Title of Invention

"A DISTRIBUTION REACTIVE VOLT-AMPERE COMPENSATOR WITHOUT SENSING LOAD REACTIVE POWER"

Abstract

The invention relates to a distribution reactive volt-ampere compensator without sensing load reactive power comprising a three-phase voltage source converter (37) connected to the secondary of the transformer (19) through three link inductors (20,21,22) and the control block (38) to monitor the voltages and currents of the incoming feeder, characterized in that said control block (38) comprises a reference reactive power generator (1) connected to reactive power controller block (2) which alongwith DC voltage controller (3) is connected in series to low pass filter and function selector (4), power to two-phase current converter (5a), two-phase to three-phase converter (5b) to convert two-phase reference currents (i*,i*b) to three-phase reference currents (i*a, i*b, i*c) as input to the respective hysteresis comparators (6,7,8) and to lock out circuits (9,10,11) to obtain switching pattern for top and bottom switches in each of the three phases of VSC.

Full Text

This invention relates to a Distribution Reactive Volt-Ampere (VAR) Compensator without sensing load reactive power (hereafter referred to as DVC). This DVC comprises of a voltage source converter (hereafter referred to as VSC) in VAR compensation mode without sensing load reactive power. The use of VSC for reactive power and harmonics compensation has been described in published literature and disclosed in patents. Use of VSC for reactive power compensation is gaining popularity because of the availability of high power self-commutating semiconductor devices, high-speed digital computer and advanced control philosophy The VSC is connected to the grid through link inductors .The VSC is controlled in such a way that it generates leading or lagging VAR The sign of the VAR to be generated depends on the VAR consumed by the load. The control is carried out automatically in such a way that the desired objective is met dynamically. Generally, the dynamic compensation of reactive power and harmonics with voltage source two level or multilevel converters are carried out by using load reactive power or load currents in control loop. The load reactive power is calculated by using measured load voltage and current parameters.The VSC generates reactive power equal in magnitude to the reactive power consumed by the load, but of opposite sign. Patent No. US 5586018 discloses the use of voltage source converter for suppressing voltage fluctuation in a consumer network. The VSC supplies the reactive power as per the controller demand to suppress voltage fluctuations. It is also required to calculate the load reactive power based on voltage and current supplied to the load. Patent Nov. US 5751138 discloses the use of voltage source converter as power conditioner for reactive power and harmonics compensation. For controlling the converter, the reactive power consumed by the load is calculated by measuring load voltage and current.Patent No^-' US - 648894^ which discloses the use of VSC as active filter for harmonics compensation of load network, also describes the requirement of measuring load currents for determining the harmonics injected by the load and accordingly controls the VSC to generate harmonic currents to neutralize load current harmonics. One of the main disadvantages of the conventional VSC based VAR compensator for distribution purpose is that it requires measurement of reactive power consumed by the load exclusively. In industrial premises, the loads are distributed and for compensating the reactive power, the control demands segregation of load bus bar and VAR compensator bus bar. The requirement of segregation of load bus bar and VSC based VAR compensator bus bar demands additional investment in terms of substation equipment and space. This also demands the installation of VAR compensator at the incoming substation. Therefore, the main objective of the present invention is to provide a DVC such that the segregation of load bus bar and DVC bus bar is avoided and thus the system becomes cost effective. In DVC, the reference reactive power is calculated based on power factor demanded by the utility and the active power drawn from the source; and accordingly, the DVC controls the VSC to generate the demanded VAR dynamically to maintain power factor at the desired value. The object of the present invention is using the DVC, to operate it in a closed-loop without measuring load voltage, current or reactive power, and thus avoiding major changes in the distribution network of the consumer. With the existing control scheme for static compensator, it is required to measure the reactive power and harmonics generated by the electrical loads installed at the consumer premises.The control loop of the conventional VSC based VAR compensator monitors the reactive power consumed by the load, compares it with the reference reactive power and accordingly changes the pulse width modulation (P M) pattern. In practice, it is difficult to segregate the load bus bar and VSC based VAR Compensator bus bar, and measure the reactive power consumed. The novel VSC based distribution VAR compensator control scheme developed here does not require the reactive power consumed by the load. Rather, it monitors the incoming feeder currents and voltages, which are always available in the sub-station of the consumer. These signals are used in such a way that the input power factor is maintained at the desired value. Another object of this invention is to provide novel control scheme for DVC, with which, it can be installed anywhere in the consumer premises. A further objective of this invention is that, by suitable selection of control parameters in the invented control scheme, the power factor of the consumer can be set at any desired value. An another objective of this invention is to avoid excessive voltage drop in consumer premises by reducing the reactive power consumption. Yet another objective of the present invention is to compensate for the harmonics generated by the consumer equipment dynamically, such that the harmonics injected to the grid are within tolerable limits. The other advantage with the invented control scheme of the DVC is that it can also work satisfactorily under unbalanced conditions. According to the present invention there is provided a distribution reactive volt-ampere compensator without sensing load reactive power comprising a three-phase voltage source converter connected to the secondary of the transformer through three link inductors and the control block to monitor the voltages and currents of the incoming feeder, characterized in that said control block comprises a reference reactive power generator connected to reactive power controller block which alongwith DC voltage controller is connected in series to low pass filter and function selector, power to two-phase current converter, two-phase to three-phase converter to convert two-phase reference currents to three-phase reference current as input to the respective hystersis comparators and to lock out circuits to obtain switching pattern for top and bottom switches in each of the three phases of VSC. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings. Figure 1- control block diagram of distribution reactive volt-ampere compensator; Figure 2- power circuit of distribution reactive volt-ampere compensator; Figure 3- single line diagram of power distribution network; Figure 4- single line diagram of the substation with conventional reactive volt-ampere compensator; Figure 5- single line diagram of the substation with the invented distribution reactive volt-ampere compensator (DVC). According to this invention, the DVC comprising of the three-phase VSC (37) connected to the secondary of transformer (19) through three link inductors (20, 21, 22) and the control block (38), which monitors the voltages and currents of the incoming feeder, and avoids monitoring of load voltage, current, or power. The control block (38), after processing the system parameters, releases PWM switching pattern for the VSC ( 37) in such a way that the power factor at the consumer premises is maintained at the desired value. Further, it can dynamically compensate for the harmonics generated by the loads installed at the consumer premises. DETAILED DESCRIPTION Fig. 1 shows the control block diagram of the DVC ( 38). The main objective of the invented DVC is to compensate the reactive power drawn and to maintain the power factor at the incoming feeder at the desired level. The reference reactive power generator block 1 receives the signal (p ) proportional s to the active power drawn through incoming feeder and the desired power factor (p*f ). Taking these two signals, it calculates the reference reactive power (qref )• The reference reactive power ( q ref ) so obtained is compared with the actual reactive power (qs) drawn through the incoming feeder. Please note that when the power factor is to be maintained at unity, the reference reactive power (qref ) has to be set at zero.The reactive power error signal (qerr) is processed by block 2 to generate a signal qref1 To operate the DVC, for power factor improvement, the DC link voltage ( voltage across filter capacitor) has to be maintained at a particular level and the PWM switching pattern has to be controlled such that the desired reactive power compensation is obtained. The reference DC link voltage (V*dc ) is compared with actual DC link voltage (Vdc ) and the error signal (Vderr) is processed by the block 3 to obtain a signal (Pref 1 ) proportional to the reference active power to be drawn from the incoming feeder. As mentioned earlier, one of the claims is also to compensate the harmonics generated by the loads installed at consumer premises. The harmonics in current get reflected in the reference reactive and active power as higher frequency components. Depending on the objective function of the equipment, the active power and reactive power signals are to be filtered in block 4 and accordingly, the power components to be compensated are selected. The active power and reactive power, after being processed through filter and function selector block 4, are taken as ultimate active and reactive power signals (p* and q* ) to be compensated. These two signals are processed through power-to-current calculator block 5a to obtain the reference currents. This block also receives the voltage signals (Viα and V iß ) at the input of link inductors in two axes stationary reference frame.The voltage source converter has to be controlled such that it generates the phase currents, which closely follow the reference currents both in phase and magnitude. There are well-known techniques to achieve this objective, e.g. (a) hysteresis, (b) ramp comparison, and (c) space vector type current controllers. The invented control scheme has adopted the hysteresis type current control. The active power and reactive power of incoming feeder can be calculated by measuring three-phase instantaneous voltage and current signals. The three-phase voltage and current signals (vs and is )after signal conditioning, in blocks 12 and 13, respectively, are transferred to the respective two axes stationary frame in blocks 15 and 16.These voltage and current signals are processed in power calculator block 18 to obtain the instantaneous active power and reactive power (p5 , qs) in incoming feeder. The three-phase voltage signals (Vi ) at the point of connection of link inductors are also signal conditioned in block 14 and transformed to two axes stationary reference frame (viα and viß ) in block 17 and used in power-to-current calculator block 5a to generate the two phase reference currents i *α and i*ß,which are converted to three-phase reference currents (i *a , i*b and i*c ) in block 5b. The phase currents (i *a , i*b and ic ) are subtracted from the respective reference phase currents (i *a , i*b and i*c ) and the error in phase currents are processed by hysteresis type comparators in blocks 6, 7 and 8 to obtain the switching instants.The output of the blocks 6, 7 and 8 are ;. processed by blocks 9, 10 and 11 to obtain switching pattern for top and bottom switches in each of the three phases of VSC.. Fig 2 shows the power circuit of the DVC. Blocks 23 to 28 are the power semiconductor devices ( for example, insulated gate bipolar transistors (IGBTs))connected to form a three-phase VSC.The P M switching pattern is generated such that the three-phase VSC acts as a leading or lagging VAR generator.The three-phase terminals of the VSC are connected to the secondary of transformer ( block 19) through three link inductors ( blocks 20, 21 and 22). The primary of the transformer is connected to the mains. The DC terminals of the VSC are connected to a filter capacitor bank ( block 29). Fig. 3 shows the single line diagram of. a typical power distribution network in consumer premises. The incoming voltage is received at high voltage, and stepped- down by transformer ( block 30). The output of the transformer is connected to the substation bus through oil circuit breaker (OCB, block 31). The loads are connected to the substation bus through OCBs (blocks 32 and 33). Fig. 4 shows the connections of the VSC based VAR compensator with the conventional control at the substation. An additional bus as shown in the figure has to be mounted for installing the conventional VAR compensator. The substation bus is connected to the additional bus through OCB ( block 34). The conventional VAR compensator is connected to the additional bus through OCB (block 35). Please note that the layout has to be changed at incoming substation for reliability and ease of maintenance. These modifications are required for measurement of load active power and reactive power. Fig. 5 shows the distribution network at the consumer premises with the invented DVC. The invented DVC can be connected to the substation bus through OCB(block 36).Thus, with DVC, the installation of additional bus and OCB (block 34) can be avoided. This invented DVC control does not require signal proportional to the load active power and reactive power. Rather, it requires the reactive power signal in incoming feeder,which is available easily. The DVC can be connected to the network anywhere in consumer premises. Thus, major modification in substation layout can be avoided, as is evident from the figure. DETAILED DESCRIPTION OF THE OPERATION A Distribution VAR Compensator (DVC) comprises of a voltage source converter consisting of self-commutating power semiconductor devices with accompanied freewheeling diode connected in three-phase Gratez bridge configuration. Three AC link inductors provided to connect the VSC (37) to the mains. The control block ( 38) dynamically sets the reference reactive power based on active power drawn and set power factor. The reactive power control block (2) decides the response of the DVC to change in reactive power demand.The DC voltage controller (3) maintains the DC link voltage (Vdc) at the set value at all operating conditions.A low pass filter and function selector (4) isolates the DC components and selects instantaneous active and reactive power components (p ,q ) depending on objective function. An instantaneous power to current converter (5a) generates two - phase reference currents ( i*α , i*ß ) by taking into account two-phase voltages. The two-phase to three-phase converter block (5b) transforms two-phase currents to three-phase currents. Hysteresis comparators ( 6, 7, 8) compare the reference phase currents ( i*a .i*b ,i*c ) with respective phase currents (ia ,ib , ic ) for generating VSC switching signals. Lockout circuits (9, 10, 11) generate switching signals for power semiconductor devices connected to positive and negative DC bus. Signal conditioning circuits (12,13) provide conditioned signals of three-phase voltages and currents to the controller. Three-phase to two-phase signal conversion blocks (15, 16) convert three-phase voltages and currents to equivalent two - phase voltages and currents. Power calculator block ( 18) calculates the active power and reactive power drawn through incoming feeder. The DVC dynamically sets the reactive power to be drawn from the source depending on the active power drawn and set power and controls the voltage source converter to generate the set reactive power. The DVC monitors the incoming voltages and currents only and does not monitor the load voltages and currents for dynamic compensation of reactive power; and thus avoids the segregation of load bus and DVC bus. The voltage source converter (37) consists of self- commutating power semiconductor devices ( 23, 24, 25, 26, 27, 28) with accompanied fast recovery diodes connected in three-phase voltage source inverter configuration. The AC link inductor ( 20, 21, 22) are iron core AC inductors, which are used to connect the AC terminals of the converter to the mains. The DVC can also be connected to the mains through a high reactance transformer. The reference reactive power generator (1) is the control block to generate the instantaneous reference reactive power as per the active power drawn through the incoming feeder and the reference power-factor set by the operator. The reactive power controller (2) is the control block that processes the error between the reference reactive power and the actual reactive power drawn through the incoming feeder and gives out a signal to control the DVC such that the error in reactive power is driven towards zero. The DC voltage controller(3) is the control block which maintains the DC link capacitor voltage at the set value by generating reference active power signal.The power to current converter (5a) is the control block for generating two - phase reference current signals from the reference active and reactive power signals taking into account the inverter voltages in two axes reference frame. The low pass filter and function selector (4) is the control block which isolates the DC component and high frequency component in reference active and reactive power signals (p* ,q* ) and selects the components of active power and reactive power to be compensated by the DVC. The two-phase to three - phase converter block (5b) is the control block to transform two-phase currents to three-phase currents. The hysteresis comparator ( 6,7,8) is the electronic circuit for comparing the reference phase currents with the actual phase currents and generate switching signals for the three phase in VSC. The lock-out circuit (9, 10, 11) is the electronic circuit for generating P M switching signals for the three phases in VSC while taking into account the turn-on and turn-off time requirements of power semiconductor/ devices (23, 24, 25, 26, 27, 28) and thus avoiding shoot-through fault. The signal conditioning circuit (12, 13, 14) is the electronic circuit or eliminating noise is measured current and voltage signals. The three-phase to two-phase signal converter (15, 16, 17) is the control block for converting three-phase voltage or current signals to equivalent two-phase voltage signals or control purpose. The power calculator (18) is the control block for calculating instantaneous active power and reactive power based on two-phase instantaneous voltages and currents. two-phase instantaneous voltages and currents. The invention described hereinabove is in relation to a non-limiting embodiment and as defined by the accompanying claims. WE CLAIM; 1. A distribution reactive volt-ampere compensator without sensing load reactive power comprising a three-phase voltage source converter (37) connected to the secondary of the transformer (19) through three link inductors (20,21,22) and the control block (38) to monitor the voltages and currents of the incoming feeder, characterized in that said control block (38) comprises a reference reactive power generator (1) connected to reactive power controller block (2) which alongwith DC voltage controller (3) is connected in series to low pass filter and function selector (4), power to two-phase current converter (5a), two- phase to three-phase converter (5b) to convert two-phase reference currents (i*a,i*p) to three-phase reference current (i*a, i*b, i*c) as input to the respective hystersis comparators (6,7,8) and to lock out circuits (9,10,11) to obtain switching pattern for top and bottom switches in each of the three phases of VSC. 2. The distribution reactive volt-ampere compensator as claimed in claim 1 wherein said low pass filter and function selector (4) are connected to blocks (2,3) for an input of reference reactive power (qref) and actual reactive power (qs) to process and generate ultimate active and reactive power signals (p*., q*). 3. A distribution reactive volt-ampere compensator without sensing load reactive power as herein described and illustrated with the accompanying drawings.

Documents:

171-del-2001-abstract.pdf

171-del-2001-claims.pdf

171-del-2001-correspondence-others.pdf

171-del-2001-correspondence-po.pdf

171-del-2001-description (complete).pdf

171-del-2001-drawings.pdf

171-del-2001-form-1.pdf

171-del-2001-form-19.pdf

171-del-2001-form-2.pdf

171-del-2001-form-26.pdf

171-del-2001-form-3.pdf