Title of Invention	A SINE WAVE STEPLESS DUMP LOAD FOR FREQUENCY CONTROL OF WEAK GRID
Abstract	A novel sine wave step-less dump load for frequency control of weak grid (R-Y-B) formed by hybrid wind, diesel and solar photovoltaic (SPV) generators comprising of: - diode bridge (4.1) connected across R-Y lines; - inductor-capacitor filter (L1, C1) connected across diode bridge (4.1); - resistor R1 connected across the capacitor (C1) through a series connected IGBT or MOSFET (4.4); and - identical configuration of diode bridges (4.2, 4.3), inductor- capacitor filters (L2,C2, L3, C3) and resistors R2 , R3 corrected across capacitors (C2, C3) through series connected IGBT or MOSFET (4.5, 4.6) across lines Y-B and B-R, - Characterized in that in 3-phase mode almost pure sine currents are obtained due to cancellation of harmonics, a small 3rd harmonic filter is added for 1-ph systems and the dump load can be varied steplessly by changing the modulation index as a function of frequency, hence can be integrated easily with the overall frequency without injecting harmonics in the said weak grid.

Title of Invention

A SINE WAVE STEPLESS DUMP LOAD FOR FREQUENCY CONTROL OF WEAK GRID

Abstract

A novel sine wave step-less dump load for frequency control of weak grid (R-Y-B) formed by hybrid wind, diesel and solar photovoltaic (SPV) generators comprising of: - diode bridge (4.1) connected across R-Y lines; - inductor-capacitor filter (L1, C1) connected across diode bridge (4.1); - resistor R1 connected across the capacitor (C1) through a series connected IGBT or MOSFET (4.4); and - identical configuration of diode bridges (4.2, 4.3), inductor- capacitor filters (L2,C2, L3, C3) and resistors R2 , R3 corrected across capacitors (C2, C3) through series connected IGBT or MOSFET (4.5, 4.6) across lines Y-B and B-R, - Characterized in that in 3-phase mode almost pure sine currents are obtained due to cancellation of harmonics, a small 3rd harmonic filter is added for 1-ph systems and the dump load can be varied steplessly by changing the modulation index as a function of frequency, hence can be integrated easily with the overall frequency without injecting harmonics in the said weak grid.

Full Text	Field of the invention: The invention relates in general to generation of green power using solar photovoltaic (SPV) and wind generators, and in particular to a novel step-less dump load for frequency control of weak grids formed by hybrid wind, diesel and SPV generation. Background and prior art: The world over research is going on to generate green power using wind generators and solar photovoltaic inverters. These systems work well if grid power is available at the site. For remote sites, where grid power is not available but there is a lot of wind and sun, it is difficult to form a stand alone grid with only wind generators and SPV inverters, and diesel generators are also required to be used to form a weak grid. In such installations the frequency of the grid will have to be controlled using dump load. Ideally the dump loads should be step-less and should not generate harmonics, as harmonics may excite resonances in the weak grid. The control action should not cause generation of reactive VARS and it should be easy to integrate the action with the overall frequency control. This invention addresses these problems. There are many methods reported for designing a dump load. A US patent 4,511,807 'ELECTRIC GENERATOR CONTROL SYSTEM' uses simple switches in series with resistors. A French patent FR2786955/FR2786955 'LINEARLY VARIABLE LOADING BANK FOR HIGH-POWER 3- PHASE ELECTRICAL NETWORK, COMPRISES RESISTOR BANK ETC SWITCHED BY THYRISTORS PAIRS ,ONE REVERSE /PARALLEL PAIR PER PHASE'. The patent suggests firing angle control which is very much known to generate harmonics. US patent pub no. 2007/0164567 'WIND TURBINE DUMP LOAD SYSTEM AND METHOD \ Uses a thyristorised dump load in combination with mechanically switched dump loads for Permanent Magnet Generators. A US patent 4,563,630 'METHOD OF GOVERNING A GENERATOR AND/OR APPARATUS FOR GOVERNING A GENERATOR' suggests a method using Triacs (or thyristors in back to back mode) for switching resistors in R,2R,4R,8R steps to give 15 equal steps. Yet another US patent pub. no. US2007/0246943 'STAND ALONE WIND TURBINE SYSTEM ,APPARATUS AND METHOD SUITABLE FOR OPERATING THE SAME' mentions about using IGBT for step less dump control as per Fig-3.This system injects major characteristic harmonics in the lines as the line currents are trapezoidal. In prior art, control logic is to be implemented for discrete switching, which does not go well with the closed loop frequency control The present invention seeks to overcome the above mentioned drawbacks of the prior art. Objects of the invention: The major objectives of present invention are as follows: An object of the invention is to provide a 3-ph dump load which can be controlled in a step-less manner without injecting harmonics in the weak grid formed by stand-alone wind generators, diesel generators, and solar photovoltaic inverters. Another object of the invention is to provide a dump load which does not inject reactive VARS (pf to be unity) into the weak grid. Yet another object is to integrate the control with overall closed loop frequency control in a seamless manner. A further object is to provide a system for 1-phase load also by adding a small third harmonic filter. Brief description of the accompanying drawings: Fig.1 shows a block diagram of thyristorized phase angle control as in prior art. Fig.2 shows thyristorized step switching of dump load as in the prior art. Fig.3 shows unit configuration of the IGBT based stepless dump load according to prior art. Fig.4 shows complete three-phase configuration of step-less dump load as invented. Fig.5 shows the block diagram for the control circuit according to the invention. The invention will now be described in detail in an exemplary embodiment as depicted in the accompanying drawings. There can however be other embodiments of this invention, all of which are deemed covered by this description. Description A circuit configuration has been invented which PROVIDES results in to a simple cost effective step-less control of dump load without injecting harmonics in to the weak grid formed by Wind Generators, Diesel Sets and Solar Photovoltaic Inverters. The conventional methods are deficient as step-less dump loads generate a lot of harmonics which can excite resonances in the weak grid. As per the invention, an Inductor-Capacitor filter is connected through a single phase diode bridge between the two lines of 3 phase supply and is dimensioned in such a way that high frequency sine modulated switching of one IGBT across the capacitor results in to a sine AC current with predominant 3rd harmonic and a small amount of 5th harmonic . This process is repeated for other two phases The resultant line current is almost pure sine due to cancellation of 3rd harmonic and 5th harmonic to some extent. The controller is implemented in such a way that unity power factor is obtained. The frequency control signal causes a step-less change in the modulation index, resulting into a step less dump load. Fig.4 shows the configuration of the step-less dump load according to the invention, which is connected across a 3-phase grid designated as R-Y-B. The dump load is divided in three parts, connected to R-Y, Y-B and B-R phases respectively. The following description holds good for R-Y line, it is repeated for Y-B and B-R lines in exactly the same manner. 1. The weak grid is designated as R-Y-B lines. 2. A diode bridge (4.1) is connected across R-Y lines. 3. An Inductor-Capacitor filter(L1-C1) is connected across the diode bridge. Inductor L1 is preferably connected split as shown in the figure(both halves can be coupled or uncoupled) to reject common mode noise generated by IGBT switching. 4. A resistor'Rl' is connected across the capacitor through a series connected IGBT or MOSFET(in the figure is shown as N channel MOSFET) 5. Resistor R1 is selected so as to load 1.4 to 1.5 times the required power per phase (line to line) assuming the IGBT is fully on. This is based on the effect of third harmonic cancellation in 3-ph system and assuming 20% over modulation is resorted to. 6. Capacitor is C1 selected so that RC time constant has near the time period of carrier frequency in the range 0.5 to 0.7 times. 7. Inductor L1 is chosen in conjunction with C1 to block switching carrier frequency to achieve a tuning frequency which is approx 10 % of carrier frequency. 8. This configuration is implemented in other two lines, resulting in cancellation of 3rd harmonics currents generated by each phase. Hence the resultant network line current is almost a pure sine wave at unity power factor. 9. Referring to Fig.-5, IGBT is triggered by sine modulated voltage derived from R-Y. As the weak grid voltage is susceptible to harmonic distortion R-Y voltage is attenuated and filtered to remove harmonics from other sources(specially SPV inverters),giving a voltage lag of G.This lag is compensated by 61 which also compensates the lag introduced by carrier frequency fc. Hence 61=6+1/fc is selected, to get unity power factor. The resultant signal is rectified and is multiplied with modulation Index (MI) .MI is derived as a function of frequency control signal VC. The output of multiplier is modulated with a high frequency carrier (at frequency =fc).The final output of the comparator is the desired Pulse Width Modulated (PWM) signal which goes to the gate of the IGBT(or MOSFET) The preferred carrier frequency is more than 3600HZ,though lower frequency can also be used with the trade off as in that case higher values of filter 'L1' and 'C1' shall be required. 10. For single phase generators, the same configuration can be used with an additional 3rd harmonic filter. 11. The load is varied steplessly by changing the modulation index as a function of frequency, hence can be integrated easily with the overall frequency control. WE CLAIM 1. A novel sine wave step-less dump load for frequency control of weak grid (R-Y-B) formed by hybrid wind, diesel and solar photovoltaic (SPV) generators comprising of: - diode bridge (4.1) connected across R-Y lines; - inductor-capacitor filter (L1, C1) connected across diode bridge (4.1); - resistor R1 connected across the capacitor (C1) through a series connected IGBT or MOSFET (4.4); and - identical configuration of diode bridges (4.2, 4.3), inductor- capacitor filters (L2,C2, L3, C3) and resistors R2 , R3 corrected across capacitors (C2, C3) through series connected IGBT or MOSFET (4.5, 4.6) across lines Y-B and B-R, characterized in that the dump load can be varied steplessly by changing the modulation index as a function of frequency, hence can be integrated easily with the overall frequency without injecting harmonics in the said weak grid. 2. The step-less dump load as claimed in claim 1, wherein the inductors L1, L2 and L3 each have two halves, which can be coupled or decoupled to reject common mode noise generated by IGBT switching. 3. The step-less dump load as claimed in claim 1, wherein resistors R1, R2 and R3 are selected so as to load 1.4 to 1.5 times the required power per phase. 4. The step-less dump load as claimed in claim 1, wherein capacitors C1, C2 C3 are selected so that the RC time constant lies near the time period of carrier frequency in the range of 0.5 to 0.7 times. 5. The step-less dump load as claimed in claim 1, wherein inductors L1, L2, L3 are chosen in conjunction with C1,C2, C3 to block switching carrier frequency to achieve a tuning frequency which is approximately 10% of carrier frequency. 6. The step-less dump load as claimed in claims 1 and 4 wherein the preferred value of the said carrier frequency is more than 3600 Hz. 7. The step-less dump load as claimed in claims 1 and 6, wherein when the said carrier frequency is less than 3600Hz, higher value of inductor-capacitor filters (L1-C1, L2-C2, L3-C3) shall be required. 8. The step-less dump load as claimed in claim 1, wherein for single phase generators, the same configuration of the said dump load is applicable with an additional 3rd harmonic filter. A novel sine wave step-less dump load for frequency control of weak grid (R-Y-B) formed by hybrid wind, diesel and solar photovoltaic (SPV) generators comprising of: - diode bridge (4.1) connected across R-Y lines; - inductor-capacitor filter (L1, C1) connected across diode bridge (4.1); - resistor R1 connected across the capacitor (C1) through a series connected IGBT or MOSFET (4.4); and - identical configuration of diode bridges (4.2, 4.3), inductor- capacitor filters (L2,C2, L3, C3) and resistors R2 , R3 corrected across capacitors (C2, C3) through series connected IGBT or MOSFET (4.5, 4.6) across lines Y-B and B-R, - Characterized in that in 3-phase mode almost pure sine currents are obtained due to cancellation of harmonics, a small 3rd harmonic filter is added for 1-ph systems and the dump load can be varied steplessly by changing the modulation index as a function of frequency, hence can be integrated easily with the overall frequency without injecting harmonics in the said weak grid.

Full Text

Field of the invention:
The invention relates in general to generation of green power using
solar photovoltaic (SPV) and wind generators, and in particular to a
novel step-less dump load for frequency control of weak grids
formed by hybrid wind, diesel and SPV generation.
Background and prior art:
The world over research is going on to generate green power using wind
generators and solar photovoltaic inverters. These systems work well if
grid power is available at the site. For remote sites, where grid power is
not available but there is a lot of wind and sun, it is difficult to form a
stand alone grid with only wind generators and SPV inverters, and diesel
generators are also required to be used to form a weak grid. In such
installations the frequency of the grid will have to be controlled using
dump load. Ideally the dump loads should be step-less and should not
generate harmonics, as harmonics may excite resonances in the weak
grid. The control action should not cause generation of reactive VARS and
it should be easy to integrate the action with the overall frequency
control. This invention addresses these problems.
There are many methods reported for designing a dump load.
A US patent 4,511,807 'ELECTRIC GENERATOR CONTROL SYSTEM' uses
simple switches in series with resistors.
A French patent FR2786955/FR2786955 'LINEARLY VARIABLE LOADING
BANK FOR HIGH-POWER 3- PHASE ELECTRICAL NETWORK, COMPRISES
RESISTOR BANK ETC SWITCHED BY THYRISTORS PAIRS

,ONE REVERSE /PARALLEL PAIR PER PHASE'. The patent suggests firing
angle control which is very much known to generate harmonics.
US patent pub no. 2007/0164567 'WIND TURBINE DUMP LOAD SYSTEM
AND METHOD \ Uses a thyristorised dump load in combination with
mechanically switched dump loads for Permanent Magnet Generators.
A US patent 4,563,630 'METHOD OF GOVERNING A GENERATOR AND/OR
APPARATUS FOR GOVERNING A GENERATOR' suggests a method using
Triacs (or thyristors in back to back mode) for switching resistors in
R,2R,4R,8R steps to give 15 equal steps.
Yet another US patent pub. no. US2007/0246943 'STAND ALONE WIND
TURBINE SYSTEM ,APPARATUS AND METHOD SUITABLE FOR OPERATING
THE SAME' mentions about using IGBT for step less dump control as per
Fig-3.This system injects major characteristic harmonics in the lines as
the line currents are trapezoidal.
In prior art, control logic is to be implemented for discrete switching,
which does not go well with the closed loop frequency control
The present invention seeks to overcome the above mentioned drawbacks
of the prior art.
Objects of the invention:
The major objectives of present invention are as follows:
An object of the invention is to provide a 3-ph dump load which can
be controlled in a step-less manner without injecting harmonics in
the weak grid formed by stand-alone wind generators, diesel
generators, and solar photovoltaic inverters.

Another object of the invention is to provide a dump load which
does not inject reactive VARS (pf to be unity) into the weak grid.
Yet another object is to integrate the control with overall closed
loop frequency control in a seamless manner.
A further object is to provide a system for 1-phase load also by
adding a small third harmonic filter.
Brief description of the accompanying drawings:
Fig.1 shows a block diagram of thyristorized phase angle control as in
prior art.
Fig.2 shows thyristorized step switching of dump load as in the prior art.
Fig.3 shows unit configuration of the IGBT based stepless dump load
according to prior art.
Fig.4 shows complete three-phase configuration of step-less dump load
as invented.
Fig.5 shows the block diagram for the control circuit according to the
invention.
The invention will now be described in detail in an exemplary embodiment
as depicted in the accompanying drawings. There can however be other
embodiments of this invention, all of which are deemed covered by this
description.

Description
A circuit configuration has been invented which PROVIDES results in to a
simple cost effective step-less control of dump load without injecting
harmonics in to the weak grid formed by Wind Generators, Diesel Sets
and Solar Photovoltaic Inverters. The conventional methods are deficient
as step-less dump loads generate a lot of harmonics which can excite
resonances in the weak grid. As per the invention, an Inductor-Capacitor
filter is connected through a single phase diode bridge between the two
lines of 3 phase supply and is dimensioned in such a way that high
frequency sine modulated switching of one IGBT across the capacitor
results in to a sine AC current with predominant 3rd harmonic and a
small amount of 5th harmonic . This process is repeated for other two
phases The resultant line current is almost pure sine due to cancellation
of 3rd harmonic and 5th harmonic to some extent. The controller is
implemented in such a way that unity power factor is obtained. The
frequency control signal causes a step-less change in the modulation
index, resulting into a step less dump load.
Fig.4 shows the configuration of the step-less dump load according to the
invention, which is connected across a 3-phase grid designated as R-Y-B.
The dump load is divided in three parts, connected to R-Y, Y-B and B-R
phases respectively.
The following description holds good for R-Y line, it is repeated for Y-B
and B-R lines in exactly the same manner.

1. The weak grid is designated as R-Y-B lines.
2. A diode bridge (4.1) is connected across R-Y lines.
3. An Inductor-Capacitor filter(L1-C1) is connected across the diode
bridge. Inductor L1 is preferably connected split as shown in the
figure(both halves can be coupled or uncoupled) to reject common
mode noise generated by IGBT switching.
4. A resistor'Rl' is connected across the capacitor through a series
connected IGBT or MOSFET(in the figure is shown as N channel
MOSFET)
5. Resistor R1 is selected so as to load 1.4 to 1.5 times the required
power per phase (line to line) assuming the IGBT is fully on. This is
based on the effect of third harmonic cancellation in 3-ph system and
assuming 20% over modulation is resorted to.
6. Capacitor is C1 selected so that RC time constant has near the time
period of carrier frequency in the range 0.5 to 0.7 times.
7. Inductor L1 is chosen in conjunction with C1 to block switching carrier
frequency to achieve a tuning frequency which is approx 10 % of
carrier frequency.
8. This configuration is implemented in other two lines, resulting in
cancellation of 3rd harmonics currents generated by each phase.
Hence the resultant network line current is almost a pure sine wave at
unity power factor.

9. Referring to Fig.-5, IGBT is triggered by sine modulated voltage
derived from R-Y. As the weak grid voltage is susceptible to
harmonic distortion R-Y voltage is attenuated and filtered to
remove harmonics from other sources(specially SPV
inverters),giving a voltage lag of G.This lag is compensated by 61
which also compensates the lag introduced by carrier frequency fc.
Hence 61=6+1/fc is selected, to get unity power factor. The
resultant signal is rectified and is multiplied with modulation Index
(MI) .MI is derived as a function of frequency control signal VC. The
output of multiplier is modulated with a high frequency carrier (at
frequency =fc).The final output of the comparator is the desired
Pulse Width Modulated (PWM) signal which goes to the gate of the
IGBT(or MOSFET) The preferred carrier frequency is more than
3600HZ,though lower frequency can also be used with the trade
off as in that case higher values of filter 'L1' and 'C1' shall be
required.
10. For single phase generators, the same configuration can be used
with an additional 3rd harmonic filter.
11. The load is varied steplessly by changing the modulation index as a
function of frequency, hence can be integrated easily with the overall
frequency control.

WE CLAIM
1. A novel sine wave step-less dump load for frequency control of
weak grid (R-Y-B) formed by hybrid wind, diesel and solar
photovoltaic (SPV) generators comprising of:
- diode bridge (4.1) connected across R-Y lines;
- inductor-capacitor filter (L1, C1) connected across diode bridge
(4.1);
- resistor R1 connected across the capacitor (C1) through a series
connected IGBT or MOSFET (4.4); and
- identical configuration of diode bridges (4.2, 4.3), inductor-
capacitor filters (L2,C2, L3, C3) and resistors R2 , R3 corrected
across capacitors (C2, C3) through series connected IGBT or
MOSFET (4.5, 4.6) across lines Y-B and B-R,
characterized in that the dump load can be varied steplessly by
changing the modulation index as a function of frequency, hence
can be integrated easily with the overall frequency without injecting
harmonics in the said weak grid.
2. The step-less dump load as claimed in claim 1, wherein the
inductors L1, L2 and L3 each have two halves, which can be
coupled or decoupled to reject common mode noise generated by
IGBT switching.
3. The step-less dump load as claimed in claim 1, wherein resistors
R1, R2 and R3 are selected so as to load 1.4 to 1.5 times the
required power per phase.

4. The step-less dump load as claimed in claim 1, wherein capacitors
C1, C2 C3 are selected so that the RC time constant lies near the
time period of carrier frequency in the range of 0.5 to 0.7 times.
5. The step-less dump load as claimed in claim 1, wherein inductors
L1, L2, L3 are chosen in conjunction with C1,C2, C3 to block
switching carrier frequency to achieve a tuning frequency which is
approximately 10% of carrier frequency.
6. The step-less dump load as claimed in claims 1 and 4 wherein the
preferred value of the said carrier frequency is more than 3600 Hz.
7. The step-less dump load as claimed in claims 1 and 6, wherein
when the said carrier frequency is less than 3600Hz, higher value of
inductor-capacitor filters (L1-C1, L2-C2, L3-C3) shall be required.
8. The step-less dump load as claimed in claim 1, wherein for single
phase generators, the same configuration of the said dump load is
applicable with an additional 3rd harmonic filter.

A novel sine wave step-less dump load for frequency control of weak
grid (R-Y-B) formed by hybrid wind, diesel and solar photovoltaic
(SPV) generators comprising of:
- diode bridge (4.1) connected across R-Y lines;
- inductor-capacitor filter (L1, C1) connected across diode bridge
(4.1);
- resistor R1 connected across the capacitor (C1) through a series
connected IGBT or MOSFET (4.4); and
- identical configuration of diode bridges (4.2, 4.3), inductor-
capacitor filters (L2,C2, L3, C3) and resistors R2 , R3 corrected
across capacitors (C2, C3) through series connected IGBT or
MOSFET (4.5, 4.6) across lines Y-B and B-R,
- Characterized in that in 3-phase mode almost pure sine currents
are obtained due to cancellation of harmonics, a small 3rd harmonic
filter is added for 1-ph systems and the dump load can be varied
steplessly by changing the modulation index as a function of
frequency, hence can be integrated easily with the overall
frequency without injecting harmonics in the said weak grid.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=u21lJz5T5+js2qH6tG6peg==&loc=wDBSZCsAt7zoiVrqcFJsRw==

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Patent Number

278863

Indian Patent Application Number

170/KOL/2010

PG Journal Number

01/2017

Publication Date

06-Jan-2017

Grant Date

31-Dec-2016

Date of Filing

23-Feb-2010

Name of Patentee

BHARAT HEAVY ELECTRICALS LIMITED

Applicant Address

REGIONAL OPERATIONS DIVISION (ROD), PLOT NO: 9/1, DJBLOCK 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA-700091, HAVING ITS REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	BRAHMA SWARUP GUPTA	BHEL-CORP. RESEARCH & DEVELOPMENT VIKASNAGAR HYDERABAD-500093, A.P., INDIA
2	AMBATIPUDI RAMACHANDRA PRABHU	BHEL-CORP. RESEARCH & DEVELOPMENT VIKASNAGAR HYDERABAD-500093, A.P., INDIA
3	SAMAVEDULA VENKAT RAM SARMA	BHEL-CORP. RESEARCH & DEVELOPMENT VIKASNAGAR HYDERABAD-500093, A.P., INDIA
4	POTTURI VENKATA RAMA GOPAL	BHEL-CORP. RESEARCH & DEVELOPMENT VIKASNAGAR HYDERABAD-500093, A.P., INDIA

PCT International Classification Number

H03L7/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA