A kind of grounding transformer impedance defining method
Technical field
The present invention relates to a kind of electric power system power distribution field based on modular multilevel (ModularMultilevelConverter; MMC) the flexible direct current power transmission system grounding transformer parameter designing of technology and zero-sequence current protection action threshold value determination method, in particular to a kind of grounding transformer impedance defining method.
Background technology
6kV, 10kV, 35kV electrical network of AC electric power systems is generally adopted the method for operation of isolated neutral.In electrical network, main transformer low-pressure side (distribution voltage side) is generally delta connection, hence without the neutral point being available for earth resistance.When isolated neutral system generation singlephase earth fault, line voltage triangle remains in that symmetry, and impact that user is worked on is little, and when capacitance current is smaller, some instantaneity earth faults can be died away.Therefore, isolated neutral system serves very important effect to improving power supply reliability.But along with the urban electric power user fast development to electricity needs, urban electric power corridor is nervous, transmission line of electricity is frequently with taking up an area few power cable mode, thus the capacitance current of distribution system leads to the permissible value (10A) having often exceeded power system.In order to reduce the capacity current of distribution system, the system that system designer adopts grounding transformer to be △ type wiring or Y type wiring neutral point cannot be drawn is thought and structure neutral point is reduced the capacity current of distribution system by the method for adjunction arc suppression coil.Therefore, carry out the Calculation and Study of the positive sequence of grounding transformer, negative phase-sequence, zero sequence impedance, the calculating of distribution system capacity current and the determination of arc suppression coil parameter are had important engineering significance.
Grounding transformer is also based on a kind of key equipment of the grounding device of MMC flexible direct-current transmission system, can occur that the electric current that asymmetrical voltage produces provides low impedance path for the offset voltage of the DC side of flexible direct current power transmission system or AC system.Wherein by name " a kind of modular multi-level flexible direct-current transmission Grounding design scheme ", application number be 201210524468.1 patent documentation disclose technical scheme and the advantage thereof of the grounding device being made up of grounding transformer, but this invention does not provide the defining method of the positive sequence of grounding transformer, negative phase-sequence, zero sequence impedance.Along with flexible DC power transmission engineering Rapid Popularization is applied, the research of the impedance computation of the grounding transformer that can be used for flexible direct current power transmission system also there are important theory and engineering significance.
Grounding transformer is widely used in alternating-current system and the flexible direct current power transmission system based on MMC technology.The impedance parameter of reactive power, active power and grounding transformer that grounding transformer consumes is closely related.Simultaneously; zero-sequence current is presented Low ESR by grounding transformer; the voltage on impedor (being generally resistance) being connected with grounding transformer neutral point protects signal frequently as system zero sequence voltage (or zero-sequence current), and the zero sequence impedance of grounding transformer is one of key factor affecting zero-sequence component protection definite value.Although general at present, grounding transformer is interpreted as, and the positive sequence of grounding transformer, negative sequence impedance are very big, and zero sequence impedance is very little, but the defining method of each sequence impedance of a kind of grounding transformer need to be provided, with the needs of adaptive technique development.
Summary of the invention
For the deficiencies in the prior art, it is an object of the invention to provide a kind of grounding transformer impedance defining method, the method each sequence impedance computation for the grounding transformer in distribution system, based on the grounding transformer in the flexible direct current power transmission system of MMC technology can provide method, can for providing reference by the action threshold value of the zero-sequence component of grounding transformer.
It is an object of the invention to adopt following technical proposals to realize:
The present invention provides a kind of grounding transformer impedance defining method, and described grounding transformer is made up of 3 single-phase transformers;The first side winding of described single-phase transformer is identical with the secondary side winding number of turn;It thes improvement is that, positive-negative sequence equivalent circuit and the zero sequence equivalent circuit of described method grounding transformer realize impedance computation;Described method comprises the steps:
Step 1: determine grounding transformer leakage impedance;
Step 2: determine grounding transformer exciting impedance;
Step 3: determine grounding transformer positive sequence, negative phase-sequence and zero sequence total impedance.
Further, the rated capacity of described single-phase transformer is S1;Rated exciting current I0;Specified phase voltage is U1;Specified phase current is I1;Single-phase transformer primary side and the secondary side winding number of turn are N1;Primary side and secondary side winding resistance are r1;Primary side, the equal x of secondary side winding leakage reactance1σ;Excitatory resistance is R1m;Excitatory reactance is x1m;
Described positive-negative sequence equivalent circuit is by three phase compositions, every by the resistance sum r of 2 windings, 2 winding positive sequences, negative phase-sequence equivalent leakage reactance xσ;The iron loss resistance R of core limbmAnd the excitatory reactance x of core limbmIt is composed in series;
When for positive-negative sequence equivalent circuit, 6 umber of turns are N1The positive sequence of grounding transformer, negative sequence impedance parameter and 3 primary sides, secondary side winding be all N1Single-phase transformer compare, iron loss resistance and excitatory reactance all to square being directly proportional of umber of turn, the winding impedance relation of grounding transformer and single-phase transformer meets:
Described zero sequence equivalent circuit is by three phase compositions, often by the resistance sum r of 2 windings and 2 winding positive sequences, negative phase-sequence equivalent leakage reactance xσIt is composed in series;
When for zero sequence equivalent circuit, 6 umber of turns are all the positive sequence of grounding transformer of N1, negative sequence impedance parameter is all N with 3 primary sides, secondary side winding1Single-phase transformer compare, iron loss resistance and excitatory reactance all to square being directly proportional of umber of turn, the winding impedance relation of grounding transformer and single-phase transformer meets:
Further, in described step 1, the leakage impedance calculating grounding transformer includes calculating its primary side, secondary side winding resistance r1And primary side, secondary side winding leakage reactance x1σ;Two winding all-in resistance r on the same core limb of grounding transformer are 2r1;The leakage reactance x of grounding transformerσOn same core limb, two identical windings unshakable in one's determination produce equal in magnitude and difference 60 ° magnetic potential when flowing through each sequence electric current, and total magnetic potential of synthesis is single winding correspondence magnetic potentialTimes;
The impedance of single-phase transformer or grounding transformer is frequently with perunit value;The impedance perunit value Z of single-phase transformerbFor:
Wherein, SNRated capacity for single-phase transformer;UNSpecified phase voltage for single-phase transformer.
The leakage impedance actual value of grounding transformer or perunit value be:
Or
Wherein, r*For grounding transformer winding resistance perunit value;xσ*For grounding transformer winding leakage reactance perunit value.
Further, in described step 2, when identical forward-order current and negative-sequence current flow through two windings on the same core limb of grounding transformer, the composite magnetic power of generation is identical, and is the corresponding excitatory magnetic potential of single windingTimes, forward-order current and negative-sequence current are presented high impedance by grounding transformer;
The exciting impedance Z of single-phase transformer1m*=R1m*+jx1m*, wherein:
Wherein, R1mPerunit value for single-phase transformer excitatory loss correspondence resistance unshakable in one's determination;x1mFor single-phase transformer excitatory reactance unshakable in one's determination.
Further, described step 2 comprises the steps:
Step 2.1, determine positive sequence and negative phase-sequence exciting impedance:
The exciting impedance actual value of grounding transformer or perunit value be:
Or
Wherein, RmAnd Rm*The respectively actual value of grounding transformer excitatory loss correspondence resistance unshakable in one's determination or perunit value;xmAnd xm*The respectively actual value of grounding transformer excitatory reactance unshakable in one's determination or perunit value;The leakage reactance of grounding transformer existsBetween.
Step 2.2, determine zero sequence exciting impedance:
When zero-sequence current flows through two windings on the same core limb of grounding transformer, two windings produce excitatory magnetic potential equal in magnitude and in opposite direction, and its composite magnetic power is zero, and the exciting impedance of grounding transformer is zero.
Further, described step 3 comprises the steps:
Step 3.1, determine grounding transformer positive sequence and negative phase-sequence total impedance:
Grounding transformer positive sequence, negative phase-sequence total impedance are equal, and are grounding transformer leakage impedance and grounding transformer exciting impedance sum:
Or
Wherein rTAnd rT*The respectively actual value of grounding transformer positive-negative sequence all-in resistance or perunit value;xTAnd xT*The respectively actual value of the total reactance of grounding transformer positive-negative sequence or perunit value;
Step 3.2, determine Zero-sequence Impedance for Earthing Transformer:
The zero sequence impedance of grounding transformer is equal with the leakage reactance of grounding transformer, calculates the zero-sequence impedance parameter of grounding transformer with reference to formula (4), and Zero-sequence Impedance for Earthing Transformer actual value or perunit value be:
Or
Wherein, r*For grounding transformer winding resistance perunit value;xσ*For grounding transformer winding leakage reactance perunit value;ZbImpedance perunit value for single-phase transformer;x1σFor grounding transformer primary side, secondary side winding leakage reactance;r1For grounding transformer primary side, secondary side winding resistance.
Compared with immediate prior art, the excellent effect that technical scheme provided by the invention has is:
Constituting grounding transformer single-phase transformer (primary side, the secondary side winding number of turn are identical) parameter: rated capacity is S1;Rated exciting current I0;Specified phase voltage is U1;Specified phase current is I1;Primary side, the secondary side winding number of turn are N1;Primary side, secondary side winding resistance are r1, primary side, the equal x of secondary side winding leakage reactance1σ;Excitatory resistance is R1m;Excitatory reactance is x1mBasis on, with reference to the present invention propose grounding transformer impedance computation method, it is possible to calculate each sequence impedance of grounding transformer rapidly.
Method provided by the invention each sequence impedance computation for the grounding transformer in distribution system, based on the grounding transformer in the flexible direct current power transmission system of MMC technology can provide method, can for providing reference by the action threshold value of the zero-sequence component of grounding transformer.
Accompanying drawing explanation
Fig. 1 is grounding transformer provided by the invention and earth resistance schematic diagram;
Fig. 2 is wiring diagram and the phasor diagram of Znyn grounding transformer provided by the invention, and wherein (a) is wiring diagram;B () is phasor diagram;
Fig. 3 is grounding transformer positive-negative sequence equivalent circuit diagram provided by the invention;
Fig. 4 is grounding transformer zero sequence equivalent circuit diagram provided by the invention;
Fig. 5 is positive sequence provided by the invention, negative phase-sequence, zero sequence impedance calculation flow chart.
Detailed description of the invention
Below in conjunction with accompanying drawing, the specific embodiment of the present invention is described in further detail.
Grounding transformer, in distribution system or when applying based on MMC flexible direct-current transmission system, generally concatenates a resistance to limit the zero-sequence current of grounding transformer, as shown in Figure 1 between the neutral point of grounding transformer and ground.
The number of turn assuming three windings directly connect with power supply is N1, three umber of turns being joined directly together with O point are N2.Having 2 windings on each core limb, each phase winding is connected by 2 the winding reversed polarity being set on different core limb respectively, and wherein, phasor diagram is determined according to Z-type transformator positive sequence phase relation.
Winding on A phase core limb, no-load voltage ratio k=N1/N2.Phase shifting angle α and N1、N2Relation be:
By the cosine law it can be seen that two windings on same core limb convert the equivalent number of turn N connecting mains side3For
From above formula it can be seen that N3With N1Change with no-load voltage ratio k.Therefore, when grounding transformer line end adds the positive and negative sequence voltage of three-phase, the magnetic potential that each core limb of grounding transformer produces is shown in composite magnetic power such as Fig. 2 (a) of two phase windings and (b).Composite magnetic power on three core limbs differs 120 ° successively, is one group of three-phase symmetrical phasor.Three-phase magnetic flux can form mutually flux path on three core limbs, and magnetic resistance is little, magnetic flux is big, induced potential is big, presents very big excitation impedance.
When grounding transformer passes through a certain size zero-sequence current I0Time, on each core limb, the sense of current of 2 windings is contrary, can reduce the effect of zero-sequence current magnetic potential, and therefore the zero sequence impedance of grounding transformer is less, and zero-sequence current can not be risen choking effect.
Therefore, when grounding transformer connects asymmetrical voltage operation, it is high impedance to its positive sequence, negative-sequence current, to zero-sequence current then in Low ESR.For more effective reduction residual voltage, generally take the no-load voltage ratio k=1 of grounding transformer.Analysis below carries out according to this no-load voltage ratio, namely
In order to calculate grounding transformer positive sequence impedance, negative sequence impedance and zero sequence impedance, the grounding transformer that three single-phase transformers (primary side, the secondary side winding number of turn are identical) are constituted by the present invention according to accompanying drawing 2 wiring, it is assumed that the rated capacity of single-phase transformer is S1;Rated exciting current I0;Specified phase voltage is U1;Specified phase current is I1;Primary side, the secondary side winding number of turn are N1;Primary side, secondary side winding resistance are r1, primary side, the equal x of secondary side winding leakage reactance1σ;Excitatory resistance is R1m;Excitatory reactance is x1m, analyze positive sequence, negative phase-sequence and zero sequence equivalent circuit, and calculate corresponding equiva lent impedance.
Resistance for each winding of grounding transformer is not unrelated with the electric current sequence passed through, therefore grounding transformer primary side, secondary side winding resistance equal.
Leakage reactance for grounding transformer reflects the tightness degree of coupling between primary side, secondary side winding, and the path of leakage magnetic flux is not unrelated with an alive sequence, and therefore grounding transformer primary side, the positive sequence of secondary side winding, negative phase-sequence and zero sequence leakage reactance are also equal.
Exciting impedance for grounding transformer depends on core loss correspondence resistance and excitatory reactance.On the iron loss core limb of grounding transformer, two winding composite magnetic powers are relevant.When passing to positive sequence and negative-sequence current respectively when grounding transformer, the path of main flux is identical, and therefore positive sequence, negative sequence voltage correspondence iron loss resistance are identical with excitatory reactance.When grounding transformer passes to zero-sequence current, synthesis main flux is zero, and therefore residual voltage correspondence iron loss resistance and the excitatory reactance of zero sequence are all approximately zero.The present invention carries out impedance analysis from positive-negative sequence equivalent circuit and zero sequence equivalent circuit respectively.
(1) positive-negative sequence equivalent circuit
Wiring diagram according to grounding transformer, it can be seen that magnetic potential produced by two coil positive sequences, negative-sequence currents on same core limb differs 60 °.Therefore, the positive-negative sequence equivalent circuit of grounding transformer is as shown in Figure 3.Wherein, r represents the resistance sum of 2 windings;xσIt is 2 winding positive sequences, negative phase-sequence equivalent leakage reactance;RmRepresent the iron loss resistance of core limb;xσExcitatory reactance for core limb.According to analysis above, 6 umber of turns are all N1The positive sequence of grounding transformer, negative sequence impedance parameter and 3 primary sides, secondary side winding be all N1Single-phase transformer compare, iron loss resistance and excitatory reactance all to square being directly proportional of the number of turn, therefore have
(2) zero sequence equivalent circuit
The zero-sequence current composite magnetic power of grounding transformer is zero, and corresponding zero sequence main flux is zero, and residual voltage correspondence iron loss resistance and the excitatory reactance of zero sequence all can be ignored.Therefore zero sequence equivalent circuit is as shown in Figure 4.
Zero-sequence Impedance for Earthing Transformer is only winding resistance and leakage reactance composition.6 umber of turns are all N1The positive sequence of grounding transformer, negative sequence impedance parameter and 3 primary sides, secondary side winding be all N1Single-phase transformer compare, iron loss resistance and excitatory reactance all to square being directly proportional of the number of turn, the impedance relationship of grounding transformer and single-phase transformer meets
The technical scheme is that a kind of grounding transformer positive sequence, negative phase-sequence and zero sequence impedance computational methods.The method mainly comprises the steps that
Step 1: the calculating of grounding transformer leakage impedance.With reference to the method that single-phase transformer measures leakage impedance at low pressure short circuit in winding, calculate the leakage impedance of grounding transformer, including primary side, secondary side winding resistance r1, primary side, secondary side winding leakage reactance x1σ.Owing to the winding resistance of grounding transformer is not unrelated with electric current sequence, therefore two winding all-in resistance r on the same core limb of grounding transformer are 2r1.The leakage reactance x of grounding transformerσBeing that on same core limb, two identical windings unshakable in one's determination produce equal in magnitude when flowing through each sequence electric current, differ the magnetic potential of 60 °, total magnetic potential of synthesis is single winding correspondence magnetic potentialTimes.
In order to the impedance of convenience of calculation, transformator or grounding transformer is frequently with perunit value.For single-phase transformer, its impedance perunit value ZbFor:
Wherein, SNRated capacity for single-phase transformer;UNSpecified phase voltage for single-phase transformer.
The leakage impedance (actual value or perunit value) of grounding transformer is
Or
Wherein, r*For grounding transformer winding resistance perunit value;xσ*For grounding transformer winding leakage reactance perunit value.
The perunit value of usual single-phase transformer leakage reactance is about 0.1, relevant with factors such as the capacity of transformator, structures.Winding resistance, compared with winding leakage reactance, generally can be ignored.Therefore, the leakage reactance of grounding transformer existsLeft and right.
Step 2: the calculating of exciting impedance.When flowing through grounding transformer because of identical forward-order current and negative-sequence current on same core limb during two windings, the composite magnetic power of generation is identical, and is the corresponding excitatory magnetic potential of single windingTimes, therefore positive sequence, negative-sequence current are presented high impedance by grounding transformer.Zero-sequence current flows through during grounding transformer on same core limb during two windings, and owing to two windings produce excitatory magnetic potential equal in magnitude, in opposite direction, its composite magnetic power is approximately zero.Therefore, the exciting impedance of grounding transformer is approximately zero.
The exciting impedance Z of single-phase transformer1m*=R1m*+jx1m*, Qi Zhongwei
Wherein, R1mPerunit value for single-phase transformer excitatory loss correspondence resistance unshakable in one's determination;x1mFor single-phase transformer excitatory reactance unshakable in one's determination.
Step 2.1 positive sequence, negative phase-sequence exciting impedance:
According to above-mentioned analysis, the exciting impedance (actual value or perunit value) of grounding transformer is:
Or
Wherein, Rm、Rm*The respectively actual value of grounding transformer excitatory loss correspondence resistance unshakable in one's determination or perunit value;xm、xm*The respectively actual value of grounding transformer excitatory reactance unshakable in one's determination or perunit value.
The excitatory per unit reactance of single-phase transformer is generally between 50-100, relevant with factors such as the capacity of transformator, structures.Core loss correspondence resistance perunit value, compared with excitatory per unit reactance, generally can be ignored.Therefore, the leakage reactance of grounding transformer existsBetween.
Step 2.2 zero sequence exciting impedance:
Being zero according to the electric current composite magnetic power that on grounding transformer two windings unshakable in one's determination flow through, therefore the zero sequence impedance perunit value of grounding transformer is for being approximately zero.
Step 3: calculate each sequence total impedance of grounding transformer:
According to step 1 and step 2, it is possible to calculate each sequence total impedance of grounding transformer.
3.1: grounding transformer positive sequence, negative phase-sequence total impedance:
Grounding transformer positive sequence, negative phase-sequence total impedance are equal, and are grounding transformer leakage impedance and grounding transformer exciting impedance sum:
Or
Wherein rT、rT*The respectively actual value of grounding transformer positive-negative sequence all-in resistance or perunit value;xT、xT*The respectively actual value of the total reactance of grounding transformer positive-negative sequence or perunit value.
Owing to the exciting impedance of grounding transformer is much larger than its leakage reactance, when engineering calculates positive sequence, negative sequence impedance, its leakage reactance can be ignored.
3.2: Zero-sequence Impedance for Earthing Transformer:
The zero sequence impedance of grounding transformer and the leakage reactance approximately equal of grounding transformer, namely can refer to formula (4) and calculate the zero-sequence impedance parameter of grounding transformer.
In sum, the impedance computation flow process of grounding transformer is as shown in Figure 5.
Embodiment
If the transmission capacity of flexible direct current power transmission system and the rated voltage respectively 1000MW of DC line and ± 320kV based on MMC technology.In order to calculate each sequence (positive sequence, negative phase-sequence, the zero sequence) impedance of grounding transformer, the present invention illustrates with example: MMC converter valve side alternating voltage UTNFor 360kV, the leakage impedance r of the single-phase winding of composition grounding transformer1+jx1σFor 4+j10 Europe, exciting impedance R1m+jx1mFor 1000+j2500 Europe.According to grounding transformer impedance computation flow chart 5, from formula (4): the zero sequence impedance of grounding transformer is r+jxσFor 8+j17.3 Europe;
From formula (6): the positive-negative sequence exciting impedance R of grounding transformerm+jxmFor 3000+j7500 Europe, in conjunction with the leakage impedance r+jx of grounding transformerσ, utilizing formula (7), the positive-negative sequence impedance of grounding transformer is 3008+j7517.3 Europe.Positive-negative sequence impedance according to grounding transformer, when based on MMC technology flexible direct current power transmission system (transmission capacity be 1000MW, DC line rated voltage be ± 320kV) properly functioning time, the reactive power of grounding transformer consumption is 16Mvar.
Finally should be noted that: above example is only in order to illustrate that technical scheme is not intended to limit; although the present invention being described in detail with reference to above-described embodiment; the specific embodiment of the present invention still can be modified or equivalent replacement by those of ordinary skill in the field; these are without departing from any amendment of spirit and scope of the invention or equivalent replace, within the claims of the present invention all awaited the reply in invention.