WO1996027802A2

WO1996027802A2 - Measuring device for measuring a current flowing in the overhead wires of a medium-voltage line

Info

Publication number: WO1996027802A2
Application number: PCT/FI1996/000138
Authority: WO
Inventors: Timo JÄMIÄ
Original assignee: Powex Oy
Priority date: 1995-03-08
Filing date: 1996-03-07
Publication date: 1996-09-12
Also published as: FI951071A0; AU4833596A; WO1996027802A3; FI100828B; FI951071A

Abstract

The invention relates to a measuring device for measuring a current flowing in medium-voltage lines from the ground by means of a measuring device remote from the wires. The measuring device includes a substantially horizontally held coil (1) for supplying a voltage (E) which is proportional to the variations of flux density (ζ) passing through a surface area (A) defined by the coil (1), elements (4, 5, 6) for supplying a signal which is proportional to a distance between the measuring device and the wires, elements (7, 8, 9) for supplying a signal which is proportional to a gap between the wires, and elements (3) for correcting said voltage (E) on the basis of the distance and gap signals, such that the corrected voltage represents a current flowing in the wires.

Description

Measuring device for measuring a current flowing in the overhead wires of a medium-voltage line.

The present invention relates to a measuring device of the type defined in the preamble of claim 1 for measuring a current flowing in the overhead wires of a medium-voltage line from the ground by means of a measuring device located at a distance from the wires.

Thus far, there has not been available a measuring device capable of measuring currents flowing in the overhead wires of medium-voltage lines from the ground, e.g. at a distance of 10 m from the wires. However, there is clearly a major demand for such measuring as the indicator can be used e.g. for the following applications:

1. Measuring of short-circuit currents from line branches in fault detection (measuring of a short-circuit current with a duration of 100-200 ms). 2. Control measurements to be carried out prior to the opening of disconnectors, such that the opening is not effected at an excessively high load.

3. Controlling the loads of exceptional supply loops made because of fault and correction situations in order not to exceed the current-bearing capacity of the wires.

4. Monitoring the power in special situations and control¬ ling e.g. the assessed currents of mains data systems.

5. Finding the so-called "0"-point between transforming and switching stations from trunk lines (e.g. in a switching situation) .

6. Confirmation of an asymmetry between conduction currents (e.g. a cut-off wire).

An object of the invention is to provide a measuring device which is capable of measuring a current flowing in the overhead wires of a medium-voltage line from the ground without contacting the lines. This object is achieved on the basis of the characterizing features set forth in the annexed claim 1.

Claim 2 discloses a preferred embodiment of the invention which is also capable of confirming an asymmetry between conduction currents.

The invention will now be described in more detail with reference made to the accompanying drawings, in which

fig. 1 shows a functional block diagram for a measuring device of the invention;

fig. 2 shows a cross-section for a line to be measured, depicting the effect of flux densities produced by conduction currents on a measuring device located at point P, when the instantaneous values of phase currents are equal to those in fig. 3 at moment t;

fig. 3 shows phase currents for three phase advances in a line; and

fig. 4 is a curve illustrating a vertical summation compo¬ nent B for a flux density produced by phase cur- rents.

Examined first is the type of flux density (direction and strength) produced at a measuring point P by currents flow¬ ing in phase advances. In the case of fig. 2, a horizontal crossar 13 included in a column 12 supports three phase advances in parallel. As shown in fig. 3, there is a phase shift of 120° between phase currents i*-, i₂ and i₃. At an examination moment t, the currents i*_| and i₃ are equal but reversed and the current i₂ is 0. At the examination moment, the currents i*- and i₃ produce at point P flux densities B-, and B₃, the flux density indicated by a vertical arrow B being created as a sum thereof. A flux density produced by the phase current i₂ at point P is horizontal but, at the examination moment t, the current i₂ and, respectively, the flux density B₂ is 0.

Since the sum vector B of a flux density produced by the end wires or advances is vertical, the current of these wires must be measured with a coil, having its plane of winding in a horizontal plane. Relative to such a coil, the current flowing in each current conductor produces the following flux densities:

_B, - _{μ l_) __g__ ^μ- ⁴ - ^{π •} 1^0"'

2πr₁ α = arctn (1/r) r₁ = r/cos α r₂ = r

B₂ = (μ ^ ) / 0° ^r3 - ^r/^{cos σ}

2πr₂

B₃ = (μ

2πr-

The sum vector of flux density at point P is

B = B*-, + B₂ + B₃ = ' ____ + B₂ /0° + B₃ ____ .

When the coil has a surface area A, the flux density passing therethrough is = B/A and the coil has an inducing voltage which is

Δ Φ f = 50 Hz =■ Δt = 5 ms E = N • —— ₍₃₆₀o _{= 20 ms)}

Δ t Thus, a coil, having a number of turns which is N and a loop surface area of A, produces at measuring point P a voltage

(B/A - 0)

E = N

5 ms

This voltage induced in a winding is amplified in an ampli¬ fier 2 and then corrected in a unit 3 so as to take into account a distance r of the measuring point P and a gap 1 between the conductors. The distance r can be indicated manually by means of a potentiometer 4 or automatically by means of an ultrasonic unit 5. Measuring devices such as the ultrasonic unit 5 are commercially available. Until now, however, such devices have only been used for measuring the height of conductors based on a time spent for the echo of an ultrasonic signal emitted from the measuring device. The distance information is processed in a unit 6, which deliv¬ ers to the unit 3 a suitable correction signal for correct- ing the voltage received from amplifier 2.

As revealed by the above examination, the relative distance 1 between the conductors (because of an angle α) has also an effect on the vertical component B of flux density. In order to make a necessary correction to a voltage delivered by the amplifier 2, the information relevant to distance 1 is produced manually by means of a potentiometer 7. This infor¬ mation is generally available since the dimensions of a crossarm structure are usually known. If necessary, the measurement of distance 1 can also be effected automatical¬ ly, in which case the measuring device includes an electric- field directional measuring unit 8, capable of determining the angle α on the basis of the direction of a force experi¬ enced by the electric charge. A unit 9 is used for convert- ing the information relating to distance 1 into a signal suitable for correcting a voltage that is based on a voltage E received from the coil 1 and boosted by the amplifier 2. From the correction unit 3 the signal is carried by way of an AD-converter 10 to an LCD-display 11 scaled directly to amperes. For example, the measuring result of a continuous load current represents the RMS (Root Mean Square) value of the current. The measuring result of a short-circuit current represents an instantaneous peak value.

In order to use the measuring device also for detecting an asymmetry appearing between various phases (e.g. when a wire is cut off), said device is preferably provided with a second coil, having a plane of winding which is substantial¬ ly perpendicular to the plane of coil 1. Thus, the second coil supplies a voltage which is proportional to variations of the horizontal flux density component B₂ and produced in the exemplary case by the current i₂ of the middle conduc¬ tor. Following the amplification, this voltage is added to a signal downstream of the amplifier 2, the sum signal being based on the flux density

B = Bi + B₂ + B₃.

Then, if the phase currents indicate asymmetry, it will respectively show as an asymmetry relative to the zero level in the curve of flux density sum vector B depicted in fig. 4. Naturally, the existence of a second-coil signal can also be monitored directly prior to addition to the signal of coil 1 , the appearance of a second-coil signal indicating a load asymmetry since the symmetrical phase currents have a very small horizontal flux density component regardless of the moment it is examined.

In addition to its main application, the invention can be naturally used for other applications as well, such as for locating live ground cables and for finding the zero faults in AMKA-conductors.

Claims

1. A measuring device for measuring a current flowing in the overhead wires of a medium-voltage lines from the ground by means of a measuring device remote from the wires, c h a r a c t e r i z e d in that the measuring device includes a substantially horizontally held coil (1) for supplying a voltage (E) which is proportional to the varia¬ tions of flux density (Φ) passing through a surface area (A) defined by the coil (1), elements (4, 5, 6) for supplying a signal which is proportional to a distance between the measuring device and the wires, elements (7, 8, 9) for supplying a signal which is proportional to a gap between the wires, and elements (3) for correcting said voltage (E) on the basis of the distance and gap signals, such that the corrected voltage represents a current flowing in the wires.

2. A measuring device as set forth in claim 1, c h a r ¬ a c t e r i z e d in that the device includes a second coil, having a plane of winding which is substantially perpendicular relative to said horizontally held coil (1), the second coil supplying a voltage which is proportional to the variations of a horizontal flux density component.