JP2013055288A - Transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a transformer in which the impact of voltage drop on a system is below a fixed level at the time of power-up of the transformer and loss evaluation is enhanced by low loss.SOLUTION: A low loss transformer is designed so that the scale factor of an excitation rush current to a rated current is less than a predetermined vale while limiting the voltage drop rate to a design value, and the short circuit impedance has a value within a standard range determined by the voltage rank of the transformer or a value of the same level as that of an existing transformer. The rated flux density of the transformer is estimated to be less than 1.2 Tesla.

Description

  The present invention relates to a power transmission / distribution transformer.

Conventional transformers for power transmission / distribution have been designed and produced based on the JEC standard shown in Non-Patent Document 1 and the JIS standards shown in Non-Patent Documents 2 and 3 for 6 kV class transformers. Standard rated capacity, standard rated voltage, etc. are defined in Non-Patent Document 1, but short-circuit impedance is defined in Cited Document 4.
Regarding the loss of the transformer, the top runner method is adopted in the JIS standard and is oriented toward low loss, but the transformer loss is not defined in the JIS standard.

  On the other hand, regarding the inrush current of the transformer, there is no provision from the viewpoint of mitigating the influence of the power quality on the power system and other customer facilities. Furthermore, in any of the transformers, there are many cases where they are produced with the aim of downsizing according to the demand for space saving, which is a cause of concern that the magnetizing inrush current may increase.

Electrical Standards Investigation Committee Standard JEC2200-1995 Transformer Japanese Industrial Standard JISC4304-2005 6kV oil-filled transformer for power distribution Japanese Industrial Standard JISC4306-2005 6kV molded transformer for power distribution Electric Cooperative Research Vol.35, No.4, Standardization of Tap Switching Transformer for Load Distribution, page 6 Guidelines for grid interconnection technical requirements for ensuring power quality October 2004, pages 6-12 of the Agency for Natural Resources and Energy IEEJ Electrical Engineering Handbook 6th edition 2001 705 pages

As described above, in a transformer that conforms to the characteristics of the current JEC standard or JIS standard, there is no definition of a prescribed value for the magnetizing inrush current that occurs when the transformer is inserted into the power system (during charging).
Power supply system connected to the primary side of the transformer due to the magnetic flux density remaining in the transformer core when the transformer is turned on and depending on the voltage phase at the time of turning on, the transformer core is magnetically saturated and a large inrush current is generated. Causes a sudden voltage drop. As a result, a voltage drop also occurs on the secondary side of other transformers connected to the system, which may adversely affect the operation of equipment connected to the transformer secondary side, for example, precision equipment.
In this case, the magnitude of the voltage drop rate (maximum voltage drop) generated in the power supply system is a problem, not just the magnitude of the magnetizing inrush current.

  Regarding the degree of the voltage drop rate, for example, in Non-Patent Document 5, as a technical index, in the item of countermeasures against instantaneous voltage fluctuation of voltage fluctuation, it is also possible to deal with voltage fluctuation that occurs instantaneously at the time of parallel arrangement of power generation equipment or the like. If the voltage of the system is likely to deviate from the normal voltage by more than 10% due to the instantaneous voltage drop when the voltage is reduced in parallel, the installation of a current-limiting reactor, etc. Is presented. In other words, this indicates that the reference value may be cleared by changing the characteristics of the transformer.

In 6kV class transformers for power distribution, the protection fuse that protects the transformer may be blown off rarely due to a large instantaneous inrush current when the power is turned on, and the protective relay trips as problems during maintenance. In rare cases, replacement of protective fuses, changes in ratings, and changes in specifications of protective relays were considered.
The magnitude of the magnetizing inrush current is shown in Non-Patent Document 6 as an approximate value of the magnetizing inrush current (maximum value). FIG. 1 shows this, but the maximum peak value of the magnetizing inrush current is several times to several tens of times the transformer rated current effective value. When this value is reduced to, for example, 50%, the protective relay, the protection It can be seen that the fuse specification can be changed at the rating stage.

In order to solve these situations, the realization of a transformer having a characteristic that the voltage drop rate is 2% or less for a transmission transformer and 10% or less for a distribution transformer is considered as a direct means of solving the problem. .
A value of the transformer inrush current that satisfies this value is required, but the line impedance of the system itself is also included in the calculation factor of the voltage drop rate, so it is not determined as the limit value of the magnetizing inrush current itself.

  Furthermore, transformers that are designed to save energy while considering the global environment while reducing operating costs by reducing the loss of the transformer from the conventional ones are also required from the energy saving needs of the times. As the predetermined loss, if the loss of the conventional transformer or the loss determined from the efficiency defined in the JIS standard in the case of the primary voltage 6.6kV class transformer is the predetermined loss, the loss value is less than the predetermined loss. By doing so, an energy saving effect occurs.

  Moreover, if the replacement of the transformer is assumed as an application case, it is desirable to eliminate the need to change the protection device on the primary side of the transformer. For this purpose, it is desirable to make the short-circuit impedance of the transformer the same level as before in order to make the short-circuit current level of the transformer the same. Even in the case of a new installation, if the short-circuit impedance of the transformer is matched with the standard value, the primary-side protection device can be selected with a standard specification, and the power distribution equipment can be prepared at a low cost.

  As described above, the needs desired for the characteristics of the transformer are currently variously complicated. However, many of these problems, that is, (1) the voltage drop rate is made less than a predetermined value. (2) Make the magnetizing inrush current less than a predetermined value. (3) The loss is made less than a predetermined value. (4) Adjust the short-circuit impedance to a predetermined value. No transformer has been realized that has the ability to solve all these problems at once.

  In order to solve the above-described problems, in the present invention, the transformer short-circuit impedance is set to the conventional value level, the transformer short-circuit current is set to the conventional level, the magnetizing inrush current is suppressed, and the voltage drop of the system is suppressed to a predetermined value or less. An energy-saving transformer that reduces unnecessary loss and eliminates unnecessary operation of fuses and protective relays.

  The transformer according to the present invention can be expected to mitigate the influence on the power system and other customer facilities at the time of turning on the transformer, and good power quality with little disturbance in the power system can be obtained. In addition, unnecessary operation of the protective fuse and the protective relay can be prevented. In addition, the operating cost can be reduced by reducing the loss during operation.

An approach for realizing the problem of the present invention will be described below.
The transformer of this invention is implement | achieved by the design which added various design conditions.
(1) The voltage drop rate when the transformer is turned on is designed to be less than 2% for the high voltage system including the primary voltage 66 kV class and less than 10% for the high voltage system including the 6.6 kV primary voltage. .
This is a value that does not cause an abnormal operation in the equipment connected to the secondary side of another transformer due to voltage fluctuation of the primary system when the voltage of the transformer is turned on.

  When this value is realized, the value of the short-circuit impedance is set to the conventional level. Therefore, the design items and values used as a design guideline are less than a predetermined value when the maximum excitation inrush current value is approximately 50% or less. It will be confirmed whether it is a standard value for the rate. Note that the standard value of the short-circuit impedance is shown in Non-Patent Document 4.

The voltage drop rate is a value calculated by Equation 1 from the value obtained by adding the impedance of the upper system including the transformer primary side line to the impedance of the transformer winding when considering the simple equivalent circuit of FIG. is there.
Since the transformer core is magnetically saturated when the magnetizing inrush current is generated, the inductance value of the transformer represented by L _L changes from the magnetizing inductance value Lm to the air-core inductance value Lair.


Explanation of variables • R _B , L _B : Impedance of upper system • r ₁ : Primary winding resistance • L _C : Leakage inductance of transformer • L _L : Inductance reflecting the magnetic saturation characteristics of the transformer
(Lm → Lair)
Lm: Excitation inductance
Lair: Air-core inductance • V _S : Power supply voltage (effective value: Vs_rms)
· _{V p:} voltage of the interconnection points (effective value: Vp_rms)
I: Current flow

If R = R _B + r ₁ and L = L _B + L _C + L _L , then the basic equation of this simple equivalent circuit can be expressed as V _S = L (di / dt) + Ri. Then Vp is
From di / dt = (V _S −Ri) / L to V _p = V _S −L _B (di / dt) −R _{B ·} i
= (1−L _B / L) V _S + (L _B · R / L−R _B ) i
Here, the voltage drop rate [Delta] V from the effective value _{V p _rms} of _{V p} can be calculated as _{ΔV = (1-V p _rms} / V S_ rms) × 100%.

(2) The transformer short-circuit impedance is at the conventional level.
The value of the short-circuit impedance limits the short-circuit current in the transformer or in the secondary circuit, but the value is the same level as the standard value of Non-Patent Document 4 and the value of the transformer before update. This is because system protection and measurement can be applied to the conventional transformer, that is, the protective relay and measuring instrument can be applied as standard using the standard products and the set values and set values can be applied as before even after the transformer is updated.

(3) In order to reduce the loss of the transformer and suppress the magnetizing inrush current level to a predetermined level, the low rated magnetic flux density in the iron core and the high air core inductance in the winding are harmonized.
The short-circuit impedance of the transformer is a characteristic value that determines the short-circuit current. The first characteristic value that determines the magnetizing inrush current of the transformer is the air-core inductance of the winding.
Since the impedance of the short-circuit impedance is generally small, the short-circuit impedance is almost equal to the short-circuit reactance. When the frequency portion is separated, it becomes a short-circuit inductance and the difference in formula from the air-core inductance becomes clear.

Although both are calculated | required from the type | formula of Formula 3 and Formula 4, the view of the cross-sectional area part differs.



Where f: frequency N: number of turns, lm: average length of winding
α: gap between primary and secondary windings
d1, d2: widths of primary and secondary windings
k: Coil height correction factor
h: Winding height



Where Qc: iron core cross-sectional area
Bm: Rated magnetic flux density
Br: residual magnetic flux density
Bs: saturation magnetic flux density

The short-circuit inductance is related to the limitation of the short-circuit current, and depends on the distribution of the leakage magnetic flux in the vicinity of the transformer winding main insulation part. Is proportional to
The air-core inductance is related to the limit of the magnetizing inrush current and is proportional to the equivalent cross-sectional area of the winding on the side through which the magnetizing inrush current flows as shown in Equation 4.

  Next, the second characteristic value that determines the magnetizing inrush current is the rated magnetic flux density of the iron core. As shown in Equation 5, by reducing this, the room until the iron core reaches the saturation magnetic flux density can be increased. When residual magnetic flux is present when the transformer is turned on, the inrush current can be reduced depending on the polarity, but it is technically difficult to control the applied phase, and demagnetization is also effective in reducing the residual magnetic flux density. Since it takes time and effort, it is often performed without controlling these values.

In a normal design, Bm = 1.75 Tesla Br = Bm · 0.85 Bs = 2.02
On the other hand, in the present invention, when Bm = 1.2 Tesla, the value of Formula 5 is 2.97 to 1.40 of the normal design. That is, by setting Bm = 1.2 Tesla, the magnetizing inrush current value is about 50% of the original value, so the predetermined value of Bm may be less than 1.2 Tesla.

  If the value of the short-circuit impedance is the same as the conventional value, the equivalent cross-sectional area value of the normal leakage magnetic flux passing part is the same as in the conventional design even in the new design, but in order to limit the voltage drop rate due to the excitation inrush current, It is necessary to enlarge it.

  Furthermore, in recent years, it has become important to make the transformer loss characteristics energy-saving and to make a transformer that can be expected to reduce the environmental load. For this purpose, it is effective to reduce the no-load loss (iron loss) of the transformer that is consumed regardless of fluctuations in the load. This can be achieved by reducing the rated magnetic flux density of the iron core. If the rated magnetic flux density is set to less than Bm = 1.2 Tesla, the loss can be reduced.

The transformer according to the present invention has the advantage that the running cost is low because the voltage drop rate at the time of application is less than the allowable value and low loss compared to the conventional transformer.
As for the short-circuit impedance, the predetermined value is not a standard value but may be a conventional transformer value. This is a case where the setting conditions of the protection device selected according to the conventional transformer are not changed.

Examples designed according to the above policy are shown in the following examples.
The characteristics of the target transformer can be compared.
Note that the transformer of the present invention is designed by, for example, using the design system disclosed in Japanese Patent Application Laid-Open No. 2011-061171, which is part of the inventor of the present application, and then designing the detailed design. It is efficient to go and confirm. The voltage drop rate is numerically calculated in the design system by simulation of ATP (Alternative Transients Program).

Design example 1: Primary 66kV class distribution transformer Table 1 shows the specifications of the target transformer.
(Table 1) Target specifications (oil-filled transformer)
The design goals are as follows.
・ About influence by magnetizing inrush current,
The maximum voltage drop rate at the transformer power receiving point is targeted to be less than 2%.
・ About loss (no load loss, load loss)
[Calculation Formula] Pm = Wf + (m / 100) ² × Wrc
m: Load factor (%), here 50%
Table 2 shows the design results.
(Table 2) Design results

The analysis results on the excitation inrush current 66 kV side are as shown in FIGS. 4 and 5 when φr: 85%.
The calculated excitation inrush current (maximum peak value, absolute value) Ipmax was 647A for the actual machine and 318A for the trial design. In other words, the Ipmax value could be reduced to 49% of the actual machine by trial design.

The short-circuit capacity of the 66 kV system was 1000 MVA. The back impedance at this time is 13.48 mH.
The analysis results on the comparison 66 kV side of the voltage (effective value) are as shown in FIGS. 6 and 7 when φr: 85%.
The maximum voltage drop rate ΔVmax was 2.53% (64330V / 66000V) for the actual machine and 1.37% (65095V / 66000V) for the trial design. That is, by performing the trial design, the ΔVmax value could be reduced to 54% (less than 2%) of the actual machine.

Design example 2: Primary 6.6 kV class distribution transformer Table 3 shows the specifications of the target transformer.
(Table 3) Target specifications (oil-filled transformer)
The design goals are as follows.
・ About influence by magnetizing inrush current,
The maximum voltage drop rate at the transformer receiving point is targeted to be less than 10%.
・ About loss (no load loss, load loss)
Satisfies the energy consumption efficiency standard value (Pm) of JISC4304.
[Calculation Formula] Pm = Wf + (m / 100) ² × Wrc
m: Standard load factor (%)
40% for a capacity of 500 kVA or less
50% for capacity of 500kVA or more

Table 4 shows the design results.
(Table 4) Design results
This characteristic satisfies the standard value of the JIS standard.

The analysis results on the excitation inrush current side of 6.6 kV are as shown in FIGS. 8 and 9 when φr: 85%.
The calculated excitation inrush current (maximum peak value, absolute value) Ipmax was 1050 A for the actual machine and 464 A for the trial design. In other words, the Ipmax value could be reduced to 42% of the actual machine by trial design.

The constants of the higher system than the transformer are as follows.
・ Upper side of distribution substation 0.347mH
・ Distribution substation main transformer 1.04mH (10MVA Base, j7.5%)
・ High voltage distribution line Copper 150mm ² 0.122Ω / km, 1.04mH / km
The analysis results on the voltage drop rate 6.6 kV side are as shown in FIGS. 10 and 11 when φr is 85%.

  The maximum voltage drop rate ΔVmax was 17.9% (5418V / 6600V) for the actual machine, and 8.3% (6052V / 6600V) for the trial design. That is, by performing the trial design, the ΔVmax value could be reduced to 46% (less than 10%) of the actual machine.

Approximate range of excitation inrush current maximum peak value Simple equivalent circuit Magnetic saturation characteristics Calculation result of excitation inrush current of 66kV side actual machine 66 kV side trial design excitation inrush current calculation result 66kV side actual machine voltage drop rate calculation result 66kV side trial design voltage drop rate calculation results 6.6 kV side actual machine excitation inrush current calculation result 6.6kV side trial design excitation inrush current calculation result 6.6kV side actual machine voltage drop rate calculation results 6.6kV side trial design voltage drop rate calculation result

Claims (3)

A transformer whose short-circuit impedance is a standard range determined by the voltage class of the transformer,
The voltage drop rate is less than 2% in the extra high voltage system including the primary voltage 66 kV class, the primary voltage is less than 10% in the high voltage system including the 6.6 kV class, and the rated magnetic flux density is less than 1.2 Tesla. Transformer. It is a transformer to replace the existing transformer,
The short-circuit impedance is the same as the value of the existing transformer, the voltage drop rate is less than 2% in the high voltage system including the primary voltage 66 kV class, and the primary voltage is 10 in the high voltage system including the 6.6 kV class. Transformer with a rated magnetic flux density of less than 1.2 Tesla.   The transformer according to claim 2, wherein a loss is equal to or less than a loss of the existing transformer.