FIELD OF THE INVENTION
The present invention generally relates to a method for correcting essential parameters in the power system, more particularly for correcting power factor (PFC) and harmonic suppression in parallely connected inverter units applicable for oil rig application. The invention further relates to an improved system for correcting power factor and harmonic suppressor for oil rig.
BACKGROUND OF THE INVENTION
In the prior art system, power requirement of present AC-SCR type oil rig is typically met by 3-5 numbers DG sets with combined diesel engine rating of a lower MW. In power systems, AC supply is fed to SCR drives, for control of the DC motor speed. Due to inherent nature of SCR drives driving variable speed DC motors, the power factor of the system varies with the speed of controlled motor and is very low if the system demands operation of the motor at lower speed. Such drives also introduce large harmonics into the power system.
The prior art system includes AC-SCR type oil well drilling rig having SCR drives, wherein drilling operations demand running the motor driven mud pumps at speed lower than full speed. Drilling operations also demands several hundred repeated short cycles, lasting few minutes of SCR driven draw-work motor for lifting of string (typically having length of few kilometer) or block, from derrick floor height to crown block height hence requiring speed variation from zero speed to high speed and back to zero speed.
Moreover, an AC-SCR oil rig power system suffers from presence of very high reactive power and harmonics coupled with large and recurring fluctuations in reactive power. Power system of Oil Rig are generally powered by DG set running on continuous duty basis. Alternators coupled to engines and feeding common bus are oversized with typically 0.7 PF (power factor) rating to take care of large reactive load in oil rig power system, against 0.8 PF as generally used for power generation in other industries. Alternator with low PF is used due to low power factor demanded by Rig power system.

SCR drives controlling speed of various DC motors such as draw-work, mud-pump, rotary pump etc. are fed from common bus. Besides SCR drives, various AC loads such as lighting, air-conditioning and auxiliary motors are fed by a step down transformer.
Further, the prior art system is illustrated by accompanying Figure-1A and 1B, where Diesel engine as per system requirement ( 1a-1d) are driving alternators (2a-2d), feeding to common bus (3) through DG breakers (4a-4d). Various SCR drives (5a-5d), as per system requirement draw power from common bus (3) through SCR breakers (6a-6d) and controls the motors (7a-7d). Step Down transformer (8) is used having protection with incoming breaker (9) and outgoing breaker (10) to feed various AC load (11). Fig 1A illustrates layout where all generator breakers come together on the bus on one side & all SCR breakers come together on the bus on other side, whereas Fig-1B illustrates an arrangement in which generator and SCR breakers are alternatively connected to the common bus.
Further, Speed control of DC motor in SCR system is through chopping of voltage waveform at calculated instant depending upon set speed, thus reducing average voltage of motor and hence controlling the speed. This method induces a delay in current waveform as compared to applied voltage resulting in poor power factor and large reactive power in system coupled with harmonics due to waveform distortion. Thus disadvantages of present system is poor PF and large reactive power along with generation of harmonics.
Further, the reactive power only fluctuates between source and load without any real contribution to work, which adds to total current flow. Thus, another disadvantages of prior art system is larger current drawn from DG set. Still another disadvantage of the prior art system is disclosed as higher capital investment due to use of thicker conductor. The thicker conductor results into over sizing of alternator to take care of larger reactive current as evident from the fact that Oil Rigs generally have rated PF of alternator as 0.7 against 0.8 generally used in other industries. Still yet another disadvantages of the prior art system is higher losses, hence poor efficiency and higher operating cost due to higher

heat loss to higher current especially in alternator. Still another disadvantage of prior art system is increased excitation due to lower PF.
Further, the alternators used in Oil Rig application is rated at lower PF of 0.7, compared to 0.8 PF used in utilities, still the reactive power in the prior art system is so high that the current limit of alternator is reached prior to active power limit of engine, not allowing further loading of the engine though its real power capacity is still not fully utilized. Thus, one of the major disadvantage of the prior art system is non availability of full capacity of engines for drilling operations.
Further, to reach the current limit of alternator, running of more numbers of engines are required to meet the reactive power requirement, though active power requirement could have been met by lesser DG sets on line. Diesel engines have recommended periodic replacement schedule for lube oil, filters and consumables and have fixed recommended overhauling schedule, based on running hours. Hence, yet another disadvantage of prior art system is higher maintenance cost.
Moreover, diesel generated power is one of the costliest power among the conventional sources of electricity. The diesel engines have higher fuel efficiency while operating near full loads compared to their operation at lighter loads. Running engines at lighter loads for longer duration results in higher fuel consumption that leads another disadvantage of prior art system to increase fuel costs.
Further, oil rig itself is also a mobile equipment and is required to be transported to next well location on completion of drilling operation at one well. The rig equipment must be easily transportable, compact and easy to install during rig building and easy to disconnect during rig dismantling. One of the major constraints associated with implementation of power factor corrector and harmonic suppressor is packaging of equipment for quick installation and dismantling with ease of transportation, required in an oil rig. A further constraint associated with implementation with power factor improvement scheme is risk of overcompensation, i.e. Power factor moving towards leading side, as leading side operation of alternator may lead to unstable operation,

resulting in alternator/s falling out of step and even damaging rotor of alternator and/or shaft of engine.
To summarize disadvantages of prior art system is given below:
1. The prior art system suffers from poor and large reactive power and generation of harmonics.
2. The prior art system generates poor PF, large reactive power and also harmonics.
3. The prior art system suffers from higher losses hence poor efficiency and higher operating cost due to higher heat loss as well as higher current generation especially in alternator.
4. The prior art system generates increased excitation due to lower PF.
5. Non availability of full capacity of engines for drilling operations.
6. Maintenance cost is much higher.
7. Higher fuel cost.
8. Power factor improvement scheme is risk of overcompensation which may lead to damage of alternator and/or shaft of engine.
Hence, there is a need of a method in an improved power system in such a manner, that the above drawbacks of conventional system are addressed.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention is to address all the above mentioned problems of prior art system in the oil rig application.
Another object of the present invention is to provide an improved system for correcting the power factor and harmonic suppression in the power system of inverter unit which are connected in parallel for oil rig application.

A still another object of the present invention is to provide an improved system for correcting power factor and harmonic suppressor for oil rig, which includes automatic electronic devices with faster response in synthesizing and delivering required reactive power.
Yet another object of the present invention is to provide a method for correction of power factor and harmonic suppression in parallelly connected inverter units available for oil rig.
Still yet another object of the present invention is to provide a method for correction of power factor and harmonic suppression in parallelly connected inverter units available for oil rig, which allows to operate at desired power factor (PF).
A still another object of the present invention is to provide an improved system for correcting the power factor and harmonic suppression in the power system of inverter unit which are connected in parallel for oil rig application, which shall calculate and supply required reactive power as per user set power factor and DG set to supply active power component of current.
Still yet another object of the present invention is to conceive an improved power system with alternators and its conductors sized for desired PF, which reduces the capital investment.
Yet another object of the present invention is to reduce heat losses in alternators resulting in increased efficiency and lower operating cost and also reduces the excitation current requirement from AVR due to achieving desired PF.
Still another object of the present invention is to make more power available for drilling operation by obviating current limit condition and enabling engine operation up to its maximum power delivery capacity.
Yet another object of the present invention is to propose a method to avoid operation of alterations in unstable zone due to over compensation of reactive power without requiring any external relay for the same.

SUMMARY OF THE INVENTION
One or more drawbacks of conventional machine are overcome by placing an improved system in a power system machine and additional advantages are provided through the improved system and method as claimed in the present disclosure. Additional feature and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be part of the claimed disclosure.
The present invention generally discloses a method in an improved system for correcting power factor and harmonic suppressor for oil rig application. The proposed improved system in the power factor corrector and harmonic suppressor house is used for correcting power factor and harmonic suppressor and also control the reactive power from the abnormal condition.
In the embodiment, a method for correcting of power factor and harmonic suppression in parallelly connected inverter units available for oil rig, comprising : providing a Digital Signal Processor (DSP) based controller(014) for each unit of the inverter; feeding multiple category of signals corresponding to grid voltage signal (010), summation of total current signal(011), individual inverter current signal (012), individual DC link capacitor (013) voltage, to Digital Signal Processor (DSP) based controller (014) of individual voltage source inverter (VSI)(008); controlling the voltage source inverter (VSI) (008) units through pulses generated by the Digital Signal Processor (DSP), based on the fed signals; comparing the system voltage (grid voltage) with the voltage of voltage source inverter (VSI) (008) unit; and delivering based on the result of comparison, the reactive power to the load or absorbing the reactive power when the result of comparison is high or less respectively.
In another embodiment, an improved system for correcting power factor and harmonic suppressor for oil rig comprising: at least four number of IGBT based voltage source inverter each assigned with one Digital Signal Processor (DSP) and connected in parallel;

a controller(14) for each individual voltage source inverter (VSI) unit for calculating VSI voltage requirement and generating pulses for IGBT based voltage source inverter (VSI) unit; at least one power factor corrector and harmonic compensator including an air circuit breaker (ACB)(004), a voltage feedback transformer(005), a control supply transformer (007) for individual panel, a reactor(006), a voltage source inverter (VSI)(008), a current transformer (002) for sensing the current feedback from DC-PCR grid/bus bars, and a supply in DC-PCR grid/bus from generator/s; at least one grid (001) connected to both the IGBT based and Digital Signal Processing (DSP) controlled voltage source inverter unit, and a load for delivering the reactive power or absorbing the reactive power based on comparison of grid voltage(Vg) and the inverter voltage(Vi); wherein an air core reactor having two reactors (006) which are stacked one above other keeping sufficient gap in between and placed in the air conditioned area.
Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designed by like numerals throughout. The following description is intended only by way of example, and simply illustrates. Certain selected embodiments of improved system for power factor correction and harmonic suppression in parallelly connected inverter units applicable for oil rig that are consistent with the subject matter as claimed herein, wherein:
Fig 1A- illustrates layout drawing of conventional system where all generator breakers comes together on the bus on one side and all SCR breakers comes together on the bus on other side according to prior art system.

Fig 1B- illustrates layout drawing of conventional system which represents an arrangement in which generator and SCR breakers are alternatively connected to the common bus according to prior art system.
Fig 02- illustrates the topology of single line diagram of power factor corrector and harmonic suppressor according to present invention.
Fig 03- illustrates general arrangement of power factor corrector and harmonic suppressor house according to present invention.
Fig 04- illustrates general capability diagram of an alternator and methodology to avoid overcompensation according to present invention.
Fig 05- illustrates reactor stack with two reactors to limit foot step according to present invention.
Fig 06a- illustrates schematic view of voltage source inverter connected with grid and load according to present invention.
Fig 06b- illustrates the graphs of comparison between grid voltage and the inverter voltage in both leading and lagging case.
Fig-07a- illustrates output waveform of the power factor corrector and harmonic suppressor house of conventional prior art system. The waveforms are shown in figure before implementation of the improved system
Fig-07b- illustrates output waveform of the power factor corrector and harmonic suppressor house of improved system according to present invention. The waveforms are shown in figure after the implementation of the improved system.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIEMENT OF THE INVENTION
While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.
The figures illustrate only those specific details that are pertinent to understand the embodiments of the present disclosure, so as not to obscure the disclosure with details that will be clear to those of ordinary skill in the art having benefit of the description herein.
The present invention is directed towards a method and an improved system for correcting PF and harmonic suppression in parallelly connected inverters units applicable for oil rig. In the present invention, Fig-2 illustrates a method for correcting of power factor and harmonic suppression in parallelly connected inverter units available for oil rig, comprising : providing a Digital Signal Processing (DSP) based controller(014) for each unit of the inverter units; feeding multiple category of signals corresponding to grid voltage signal (010), summation of total current signal(011), individual inverter current signal (012), individual DC link capacitor (013) voltage, to Digital Signal Processing (DSP) based controller (014) of individual voltage source inverter (VSI)(008);
controlling the voltage source inverter (VSI) (008) units through pulses generated by the Digital Signal Processing (DSP), based on the fed signals; comparing the system voltage (grid voltage) with the voltage of voltage source inverter (VSI) (008) unit; delivering the reactive power based on the result of comparison, to the load or absorbing the reactive power when the result of comparison is high or less respectively. Further, based on the comparison of two voltage Vg (grid voltage) and Vi (inverter voltage) as given in the following relationship,
when ViVg, Power Factor Corrector & Harmonic Suppressor draws leading current, where Vi= inverter voltage and Vg=grid voltage and wherein, the difference in magnitude of voltage between PFC & HS and grid will result in reactive power exchange between grid (001) and voltage source inverter(008).
In the present invention, Fig-2 illustrates an improved system for correcting power factor and harmonic suppressor for oil rig comprising: at least four number of insulated-gate bipolar transistor (IGBT) based voltage source inverter each assigned with one Digital Signal Processor (DSP) are connected in parallel; the Digital Signal Processor (DSP) based controller(014) for each individual voltage source inverter (VSI) unit for calculating VSI voltage requirement and generating pulses for IGBT based voltage source inverter (VSI) unit; at least one unit of power factor corrector and harmonic compensator including an air circuit breaker (ACB)(004), a voltage feedback transformer(005), a control supply transformer (007) for individual panel, a reactor(006), a voltage source inverter (VSI)(008), a current transformer (002) for sensing the current feedback from DC-PCR grid/bus bars, and a supply in DC-PCR grid/bus from generator/s; at least one grid(001) out of a plurality of grids which is connected the IGBT based and Digital Signal Processing (DSP) controlled voltage source inverter unit and load for delivering the reactive power or absorbing the reactive power based on comparison of grid voltage(Vg) and the inverter voltage(Vi); wherein an air core reactor stack having two reactors (006) which are stacked one above other keeping sufficient gap in between and placed in the air conditioned area.
Further, Fig -2 illustrates the improved system involves the power supply, used in DC-PCR grid/bus is fed from generator/s (001). In DC-PCR bar air circuit breakers (003) are provided to connect generator to bus bar and connect the grid to power factor corrector and harmonic suppressor. The transformer (009) which is 415V/230V transformer (ACPCR), has been used for lighting and generating AC supply one 25 KVA.
Further, fig-2 illustrates the variation of reactive power is performed by means of an IGBT based voltage source inverter (VSI) (008). The VSI (008) uses power electronic device insulated-gate bipolar transistor (IGBT) to synthesize a voltage from a DC voltage source

(capacitor). Switching of inverters online/offline, depending upon load, is controlled through programmable logic controller (PLC) which is fully automatic.
In the present invention, Fig-3 illustrates the improved system, which is containerized in a fabricated house for weather protection and easy transportability, is accommodated in the fabricated house. The house is based upon multi-runner skid. At both ends skid shall be provided with two heavy duty pipes across the whole width with collars for the purpose of lifting and dragging at oil fields for making cluster well or loading/unloading on trailer for transportation.
Further illustrating the Fig- 3, the complete house is divided in two parts. Out of the two parts, the first part is having sophisticated control DSP and inverters which are accommodated in the air-conditioned area and second part is having reactors which are accommodated in non-air conditioned area. In air-conditioned area, plural walls at sides are sandwiched with thermal insulation, so that it is provided thermal insulation and maintaining inside temperature within limits. During operation, different external control electrical connection are used for safety purpose during transportation and rain protection. External control electrical connection comprises of the sockets installed on metallic socket board, power sockets installed on epoxy socket board. The said electrical devices are protected by socket board doors (020) serving as dual safety purpose during transportation and rain protection. Further, during operation, socket board door are kept open and hinge operated rain protection cover rests over it. Above the epoxy socket board, a metallic socket board is also provided for accommodating the control sockets such as multi pin socket and suitable rating sockets for lighting & air conditioning purposes etc. In the back side of epoxy socket board bus bar arrangement, separate R Y B phases have been provided for feeding to reactors and thereafter inverters through cables having flat Lug. The improved system is accommodated with air conditioners (021) having 100% percentage redundancy to keep temperature of electronic controls in safe zone.
Further, there are two safety doors which are also provided with anti-panic lock as shown in the figure 3. ISMC (Indian standard medium channel) channels are used as columns

to support the fabricated roof assembly filled with thermal insulation between top and bottom, has gradient at two sides at the top to avoid water accumulation. Roof assembly is bolted on columns and welded thereafter on beam for proper strength and perfect sealing. IGBT based inverters have been placed in air conditioned area in panel, for discharging of hot air using 02 numbers Air Duct which has been provided. For internal lighting, 25kVA transformer is used with RCBO box protection. All cables are routed through cable tray. MCB box has been used for lighting purpose. To avoid black out, emergency tube lights are provided on lighting bracket for easy dismantling.
Further, the inverters are envisaged to be connected to the grid through air/iron core reactor. There are Air core reactor stack which is uniquely designed. The air core reactor stack is used for 12 nos reactors, where 03 numbers are for each VSI. Out of three reactors, two reactors which are stacked one above other keeping sufficient gap and placed in air conditioned area and one further is disposed in the non-air conditioned area.
As shown in the fig-5, the four inverters, connected in parallel, has been conceived to obviate mutual flux linkage in adjoining reactor, as well as MS structure of container, and also to optimize footage area. For supporting cable, the reactors SS Rods (025) has been provided instead of MS material.
As illustrated in fig-5, the two reactors are mounted one above the other with sufficient gap in between created through non-magnetic material. The base and top of reactor stack are at sufficient distance from MS structure and clamps are provided at top and bottom for holding of the reactor stacks.
As illustrated in Fig-4, it shows general capability diagram of alternator with curve showing/tracing zone of stable operation. In the present invention, a simple straight line is drawn close to boundary of stable operating zone in such a manner, it is within stable operating zone. During operations, the operating point in KW v/s KVAR plane shall be continuously tracked. If the operating point is at right side of line the operation are in stable zone. In case, it approaches/breaches safe line, the improved system shall have

provision to withdraw the reactive power compensation provided by power factor corrector (PFC).
Thus, the improved system is used for identification and elimination of overcompensation which also performs compensation elimination requirement of external relay for protection. According to the figure-4, use of straight lines as boundary for stable operation instead of actual curve, makes detection simpler than using the curve.
Moreover, Fig-5 illustrates about the brief arrangement of air core reactor stack mounted in two tier. These reactors has been mounted with bottom surface, stainless-steel at middle and roof a top of the container. Two tier arrangements are used to save the space. These reactors are placed in distance with 2r (2 radius) with each other for avoiding the mutual inductance between each other. In nearby areas of reactors stainless steel material has been used as mesh and rods for avoiding the mutual inductance. The design is preferred when air core reactors are used, alternatively iron core reactors can also be used.
In the present invention, Fig-6a illustrates about the schematic diagram of circuit which comprises grid, voltage source inverter (VSI) connected with the load. The leading and lagging current are shown in the figure based on the comparison of system voltage (grid voltage (Vg)) and inverter voltage (Vi). The fig-6b shows the graph of the comparison result of both the grid voltage (Vg) and the inverter voltage (Vi) based on leading current and lagging current drawn by the power factor corrector and harmonic suppressor. The difference in phase angle between grid and inverter will result in active power exchange. The comparison of said two voltage is given by following relationship,
when ViVg, Power Factor Corrector & Harmonic Suppressor draws leading current.
The difference in magnitude of voltage between Power Factor Corrector & Harmonic Suppressor and grid, will result in reactive power exchange between grid (001) and voltage source inverter(008).

In the present invention, Fig-7a illustrates the waveform of the conventional system and fig 7b illustrates the waveform of the present invention where harmonics are suppressed. The test results of the present invention is tabulated in the following comparison table. The test table data are tabulated for prior art system and improved system.

Sl.no Power factor of conventional system Power factor of improved system
1 0.566 0.982

WE CLAIM :
1. A method for correcting of power factor and harmonic suppression in parallelly
connected inverter units available for oil rig, comprising :
- providing a Digital Signal Processor (DSP) based controller(014) for each unit of the inverter;
- feeding multiple category of signals corresponding to grid voltage signal (010),
- summation of total current signal(011), individual inverter current signal (012), individual DC link capacitor (013) voltage, to the Digital Signal Processor (DSP) based controller (014) of individual voltage source inverter (VSI)(008);
- controlling the voltage source inverter (VSI) (008) unit through pulses generated by the Digital Signal Processor (DSP), based on the fed signals;
- comparing the system voltage (grid voltage) with the voltage of voltage source inverter (VSI) (008) unit; and
- delivering based on the result of comparison, the reactive power to the load or absorbing the reactive power when the result of comparison is high or less respectively.
2. The method as claimed in claim 1, wherein the system voltage is grid voltage and the grid voltage is compared with the inverter voltage, wherein the comparison of said two voltage is given by following relationship,
when ViVg, Power Factor Corrector (PFC) & Harmonic Suppressor (HS) draws leading current, where Vi= inverter voltage and Vg=grid voltage and wherein, the difference in magnitude of voltage between Power Factor Corrector & Harmonic Suppressor and grid will result in reactive power exchange between grid (001) and voltage source inverter(008).

3. An improved system for correcting power factor and harmonic suppressor for oil
rig comprising:
- at least four number of insulated-gate bipolar transistor (IGBT) based voltage source inverter each assigned with one Digital Signal Processor (DSP) and connected in parallel;
- a controller(14) for each individual voltage source inverter (VSI) unit for calculating VSI voltage requirement and generating pulses for IGBT based voltage source inverter (VSI) unit;
- at least one power factor corrector and harmonic compensator including an air circuit breaker (ACB)(004), a voltage feedback transformer(005), a control supply transformer (007) for individual panel, a reactor(006), a voltage source inverter (VSI)(008), a current transformer (002) for sensing the current feedback from DC-PCR grid/bus bars, and a supply in DC-PCR grid/bus from generator/s;
- at least one grid (001) connected to both the IGBT based and Digital Signal Processor (DSP) controlled voltage source inverter unit, and a load for delivering the reactive power or absorbing the reactive power based on comparison of grid voltage(Vg) and the inverter voltage(Vi), wherein
an air core reactor stack having two reactors (006) which are stacked one above other keeping sufficient gap in between and placed in the air conditioned area.
4. The improved system as claimed in claim 3, wherein the improved system is
accommodated in a containerized unit which is a multi-runner skid based house,
which is divided into two parts such as air conditioned area and non-air conditioned
area.

5. The improved system as claimed in claim 3, wherein the switching of inverter online/offline, depending upon load, is controlled through an automatic process using a programmable logic controller (PLC).
6. The improved system as claimed in claim 3, wherein epoxy socket board bus bars have been provided separate for R Y B phases for feeding to the reactors and thereafter inverters through cables having flat lug.