S.S.Deswal et. al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 2445-2455
IMPROVED PERFORMANCE OF AN ADJUSTABLE SPEED DRIVES DURING VOLTAGE SAG CONDITION S.S.DESWAL* Assistant Professor, EEE Department, Maharaja Agrasen Institute of Technology, Sector-22, Rohini, Delhi-110086, India
[email protected] RATNA DAHIYA Associate Professor, Department of Electrical Engineering, NIT,Kurukshetra, kurukshetra, Haryana-136119, India
[email protected] D.K.JAIN Director(Technical), Guru Premsukh Memorial College of Engineering, Budhpur, Delhi-110036, India
[email protected] Abstract: Voltage sags normally do not cause equipment damage but can easily disrupt the operation of sensitive loads such as electronic Adjustable Speed Drives (ASD’s). Voltage sags cause a momentary decrease in DC-link voltage triggering an under voltage trip leading to nuisance tripping of adjustable speed drives (ASD’s) employed in continuous-process industries which contributes to loss in revenue. A practical ride-through scheme for an adjustable speed drives based on supercapacitor during voltage sag has been presented in this paper. The supercapacitor maintains the ASD dc bus voltage under voltage sag condition. Energy storage module is connected to support the DC-link voltage during power system faults. The performance of ASD’s under normal and power system faults is first simulated in MATLab Simulink and then experimentally verified. The Data AcQuisition boards (DAQ) of National Instruments along with LabVIEW software have been used to record the observed waveforms. Keywords : Adjustable speed drives, Low voltage ride-through capability, Voltage sags, Supercapacitor, Ultracapacitor.
1.
Introduction
Adjustable Speed Drives (ASD’s) used in a wide variety of industrial applications. The benefits that might be provided by the ASD’s are the reason for their widespread use by the industry. Despite of its importance to the process operation, the ASD’s are sensitive to voltage sags. Undervoltage and overcurrent often follow voltage sags which may cause the ASD’s to trip bringing about the halt of the productive process and revenue losses. The ASD’s may also operate inappropriately resulting on load torque and load speed variations since the control of the current and of the output voltage are dependent on the inverter DC voltage level which decays during voltage sag [1], as shown in eqn(1).
V dcC
d V dc T L r dt m o t i n v
(1)
Thus, the decrease rate of the dc bus voltage dVdc/dt depends on the capacitance C, the voltage Vdc across the capacitor at the beginning of the voltage sag, the load torque TL, the motor speed ωr, the motor efficiency ηmot . Different approaches to improve the ASD’s ride through by increasing the average voltage of the DC-link have been proposed [1], [5], [6], [7], [8]. The methods include the addition of capacitors to the DC- link [6], the regenerative mitigation which converts the kinetic energy from the motor and load into electric energy transferring it to the ASD’s DC-link [1], the connection of the neutral conductor of the supply source to the midpoint of the DC-link through a controlled switch [8], and the application of boost converters [1], [5], [7].
ISSN: 0975-5462
2445
S.S.Deswal et. al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 2445-2455 This paper presents a proposed topology to improve the low voltage ride-through capability of an adjustable speed drive via experimental and simulation results. The system is tested under symmetrical and asymmetrical voltage sag conditions in order to assess the contribution of the supercapacitor as an energy storage device to improve the ASD’s operation under voltage sags. The typical duration of voltage sags are between 0.5 to 30 cycles or 8ms to 0.5s. Voltage sags, classified as type A, are the most severe ones as they cause the larger amount of energy withdraw from the dc bus, and are more likely to trip the ASD’s under voltage protection. The asymmetric voltage sags usually have at least one line supply voltage which keeps the DC-link voltage above the under voltage protection level. Nevertheless, voltage sag type A is the least severe as far as the over current level is concerned. On the other hand, voltage sags type B, caused by one-phase faults, are accountable for the most severe sags as far as over current are concerned and the least severe as for the dc bus under voltage threshold level [5], [10]. It has been withdrawn from [5] that tests with voltage sag type A can set the under voltage protection level and tests with voltage sag type B can set the over current protection level of an ASD’s.[11-12] 2.
Energy Storage Systems
Energy storage systems, also known as restoring technologies are used to provide the electric loads with ridethrough capability in poor Power Quality (PQ) environment. Recent technological advances in power electronics and storage technologies are turning the restoring technologies one of the premium solutions to mitigate PQ problems. The first energy storage technology used in the field of PQ, yet the most used today, is electrochemical battery. Although new technologies, such as flywheels, supercapacitors and superconducting magnetic energy storage (SMES) present many advantages, electrochemical batteries still rule due to their low price and mature technology.[8-9,13-14] 2.1. Flywheels A flywheel is an electromechanical device that couples a rotating electric machine (motor/generator) with a rotating mass to store energy for short durations. The motor/generator draws power provided by the grid to keep the rotor of the flywheel spinning. During a power disturbance, the kinetic energy stored in the rotor is transformed to DC electric energy by the generator, and the energy is delivered at a constant frequency and voltage through an inverter and a control system. Traditional flywheel rotors are usually constructed of steel and are limited to a spin rate of a few thousand revolutions per minute (RPM). Advanced flywheels constructed from carbon fiber materials and magnetic bearings can spin in vacuum at speeds up to 40,000 to 60,000 RPM. The stored energy is proportional to the moment of inertia and to the square of the rotational speed. High speed flywheels can store much more energy than the conventional flywheels. The flywheel provides power during a period between the loss of utility supplied power and either the return of utility power or the start of a back-up power system (i.e., diesel generator). Flywheels typically provide 1-100 seconds of ride-through time, and backup generators are able to get online within 5-20 seconds.[9,15-16] 2.2. Supercapacitors Supercapacitors (also known as ultracapacitors) are DC energy sources and must be interfaced to the electric grid with a static power conditioner, providing energy output at the grid frequency. A supercapacitor provides power during short duration interruptions or voltage sags. Medium size supercapacitors (1 MJoule) are commercially available to implement ride-through capability in small electronic equipment. 2.3. SMES A magnetic field is created by circulating a DC current in a closed coil of superconducting wire. The path of the coil circulating current can be opened with a solid-state switch, which is modulated on and off. Due to the high inductance of the coil, when the switch is off (open), the magnetic coil behaves as a current source and will force current into the power converter which will charge to some voltage level. Proper modulation of the solidstate switch can hold the voltage within the proper operating range of the inverter, which converts the DC voltage into AC power. Low temperature SMES cooled by liquid helium is commercially available. High temperature SMES cooled by liquid nitrogen is still in the development stage and may become a viable commercial energy storage source in the future due to its potentially lower costs. SMES systems are large and generally used for short durations, such as utility switching events. The high speed flywheel is in about the same cost range as the SMES and supercapacitors and about 5 times more expensive than a low speed flywheel due to its more complicated design and limited power rating. Electrochemical battery has a high degree of mature and a simple design. Below a storage time of 25 seconds the low speed flywheel can be more cost effective than the battery. Table 1, shows a comparison of the different storage technology in terms of specific power and specific energy. [3] Table-I Comparison of different ASD’s Ride-through Alternatives
ISSN: 0975-5462
2446
S.S.Deswal et. al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 2445-2455 Besides energy storage systems, some other devices may be used to solve PQ problems. Using proper interface devices, one can isolate the loads from disturbances deriving from the grid. 2.4. DVR ASD Ride- Through Alternatives
Cost Rs/KW
Ride-Through Duration Limit
Power Range
Efficiency
Cycle Life
Charging Time
Additional Capacitors*
30000
0.1sec.
100kw
95%
10000
Seconds
Load Inertia
≈0
2.0 sec.
1kw-1mw
---
---
Continues
Reduced Speed/Load
≈0
0.01 sec.
5-10kw
---
---
---
Lower Voltage Motors*
≈0
0.01 sec.
5-10kw
---
---
---
Boost Converter**
500010000
5.0 sec.
5-200kw
90%
---
---
Active Rectifier**
500010000
5.0 sec.
5-200kw
---
---
---
Battery Backup*
500010000
5.0 sec.,1hr.
5kw-1MW
70-90%
2000
Hours
Ultra Capacitors*
1500020000
5.0 sec.
5-100kw
90%
10000
Seconds
Motor-Generator Sets*
1000015000
15.0 sec.
100kw
70%
---
---
FlyWheels*
1000015000
15.0 sec.,1hr.
1kw10MW
90%
10000
Minutes
SMES*
3000040000
10.0 sec.
3001000KW
95%
10000
Minuteshours
Fuel Cells*
75000
1 hr.
10kw2MW
40-50%
continues
continues
* provides Full-power ride-through ** provide full-power ride-through for single-phase sags
