CN104391198A - Low-voltage power grid safety monitoring method - Google Patents

Low-voltage power grid safety monitoring method Download PDF

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Publication number
CN104391198A
CN104391198A CN201410690865.5A CN201410690865A CN104391198A CN 104391198 A CN104391198 A CN 104391198A CN 201410690865 A CN201410690865 A CN 201410690865A CN 104391198 A CN104391198 A CN 104391198A
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phase
voltage
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load
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CN104391198B (en
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杨琦
杨桦
罗建
吉畅
罗明才
代熲
宋洪宾
曾礼强
徐川
曾敏
杨光学
吴涛
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State Grid Corp of China SGCC
State Grid Sichuan Electric Power Co Ltd
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State Grid Corp of China SGCC
State Grid Sichuan Electric Power Co Ltd
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Abstract

The invention relates to power grid safety, in particular to a low-voltage power grid safety monitoring method which includes the steps: acquiring three-phase transient current signals iA (t), iB (t) and iC (t) and three-phase transient voltage signals uA (t), uB (t) and uC (t) of a head end of a low-voltage power grid circuit; taking any two phases as a loop to form a high-order differential equation; calculating a coefficient of the high-order differential equation in the second step; solving an equivalent parameter by the aid of the relationship between the transient high-order differential equation coefficient and a stable equivalent parameter; extracting characteristics such as fundamental frequency components in the acquired three-phase transient current signals and three-phase transient voltage signals. The data sampling frequency of the current signals and the voltage signals is 10kHz, and a time window is a disturbed half cycle. The low-voltage power grid safety monitoring method can judge whether a power grid is safe or not according to the impedance of a neutral wire and practical degree of unbalance of a load.

Description

A kind of low pressure electrical network safety monitoring method
Technical field
The present invention relates to power grid security, be specifically related to a kind of low pressure electrical network safety monitoring method.
Background technology
Low pressure power utilization network, as last ring of whole electrical network, is directly connected with user, and its security is subject to extensive concern.National regulations needs to ensure that on three-phase load mean allocation, the neutral line, impedance is zero in three phase supply situation.But three-phase load is unbalanced often under actual motion, the existence of neutral line impedance in addition, makes load side dummy neutral voltage occur skew, the rising of at least one phase load terminal voltage, namely produces neutral point overvoltage.For user, electrical equipment runs and easily burns electrical equipment under over-voltage condition; Secondly, the zero-sequence current that uncompensated load produces conducts back system side by the neutral line, once the neutral line, because the reason such as circuit aging, neutral line junction contacts is bad causes, resistance value is abnormal to be increased, and just there is overheated danger at the position that on the neutral line, resistance increases, thus becomes disaster hidden-trouble.Therefore the safety monitoring of degree of unbalancedness and voltage protection is carried out to low pressure electrical network most important.
The data that current low pressure electrical network safety monitoring uses are mainly three phase-to-ground voltages, the current information at points of common connection (PCC) place.But PCC point voltage-to-ground is subject to superior voltage and earthy restriction, is three-phase symmetric voltage under normal operating condition.Deposit in case at load side dummy neutral voltage, the voltage-to-ground of PCC point cannot represent the true pressure drop of user side and the degree of unbalancedness of load.Therefore, under the non-faulting state of dummy neutral voltage existence, the safety monitoring problem of low pressure electrical network is realized mainly through three-phase current.But, only use current information cannot characterize the pressure drop on load equally, and when monitoring for three-phase load unbalance degree, because three-phase current is subject to the impact of impedance on the neutral line, under same load condition, neutral line impedance changes, the load unbalanced degree of gained changes thereupon, and it is inadequate to be therefore used alone current information cogency.In addition, cannot centering line impedence and change thereof to carry out monitoring also be existing methodical deficiency.
Summary of the invention
The object of the present invention is to provide a kind of low pressure electrical network safety monitoring method, when asymmetric situation, the stable state port information existed with neutral line impedance of threephase load retrains, transient information identification Equivalent Model parameter can be collected.
For solving above-mentioned technical matters, the present invention by the following technical solutions:
A kind of low pressure electrical network safety monitoring method, comprises the following steps:
Step one, gathers the three-phase transient current signal i of low pressure power network line head end a(t), i b(t), i c(t) and three
Phase transient voltage signal u a(t), u b(t), u c(t), the data sampling frequency of current signal and voltage signal is 10kHz,
Time window is a half cycles (30ms) after disturbance;
Step 2, using any two-phase as a loop, forms differential equation of higher order
Σ k = 0 K a k U AB ( t ) ( k ) = Σ k = 0 K b k i A ( t ) ( k ) + Σ k = 0 K c k i B ( t ) ( k )
Wherein parameter a 0, a 1..., a kand b 0, b 1..., b k, a and b is the coefficient to be identified of the differential equation, and K is called equivalent exponent number, U in equation aB(t) (k)voltage microvariations signal U aBthe k order derivative of (t), i a(t) (k), i b(t) (k)electric current microvariations signal i a(t), i bthe k order derivative of (t), K is the equivalent exponent number of load model;
Step 3, asks for differential equation of higher order coefficient in step 2;
Step 4, utilizes the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters to solve equivalent parameters, described transient state differential equation of higher order coefficient and stable state equivalent parameters, i.e. transient state differential equation of higher order coefficient a 0, a 1..., a k, b 0, b 1..., b k, c 0, c1 ..., c kwith stable state equivalent parameters R aeq, R beq, R ceqbetween relation as follows
When K is even number, order calculate
A 0 = Σ n = 0 N ( - 1 ) n a 2 n ω 2 n
A 1 = Σ n = 0 N - 1 ( - 1 ) n a 2 n + 1 ω 2 n + 1
B 0 = Σ n = 0 N ( - 1 ) n b 2 n ω 2 n
B 1 = Σ n = 0 N - 1 ( - 1 ) n b 2 n + 1 ω 2 n + 1
C 0 = Σ n = 0 N ( - 1 ) n c 2 n ω 2 n
C 1 = Σ n = 0 N - 1 ( - 1 ) n c 2 n + 1 ω 2 n + 1
When K is odd number, order calculate
A 0 = Σ n = 0 N ( - 1 ) n a 2 n ω 2 n
A 1 = Σ n = 0 N ( - 1 ) n a 2 n + 1 ω 2 n + 1
B 0 = Σ n = 0 N ( - 1 ) n b 2 n ω 2 n
B 1 = Σ n = 0 N ( - 1 ) n b 2 n + 1 ω 2 n + 1
C 0 = Σ n = 0 N ( - 1 ) n c 2 n ω 2 n
C 1 = Σ n = 0 N - 1 ( - 1 ) n c 2 n + 1 ω 2 n + 1
Thus, the A phase stable state equivalent parameters R of load model is drawn aeqand L aeqand B phase stable state equivalent parameters R beqand L beq:
R Aeq = A 1 B 1 + A 0 B 0 A 0 2 + A 1 2
L Aeq = A 1 B 0 - A 0 B 1 ω ( A 0 2 + A 1 2 )
R Beq = A 1 C 1 + A 0 C 0 A 0 2 + A 1 2
L Beq = A 1 C 0 - A 0 C 1 ω ( A 0 2 + A 1 2 ) ;
Step 5, extracts wherein comprised fundamental frequency component, and is written as the expression formula of sin cos functions from the three-phase transient current signal collected and three-phase transient voltage signal, as follows
U Awen=a 1cosωt+a 2sinωt
U Bwen=a 3cosωt+a 4sinωt
I Awen=b 1cosωt+b 2sinωt
I Bwen=b 3cosωt+b 4sinωt
U Nwen=c 1cosωt+c 2sinωt;
Step 6, represents the equivalent impedance parameter obtained in step 4 with the coefficient of the sine and cosine expression formula obtained, as follows
c 1=a 1+R Ab 1-ωb 2L A
c 2=a 2-R Ab 2-ωb 1L A
c 1=a 3+R Bb 3-ωb 4L B
c 2=a 4+R Bb 4+ωb 3L B
L A = a 2 b 1 + b 2 c 1 - b 2 a 1 - b 1 c 2 ω ( b 1 2 + b 2 2 )
R A = a 2 b 2 - b 2 c 2 + b 1 a 1 - b 1 c 1 b 1 2 + b 2 2
L B = a 3 b 4 - b 4 c 1 - b 3 a 4 + b 3 c 2 ω ( b 3 2 + b 4 2 ) ;
R B = b 4 c 2 - a 4 b 4 - b 3 a 3 + b 3 c 1 b 1 2 + b 2 2
Step 7, utilize the expression formula obtained in step 6 as constraint condition, the parameter calculated in step 4, as initial value, is optimized, and obtains each equal value impedance exact value;
Step 8, passes through
U N = i O R O + L O di o dt U N = U A - I A * Z A
The drift voltage of calculated load side dummy neutral;
Step 9, calculates three-phase load unbalance degree, together with three-phase load size and dummy neutral drift voltage, characterizes the security of low pressure electrical network, voltage unbalance factor ε u, voltage unbalance factor ε icomputing formula as follows:
ϵ u = U 2 U 1 × 100 %
Wherein: U 1the positive-sequence component amplitude root mean square of-three-phase voltage;
U 2the negative sequence component amplitude root mean square of-three-phase voltage;
ϵ i = I 2 I 1 × 100 %
Wherein: I 1the positive-sequence component amplitude root mean square of-three-phase current;
I 2the negative sequence component amplitude root mean square of-three-phase current;
I 2the negative sequence component amplitude root mean square of-three-phase current;
First the positive-negative sequence component of the actual pressure drop on three-phase load is asked for:
U 1 N · U 2 N · U 0 N · = 1 3 1 a a 2 1 a 2 a 1 1 1 * U · A - U · N U · B - U · N U · C - U · N
Wherein: a=e j120 °;
positive and negative, the zero-sequence component of-three-phase voltage,
for the voltage phasor utilizing the matching of stable state sampled point to obtain, for the dummy neutral voltage phasor that the corresponding time is obtained,
Then positive-negative sequence amplitude is got ask degree of unbalancedness,
ε uN, ε u, ε ivalue larger, the degree of unbalancedness of load is larger, larger to the threat of the safe operation with electrical network, current unbalance factor ε iwhen calculating identical, improve voltage unbalance factor ε ucalculating, utilize the actual load pressure drop obtained in preceding step to ask for degree of unbalancedness ε uN, with a high credibility.
Calculate the method for low pressure network load equivalent impedance with transient state component, its feature is existence that is uneven due to threephase load and neutral line impedance, and dummy neutral voltage and transformer neutral point voltage have skew, are called drift voltage.Now equivalent circuit can not be split as single-phase circuit, and drift voltage is subject to the impact of each phase load, can not directly cancellation.
This patent feature establishes an equation for directly using any two-phase in three-phase, separate between biphase current, does not have the parameter of other phases, without the need to decoupling zero in equation.
Compared with prior art, the invention has the beneficial effects as follows: combining with the actual degree of unbalancedness of neutral line impedance magnitude and load judges by power grid security whether safety, utilize and calculate dummy neutral voltage, obtain the actual electrical pressure reduction (voltage difference between PCC point and load side dummy neutral) in power load, voltage unbalance factor is asked for by national standard method, together with the size of resistance value on the neutral line, the security of sign electrical network; In addition, this patent directly obtains the equivalent impedance value of three-phase load, directly can find out whether three-phase balances.
Accompanying drawing explanation
Fig. 1 is basic three-phase four-wire system network equivalence circuit diagram.
Fig. 2 is the schematic flow sheet of a kind of low pressure electrical network of the present invention safety monitoring method.
Fig. 3 is simple structure analogous diagram.
Fig. 4 is simple structure simulation result.
Fig. 5 is neutral line change in the instantaneous impedance analogous diagram.
Fig. 6 is neutral line change in the instantaneous impedance simulation result figure.
Fig. 7 is neutral line impedance transition mechanism analogous diagram.
Fig. 8 is neutral line impedance transition mechanism simulation result figure.
Embodiment
In order to make object of the present invention, technical scheme and advantage clearly understand, below in conjunction with drawings and Examples, the present invention is further elaborated.Should be appreciated that specific embodiment described herein only in order to explain the present invention, be not intended to limit the present invention.
Fig. 1 and Fig. 2 shows an embodiment of a kind of low pressure of the present invention electrical network safety monitoring method: a kind of low pressure electrical network safety monitoring method, comprises the following steps:
Step one, gathers the three-phase transient current signal i of low pressure power network line head end a(t), i b(t), i c(t) and three
Phase transient voltage signal u a(t), u b(t), u c(t), the data sampling frequency of current signal and voltage signal is 10kHz,
Time window is a half cycles (30ms) after disturbance;
Step 2, using any two-phase as a loop, forms differential equation of higher order
Σ k = 0 K a k U AB ( t ) ( k ) = Σ k = 0 K b k i A ( t ) ( k ) + Σ k = 0 K c k i B ( t ) ( k )
Wherein parameter a 0, a 1..., a kand b 0, b 1..., b k, a and b is the coefficient to be identified of the differential equation, and K is called equivalent exponent number, U in equation aB(t) (k)voltage microvariations signal U aBthe k order derivative of (t), i a(t) (k), i b(t) (k)electric current microvariations signal i a(t), i bthe k order derivative of (t), K is the equivalent exponent number of load model;
Step 3, asks for differential equation of higher order coefficient in step 2;
Step 4, utilizes the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters to solve equivalent parameters, described transient state differential equation of higher order coefficient and stable state equivalent parameters, i.e. transient state differential equation of higher order coefficient a 0, a 1..., a k, b 0, b 1..., b k, c 0, c 1..., c kwith stable state equivalent parameters R aeq, R beq, R ceqbetween relation as follows:
When K is even number, order
A 0 = Σ n = 0 N ( - 1 ) n a 2 n ω 2 n
A 1 = Σ n = 0 N - 1 ( - 1 ) n a 2 n + 1 ω 2 n + 1
B 0 = Σ n = 0 N ( - 1 ) n b 2 n ω 2 n
B 1 = Σ n = 0 N - 1 ( - 1 ) n b 2 n + 1 ω 2 n + 1
C 0 = Σ n = 0 N ( - 1 ) n c 2 n ω 2 n
C 1 = Σ n = 0 N - 1 ( - 1 ) n c 2 n + 1 ω 2 n + 1
When K is odd number, order calculate
A 0 = Σ n = 0 N ( - 1 ) n a 2 n ω 2 n
A 1 = Σ n = 0 N ( - 1 ) n a 2 n + 1 ω 2 n + 1
B 0 = Σ n = 0 N ( - 1 ) n b 2 n ω 2 n
B 1 = Σ n = 0 N ( - 1 ) n b 2 n + 1 ω 2 n + 1
C 0 = Σ n = 0 N ( - 1 ) n c 2 n ω 2 n
C 1 = Σ n = 0 N - 1 ( - 1 ) n c 2 n + 1 ω 2 n + 1
Thus, the A phase stable state equivalent parameters R of load model is drawn aeqand L aeqand B phase stable state equivalent parameters R beqand L beq:
R Aeq = A 1 B 1 + A 0 B 0 A 0 2 + A 1 2
L Aeq = A 1 B 0 - A 0 B 1 ω ( A 0 2 + A 1 2 ) ;
R Beq = A 1 C 1 + A 0 C 0 A 0 2 + A 1 2
L Beq = A 1 C 0 - A 0 C 1 ω ( A 0 2 + A 1 2 )
Step 5, extracts wherein comprised fundamental frequency component, and is written as the expression formula of sin cos functions from the three-phase transient current signal collected and three-phase transient voltage signal, as follows
U Awen=a 1cosωt+a 2sinωt
U Bwen=a 3cosωt+a 4sinωt
I Awen=b 1cosωt+b 2sinωt
I Bwen=b 3cosωt+b 4sinωt
U Nwen=c 1cosωt+c 2sinωt;
Step 6, represents the equivalent impedance parameter obtained in step 4 with the coefficient of the sine and cosine expression formula obtained, as follows
c 1=a 1+R Ab 1-ωb 2L A
c 2=a 2-R Ab 2-ωb 1L A
c 1=a 3+R Bb 3-ωb 4L B
c 2=a 4+R Bb 4+ωb 3L B
L A = a 2 b 1 + b 2 c 1 - b 2 a 1 - b 1 c 2 ω ( b 1 2 + b 2 2 )
R A = a 2 b 2 - b 2 c 2 + b 1 a 1 - b 1 c 1 b 1 2 + b 2 2
L B = a 3 b 4 - b 4 c 1 - b 3 a 4 + b 3 c 2 ω ( b 3 2 + b 4 2 ) ;
R B = b 4 c 2 - a 4 b 4 - b 3 a 3 + b 3 c 1 b 1 2 + b 2 2
Step 7, utilize the expression formula obtained in step 6 as constraint condition, the parameter calculated in step 4, as initial value, is optimized, and obtains each equal value impedance exact value;
Step 8, passes through
U N = i O R O + L O di o dt U N = U A - I A * Z A
The drift voltage of calculated load side dummy neutral;
Step 9, calculates three-phase load unbalance degree, together with three-phase load size and dummy neutral drift voltage, characterizes the security of low pressure electrical network, voltage unbalance factor ε u, voltage unbalance factor ε icomputing formula as follows:
ϵ u = U 2 U 1 × 100 %
Wherein: U 1the positive-sequence component amplitude root mean square of-three-phase voltage;
U 2the negative sequence component amplitude root mean square of-three-phase voltage;
ϵ i = I 2 I 1 × 100 %
Wherein: I 1the positive-sequence component amplitude root mean square of-three-phase current;
I 2the negative sequence component amplitude root mean square of-three-phase current;
I 2the negative sequence component amplitude root mean square of-three-phase current;
First the positive-negative sequence component of the actual pressure drop on three-phase load is asked for:
U 1 N · U 2 N · U 0 N · = 1 3 1 a a 2 1 a 2 a 1 1 1 * U · A - U · N U · B - U · N U · C - U · N
Wherein: a=e j120 °;
positive and negative, the zero-sequence component of-three-phase voltage,
for the voltage phasor utilizing the matching of stable state sampled point to obtain, for the dummy neutral voltage phasor that the corresponding time is obtained,
Then positive-negative sequence amplitude is got ask degree of unbalancedness ε uN, ε u, ε ivalue larger, the degree of unbalancedness of load is larger, larger to the threat of the safe operation with electrical network, current unbalance factor ε iwhen calculating identical, improve voltage unbalance factor ε ucalculating, utilize the actual load pressure drop obtained in preceding step to ask for degree of unbalancedness ε uN, with a high credibility.
The method of low pressure network load equivalent impedance is calculated with transient state component, its feature is existence that is uneven due to threephase load and neutral line impedance, dummy neutral voltage and transformer neutral point voltage have skew, be called drift voltage, now equivalent circuit can not be split as single-phase circuit, drift voltage is subject to the impact of each phase load, can not directly cancellation.
This patent feature establishes an equation for directly using any two-phase in three-phase, separate between biphase current, does not have the parameter of other phases, without the need to decoupling zero in equation.
The degree of unbalancedness of true pressure drop on three-phase load calculates:
Ask for the positive-negative sequence component of the actual pressure drop on three-phase load:
U 1 N · U 2 N · U 0 N · = 1 3 1 a a 2 1 a 2 a 1 1 1 * U · A - U · N U · B - U · N U · C - U · N
Wherein: a=e j120 °;
positive and negative, the zero-sequence component of-three-phase voltage.
for the voltage phasor utilizing the matching of stable state sampled point to obtain, for the dummy neutral voltage phasor that the corresponding time is obtained.
Then positive-negative sequence amplitude U is got 1N, U 2Nask degree of unbalancedness ε uN:
ϵ uN = U 2 N U 1 N × 100 %
Its result is as table 2:
The load unbalanced degree of table 2
ε ui
Classic method 0/125.24%
Patented method 14.3%/125.24%
The invention will be further described now to combine emulation:
Network during transient state three rank circuit are as shown in Figure 3 represented, R1=100 Ω in Fig. 3 by the low pressure power system simulation model under 1, utilizing MATLAB simulation software to set up three-phase four-wire system situation herein, L1=0.003H, L2=0.002H, C1=1uF, RB=400 Ω, LB=0.25H, R2=500 Ω, L3=0.001H, R3=400 Ω, R0=4 Ω, L0=0.03H.
Step 1: list differential equation of higher order:
R 1 ( i A - c 1 d ( u A - u B - L 2 i A ′ - R b i B - L b di B dt ) dt ) + L 1 d ( i A - c 1 d ( u A - u B - L 2 i A ′ - R b i B - L b di B dt ) dt ) dt = u A - u B - L 2 i A ′ - R b i B - L b di B dt
Obtain through arranging:
U aB+ R 1c 2u' aB+ L 1c 1u aB"=R 1i a+ (L 1+ L 2) i' a+ R 1c 1l 2i a"+L 1c 1l 2i a' "+R bi b+ (R 1c 1r b+ L b) i' b+ (R 1c 1l b+ L 1r bc 1) i b"+L 1l bc 1i b' " can find out K=3 from the above-mentioned differential equation, identification model is:
Σ k = 0 3 a 3 U ( t ) ( 3 ) = Σ k = 0 3 b k i A ( t ) ( 3 ) + Σ k = 0 3 c k i B ( t ) ( 3 )
The parameter of identification is needed to have 3* (3+1)-1=11.
Make a 0=1, equation is written as: the matrix of Y=AX form.
a 0 U AB ( t ) = b 0 i A ( t ) + b 1 di A ( t ) dt + b 2 d 2 i A ( t ) dt 2 + b 3 d 3 i A ( t ) dt 3 + c 0 i B ( t ) + c 1 di B ( t ) dt + c 2 d 2 i B ( t ) dt 2 + c 3 d 3 i B ( t ) dt 3 - a 1 dU AB ( t ) dt + a 2 d 2 U AB ( t ) dt 2 + a 3 d 3 U AB ( t ) dt 3
Adopt least square method to carry out identification above formula herein, 11 coefficient: a can be obtained 0-a 3, b 0-b 3, c 0-c 3.
Have again: A 0 = a 0 - a 2 ω 2 A 1 = a 1 ω - a 3 ω 3
B 0 = b 0 - b 2 ω 2 B 1 = b 1 ω - b 3 ω 3
C 0 = c 0 - c 2 ω 2 C 1 = c 1 ω - c 3 ω 3
Afterwards according to formula (2) (3), be translated into stable state equivalent impedance:
R Aeq = A 1 B 1 + A 0 B 0 A 0 2 + A 1 2
L Aeq = A 1 B 0 - A 0 B 1 ω ( A 0 2 + A 1 2 )
R Beq = A 1 C 1 + A 0 C 0 A 0 2 + A 1 2
L Beq = A 1 C 0 - A 0 C 1 ω ( A 0 2 + A 1 2 )
In like manner can obtain C phase parameter.
Then have: Z Aeq = R Aeq + j L Ae Z Beq = R Beq + j L Be Z Ceq = R Ceq + j L Ce
Step 2: get sampled value i when stable state o, i a, u aand calculated value di/dt is according to formula (5), obtains L accurately o, R o.
u N = u A - i A * Z A u N = i O R O + L O di o dt - - - ( 5 )
Now list simulation result in following table 1:
Table 1 rank circuit identification result
The voltage waveform calculating load side dummy neutral contrasts as Fig. 4 with the sample waveform in emulation.
Step 3: the degree of unbalancedness of true pressure drop on three-phase load calculates:
Ask for the positive-negative sequence component of the actual pressure drop on three-phase load:
U 1 N · U 2 N · U 0 N · = 1 3 1 a a 2 1 a 2 a 1 1 1 * U · A - U · N U · B - U · N U · C - U · N
Wherein: a=e j120 °;
positive and negative, the zero-sequence component of-three-phase voltage.
for the voltage phasor utilizing the matching of stable state sampled point to obtain, for the dummy neutral voltage phasor that the corresponding time is obtained.
Then positive-negative sequence amplitude U is got 1N, U 2Nask degree of unbalancedness ε uN:
ϵ uN = U 2 N U 1 N × 100 %
Its result is as table 2:
The load unbalanced degree of table 2
2, neutral line change in the instantaneous impedance emulation:
As shown in Figure 5, in Fig. 5, the unit of resistance is Ω, and the unit of inductance is H, emulation breaker in middle disconnects suddenly at t=0.1s, and on the neutral line, impedance is undergone mutation, and causes microvariations at monitoring port, the neutral line impedance of unexpected access is 4+j0.5, by identification result typing following table 3:
Table 3 neutral line change in the instantaneous impedance emulates
After change, dummy neutral voltage and sampled voltage comparison diagram are as shown in Figure 6.Emulation illustrates, when the neutral line suddenlys change, institute's extracting method still has good identification result to impedance on virtual earth, the neutral line and three-phase equivalent parameters herein.
3, neutral line impedance transition mechanism emulation:
As shown in Figure 7, in figure, the unit of resistance is Ω to realistic model, and the unit of inductance is H.R 0constant interval be [0-5] Ω, rate of change is 0.002 Europe/second, R 0constant interval be [0-5] Europe, rate of change is 0.002 Europe/second.Because rate of change is lower, only rely on the change of neutral line impedance cannot disturbing signal be detected in port, but in low pressure electrical network, inner and outside microvariations often occur, and program still can be run.Information after double disturbance is contrasted, easily obtains its rate of change.
Now list simulation result in following table 4:
Table 4 neutral line impedance transition mechanism emulates
Dummy neutral voltage and sampled voltage comparison diagram are as shown in Figure 8.
Although with reference to multiple explanatory embodiment of the present invention, invention has been described here, but, should be appreciated that, those skilled in the art can design a lot of other amendment and embodiment, these amendments and embodiment will drop within spirit disclosed in the present application and spirit.More particularly, in the scope of, accompanying drawing open in the application and claim, multiple modification and improvement can be carried out to the building block of subject combination layout and/or layout.Except the distortion carried out building block and/or layout and improving, to those skilled in the art, other purposes also will be obvious.

Claims (1)

1. a low pressure electrical network safety monitoring method, is characterized in that comprising the following steps:
Step one, gathers the three-phase transient current signal i of low pressure power network line head end a(t), i b(t), i c(t) and three-phase transient voltage signal u a(t), u b(t), u c(t), the data sampling frequency of current signal and voltage signal is 10kHz, and time window is a half cycles after disturbance;
Step 2, using any two-phase as a loop, forms differential equation of higher order
Σ k = 0 K a k U AB ( t ) ( k ) = Σ k = 0 K b k i A ( t ) ( k ) + Σ k = 0 K c k i B ( t ) ( k )
Wherein parameter a 0, a 1..., a kand b 0, b 1..., b k, a and b is the coefficient to be identified of the differential equation, and K is called equivalent exponent number, U in equation aB(t) (k)voltage microvariations signal U aBthe k order derivative of (t), i a(t) (k), i b(t) (k)electric current microvariations signal i a(t), i bthe k order derivative of (t), K is the equivalent exponent number of load model;
Step 3, asks for differential equation of higher order coefficient in step 2;
Step 4, utilizes the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters to solve equivalent parameters, and between described transient state differential equation of higher order coefficient and stable state equivalent parameters, relation is as follows
When K is even number, order calculate
A 0 = Σ n = 0 N ( - 1 ) n a 2 n ω 2 n
A 1 = Σ n = 0 N - 1 ( - 1 ) n a 2 n + 1 ω 2 n + 1
B 0 = Σ n = 0 N ( - 1 ) n b 2 n ω 2 n
B 1 = Σ n = 0 N - 1 ( - 1 ) n b 2 n + 1 ω 2 n + 1
C 0 = Σ n = 0 N ( - 1 ) n c 2 n ω 2 n
C 1 = Σ n = 0 N - 1 ( - 1 ) n c 2 n + 1 ω 2 n + 1
When K is odd number, order calculate
A 0 = Σ n = 0 N ( - 1 ) n a 2 n ω 2 n
A 1 = Σ n = 0 N ( - 1 ) n a 2 n + 1 ω 2 n + 1
B 0 = Σ n = 0 N ( - 1 ) n b 2 n ω 2 n
B 1 = Σ n = 0 N ( - 1 ) n b 2 n + 1 ω 2 n + 1
C 0 = Σ n = 0 N ( - 1 ) n c 2 n ω 2 n
C 1 = Σ n = 0 N - 1 ( - 1 ) n c 2 n + 1 ω 2 n + 1
Thus, the A phase stable state equivalent parameters R of load model is drawn aeqand L aeqand B phase stable state equivalent parameters R beqand L beq:
R Aeq = A 1 B 1 + A 0 B 0 A 0 2 + A 1 2
L Aeq = A 1 B 0 - A 0 B 1 ω ( A 0 2 + A 1 2 )
R Beq = A 1 C 1 + A 0 C 0 A 0 2 + A 1 2
L Beq = A 1 C 0 - A 0 C 1 ω ( A 0 2 + A 1 2 ) ;
Step 5, extracts wherein comprised fundamental frequency component, and is written as the expression formula of sin cos functions from the three-phase transient current signal collected and three-phase transient voltage signal, as follows
U Awen=a 1cosωt+a 2sinωt
U Bwen=a 3cosωt+a 4sinωt
I Awen=b 1cosωt+b 2sinωt
I Bwen=b 3cosωt+b 4sinωt
U Nwen=c 1cosωt+c 2sinωt;
Step 6, represents the equivalent impedance parameter obtained in step 4 with the coefficient of the sine and cosine expression formula obtained, as follows
c 1=a 1+R Ab 1-ωb 2L A
c 2=a 2-R Ab 2-ωb 1L A
c 1=a 3+R Bb 3-ωb 4L B
c 2=a 4+R Bb 4+ωb 3L B
L A = a 2 b 1 + b 2 c 1 - b 2 a 1 - b 1 c 2 ω ( b 1 2 + b 2 2 )
R A = a 2 b 2 - b 2 c 2 + b 1 a 1 - b 1 c 1 b 1 2 + b 2 2
L B = a 3 b 4 - b 4 c 1 - b 3 a 4 + b 3 c 2 ω ( b 3 2 + b 4 2 ) ;
R B = b 4 c 2 - a 4 b 4 - b 3 a 3 + b 3 c 1 b 1 2 + b 2 2
Step 7, utilizes the expression formula obtained in step 6 as constraint condition, and the parameter calculated in step (4), as initial value, is optimized, and obtains each equal value impedance exact value;
Step 8, passes through
U N = i O R O + L O di o dt U N = U A - I A * Z A
The drift voltage of calculated load side dummy neutral;
Step 9, calculates three-phase load unbalance degree, together with three-phase load size and dummy neutral drift voltage, characterizes the security of low pressure electrical network, voltage unbalance factor ε u, voltage unbalance factor ε icomputing formula as follows:
ϵ u = U 2 U 1 × 100 %
Wherein: U 1the positive-sequence component amplitude root mean square of-three-phase voltage;
U 2the negative sequence component amplitude root mean square of-three-phase voltage;
ϵ i = I 2 I 1 × 100 %
Wherein: I 1the positive-sequence component amplitude root mean square of-three-phase current;
I 2the negative sequence component amplitude root mean square of-three-phase current;
I 2the negative sequence component amplitude root mean square of-three-phase current;
First the positive-negative sequence component of the actual pressure drop on three-phase load is asked for:
U 1 N • U 2 N • U • 0 N = 1 3 1 a a 2 1 a 2 a 1 1 1 * U A • - U N • U B • - U N • U C • - U N •
Wherein: a=e j120 °;
positive and negative, the zero-sequence component of-three-phase voltage,
for the voltage phasor utilizing the matching of stable state sampled point to obtain, for the dummy neutral voltage phasor that the corresponding time is obtained,
Then positive-negative sequence amplitude is got ask degree of unbalancedness ε uN, ε u, ε ivalue larger, the degree of unbalancedness of load is larger, larger to the threat of the safe operation with electrical network, current unbalance factor ε iwhen calculating identical, improve voltage unbalance factor ε ucalculating, utilize the actual load pressure drop obtained in preceding step to ask for degree of unbalancedness ε uN, with a high credibility.
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