant Current Charge Method The pack of supercapacitors used is BOSTCAP BMOD0050 E015 B1 (5.8 F, 150 V, 10 modules × 10 cells in series). The working test bench to determinate the supercapacitors parameters contain: the pack of batteries, 23 cells in series, 12 V/100 Ah each; the bidirectional current converter, that allows current with values between 10 and 400 A. Reference current of converter - by a low-pass filter strategy of a DC bus current; resistances (Load); voltage/current sensors; 280 V DC bus.

supercapacitors pack BPAK0058 E015 B1 parameters

Experimental characteristics obtained by the constant current (50A) charge method for BPAK0058 E015 B1 module Simulations characteristics obtained by the constant current (50A) charge method for BPAK0058 E015 B1 module

Circuit diagram of SCESS

SCESS CONTROL Control concept for boost mode Control concept for Buck mode

Current control Design Vdc link control Design Current control Design

Control of STSTCOM plus SCESS (MODE 2)

Controlof STSTCOM plus SCESS (MODE 3)

Experimental Results Vdc link, Vsc, Va, and Ia during STSTCOM plus SCESS inject real power to grid

Vdc link, Vsc, and Isc during STSTCOM plus SCESS inject and absorb real power

Various Approaches for Power Quality Improvement by SCES

Conclusion Through a critical survey of the literature for the energy storage system especially for the Supercapacitors energy storage system for improvement of power quality of the different systems; an overview has been presented. Various aspects of the problem, such as to provide ride through, stabilization of power system, to make undispatchable power into dispatchable, to improve power quality of weak transportation system, of aircraft distribution system UPS, elevator and PDS adopting PI control technique, dynamic voltage restorer in the island mode, switching transients mode, grid connected mode, in stand alone for the short term outage. Therefore it would prove a good energy storage option and power quality maintenance purpose with power conditioning system as the cost falls down being the life and its efficiency is very high.