FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
AND
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2005
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A system for improved performance of shunt active harmonic filter in presence of PFC capacitors.
APPLICANT(S):
Crompton Greaves Limited, CG House, 6th Floor, Dr. Annie Besant Road, Worli, Mumbai - 400030, Maharashtra, India; an Indian Company.
INVENTOR (S):
Vaidya Tushar of Crompton Greaves Ltd, CG Global R&D Centre, Crompton Greaves Limited, Kanjur Marg, Mumbai 400 042, Maharashtra, India; an Indian National.
PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION:
This invention relates to the field of electronics engineering.
Particularly, this invention relates to the field of power grids and networks, power supply, and filters and controller, thereof
Specifically, this invention relates to a system for improved performance of shunt active harmonic filter in presence of PFC capacitors.
BACKGROUND OF THE INVENTION:
A distribution AC network has to carry load current which normally consists of active as well as reactive component. The reactive component of the current does not transfer any usable power but it contributes towards increasing the transmission losses. In order to avoid these losses, the reactive power is needed to be supplied locally at the point of common coupling (PCC). The reactive power support near the load also helps in improving the profile of the grid voltage.
Distribution utilities impose financial penalties for drawing reactive power from the grid to promote the use of reactive power support equipments by industries. These equipment typically include power factor correction capacitors (PFCs), Static VAr compensator (SVCs), and Static VAr Generators (SVGs). Another challenge being faced by utilities is to supply clean power. Increasing use of power electronic loads have raised severe concerns as far as quality of power is concerned. Utilities have started to implement the harmonic standards such as IEEE 519 for industrial and large commercial customers. The standard states the maximum limits on Total Demand Distortion (TDD) as well as individual current harmonics in % of maximum demand load current. In order to keep these harmonic levels under check, passive harmonic filters have traditionally been used. Though the passive filters have low cost and are highly efficient, the supply impedance strongly influences their compensation characteristics. Also, passive filters are highly prone to series parallel resonance with supply. Active harmonic filter (AHF) solutions mitigate the problems of passive filters. The AHF acts as a programmable source of harmonic voltage or harmonic current depending on the type of connection, series or shunt respectively. Shunt AHFs are more popular because of their comparatively lower cost as well as the fact that majority of the loads act as a current source of harmonics to which shunt AHF suits well. While supplying the harmonic current, the AHF is also capable of supplying the fundamental reactive component of the load current, thus eliminating the need of PFC capacitors from the network. However, assigning this dual role to AHF increases the power rating and cost of the AHF system manifold. Therefore, for practical applications, it makes economic sense to use PFC capacitors for fundamental reactive power support and use AHF only for harmonic compensation. PFC capacitors and AHF may therefore co-exist in a typical distribution network. However, their relative positioning within the network, affects the effectiveness of AHF. The presence of downstream PFC capacitors drastically reduces the effective bandwidth of the AHF system, so that certain harmonic frequencies get amplified.
Gist of the analysis reported in the prior art is as follows. The presence of large value of PFC capacitors lowers

the resonant frequency of the network and brings it down in the operating frequency range of AHF. If the AHF happens to operate at a frequency greater than the resonant frequency of the network, then the phase reversal of the system current causes the harmonic current to get amplified instead of getting compensated.
Prior art involves disclosures in "Xiaofeng Sun, Ningning Li, Baocheng Wang, Xin Li, IEEE 6th International Power Electronics and Motion Control Conference, 2009. IPEMC '09", "Yan Xiaoqing , yang jun, wang zhaon, The Mathematical Model and Stability Analysis of Shunt Active Power Filter,Transations of China Electrotechnical Sosciety, Vol. 13, pp 41-45, Feb 1998", and "Paolo Mattavelli, A closed-loop selective harmonic compensation for active filter, IEEE Transaction on Industrial Applications, Vol. 37, NO. 1, pp 81-89, January 2001". The philosophy of solutions reported in these prior art documents revolves around adding the phase boost at higher order harmonic frequencies. Improvement in stability margin has been discussed in Prior art involves disclosures in "Xiaofeng Sun, Ningning Li, Baocheng Wang, Xin Li, IEEE 6th International Power Electronics and Motion Control Conference, 2009. IPEMC '09", where a single order phase lead correction method is applied. However, adding the phase boost reduces the gain at lower frequencies leading to lower inhibitions to low frequency harmonics which are relatively high in the system.
To overcome this problem, selective harmonic detection using a modified band pass filter has been proposed in Yan Xiaoqing , yang jun, wang zhaon, The Mathematical Model and Stability Analysis of Shunt Active Power Filter,Transations of China Electrotechnical Sosciety, Vol. 13, pp 41-45, Feb 1998". Although, not mentioned explicitly, the reported solutions have one assumption in common, that the value of PFC capacitor connected downstream is known and it does not change. This makes the task simple enough in a sense that, it fixes the resonant frequency of the network. Now with the known phase lag angles, the phase boost circuitry of prefixed value can be designed. However, practical scenario is such that, value of PFC capacitors keep on changing because of presence of switched capacitor banks. Large variation of PFC capacitor value in tern changes the resonant frequency over larger band, making it impossible to have a fixed value of phase booster circuit.
OBJECTS OF THE INVENTION:
An object of the invention is to provide a system and method, for improved performance of shunt active harmonic filter in presence of PFC capacitors, which adaptively calculates the amount of phase boost required, in a network grid, on a real time basis
Another object of the invention is to provide a system and method, for improved performance of shunt active harmonic filter in presence of PFC capacitors, in a network grid, where value of PFC capacitors keep on changing because of presence of switched capacitor banks
Yet another object of the invention is to provide a system and method, for improved performance of shunt active harmonic filter in presence of PFC capacitors, in a network grid, which ensures stable operation of Active Harmonic Filter under all conditions.
Still another object of the invention is to provide a system and method, for improved performance of shunt active harmonic filter in presence of PFC capacitors, in a network grid, which saves efforts and pain involved in

re-positioning of loads and Power Factor Correction capacitors.
An additional object of the invention is to provide a system and method, for improved performance of shunt active harmonic filter in presence of PFC capacitors, in a network grid, which makes Active Harmonic Filters independent of load characteristics.
SUMMARY OF THE INVENTION:
According to this invention, there is provided a system for improved performance of shunt active harmonic filter
in presence of power factor correction capacitors applied to a network, said system comprises:
i. at least a harmonic signal current signal injecting mechanism in order to inject a stimulus signal which is a
harmonic current signal with a pre-determined magnitude and a pre-determined phase angle; ii. at least a sensing mechanism adapted to sense response to said injected signal, on supply side of said
network, said sensed response comprising extracted magnitude and phase angle information; iii. at least an odd harmonic extraction mechanism adapted to extract odd harmonic current signal from said
network; iv. at least a first even harmonic reference generation mechanism in order to generate a first even harmonic
reference signal, by injecting at least a first even harmonic, corresponding to said extracted odd harmonic
current signal, in that, said first even harmonic reference generation mechanism generating a preceding
even harmonic current signal; v. at least a second even harmonic reference generation mechanism in order to generate a second even
harmonic reference signal, by injecting at least a second even harmonic, corresponding to said extracted
odd harmonic current signal, in that, said second even harmonic reference generation mechanism
generating a successive even harmonic current signal; vi. at least a magnitude and phase lag computation mechanism in order to compute magnitude and phase angle
lag based on sensed current data obtained from said sensing mechanism, said phase angle lag being
computed for said extracted odd harmonic; vii. at least a plotting mechanism adapted to plot magnitude and phase angle lag for each of said injected first
even harmonic reference signals and for each of said injected second even harmonic reference signals; viii. at least an interpolation mechanism in order to interpolate a compensation magnitude and phase angle for
said extracted odd harmonic signal from said plotted data of said even harmonic signals which are injected
in to said network; ix. at least a phase addition mechanism in order to add compensatory interpolated phase angle for said
extracted odd harmonic signal; and x. at least a current control mechanism in order to control current in said network based on said compensatory
phase lag and magnitude to said extracted odd harmonic.
Typically, said harmonic signal current signal injecting mechanism is controlled by an Active Harmonic Filter controller.
Typically, said sensing mechanism is a Current Transformer.

Typically, said odd harmonic is a 11th harmonic signal, said first even harmonic signal is a (n-1)1h harmonic signal, and said second even harmonic signal is a (n+1)th harmonic signal.
Typically, magnitude of said first even order harmonic stimulus signal is small enough so as to honour the guidelines given by IEEE 519 for harmonic injection.
Typically, magnitude of said second even order harmonic stimulus signal is small enough so as to honour the guidelines given by IEEE 519 for harmonic injection.
Typically, said system comprises at least a comparator in order to compare if said compensatory phase angle is smaller than required phase angle for a given extracted odd harmonic, and only if it is true, then said phase addition mechanism is activated.
According to this invention, there is also provided a method for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network, said method comprises the steps of:
a) injecting a stimulus signal which is a harmonic current signal with a pre-determined magnitude and a predetermined phase angle;
b) sensing response to said injected signal, on supply side of said network, said sensed response comprising extracted magnitude and phase angle information;
c) extracting odd harmonic current signal from said network;
d) generating a first even harmonic reference signal, by injecting at least a first even harmonic, corresponding to said extracted odd harmonic current signal, in that, said first even harmonic reference generation mechanism generating a preceding even harmonic current signal;
e) generating a second even harmonic reference signal, by injecting at least a second even harmonic, corresponding to said extracted odd harmonic current signal, in that, said second even harmonic reference generation mechanism generating a successive even harmonic current signal;
f) computing magnitude and phase angle lag based on sensed current data obtained from said sensing mechanism, said phase angle lag being computed for said extracted odd harmonic;
g) plotting magnitude and phase angle lag for each of said injected first even harmonic reference signals and for each of said injected second even harmonic reference signals;
h) interpolating a compensation magnitude and phase angle for said extracted odd harmonic signal from said
plotted data of said even harmonic signals which are injected in to said network; i) adding compensatory interpolated phase angle for said extracted odd harmonic signal; and j) controlling current in said network based on said compensatory phase lag and magnitude to said extracted odd harmonic.
Typically, said response signal is distinguishable from other probable harmonic current signals present on supply side of aid signal, said stimulus signal satisfies the following properties: I. said stimulus signal having a frequency which is not present in said network; II. said stimulus signal being small enough in magnitude so that it does not pollute said network; and

III. said stimulus signal being 'even' order harmonic frequency current signal.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 illustrates circuit arrangement for the system with 'load side' sensing;
Figure 2 illustrates block diagram of control system with 'load side' sensing;
Figure 3 illustrates frequency response of loop transfer function for 'load side' sensing;
Figure 4 illustrates simulation results for case 1, all R-phase qualities: (a) grid voltage (200 V/div); (b) supply side current (200 A/div); (c) load side current (200 A/div); (d) AHF current (20 A/div).
Figure 5 illustrates simulation results for case 2, all R-phase qualities: (a) grid voltage (200 V/div); (b) supply side current (200 A/div); (c) load side current (200 A/div); (d) AHF current (20 A/div).
Figure 6 illustrates simulation results for case 3, all R-phase qualities: (a) grid voltage (200 V/div); (b) supply side current (200 A/div); (c) load side current (200 A/div); (d) AHF current (20 A/div).
Figure 7 illustrates results for case 4, all R-phase qualities: (a) grid voltage (200 V/div); (b) supply side current (500 A/div); (c) load side current (500 A/div); (d) AHF current (50 A/div).
Figure 8 illustrates an exemplary case demonstrating need for 'supply side' sensing;
Figure 9 illustrates circuit arrangement for 'supply side' current sensing;
Figure 10 illustrates control model for 'supply side' sensing;
Figure 11 illustrates frequency response of loop transfer function for 'supply side' sensing;
Figure 12 illustrates root locus of the system with PFC capacitors (70 kVAr);
Figure 13 illustrates simulation result with gain K=l:5; magnitude of 11th harmonic current on supply side (2 A/div); and
Figure 14 illustrates simulation result with gain K = 40; magnitude of 11th harmonic current on supply side (10 A/div).
The invention will now be described in relation to the accompanying drawings, in which: Figure 15 illustrates a schematic of the system applied to a grid;
Figure 16 illustrates simulation results: (a) R-phase 11th harmonic current on supply side (20 A/div); (b) Time-

zoomed view for zoom window-l of a before harmonic compensation is tuned ON; (c) Time-zoomed view for zoom window-2 of'a' after compensation is tuned ON along with proposed logic; and
Figure 17 illustrates Experimental results: (a) R-phase 11th harmonic source current (10 A/div); (b) Time-zoomed view for zoom window-1 of 'a' before harmonic compensation is tuned ON; (c) Time-zoomed view for zoom window-2 of 'a' after compensation is tuned ON along with proposed logic.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The circuit arrangement for the system with 'load side' sensing is shown in Figure 1. Reference numeral IC refers to current controller. Reference numeral CC refers to control circuit. The load is modeled as parallel combination of resistor (R) representing active component of total load, inductor (L) representing inductive reactive component of total load and harmonic current source representing on linear loads. Applying Kirchoff s current law at node 'A', for the current convention as shown in Figure 1,
Isupply =Iload -I AH F , which implies that, the harmonic current on the supply side becomes zero,
only if the AHF is made to produce harmonic current which is exactly equal in magnitude and equal in phase as that of the load current. Thus, it makes sense to apply control law as given in (1), in order to control AHF current.

Where,

is the harmonic current reference signal to be given to the AHF controller, and (Iload)h is

the harmonic current flowing into the parallel combination of load and PFC capacitors.
The current controller of AHF is designed so as to track the reference signal fairly accurately over a large range of frequencies. However, switching frequency of the inverter puts the upper limit on the maximum bandwidth of the current controller.
Designing the current controller with the bandwidth, an order less than the switching frequency is common practice for most of the practical realizations of AHF. The transfer function of the AHF current controller can be generalized as that of a single order lags system [14] as given in (2).

The magnitude and phase plot of the inverter with KAHF =1 and bandwidth of around 1 kHz is as shown in Fig.
2.
(TAHF = 0.16mSec for 1 kHz bandwidth)
Due to presence of PFC capacitors downstream to the point of connection of AHF, at higher order harmonic frequencies, the impedance, on the load side, becomes comparable with that on the supply side. Thus, significant portion of the harmonic current produced by AHF also flows on the load side (in the downstream PFC capacitors). More importantly, this current flows through the path from where the current is being sensed.


Thus, it becomes crucial to know how harmonic current produced by AHF manifests itself on the sensing side. The transfer function given in (3) relates AHF current to load current (sensed current).
where,
x = RILtC
y = LLt + BrLC
z = Br
k = RLt + Lr
m = BLt + Lr + RL
In the domain of control system engineering, the transfer function G, can also be referred to as a plant transfer function. For the purpose of stability analysis, the harmonic load acting as a disturbance signal can safely be ignored. The system shown in Figure 1 is redrawn taking transfer function of each block into account.
Figure 2 represents control block diagram of the system. Ginverter produces the AHF current signal which when applied to plant G, produces load current, which in turn acts as reference input current for Ginverter again producing inverter current signal.
The system can become unstable if the loop gain becomes more than unity for the phase angle less than 180 degrees. The loop transfer function of the system is given as (4).
G{loop) = Ginvzrter ' G (4)
The loop transfer function is plotted for the source-load parameters taken from a real life case study. Table I lists the system parameters.
Figure 3 shows magnitude and phase plots for two cases, viz: 1) with PFC capacitor; and 2) without PFC capacitor. It shows that for the case of network without PFC capacitors (denoted by dotted line), the loop gain never becomes more than unity keeping the system always stable. However, in case of PFC capacitors downstream to the point of connection of AHF (denoted by continuous line), the loop gain becomes more than unity for certain higher order harmonic frequencies. Fig. 4 shows that for a 50 Hz system, the loop gain at 5th harmonic frequency is less than unity while the loop gain for 13th harmonic frequency is more than unity. Therefore, the system shall become oscillatory, when AHF tries to compensate for 13th harmonic current produced by the load.
TABLE I SYSTEM PARAMITTERS

Parameter Symbol Value Unit
Power ratine or transformer i! 150 kvA
Source Impedance - 7 %
System phase vollage Vsource 240 V
Connected load: Active Power P 100 k IV"
Connected load: Reactive Power Q 60 kVAr
PFC capacitor hank QPFG 70 kV Ar
Validation of this stability analysis of the AHF system is carried out in MATLAB/SIMULINK simulation. The-parameters in the simulation are same as those given in Table I. Method of selective harmonic elimination using

synchronous reference frame technique is employed. It is known from control theory that even a noise signal is sufficient to create oscillatory behavior in an inherently unstable system. Therefore, only RLC parallel load is connected in a simulation model and no non-linear load is connected. Following four cases are simulated to validate the analysis.
1) AHF compensates only for 5th harmonic current and PFC capacitor is not connected.
2) AHF compensates only for 5th harmonic current and PFC capacitor is connected.
3) AHF compensates only for 13th harmonic current and PFC capacitor is not connected.
4) AHF compensates only for 13th harmonic current and PFC capacitor is connected.
Figure 4 illustrates simulation results for case 1, all R-phase qualities; (a) grid voltage (200 V/div). (b) supply side current (200 A/div). (c) load side current (200 A/div). (d) AHF current (20 A/div).
Figure 5 illustrates simulation results for case 2, all R-phase qualtities; (a) grid voltage (200 V/div). (b) supply side current (200 A/div). (c) load side current (200 A/div). (d) AHF current (20 A/div).
Figure 6 illustrates simulation results for case 3, all R-phase qualtities; (a) grid voltage (200 V/div). (b) supply side current (200 A/div). (c) load side current (200 A/div). (d) AHF current (20 A/div).
It is evident from Figures 4, 5, and 6 that in either cases the AHF remains stable and keeps on taking the current just sufficient to account for losses in the AHF system. The behavior is justified because, although the harmonic compensation loop is running in the controller of AHF, the network does not really have any harmonic current as such to be compensated.
Figure 7 illustrates results for case 4, all R-phase qualtities; (a) grid voltage (200 V/div). (b) supply side current (500 A/div). (c) load side current (500 A/div). (d) AHF current (50 A/div).
However, Figure 7 shows that when AHF is made to compensate 13th harmonic current in presence of PFC capacitor, the instability creeps in, making the AHF supply ever increasing harmonic current as shown in Fig. 8(d). This validates the stability analysis of AHF system with 'load side' current sensing.
For most of the industrial distribution networks, the non linear loads are dispersed over a wider area. In such cases the 'load side' sensing of grid current may not be feasible to compensate harmonic currents produced by all the loads. This point can be better appreciated by considering a fictitious yet very common case shown in Figure 9. The distribution system comprises an AFIF system installed to compensate the harmonic current produced by four non linear loads. It is evident that if current sensing CT is connected at Position-1, the AHF shall never be able to compensate for the harmonic currents produced by Load-1 and Load-2. Similarly if the CT is placed as shown in Position-2, AHF shall miss out on compensating the harmonic current produced by Load-3 and Load-4. However, if the current sensing mechanism is changed to the one shown as Position-3, then the AHF can compensate entire system load as long as it is rated to do so. This mechanism of current sensing is referred to as 'supply side' sensing. A single phase equivalent circuit arrangement is shown in Figure 9.


Figure 8 illustrates an exemplary case demonstrating need for 'supply side' sensing The control law to be applied for 'supply side' sensing is given in (5).
Where, K is a proportionality constant. In order to completely compensate the harmonic current, K = 1 is the theoretical value of the proportionality constant. However, stability margin of the system puts upper limit on value of K. Following analysis justifies this argument.
Figure 9 illustrates circuit arrangement for 'supply side' current sensing.
Figure 10 illustrates control model for 'supply side' sensing
At harmonic frequencies, the mathematical model of the system is shown in Figure 10.

The loop transfer function of the system as given in (7), helps to analyze the stability of the system.

Transfer function of Ginverter is given in (2). Gplant is the transfer function given in (6) which relates current on supply side to the current produced by AHF.
The bode plots shown in Figure 11 are plotted for gain K = 50 for two cases, (viz), with PFC capacitor and without PFC capacitor in the network. It is seen from the plots that when PFC capacitors are not connected, the system shows stable behavior with phase margin of nearly 25 degrees. Whereas, when PFC capacitor of the value mentioned in Table I is connected, the phase crossover frequency becomes less than the gain crossover frequency making the system unstable.
One of the ways to make the system stable is to reduce the gain K to such a value so as to make gain crossover frequency less than the phase crossover frequency. The method of plotting locus of roots of the characteristic equation is followed to know the maximum allowable value of gain K, beyond which the system becomes unstable. Figure 12 shows the root locus of the system with PFC capacitors present in the network. The plot shows that the roots of the characteristic equation move on right half of the plane when value of K> 1:83. Thus keeping the value of K less than 1.83, the system stability can be achieved. However, system stability is not the only criteria to be met for acceptable performance of overall system, because K = 1:83 is too small a gain to get minimal residual error at steady state.
The system is simulated in MATLAB/SIMULINK to validate the analysis with controller gain of K = 1:5. Figure 13 shows the magnitude of 11th harmonic current on the supply side when compensation is started at time t = 0:2 s. It shows although the system shows stable behavior, the 11th harmonic current on the supply side does not get compensated completely but a large portion of residual current is still left in the system because of

small gain K. Fig. 14 shows the simulation result of 11th harmonic current on supply side when gain K = 40. It shows that the system becomes unstable to enhance the 11th harmonic further after compensation is started at time t = 0:2 s.
Another method to achieve the stability of the system is to add phase boost at a higher order harmonic frequency so that the phase crossover frequency can be prolonged to get positive phase margin and hence stable system. Improvement in stability margin is known in the prior art, where a single order phase lead correction method is applied. However, adding the phase boost reduces the gain at lower frequencies leading to lower inhibitions to low-frequency harmonics which are relatively high in the system. To overcome this problem, selective harmonic detection using a modified band pass filter has been proposed in the prior art.
Both these methods have caveat that, they are based on an assumption that the AHF controller knows beforehand regarding the presence of PFC capacitor in the network and hence the need of phase correction before supplying the compensating currents. However, this assumption may not be valid in any of the practical implementations where, the value of PFC capacitor keeps on changing on account of switched capacitor banks. Adding unnecessary phase boost when not needed may make the system unstable. Thus it becomes essential to know the presence of PFC capacitors in the network to decide whether or not to add phase boost.
According to this invention, there is provided a system and method for improved performance of shunt active harmonic filter in presence of power factor correction capacitors. This invention is based on the principle of 'stimulus and response'.
Figure 15 refers to a schematic of the system applied to a grid. Reference numeral V refers to voltage and supply source to the grid. Reference numeral Z refers to impedance of the grid. Reference numeral PFC refers to power factor correction capacitor, downstream, in the grid. Reference numeral L refers to load in the grid.
In accordance with an embodiment of this invention, there is provided a harmonic signal current signal injecting mechanism (AHF) in order to inject a stimulus signal which is a harmonic current signal with a predetermined magnitude and a pre-determined phase angle. This harmonic signal current signal injecting mechanism is controlled by an Active Harmonic Filter controller (AHF).
In accordance with another embodiment of this invention, there is provided a sensing mechanism (CT) adapted to sense response to the injected stimuli, i.e. to the injected signal, on the supply side of the network. Typically, this sensing mechanism is a Current Transformer (CT). The sensed response comprises extracted magnitude and phase angle information. However, for the response signal to be distinguishable from other probable harmonic current signals present on supply side, the stimulus signal must satisfy following properties:
• The stimulus signal must be having a frequency which is not present in the network.
• The stimulus signal must be small enough in magnitude so that it should not pollute the grid.
• 'Even' order harmonic frequencies are generally not present in a typical industrial load distribution system. Thus, a small magnitude of 'even' order harmonic frequency current signal is used as the stimulus signal.

In accordance with yet another embodiment of this invention, there is provided an odd harmonic extraction mechanism (OH) adapted to extract odd harmonic current signal from the network. E.g. odd harmonic signal is an nth harmonic signal.
In accordance with still another embodiment of this invention, there is provided a first even harmonic reference generation mechanism (EH1) in order to generate a first even harmonic reference signal, by injecting at least a first even harmonic, corresponding to the extracted odd harmonic current signal, in that, the first even harmonic reference generation mechanism generates a preceding even harmonic current signal. E.g. first even harmonic signal is a (n-l)-1 harmonic signal. A plurality of such first even harmonics, one preceding the other, beyond the extracted odd harmonic, may be generated. However, it has to be ensured that the magnitude of 'even' order harmonic stimulus signals is small enough so as to honour the guidelines given by IEEE 519 for harmonic injection.
In accordance with still another embodiment of this invention, there is provided a second even harmonic reference generation mechanism (EH2) in order to generate a second even harmonic reference signal, by injecting at least a second even harmonic, corresponding to the extracted odd harmonic current signal, in that, the second even harmonic reference generation mechanism generates a successive even harmonic current signal. E.g. second even harmonic signal is a (n+l)th harmonic signal. A plurality of such second even harmonics, one succeeding the other, beyond the extracted odd harmonic, may be generated. However, it has to be ensured that the magnitude of'even' order harmonic stimulus signals is small enough so as to honour the guidelines given by IEEE 519 for harmonic injection.
In accordance with an additional embodiment of this invention, there is provided a magnitude and phase lag computation mechanism (MPLM) in order to compute magnitude and phase angle lag based on sensed current data obtained from the sensing mechanism (CT). This phase lag is computed for the extracted odd harmonic.
In accordance with another additional embodiment of this invention, there is provided a plotting mechanism (PM) adapted to plot magnitude and phase angle lag for each of injected first even harmonic reference signals and for each of injected second even harmonic reference signals.
In accordance with yet another additional embodiment of this invention, there is provided an interpolation mechanism (IM) in order to interpolate a compensation magnitude and phase angle for the odd harmonic signal from the plotted data of the even harmonic signals which are injected in to the grid.
In accordance with still another additional embodiment of this invention, there is provided a phase addition mechanism (PA) in order to add compensatory interpolated phase angle for the odd harmonic signal. A comparator compares if the compensatory phase angle is smaller than the required phase angle for a given odd harmonic, and only if it is true, then the phase addition mechanism is activated.
In accordance with another additional embodiment of this invention, there is provided a current control mechanism (CCM) in order to control current in the grid based on the compensatory phase lag and magnitude to the odd harmonic.

The system, of this invention, in a way, gives the magnitude and phase plots of the loop transfer function given by (7). Since this is being done in-line, the system accounts for the change in PFC value during system operation.
The system was simulated to obtain experimental results in order to validate the stability analysis of the system with supply side current sensing and variable PFC capacitors in the network. Detailed simulation studies were carried out on MATLAB-SIMULINK platform. The parameters of the three phase system chosen for the purpose of simulation are shown in Table I. Synchronous reference frame (SRF) theory was used for extracting the nth harmonic component in load current. The use of SRF theory requires services of phase lock loop (PLL) which are locked with the grid voltages. Indirect current controller was used for simulation purpose which forms the reference voltages to be generated by inverter. A sine triangle PWM generation method was used to generate PWM gating signals. The simulation was run for compensating 11th harmonic current when PFC capacitor was present. Waveform in Figure 16(a) depicts 11th harmonic current in R-phase on supply side. The system was deliberately kept inactive till time t=ls to validate it's effectiveness. Figure 16(b) depicts the time-zoomed view of 11th harmonic source current before compensation. It was seen in Figure 16(a) that once the compensation for 11th harmonic was started at t=0.8 s, the harmonic current on the supply side increased. At time t=ls, the system is activated. It was seen from Figure 16(a) that the 11th harmonic source current reduced and approached zero. Figure 16(c) depicts the zoomed view of the Fig. 19(a) after compensation.
In order to validate the simulated model of proposed algorithm, a laboratory prototype for AHF was built. Indirect current control method using synchronous reference frame was used in order to track the current reference. The logic built for generating the current reference was such that synchronous reference frames corresponding to 5th, 7th, 11th, and 13th harmonic were used in order to selectively compensate the load current of desired harmonic number. The implementation of control algorithm was carried out on a 32 bit digital signal processor.
Similar to the approach followed in simulation, the proposed algorithm was initially kept inactive to prove its viability. In order to display, only the 11th harmonic current component in source current, the grid currents were processed inside DSP and waveforms were displayed on scope using a Digital to Analog converter (DAC). Figure 17(a) shows the complete trace of Rphase 11th harmonic current on supply side, while Figure 17 (b) and Figure 17 (c) show their corresponding time-zoomed views before and after compensation using proposed algorithm.
While this detailed description has disclosed certain specific embodiments of the present invention for illustrative purposes, various modifications will be apparent to those skilled in the art which do not constitute departures from the spirit and scope of the invention as defined in the following claims, and it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We claim,
1. A system for improved performance of shunt active harmonic filter in presence of power factor correction
capacitors applied to a network, said system comprising:
i. at least a harmonic signal current signal injecting mechanism in order to inject a stimulus signal
which is a harmonic current signal with a pre-determined magnitude and a pre-determined phase
angle; ii. at least a sensing mechanism adapted to sense response to said injected signal, on supply side of said
network, said sensed response comprising extracted magnitude and phase angle information; iii. at least an odd harmonic extraction mechanism adapted to extract odd harmonic current signal from
said network; iv. at least a first even harmonic reference generation mechanism in order to generate a first even
harmonic reference signal, by injecting at least a first even harmonic, corresponding to said extracted
odd harmonic current signal, in that, said first even harmonic reference generation mechanism
generating a preceding even harmonic current signal; v. at least a second even harmonic reference generation mechanism in order to generate a second even
harmonic reference signal, by injecting at least a second even harmonic, corresponding to said
extracted odd harmonic current signal, in that, said second even harmonic reference generation
mechanism generating a successive even harmonic current signal; vi. at least a magnitude and phase lag computation mechanism in order to compute magnitude and phase
angle lag based on sensed current data obtained from said sensing mechanism, said phase angle lag
being computed for said extracted odd harmonic; vii. at least a plotting mechanism adapted to plot magnitude and phase angle lag for each of said injected
first even harmonic reference signals and for each of said injected second even harmonic reference
signals; viii. at least an interpolation mechanism in order to interpolate a compensation magnitude and phase
angle for said extracted odd harmonic signal from said plotted data of said even harmonic signals
which are injected in to said network; ix. at least a phase addition mechanism in order to add compensatory interpolated phase angle for said
extracted odd harmonic signal; and x. at least a current control mechanism in order to control current in said network based on said
compensatory phase lag and magnitude to said extracted odd harmonic.
2. The system for improved performance of shunt active harmonic filter in presence of power factor correction
capacitors applied to a network as claimed in claim 1, wherein said harmonic signal current signal injecting
mechanism is controlled by an Active Harmonic Filter controller.

3. The system for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network as claimed in claim 1, wherein said sensing mechanism is a Current Transformer.
4. The system for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network as claimed in claim 1, wherein said odd harmonic is a nth harmonic signal, said first even harmonic signal is a (n-1)th harmonic signal, and said second even harmonic signal is a (n+l)th harmonic signal.
5. The system for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network as claimed in claim 1, wherein magnitude of said first even order harmonic stimulus signal is small enough so as to honour the guidelines given by IEEE 519 for harmonic injection.
6. The system for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network as claimed in claim 1, wherein magnitude of said second even order harmonic stimulus signal is small enough so as to honour the guidelines given by IEEE 519 for harmonic injection.
7. The system for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network as claimed in claim 1, wherein said system comprising at least a comparator in order to compare if said compensatory phase angle is smaller than required phase angle for a given extracted odd harmonic., and only if it is true, then said phase addition mechanism being activated.
8. A method for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network, said method comprising the steps of:
a. injecting a stimulus signal which is a harmonic current signal with a pre - determined magnitude
and a pre - determined phase angle;
b. sensing response to said injected signal, on supply side of said network, said sensed response
comprising extracted magnitude and phase angle information;
c. extracting odd harmonic current signal from said network;
d. generating a first even harmonic reference signal, by injecting at least a first even harmonic,
corresponding to said extracted odd harmonic current signal, in that, said first even harmonic
reference generation mechanism generating a preceding even harmonic current signal;
e. generating a second even harmonic reference signal, by injecting at least a second even harmonic,
corresponding to said extracted odd harmonic current signal, in that, said second even harmonic
reference generation mechanism generating a successive even harmonic current signal;
f. computing magnitude and phase angle lag based on sensed current data obtained from said sensing
mechanism, said phase angle lag being computed for said extracted odd harmonic;

g. plotting magnitude and phase angle lag for each of said injected first even harmonic reference signals and for each of said injected second even harmonic reference signals;
h. interpolating a compensation magnitude and phase angle for said extracted odd harmonic signal from said plotted data of said even harmonic signals which are injected in to said network;
i. adding compensatory interpolated phase angle for said extracted odd harmonic signal; and
j. controlling current in said network based on said compensatory phase lag and magnitude to said extracted odd harmonic.
9. The method for improved performance of shunt active harmonic filter in presence of power factor correction capacitors applied to a network as claimed in claim 8; wherein said response signal being distinguishable from other probable harmonic current signals present on supply side of aid signal, said stimulus signal satisfying the following properties:
I. said stimulus signal having a frequency which is not present in said network; II. said stimulus signal being small enough in magnitude so that it does not pollute said network; and III. said stimulus signal being 'even' order harmonic frequency current signal.