CLIAMS:1. A neutral current compensator for three-phase, star connected unbalanced linear and nonlinear loads, the neutral current compensator comprising:
a plurality of primary windings having star connection therebetween;
each of the primary windings connected to separate phase of the three phase supply;
a plurality of secondary windings magnetically coupled with the respective primary winding,
the secondary windings having series connection therebetween, one end of the secondary windings having series electrically connected to the common point of primary windings with star connection, other end connected to a load neutral, the load neutral further connected to unbalance load;
a common connection from the primary windings and the secondary windings is either connected to a supply neutral point or to earth.

2. The neutral current compensator as claimed in claim 1, wherein each pair of the primary winding and a secondary winding is a single phase transformer.

3. The neutral current compensator as claimed in claim 1, wherein each pair of the primary winding and a secondary winding having a voltage ratio of 3:1.

4. The neutral current compensator as claimed in claim 1, further comprises a cooling fan for cooling the entire circuit enclosure.

5. The neutral current compensator as claimed in claim 1, further comprises a circuit breaker connected to the primary windings for isolation of supply.

6. The neutral current compensator as claimed in claim 5, wherein the circuit breaker is a molded case circuit breaker (MCCB).

7. The neutral current compensator as claimed in claim 5, wherein the circuit breaker is an air circuit breaker (ACB).

8. The neutral current compensator as claimed in claim 1, further comprises a contactor connected to the primary windings for isolation of supply.
,TagSPECI:FORM 2
THE PATENT ACT 1970
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(SEE SECTION 10 AND RULE 13)
1. TITLE OF THE INVENTION:
“Neutral Current Compensator for Three - Phase, Star Connected, Unbalanced Linear and Nonlinear Loads”
2. APPLICANT(s):

(a) NAME: Shreem Electric Limited
(b) NATIONALITY: Indian
(c) ADDRESS: Plot No. 43-46, L. K. Akiwate Industrial Estate, Jaysingpur (Dist. Kolhapur) 416144, MH India

3. PREAMBLE TO THE DESCRIPTION:
PROVISIONAL
The following specification describes the invention. COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed

Field of the invention

The present invention relates to a neutral current compensator for three - phase, star connected, unbalanced linear and nonlinear loads. More particularly, the present invention relates to a neutral current compensator for compensating neutral current (also referred as Zero Sequence Current) flowing through a neutral cable or a bus bar of a star connected unbalanced load.

Background of the invention

Over a period of time the industrial requirements have undergone substantial change and have become extremely demanding and challenging. More than linear, the nonlinear loads have proliferated. The power supply systems has to withstand fluctuating load demands causing dynamic variation in active power, reactive power, current harmonics, and unbalanced current operation particularly in star connected unbalanced loads (like welding loads, fabrication shops, distributed loads within factories, low voltage distribution transformers, arc furnaces etc.).

The dynamic reactive power variation causes variation in dynamic Volt Amperes Reactive (VAR) and hence variation in the dynamic fundamental power factor. The fundamental power factor needs to be maintained close to unity for reducing losses in the system and for reducing electricity bills. The current harmonics and hence the supply current distortion needs to be maintained below certain limits as per international standards (like IEEE 519) to restrict supply voltage distortion for given short circuit capacity at the Point of Common Coupling (herein after referred the “PCC”).

Neutral currents caused by unbalanced load currents (due to linear or nonlinear loads) can have fundamental frequency current component as well even and / or odd harmonic frequency components. They can also have sub-cycle harmonic frequency components depending on the type of nonlinear load.

Unbalance in the load currents and hence in the supply currents, can give rise to unacceptable magnitude of neutral current. It increases system losses and heating of the neutral cable or conductor. Further, it increases and even gives rise to changing neutral to earth potential / voltage. This in effect can lead to changing phase to neutral supply voltages causing disturbances to other loads connected on the same supply voltage bus. The unbalance in supply currents also leads to reduced life of the supply transformers.

This is applicable to all (low to high) types of supply voltage systems.

Unbalance in load currents, hence in the supply currents, is caused by linear as well as nonlinear loads as described in the beginning. There is no neutral current flowing back to supply neutral point in following cases.

• Supply is delta connected, load is unbalanced and delta connected
• Supply is delta connected, load is unbalanced and star connected
• Supply is star connected, load is unbalanced and delta connected
• Supply is star connected, load is unbalanced and star connected (but neutral connection not returned to supply neutral point, directly or through ground / earth).

Therefore, there is a need to provide a neutral current compensator, which is overcome the problems in the prior art.

Objects of the invention

An object of the present invention is to provide a neutral current compensator for three - phase, star connected, unbalanced linear and nonlinear loads, which can be used in low as well as high voltage transformer system.

Another object of the present invention is to provide a neutral current compensator, which works for both delta and star connected power supply.

Yet another object of the present invention is to provide a neutral current compensator, which circulates back the neutral current through the load itself avoiding flow of neutral current back on the supply side.

One more object of the present invention is to provide a neutral current compensator, which eliminates need for load neutral connection to supply neutral current in case of star connected supply.

Further object of the present invention is to provide a neutral current compensator, which works for supply source having either delta connected or star connected unbalanced load allowing the load to work with Line to Neutral or Phase to Neutral voltages (which is otherwise not possible).

Further one object of the present invention is to provide a neutral current compensator, which does not require any active device or active device based controller, but requires only transformer(s), thereby making the neutral current compensator more reliable in its operation and for maintenance.

Yet another object of the present invention is to provide a neutral current compensator, in which even if the neutral connection gets disconnected from ground / earth in case of a star connected supply, the power system and application can continue to work in normal way without any operational problems.

Summary of the invention

According to the present invention there is provided a neutral current compensator for three-phase, star connected unbalanced linear and nonlinear loads. The neutral current compensator has a plurality of primary windings and a plurality of secondary windings. The primary windings have a star connection therebetween. Each of the primary windings is connected to separate phase of the three phase supply. The secondary windings are magnetically coupled with the respective primary winding to configure three single-phase transformers. The secondary windings have series connection therebetween. One end of the secondary windings having series electrically connected is to the common point of primary windings with star connection, other end connected to a load neutral which is connected to unbalance load. A common connection from the primary windings and the secondary windings is either connected to a supply neutral point or to earth.

Brief description of the drawings

The Figures described below set out and illustrate a number of exemplary embodiments of the disclosure. Throughout the drawings, like reference numerals refer identical or functionally similar elements. The drawings are illustrative in nature and not drawn to scale.

Figure 1(a) show a equivalent circuit of Star connected supply and star connected unbalanced load of the prior art;

Figure 1(b) shows an equivalent circuit of a delta connected supply, star connected unbalanced load with the neutral current compensator in accordance with the present invention;

Figure 1(c) shows an equivalent circuit of a star connected supply, star connected unbalanced load with the neutral current compensator in accordance with the present invention;

Figure 2 shows detailed circuit of the Neutral Current Compensator (NCC) and its connection details in accordance with the present invention;

Figure 3(a) shows Matlab / Simulink model for star connected supply and star connected unbalanced load with the NCC in accordance with the present invention;

Figure 3(b) shows simulation results of voltage and current for each phase of circuit shown in figure 3(a);

Figure 3(c) shows simulation results of load current for each phase of circuit shown in figure 3(a);

Figure 4(a) shows Matlab / Simulink model for delta connected supply and star connected unbalanced load with the NCC in accordance with the present invention;

Figure 4(b) shows simulation results of load voltage and current for each phase of circuit shown in figure 4(a);

Figure 4(c) shows simulation results of load current for each phase of circuit shown in figure 4(a);

Figure 5 shows a circuit of an experimental set up having star connected supply a with real time welding plant load with the neutral current compensator in accordance with the present invention;

Figure 6 recorded waveforms for the experimental set up shown in figure 5, and

Figures 7(a) and 7(b) shows actual installation of the NCC 100 in accordance with the present invention.

Details description of the invention

For a thorough understanding of the present invention, reference is to be made to the following detailed description, including the appended claims, in connection with the above-described drawings. Although the present invention is described in connection with exemplary embodiments, the present invention is not intended to be limited to the specific forms set forth herein. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but these are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

The terms, such as “First” and “Second” do not denote any order, elevation or importance, but rather are used to distinguish placement of one element over another, and the terms such as “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

The present invention provides a Neutral Current Compensator (herein after referred as NCC) for three - phase, star connected loads. Further, the NCC can be used for bother unbalanced linear and nonlinear loads. The NCC is capable of circulating back the neutral current through the load itself thereby avoiding flow of neutral current back on the supply side. Further, the NCC eliminates the need of neutral connection in case of supply source having star connection. Moreover, the NCC also has a good use in industrial as well as utility power supply distribution as it is a low cost and reliable solution for neutral current compensation.

Figure 1(a) shows a circuit diagram having start connected supply and the unbalance load is also star connected (normally called as four wire system) of the prior art. Further, a load neutral is directly connected to supply neutral point and or load neutrals n. The supply neutral point and the load neutrals may or may not be grounded/earthed. Furthermore, in this arrangement the neutral connection in1 carries the unbalanced load current.

Figure 1(b), shows a circuit diagram having a delta connected supply and start connected unbalance load with a neutral current compensator (herein after referred as NCC 100) in accordance with the present invention. For this arrangement, the load normally has line to line or phase to phase voltages. Line to neutral or phase to neutral operation for the three-phase load is not possible, even though it may be desired at times. To make such an operation possible, the load neutral is connected to the NCC 100 to complete the neutral closed path. Details of the NCC 100 are explained ahead in conjunction with figure 2.

The proposed NCC 100 is described here deals with the above two cases in power system distribution.

As shown in figure 1(c), the NCC 100 is connected to the circuit (of figure 1) having star connected supply with start connected unbalance load 40. This arrangement eliminates the neutral current flow back into the supply neutral point. Further, the NCC 100 allows the unbalanced load 40 to work

(i) without connecting the load neutral to supply neutral point,
or
(ii) without grounding / earthing the load neutral to complete the closed path to supply neutral (if supply neutral is grounded / earthed).

The supply side neutral, however, can remain grounded / earthed so that the star point of the supply remains firmly held at ground / earth potential.

Referring again to figure 1(b), the NCC 100 allows the unbalanced load with neutral current to work with the delta connected supply using its own neutral point and completes the closed path for the neutral current through the NCC 100 thereof.

The NCC 100 does not require any active device (diode, thyristor, Gate Turn off Thyristor, MOSFET, Insulated Gate Bi-polar Transistor etc.), but mainly uses a transformer for its operation. The response of the NCC 100 is also instantaneous.

Figure 2 shows detailed circuit diagram of the NCC 100 in accordance with the present invention. The NCC 100 consists of a plurality of primary winding, for the purpose of explanation three primary windings 10a, 12a and 14a are shown, and a plurality of secondary windings, for the purpose of explanation three secondary windings 10b, 12b and 14b. Each of the primary windings 10a, 12a and 14a is magnetically coupled with the respective secondary winding 10b, 12b and 14b to configure three single-phase transformers 10, 12 and 14.

Further, the single-phase transformers 10, 12 and 14 are connected as close as possible to the unbalance load 40. One end of each of the primary windings 10a, 12a and 14a is connected to respective phase of a three-phase 30a, 30b and 30c supply. For the purpose of explanation only, the primary winding 10a is connected to a phase 30a, the primary winding 12a is connected to a phase 30b, and the primary winding 14a is connected to a phase 30c of the three phases 30a, 30b and 30c. This connection can be rearranged / interchanged with respect to the primary windings 10a, 12a and 14a and the three phases 30a, 30b and 30c, which may be obvious to a person skilled in the art. The primary windings 10a, 12a and 14a are connected with each other to confirm a star connection therebetween.

Further, the secondary windings 10b, 12b, and 14b of the respective transformer 10, 12 and 14 are connected with each other in series. One end of the secondary windings 10b, 12b, and 14b connected in series is connected with the neutral load connection. The neutral load is connected to the unbalance load 40. Other end of the secondary windings 10b, 12b, and 14b in series is connected to another common end of the primary windings 10a, 12a, and 14a with star connection. Specifically, the primary windings 10a, 12a, and 14a and the secondary windings 10b, 12b, and 14b ratio for each of the transformer 10, 12, and 14 is 3:1 (for example, if there are 3 volts on the primary winding there will be 1 volt on the secondary winding, 3-to-1). Further, a common connection from the primary windings 10a, 12a, and 14a and the secondary windings 10b, 12b, and 14b is connected to supply neutral point or to the earth.

Due to the series connection of the secondary windings 10b, 12b, and 14b, the theoretical voltage across the series connection of the secondary windings 10b, 12b, and 14b is “zero” volt (as balanced three-phase voltages always add up to “zero” volt). When the neutral current flows through the load side neutral, it circulates through the secondary windings 10b, 12b, and 14b. This is reflected as 1/3 identical (value / magnitude and phase) current in the primary three-phase windings 10a, 12a and 14a. The current is circulates through the unbalance load 40 and is then returned through the load neutral completing the loop as well as adhering to the famous Kirchhoff’s Current Law (KCL). Further, at least one MCCB (molded case circuit breaker) switch may be connected to the NCC 100 for protection against overload and to for switching “ON” and “Off” the NCC 100. In an embodiment, the MCCB can be replaced by a contactor or an ACB (air circuit breaker), which may be obvious to a person skilled in the art.

Therefore, there is no need for load neutral to complete the current path through supply neutral point connection directly or through ground / earth on incorporating the NCC 100 in the circuit of figure 1(a) and as shown in figure 1(c).

The NCC 100, hence, allows the neutral current to circulate locally and eliminates the need for load neutral connection to supply side neutral directly or through ground / earth as in figure 1(c).

As the NCC 100 is directly circulating the neutral current locally, it takes care of fundamental frequency unbalanced load current as well as any other harmonic frequency unbalanced load current.

Modeling and Simulation Results

Figure 3(a) shows matlab/simulink model for start connected supply and star connected the unbalance load with the Neutral Current Compensator in accordance with the present invention. The supply considered here is three-phase, 50 Hz, 415 V. The supply is either star connected or delta connected delivering a 100 kVA load. For 100 kVA (Kilo Volt Amperes) load, balanced current RMS (root mean square) value at 415 V is 139.12 A.

The 100 kVA “unbalanced load” considered is with following parameters.

Phase ‘a’: 33 kVA at unity power factor
Phase ‘b’: 33 kVA at 0.8 lagging power factor
Phase ‘c’: 33 kVA at 0.9 leading power factor

Unbalanced load current harmonics are as under.

Phase ‘a’: 3rd harmonic 30 A, 5th harmonic 20 A, 7th harmonic 40 A
Phase ‘b’: 3rd harmonic 20 A, 5th harmonic 40 A, 7th harmonic 30 A
Phase ‘c’: 3rd harmonic 40 A, 5th harmonic 30 A, 7th harmonic 20 A

The harmonic currents are displaced by 1200 and 2400 (corresponding to fundamental frequency).

The modeling and simulation results (MSR) are presented here for following cases.

Case MSR-1:

Supply star connected, load star connected corresponding to Case-1 stated earlier and explained in conjunction figure 1(a) and 1(c). The Matlab / Simulink model is given in figure 3(a) and the simulation results are given figure 3(b).

Specifically, figure 3(b) shows:

Ch1: Phase “a” voltage and current
Ch2: Phase “b” voltage and current
Ch3: Phase “c” voltage and current

Figure 3(c) shows simulation results of load current for each phase of circuit shown in figure 3(a).

Specifically, figure 3(c) shows:

Ch1: Phase “a” load current
Ch2: Phase “b” load current
Ch3: Phase “c” load current
Ch4: Load neutral current in1
Ch5: Supply neutral current in2

Case MSR-2:

Supply Delta connected, load star connected corresponding to Case-2 stated earlier and explained in conjunction figure 1(b). The Matlab / Simulink model is given in figure 4(a) and the simulation results are given figure 4(b).

Specifically, figure 4(b) shows:

Ch1: Phase “a” load voltage and current
Ch2: Phase “b” load voltage and current
Ch3: Phase “c” load voltage and current

Figure 4(c) shows simulation results of load current for each phase of circuit shown in figure 4(a).

Specifically, figure 4(c) shows:

Ch1: Phase “a” load current
Ch2: Phase “b” load current
Ch3: Phase “c” load current
Ch4: Load neutral current in1
Ch5: Supply neutral current in2 (Note that there is no supply neutral connection)

From the results it is clear that

There is no neutral current returned to supply neutral point for Case -1 stated earlier and explained in conjunction figure 1(a) and (c).

The NCC works with Delta connected supply corresponding to Case-2 stated earlier and explained in conjunction figure 1(b).

Experimental Results

Figure 5 shows the experimental set up. The experimental set up consists of the NCC 100 working with a real time welding plant load supplied through 11kV / 415 V, Delta-Star, 200 kVA transformer. The supply is star connected and the unbalanced welding plant load is also star connected (similar to circuit shown in figure 1(c)). For the sake of brevity, the experimental set up shown in figure 5, which is similar to the circuit shown and described along with figure 1(c), is not explained in detail. The circuit of figure 5 further includes a first MCCB is connected before the NCC 100 and before the unbalance load 40, and a second MCCB is connected to before the NCC 100 for switching “ON” and “Off” the NCC 100 and also protecting the NCC 100 by tripping in over load conditions. The MCCB can be replaced by a contactor or an ACB (air circuit breaker), which may be obvious to a person skilled in the art.

Figure 6 shows the recorded waveforms for the three load currents, the load neutral current, close to zero neutral current returned to supply neutral. From the waveforms it is clear that there is no neutral current returned to supply neutral point for Case -1 stated earlier and explained in figure 1(a) and (c) in section 2.0.

Specifically, figure 6 shows:

Ch1: Phase “a” supply current
Ch2: Phase “b” supply current
Ch3: Phase “c” supply current
Ch4: Load neutral current in1
Ch5: Supply neutral current in2

As the NCC 100 does not recognize whether the supply is star connected or delta connected, the experimental results for the delta connected supply and star connected unbalanced load are not considered essential and hence are not recorded here.

Referring now to figures 7(a) and 7(b), actual installation of the NCC 100 in accordance with the present invention is illustrated. The circuit of the NCC 100 is enclosed in an enclosure 200. Specifically, figure 7(a) shows an enclosure 200 in closed condition. The enclosure 200 having a phase indicator lights 210, a line to line system voltage indictor 212, an indicator for current from NCC 100 within a built selector switch 214, a compensator neutral current indicator 216, a load neutral current indicator 218, an ‘ON’ switch 220 for the NCC 100, an ‘OFF’ switch 222 for the NCC 100, a switch 224 and a fan 226 for cooling the enclosure 200.

Specifically, figure 7(b) shows the enclosure 200 with doors in open condition. The figure 7(b) shows a compensated neutral current 228, a control MCB 230, a current measuring CT’s 232 for the NCC 100, a meter section 234, a supply MCCB 236, single phase transformers 238, a bypass MCB 240 for the NCC 100, a MCCB 242 for the NCC 100 and a cooling fan 226.

Ratio Error

As shown in figure 2, the three numbers of single-phase transformers are required for construction of the NCC 100 having primary to secondary turns ratio of 3:1. Thus, the neutral current flowing back into the three primary side windings of the single phase transformers is 1/3.

However, if the ratios are slightly different, the reflection of secondary side current will not be identical on primary side (will be slightly different than 1/3). Thus, the small error current in2 will flow into supply neutral point if the load neutral point is connected to supply neutral (directly if the connection is direct or through the ground / earth) as shown in figure 1(c)). This current in2 could be a very small percentage of the total unbalanced neutral current in1 (refer Case-1, figure 1 (c)).

It should be noted that the NCC 100 can work without the load neutral connected to star connected supply neutral point directly or through ground / earth (as in figure 1(c)) or also with a delta connected supply (as in figure 1(b)). In both these cases, the turns ratio error still forces equal currents (in1a=in1b=in1c =in1/3) in the three primary side windings of each of the single phase transformers.

The three-phase supply to the primary side of the transformers needs to be given through either a contactor or a MCCB (molded case circuit breaker) or an ACB (air circuit breaker) based on the incoming voltage and the unbalanced neutral current expected to flow in load neutral as shown in figure 7b. The MCCB or ACB is necessary for supply isolation.

Conclusion

From the modeling and simulation results and the experimental results illustrated and described above, it is clear that the proposed NCC 100 works satisfactorily for the two different cases of supply and load connection explained, as under.

(i) Case-1: The supply is star connected, the load is unbalanced and star connected (normally called as four wire system) and the load neutral is directly connected to supply neutral point (supply neutral point may or may not be grounded or earthed) or both supply and load neutrals are grounded / earthed (refer figure 1(a)).

In this situation, the NCC 100 circulates the unbalanced neutral current through load and does not allow it to flow into supply neutral. The supply neutral connection carries only the error current (in2 as in figure 1(c)) caused by the ratio error. This also can be eliminated by disconnecting the connection between the NCC 100 to the supply neutral point (directly or through the ground / earth). If so, even if there is a ratio error, it forces equal currents (in1a=in1b=in1c =in1/3) in the three primary side windings of the single phase transformers. The supply neutral point, however, can remain grounded / earthed so that the star point of the supply remains firmly held at ground / earth potential.

(ii) Case-2: The supply is delta connected and the load is unbalanced and star connected. Under such a situation, the load normally sees line to line or phase to phase voltages. Line to neutral or phase to neutral operation for the three-phase load is not possible, even though it may be desired at times. To make such an operation possible, the load neutral is then connected to the NCC 100 to complete the neutral closed path. This is shown in figure 1(b).

In this situation, even if there is a ratio error, it forces equal currents (in1a=in1b=in1c=in1/3) in the three primary side windings of the single phase transformers.

Advantages of the present invention

The features or advantages of the proposed NCC 100 are summarized below:

• The NCC 100 reduces the earth current and supply neutral current to near zero irrespective of the unbalanced load neutral current caused by Linear as well as Non-linear loads,
• The NCC 100 enables in retaining supply neutral voltage near to earth zero potential even for large earth resistances or when supply neutral gets disconnected for some reason (which avoids tripping or damaging of sensitive loads connected to the same supply bus),
• The NCC 100 is robust in construction and reliable in operation, as the NCC 100 uses only magnetic components, that is transformers and no active devices,
• The NCC 100 is easy to manufacture and commission,
• The NCC 100 is economical in construction in compared to any active device based solution provided at present,
• Rating: Depends upon system or supply voltage and maximum unbalanced load current / neutral current already existing at the installation. However, in general could be 25 to 33% of load VA (Volt Amperes).

The transformers require some amount of magnetization current (normally 1% of its rated current). Also there will be some copper loss in the three transformers at maximum neutral current. However, these disadvantages can offset by the basic advantages the NCC 100 offers.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention.