One of the most common power quality problems today is voltage dips. A voltage dip is a short time (10 ms to 1 minute) event during which a reduction in r.m.s voltage magnitude occurs. It is often set only by two parameters, depth/magnitude and duration. The voltage dip magnitude is ranged from 10% to 90% of nominal voltage (which corresponds to 90% to 10% remaining voltage) and with a duration from half a cycle to 1 min. In a three-phase system a voltage dip is by nature a three-phase phenomenon, which affects both the phase-to-ground and phase-to-phase voltages. A voltage dip is caused by a fault in the utility system, a fault within the customer’s facility or a large increase of the load current, like starting a motor or transformer energizing. Typical faults are single-phase or multiple-phase short circuits, which leads to high currents. The high current results in a voltage drop over the network impedance. At the fault location the voltage in the faulted phases drops close to zero, whereas in the non-faulted phases it remains more or less unchanged.
Voltage dips are one of the most occurring power quality problems. Off course, for an industry an outage is worse, than a voltage dip, but voltage dips occur more often and cause severe problems and economical losses. Utilities often focus on disturbances from end-user equipment as the main power quality problems. This is correct for many disturbances, flicker, harmonics, etc., but voltage dips mainly have their origin in the higher voltage levels. Faults due to lightning, is one of the most common causes to voltage dips on overhead lines. If the economical losses due to voltage dips are significant, mitigation actions can be profitable for the customer and even in some cases for the utility. Since there is no standard solution which will work for every site, each mitigation action must be carefully planned and evaluated. There are different ways to mitigate voltage dips, swell and interruptions in transmission and distribution systems. At present, a wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these, the distribution static compensator and the dynamic voltage restorer are most effective devices, both of them based on the VSC principle. A new PWM-based control scheme has been implemented to control the electronic valves in the two-level VSC used in the D-STATCOM and DVR.
A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate voltage dips. The VSC is used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. The converter is normally based on some kind of energy storage, which will supply the converter with a DC voltage. The solid-state electronics in the converter is then switched to get the desired output voltage. Normally the VSC is not only used for voltage dip mitigation, but also for other power quality issues, e.g. flicker and harmonics.
The series voltage controller is connected in series with the protected load as shown in Fig.1. Usually the connection is made via a transformer, but configurations with direct connection via power electronics also exist. The resulting voltage at the load bus bar equals the sum of the grid voltage and the injected voltage from the DVR. The converter generates the reactive power needed while the active power is taken from the energy storage.
The energy storage can be different depending on the needs of compensating. The DVR often has limitations on the depth and duration of the voltage dip that it can compensate.
Fig.1. standard configuration for a DVR.
Fig.2. Schematic diagram of a DVR
The circuit on left hand side of the DVR represents the Thevenin equivalent circuit of the system. The system impedance Zth depends on the fault level of the load bus. When the system voltage (Vth) drops, the DVR injects a series voltage VDVR through the injection transformer so that the desired load voltage magnitude VL can be maintained.
Fig.3. Transmission Line Circuit
The aim of the control scheme is to maintain constant voltage magnitude at the point where a sensitive load is connected, under system disturbances. The control system only measures the r.m.s voltage at the load point, i.e., no reactive power measurements are required. The VSC switching strategy is based on a sinusoidal PWM technique which offers simplicity and good response. Since custom power is a relatively low-power application, PWM methods offer a more flexible option than the Fundamental Frequency Switching (FFS) methods favored in FACTS applications. Besides, high switching frequencies can be used to improve on the efficiency of the converter, without incurring significant switching losses.
The controller input is an error signal obtained from the reference voltage and the value rms of the terminal voltage measured. Such error is processed by a PI controller the output is the angle ?, which is provided to the PWM signal generator. It is important to note that in this case, indirectly controlled converter, there is active and reactive power exchange with the network simultaneously: an error signal is obtained by comparing the reference voltage with the rms voltage measured at the load point. The PI controller process the error signal generates the required angle to drive the error to zero, i.e., the load rms voltage is brought back to the reference voltage. The transmission circuit is as shown in the Fig.3.
Fig.4. PIC Microcontroller Board
Fig.5. IGBT driver board
Though wind power is considered as a very prospective energy source due to its clean and economical characteristics, output power fluctuation due to randomly varying wind speed is still a serious problem for wind turbine manufacturers and Power grid companies, especially in the case of fixed speed wind generators. Induction generators are used, in general, as fixed speed wind generator due to their superior characteristics such as brushless and rugged construction, maintenance and operational simplicity, and low cost. Therefore, many researches have been performed for smoothing output power generated from fixed speed wind generators. In a flywheel energy storage system is proposed to smooth the wind power fluctuations. Some authors have proposed superconducting magnetic energy storage (SMES) system for smoothing the wind power fluctuations. In an energy capacitor system (ECS), which consists of power electronic devices and electric double layer capacitor (EDLC), is proposed to smooth wind farm output. But a flywheel system has, in general, high standby loss within the range of 5% of its power rating. Moreover, the control scheme of flywheel system is comparatively complex. Though SMES and ECS are very good systems for the power smoothing due to their high response speed and high efficiency, their practical installation in large MW range applications is doubtful due to their large installation and maintenance cost. On the other hand, BESS is a proven technology in power system applications due to its low cost, easy replacement technique, despite of some limitation due to its chemical process. The service life of BESS has been prolonged recently with the innovation of chemical industry. When the BESS is integrated with STATCOM, it gives excellent control ability of both real and reactive power. Therefore, STATCOM/BESS topology is chosen in this study for smoothing output power and terminal voltage of fixed speed wind generator from the viewpoint of its cost and performance effectiveness. The microcontroller and driver board is as shown in the Fig 4 & 5.
The STATCOM is an electronically generated three-phase voltage source that is connected to the AC system via a shunt transformer. When operating as a pure STATCOM (without losses or SMES attached) the generated voltage is exactly in phase with the AC system voltage. If the generated voltage is greater that the terminal voltage then the device will supply VARs (i.e. is capacitive) to the system. If the internal voltage is less than the terminal voltage, the device will absorb VARS (i.e. is inductive) from the system. The control system of the STATCOM is extremely fast and therefore can rapidly change the magnitude (and phase) of the electronically generated voltage. The magnitude of AC voltage generated by the STATCOM is directly proportional to the DC bus voltage. Therefore, the generated voltage, which controls the STATCOM VARS, is controlled by changing the DC bus voltage. Changes in DC bus voltage are accomplished by supply or absorbing real power to or from the DC bus to charge or discharge the DC capacitor. This is accomplished by advancing or retarding the generated voltage angle with respect to the AC terminal voltage.
In practice, the STATCOM has a small amount of losses (e.g. resistance in windings, switching losses etc.) and to compensate for these the electronically generated AC voltage angle is slightly in retard of the AC system terminal voltage angle. The difference in angle (?) between system voltage and generated voltage allows real power to flow and hence compensate for the losses in the STATCOM circuit. When the DC chopper is employed to transfer energy to/from the SMES coil then the DC bus voltage will be charged or discharged. This change will be sensed by the STATCOM controller which will automatically adjust its control angle (?) to keep the DC voltage at the required level. In doing so it will naturally adjust to compensate for the energy request of the SMES system. The STATCOM control automatically adjusts for the power demand of the SMES and transfers the required energy to/from the AC network.
A benefit of adding a SMES system to the DC bus of a STATCOM has been considered. The power oscillation damping performance of a STATCOM has been compared with a STATCOM+SMES combination. An active line power signal conditioned to provide an auxiliary control signal to the voltage reference (in the case of STATCOM) or SMES power reference (in the case of STATCOM + SMES combination) was compared. The ability of both schemes to damp machine rotor swing oscillations at 0.5 Hz has been demonstrated. The results illustrate that a STATCOM alone (i.e. no POD) will regulate voltage in the post contingency period but will not naturally add much damping to power oscillations. The STATCOM with POD signal applied to its voltage reference may damp swing oscillations following a disturbance however this is achieved at the expense of voltage regulation. The combination of STATCOM plus SMES with POD modulating the SMES output will allow the system to both regulate voltage and provide oscillation damping. The amount of real SMES power required to damp system oscillations is quite small (20 MW) relative to the STATCOM rating (160 MVAR). The controls of the STATCOM were not specially modified to accommodate the operation of the SMES coil.
Assistant professor in lord venkateswara engineering college.I am doing phd in sathyabama university, Tamil Nadu,India.
Categories: Choppers Articles Tags: Advance, Constancy, Currents, Fault Location, Faults, Impedance, Magnitude, Mitigation Actions, Overhead Lines, Phase System, Phase To Phase, Phase Voltages, Power, Power Quality Problems, Recital, Short Circuits, Standard Solution, Statcom, System, System 13, Voltage, Voltage Dip, Voltage Dips, Voltage Drop, Voltage Levels