CN105808830A - Method for calculating thermal ageing states of cables by utilizing load current of cables - Google Patents

Abstract

The invention provides a method for calculating thermal ageing states of cables by utilizing load current of cables. The method comprises the following steps: obtaining a cable laying type and environment parameters; measuring geometric structural parameters of cables and reading heat capacities and heat resistances corresponding to layers on the cables as well as resistance parameters of metal parts on the cables, wherein the environment parameters, the heat capacities, the heat resistances and the resistance parameters are used as calculation parameters; establishing a mathematic model of relationship between the load currents and temperatures of the cables in a cable group according to the calculation parameters and the geometric structure parameters of the cable in the cable group, and calculating the real-time temperatures of the cables by utilizing a real-time iteration manner; calculating the ageing additive effect of the cables according to the obtained real-time temperatures of the cables. Compared with the conventional online monitoring method, the method disclosed in the invention can be used for improving the condition that the conventional online monitoring method is large in interference and more in false alarms, and carrying out real-time online monitoring on all the cables of power grid systems so as to lighten the load of offline test work and enable the operation and maintenance personnel to pointedly detect the cables in high-danger areas.

Description

A kind of method utilizing cable load current to calculate cable heat ageing state

Technical field

The present invention relates to high voltage and insulation technology field, specifically a kind of method utilizing cable load current to calculate cable heat ageing state.

Background technology

Cable can because causing that the situation of heat ageing occurs in XLPE insulating barrier under the temperature action of load current electric current generation in During Process of Long-term Operation.Along with the time of operation increases, cable heat ageing degree there will be accumulative effect, causes insulating properties progressively to decline, if not in time it being estimated and monitoring, finally has very big probability the situation that cable body punctures occur, causes the generation of pernicious power outage.

The on-line monitoring of existing cable has had relatively broad application, but Main Means relies on the change monitoring all kinds of weak currents to judge its state of insulation.The mode of this collection weak current is due to on-the-spot all kinds of electromagnetic interference, and the electric current that system acquisition obtains is not accurate enough, causes the situation that system is reported by mistake often occur；Simultaneously the relevant on-line monitoring equipment of new clothes is not only relatively cumbersome, costly, the problem also having certain equipment dependability.From the feedback of a line scheduling and attendant, on-line monitoring also needs to strengthen further and improve.

Off-line (power failure) detects compared to on-line monitoring, it is possible to more getting rid of excessive interference, test result is more accurate.But, one provincial capital's rank city transmission & distribution cable consumption is substantially all at more than 8000km, megalopolis cable consumption is more, according to conventional offline diagnostic means, it is all carried out periodic detection workload excessively huge, it is difficult to carry out in finding after practice, therefore can only testing for some importance circuit in actually used, coverage rate is limited.

For this present situation, a kind of method of real-time monitoring cable heat ageing state is needed in power system badly, it is possible to manage the accumulative effect of system-computed cable heat ageing based on existing load current, it is achieved the basis monitoring of state of insulation.

Summary of the invention

Present invention aims to the deficiencies in the prior art, it is provided that a kind of method utilizing cable load current to calculate cable heat ageing state, owing to only needing to access existing load current monitoring system, therefore its method is simple；By monitoring the load current of all cables, it is not necessary to personal monitoring, it is possible to effectively alleviate the burden of field personnel；Utilizing the existing load current monitoring accuracy of system and reliability, specific aim improves the shortcoming that the false alarm frequency of existing cable on-line monitoring technique is high.

A kind of method utilizing cable load current to calculate cable heat ageing state, comprises the steps:

The artificial cable laying type set of step one, basis, reads respective cable and lays the ambient parameter that type is corresponding, and described ambient parameter includes convection transfer rate а, soil near-bottom temperature T_soil, soil thermal resistivity ρ, air real time temperature T_amb；

Step 2, the reading each layer geometrical structure parameter of cable, read the resistance parameter of the corresponding thermal capacitance of cable above layers, thermal resistance and metal part, and wherein ambient parameter, thermal capacitance, thermal resistance and resistance parameter are as calculating parameter；

Step 3, according to the geometrical structure parameter of cable in described calculating parameter and Cable Group, set up the limited element calculation model of relation between load current and the temperature of each cable in Cable Group, use real-time iterative mode calculate cable real time temperature T (x, y, t)；

Step 4, when obtain each region real time temperature T of cable (x, y, after t), by read core temperature T_coreCalculate cable life loss coefficient under current Current Temperatures.

Further, described geometrical structure parameter includes conductor diameter D, conductor shielding thickness δ₁, insulation thickness t₁, insulation shielding thickness t_i2, metallic shield thickness t_s, sheath thickness t₂, its thickness δ, outer jacket thickness t₃。

Further, described step 3 particularly as follows:

Read each layer thermal capacitance of cable, thermal resistance, inverse-heat conductivity the D of thermal resistivity is obtained by resistance parameter, utilize thermal capacitance storage capacity to solve thermal diffusivity K, set up relation between resistance parameter R and caloric value Wc, shown in the such as formula of the relation between resistance parameter R and caloric value Wc (1):

Wc=I²R(1)

Wherein I is cable load current；

Resistance parameter R is when temperature difference shown in its resistance change such as formula (2) (3):

R=R'(1+Y_S+Y_P)(2)

R'=R₀(1+α₂₀(θ-20))(3)

In formula:

R ' cable core is the D.C. resistance of unit length under maximum operating temperature, unit Ω；

Y_S、Y_PThe coefficient of kelvin effect and kindred effect, is then 0 unless otherwise noted；

R₀Cable core is the D.C. resistance of unit length when 20 DEG C, unit Ω；

α₂₀Core material with 20 DEG C be benchmark temperature-coefficient of electrical resistance；

The maximum operating temperature of θ cable conductor, unit DEG C；

Caloric value Wc according to gained obtains the endogenous pyrogen heat generation rate q in face, cable core territory_v=Wc/S₀, wherein S₀For cable core region area；

Geometrical structure parameter according to each Rotating fields sets up cable domain S, and 1 x, y on the S of its cable domain, place can conduct heat formula, as shown in formula (4):

$\frac{{\∂}^{2}T}{\∂x^2}+\frac{{\∂}^{2}T}{\∂y^2}+\frac{q_v}{D}=K\frac{\∂T}{\∂t}---\left(4\right)$

Wherein T represents the temperature of cable core, and D is each region heat conductivity, and K is each region thermal diffusivity, q_vFor endogenous pyrogen heat generation rate；

According to the aforementioned ambient parameter the obtained boundary condition as formula (4) domain S, in this, as basis, it is carried out variation, obtains shown in functional I such as formula (5):

Wherein, L is the border of region S；

Functional I is divided in the S of region limited unit, and by its discretization, obtains formula (6):

$I\left(T\right)=\underset{e=1}{\overset{NE}{\Σ}}{I}_e={\stackrel{\‾}{T}}^T\stackrel{\‾}{KT}-2{\stackrel{\‾}{T}}^T\stackrel{\‾}{P}---\left(6\right)$

Wherein,For stiffness matrix,For heat source density matrix, NE is unit number, finally solve the extreme value of functional I can try to achieve each region real time temperature T of cable (x, y, t).

Further, described step 4 particularly as follows:

Paper, not drip paper, crosslinked polyethylene are impregnated for flame retardant type viscosity, shown in its relative thermal ageing rate such as formula (7), (8), (9):

$V_{oil-paper-Loss}=2^{\frac{T_{core}-60}{6}}---\left(7\right)$

$V_{paper-Loss}=2^{\frac{T_{core}-65}{6}}---\left(8\right)$

$V_{XLPE-Loss}=2^{\frac{T_{core}-90}{6}}---\left(9\right)$

According to relative thermal ageing rate, utilize following formula to calculate and obtain N number of time interval or section (t₁-t₂) interior insulant aging life-span loss L be:

$L=\underset{n=1}{\overset{N}{\Σ}}{V}_n\×t_n$ Or $L=\underset{t_1}{\overset{t_2}{\mathrm{\Σ}}}V\left(t\right)dt---\left(10\right).$

Further, also include step 5, be shown in the way of life loss duration and set alarm threshold value.

The present invention is by calling this mode of load current of monitoring system acquisition, tradition on-line monitoring can be prevented effectively from and gather the problem that weak current is subject to external disturbance, compare off-line means coverage rate wider simultaneously, be effectively improved the work efficiency of a line attendant.

By temperature, the present invention calculates that aging is that its rate of ageing increases along with temperature and increases according to Y class B insulation and following, current educational circles most people often raise 6 DEG C after thinking and exceeding running temperature, and its rate of ageing doubles.

Accompanying drawing explanation

Fig. 1 is the schematic flow sheet that the present invention utilizes the method for cable load current reckoning cable heat ageing state；

Fig. 2 is limited the discrete unit that the present invention solves cable area during temperature.

Detailed description of the invention

Below in conjunction with the accompanying drawing in the present invention, the technical scheme in the present invention is clearly and completely described.

With reference to Fig. 1, the present invention provides a kind of method utilizing cable load current to calculate cable heat ageing state, comprises the steps:

The artificial cable laying type (such as direct-burried, comb, groove or tunnel) set of step one, basis, reads respective cable and lays the ambient parameter that type is corresponding, and described ambient parameter includes convection transfer rate а, soil near-bottom temperature T_soil, soil thermal resistivity ρ, air real time temperature T_amb。

Step 2, the reading each layer geometrical structure parameter of cable, described geometrical structure parameter includes conductor diameter D, conductor shielding thickness δ₁, insulation thickness t₁, insulation shielding thickness t_i2, metallic shield thickness t_s, sheath thickness t₂, its thickness δ, outer jacket thickness t₃；Read the resistance parameter of the corresponding thermal capacitance of cable above layers, thermal resistance and metal part.Wherein, ambient parameter, thermal capacitance, thermal resistance and resistance parameter are as calculating parameter.

Step 3, according to the geometrical structure parameter of cable in described calculating parameter and Cable Group, set up the limited element calculation model of relation between load current and the temperature of each cable in Cable Group, use the mode of real-time iterative to calculate cable real time temperature.

Concrete, read each layer thermal capacitance of cable, thermal resistance, inverse-heat conductivity the D of thermal resistivity is obtained by resistance parameter, thermal capacitance storage capacity is utilized to solve thermal diffusivity K and set up relation between resistance parameter R and caloric value Wc, shown in the such as formula of the relation between resistance parameter R and caloric value Wc (1):

Wc=I²R(1)

Wherein, R is the AC resistance of conductor flat cable length, unit Ω；I is cable load current, and unit A, load current is collected by SCADA system.

Wherein, resistance parameter R is when temperature difference, and its resistance value is also varied from, as shown in formula (2) (3).

R=R'(1+Y_S+Y_P)(2)

R'=R₀(1+α₂₀(θ-20))(3)

In formula:

R ' cable core is the D.C. resistance of unit length under maximum operating temperature, unit Ω；

Y_S、Y_PThe coefficient of kelvin effect and kindred effect, is then 0 unless otherwise noted；

R₀Cable core is the D.C. resistance of unit length when 20 DEG C, unit Ω；

α₂₀Core material with 20 DEG C be benchmark temperature-coefficient of electrical resistance；

The maximum operating temperature of θ cable conductor, unit DEG C.

For the caloric value Wc of gained, it can be obtained q in face, cable core territory_vFor endogenous pyrogen heat generation rate q_v=Wc/S₀, wherein S₀For cable core region area.For cable each position temperature T (x, y, t), wherein x and y represent on the S of cable domain a bit.

Geometrical structure parameter (conductor diameter D according to each Rotating fields₀, conductor shielding thickness δ₁, insulation thickness t₁, insulation shielding thickness t_i2, metallic shield thickness t_s, sheath thickness t₂, its thickness δ, outer jacket thickness t₃) setting up cable domain S, 1 x, y on the S of its cable domain, place can conduct heat formula, as shown in formula (4):

$\frac{{\∂}^{2}T}{\∂x^2}+\frac{{\∂}^{2}T}{\∂y^2}+\frac{q_v}{D}=K\frac{\∂T}{\∂t}---\left(4\right)$

Wherein, T represent the temperature of cable core/DEG C；D is each region (i.e. the region such as conductor, each screen layer, insulating barrier, interior outer jacket, armouring, lower with) heat conductivity/W (m DEG C)^-1；K is each region thermal diffusivity/s²·m^-2；q_vFor endogenous pyrogen heat generation rate/W m³, current real-time current that endogenous pyrogen heat generation rate is obtained by SCADA system, utilize Joule's law to calculate and obtain.Wherein heat conductivity D is the inverse of thermal resistivity, and thermal diffusivity is determined by heat absorption capacity coefficient.

According to aforementioned obtain by laying the convection transfer rate а of patterns affect, soil near-bottom temperature T_soil, soil thermal resistivity ρ, air real time temperature T_ambEtc. the ambient parameter boundary condition as formula (4) domain S, in this, as basis, it is carried out variation, it is possible to obtain shown in functional I such as formula (5):

Wherein, L is the border of region S.

Functional I is divided in the S of region limited unit, with reference to Fig. 2, and by its discretization, it is possible to obtain formula (6):

$I\left(T\right)=\underset{e=1}{\overset{NE}{\Σ}}{I}_e={\stackrel{\‾}{T}}^T\stackrel{\‾}{K}\stackrel{\‾}{T}-2{\stackrel{\‾}{T}}^T\stackrel{\‾}{P}---\left(6\right)$

Wherein,For stiffness matrix,For heat source density matrix, NE is unit number, finally solve the extreme value of functional I can try to achieve each region real time temperature T of cable (x, y, t).

Step 4, when obtain each region real time temperature T of cable (x, y, after t), by read core temperature T_coreCalculate cable life loss coefficient under current Current Temperatures.

Owing to cable inner insulation material is by the combined influence of the factor such as electric, mechanical, chemical, its service life decides the service life of whole section of cable.Aging or the deterioration process temperature of the chemical property that insulant subjects always is then influence factor the most direct, most important.If only considering the impact of temperature, for conventional flame retardant type viscosity dipping paper, not drip paper, crosslinked polyethylene, shown in its relative thermal ageing rate such as formula (7), (8), (9).

$V_{oil-paper-Loss}=2^{\frac{T_{core}-60}{6}}---\left(7\right)$

$V_{paper-Loss}=2^{\frac{T_{core}-65}{6}}---\left(8\right)$

$V_{XLPE-Loss}=2^{\frac{T_{core}-90}{6}}---\left(9\right)$

According to relative thermal ageing rate, it is possible to utilize following formula to calculate and obtain N number of time interval or section (t₁-t₂) interior insulant aging life-span loss L be:

$L=\underset{n=1}{\overset{N}{\Σ}}{V}_n\×t_n$ Or $L=\underset{t_1}{\overset{t_2}{\∫}}V\left(t\right)dt---\left(10\right)$

Therefore just through type (7) to formula (10) the life loss correspondence time that under actual condition and environmental condition, cable insulation material causes can be obtained due to temperature.

Step 5, it is shown in the way of life loss duration and sets alarm threshold value.Owing to the cable depreciable life is 20 years, and its actual life is generally 30 years, therefore sets cable life alarm threshold value lost time as 1.5 times of currently running period.

Such as, if XLPE cable overload electric current continuous service 1h, when less than alarm threshold value, its maximum operating temperature is up to 93.5 DEG C；If considering, in 24h, overload electric current runs 3h, then maximum operating temperature is up to 102 DEG C.

Claims (5)

1. one kind utilizes the method that cable load current calculates cable heat ageing state, it is characterised in that comprise the steps:

The artificial cable laying type set of step one, basis, reads respective cable and lays the ambient parameter that type is corresponding, and described ambient parameter includes convection transfer rate а, soil near-bottom temperature T_soil, soil thermal resistivity ρ, air real time temperature T_amb；

Step 2, the reading each layer geometrical structure parameter of cable, read the resistance parameter of the corresponding thermal capacitance of cable above layers, thermal resistance and metal part, and wherein ambient parameter, thermal capacitance, thermal resistance and resistance parameter are as calculating parameter；

Step 3, according to the geometrical structure parameter of cable in described calculating parameter and Cable Group, set up the limited element calculation model of relation between load current and the temperature of each cable in Cable Group, use real-time iterative mode calculate cable real time temperature T (x, y, t)；

Step 4, when obtain each region real time temperature T of cable (x, y, after t), by read core temperature T_coreCalculate cable life loss coefficient under current Current Temperatures.

2. utilize the method that cable load current calculates cable heat ageing state as claimed in claim 1, it is characterised in that: described geometrical structure parameter includes conductor diameter D, conductor shielding thickness δ₁, insulation thickness t₁, insulation shielding thickness t_i2, metallic shield thickness t_s, sheath thickness t₂, its thickness δ, outer jacket thickness t₃。

3. utilize as claimed in claim 1 cable load current calculate cable heat ageing state method, it is characterised in that: described step 3 particularly as follows:

Read each layer thermal capacitance of cable, thermal resistance, inverse-heat conductivity the D of thermal resistivity is obtained by resistance parameter, utilize thermal capacitance storage capacity to solve thermal diffusivity K, set up relation between resistance parameter R and caloric value Wc, shown in the such as formula of the relation between resistance parameter R and caloric value Wc (1):

Wc=I²R(1)

Wherein I is cable load current；

Resistance parameter R is when temperature difference shown in its resistance change such as formula (2) (3):

R=R'(1+Y_S+Y_P)(2)

R'=R₀(1+α₂₀(θ-20))(3)

In formula:

R ' cable core is the D.C. resistance of unit length under maximum operating temperature, unit Ω；

Y_S、Y_PThe coefficient of kelvin effect and kindred effect, is then 0 unless otherwise noted；

R₀Cable core is the D.C. resistance of unit length when 20 DEG C, unit Ω；

α₂₀Core material with 20 DEG C be benchmark temperature-coefficient of electrical resistance；

The maximum operating temperature of θ cable conductor, unit DEG C；

Caloric value Wc according to gained obtains the endogenous pyrogen heat generation rate q in face, cable core territory_v=Wc/S₀, wherein S₀For cable core region area；

Geometrical structure parameter according to each Rotating fields sets up cable domain S, and 1 x, y on the S of its cable domain, place can conduct heat formula, as shown in formula (4):

$\frac{{\∂}^{2}T}{\∂x^2}+\frac{{\∂}^{2}T}{\∂y^2}+\frac{q_v}{D}=K\frac{\∂T}{\∂t}---\left(4\right)$

Wherein T represents the temperature of cable core, and D is each region heat conductivity, and K is each region thermal diffusivity, q_vFor endogenous pyrogen heat generation rate；

According to the aforementioned ambient parameter the obtained boundary condition as formula (4) domain S, in this, as basis, it is carried out variation, obtains shown in functional I such as formula (5):

Wherein, L is the border of region S；

Functional I is divided in the S of region limited unit, and by its discretization, obtains formula (6):

$I\left(T\right)=\underset{e=1}{\overset{NE}{\Σ}}{I}_e={\stackrel{\‾}{T}}^T\stackrel{\‾}{K}\stackrel{\‾}{T}-2{\stackrel{\‾}{T}}^T\stackrel{\‾}{P}---\left(6\right)$

Wherein,For stiffness matrix,For heat source density matrix, NE is unit number, finally solve the extreme value of functional I can try to achieve each region real time temperature T of cable (x, y, t).

4. utilize as claimed in claim 1 cable load current calculate cable heat ageing state method, it is characterised in that: described step 4 particularly as follows:

Paper, not drip paper, crosslinked polyethylene are impregnated for flame retardant type viscosity, shown in its relative thermal ageing rate such as formula (7), (8), (9):

$V_{oil-paper-Loss}=2^{\frac{T_{core}-60}{6}}---\left(7\right)$

$V_{paper-Loss}=2^{\frac{T_{core}-65}{6}}---\left(8\right)$

$V_{XLPE-Loss}=2^{\frac{T_{core}-90}{6}}---\left(9\right)$

According to relative thermal ageing rate, utilize following formula to calculate and obtain N number of time interval or section (t₁-t₂) interior insulant aging life-span loss L be:

$L=\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{V}_n\×t_n$ Or $L=\underset{t_1}{\overset{t_2}{\∫}}V\left(t\right)\mathrm{dt}---\left(10\right).$

5. utilize the method that cable load current calculates cable heat ageing state as claimed in claim 1, it is characterised in that also include step 5, be shown in the way of life loss duration and set alarm threshold value.