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65Abstract. Voltage sags caused due to unknown faults at the grid are the key power quality issues faced by industrial consumers. These sags often cause disruption in manufacturing processes and result in substantial economic losses. In order to deal with such problems a voltage sag compensator VSC consist of a voltage source inverter coupled through a series transformer with grid line is often installed since it is among the cheapest solutions against the sags to save the critical loads from damaging. However, a problem of inrush transient current occurs as the series transformer gets energized to compensate for the load voltage. This not only affects the critical loads but also triggers the circuit breakers installed to avoid overcurrent of the compensator. This paper proposes a series of voltage sag compensator based on a current-controlled voltage source inverter CRVSI with the reduced magnitude of transformer inrush current. The proposed VSC is based on a current control technique applied in a stationary frame of reference for the desired operation.

1 Introduction. The continuous power supply is every home's desire. But it becomes necessary when one is talking about industries where power failure even for a short time interval causes heavy financial loss because of interruption in manufacturing the product. Among the most common power quality issues currently being faced by the industrial consumers are the voltage sags. A voltage sag is a reduction in the magnitude of supplied voltage to 10 90 of nominal value from the grid for a time that ranges from a few milliseconds to a minute IEEE 1159. It happens due to the line fault which usually occurs due to a short circuit overload or starting of electric motors. Usually, a voltage sag compensator is being coupled to the grid with the help of a series transformer to regulate the load voltages. When any sag at grid occurs the compensator adds the reduced magnitude of load voltage through a series transformer. However, as the compensator restores the load voltage the flux of the transformer touches its level of magnetic saturation which stays too long owing to the transformer's high-efficiency nature and often causes the severe magnitude of transient inrush current. This inrush current damages the transformer's insulation even sometimes stimulates the overcurrent protection mechanism, ultimately failing the process of compensation.

The magnitude of inrush current is subjected to various origins naming transformer's own magnetic properties operating time interval of VSC or the depth of voltage dip. Higher the depth of voltage dip lowers the voltage magnitude which increases the amount of inrush current. Therefore the depth of the voltage dip is directly proportional to the magnitude of the inrush current. In literature, many techniques are available to reduce the transformer's inrush current. Oversizing the transformer is the most traditional approach used often to absorb high inrush current of the transformer which is not preferred due to various industrial limitations like size constraints and power requirements. Previously used open-loop techniques 1 3 had observed to be limited in terms of performance over specific grid faults. These get unstable on the occurrence of successive grid faults as verified in 1. A recent technique proposed in 4 uses positive and negative sequence reactive current injections which maximizes the system, additional cost and complexities. Authors in 5 10 used PWM techniques in which compensated voltage waveforms are clipped off on the detection of peak current. This changes the shape of waveforms while reducing voltage magnitude ultimately compromising the quality of sag compensation. PWM techniques are preferable for ideal AC voltage current waveforms. However real-time systems are unable to maintain ideal AC waveforms. Taking this in consideration these methods are using the synchronous frame of reference with d q transformation to convert AC signals into DC signal and then applying various PWM schemes. Though it offers satisfactory performance but the implementation of these schemes has always been complex. This paper proposes a series of voltage sag compensator consist of a PI-based current controlled voltage source inverter. The inverter of the suggested VSC uses a recently developed current control scheme implemented in a stationary frame of reference 11 12. With the purpose of validating the suggested series, VSC simulated results are obtained by implementing the controller in MATLAB considering various types of sags.

2 Operating Principle. A simplified diagram of a series VSC is illustrated in Fig 1. It consists of an inverter to generate the compensated magnitude of load voltage series transformer to add up the compensated voltage to the grid line and thyristor switches to escape the series transformer for the period of regular grid operation. At the time when the sag occurs at the grid side, the compensator generates the reduced magnitude of voltage detected by the controller. This magnitude of compensated voltage is added with the line voltages via a series transformer. The current-voltage ripples of the inverter are filtered through the capacitor and the leakage reactance of the transformer. In this manner, the load is supposed to have constant power supply. But when compensator is turned on the series transformer gets energized and inrush current is generated in the transformer's winding. This behavior of the conventional series VSC is shown in Fig 2 a b Fig 2. Simulated waveforms of conventional series VSC a compensator current and b compensator voltage. The amount of inrush current sometimes goes thrice the value of nominal current in the transformer. Therefore it triggers its own overcurrent protection mechanism eventually failing the compensation. As illustrated in the figure above a voltage sag occurs at t 0 1 sec and the compensator starts after 0 004 sec as the sag is detected. As the compensator starts injecting the compensating voltages an inrush due to the energizing of the transformer is observed. The magnitude of this inrush transient current is around 220 of the actual value. This is a very huge amount of current that is more than the designed power ratings of the critical loads hence damages the sensitive machinery.

It can cause serious damage to the transformer as well as the compensator. In addition, it can break the transformer's insulation and sometimes even breaks the circuit via stimulating overcurrent protection devices. 2 1 Implemented Control Scheme. The block diagram of the proposed series VSC has been demonstrated in Fig 3. The double-loop PI controller is used to set the high gains at a given frequency for voltage as well as current regulation. V Load and I Load are the three-phase feedback input current and voltage signals given to the controller respectively. These are compared with the reference signals to calculate error factor E's. The calculated error factor is fed to the PI controller is tuned accordingly where high gains for current regulation while low gains for voltage regulation are tuned finely. The controlled AC signal is forwarded to the transformer which is further added up with lined voltage to compensate the voltage sag. Hence providing smooth supply to the load Fig 3 Block diagram of the proposed Series VSC based on CRVSI. The control technique is applied to the stationary frame of reference. The voltage loop regulates the load voltage and produces a reference signal for the current control loop. The three-phase voltage signals are compared with the three-phase AC reference signal to calculate the error factor and then fed to the PI controller of the current control loop.

An average value model block diagram of this system given in Fig 4 In diagram V. Load's is the voltage reference signal given to the controller to compare the error factor. Here, in this case, it represents the sag detected earlier by the controller at grid VDC is the VDC is the forward gain of the amplifier used for the PWM modulator and EV s and EC's are voltage and current error factors calculated by comparing with the feedback current. I Load and feedback voltage V Load respectively. The regulated output current ILoad and voltage load of the inverter are processed ahead to attain the steady-state load current and voltage respectively Considering real-time scenario instead of the simple resistive load here the motor load is taken under observation who's back emf is given to the controller as disturbance signal D's. The proposed system's transfer function is given by equation 1 1. The controllers GV's and GC's are to compare ILoad and VLoad with I Load and V Load respectively to minimize the difference caused by the sag generation in the grid nearly equal to zero. And this is accomplished by enhancing the forward loop gain GC's, GV's VDC. Although the PI tuning becomes a bit challenging because adjusting PI gains with AC signals is quite critical.

However, the implemented control strategy, in this case, gives a satisfactory performance as that of the conventional sophisticated control techniques like dq0 rotating frame of reference and P R controllers which is well discussed and validated in 11 12 3. Simulation Results and Discussion. The behavior of the proposed series VSC is verified using simulations carried out in MATLAB Simulink. A system similar to the proposed VSC shown in Fig 3 is developed and then investigated. The parameters of the developed system are given in Table 1 as under. In the performed simulations the grid is supplying the voltage of VUility 1 p u. However when time tsag 0 096 sec a sag occurs that reduces the grid voltages to VUility 0 7 p u. The compensator detected the reduced magnitude of the load voltage and starts compensating the load voltage at t 0 1 sec after a delay of t 0 004 sec. This delay occurs due to the time required for the detection of fault and activation of bypass switches. The compensator generates a voltage of VComp 0 3 p u and injects it to the grid voltages using a series transformer to regulate the load voltage. This behaviour of the proposed VSC is depicted in Fig 5 Fig 5 a presents the utility voltage. However, the compensator output voltage and the load voltage is given in Fig 5 b and c respectively. The respective effect of proposed series VSC over inrush current is shown in Fig 6 Where the first peek of the compensator current is IComp 7 p u which is 125 of actual current. Hence the magnitude of inrush current is decreased by 105 when compared to the conventional series VSC.

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