DE10146294C1 - Network compensation determination method for tuning Petersen coil uses equations obtained from measured phase voltages and line currents for calculation of capacitive zero current - Google Patents

Abstract

The method uses measurement of the 3 line-earth voltages and the 3 line currents at a given measuring point, for calculation of symmetrical components, with adjustment of the zero impedance before re-measuring the 3 phase voltages and 3 line currents and re-calculation of the symmetrical components. A series of equations containing unknown real and imaginary components are formulated and solved for calculation of the capacitive zero current of the network, with corresponding adjustment of the Petersen coil for compensation of the latter.

Description

The present invention relates to the tuning of the earth fault canceller, according to nem inventor is also referred to as a Petersen coil or P coil, also during the single-pole earth fault in three-phase networks by measuring the conductor currents and the Lei ter-earth voltage at the connection point of the earth fault quenching coil to the three-phase network.

Methods for tuning an earth leakage coil for resonance or a desired compensation value are currently only known for healthy networks. Some procedural ren, which work according to the resonance method, require a sufficient displacement voltage U ₀ z. B. EZR2 from the company Spezielektra.

The other methods, such as. B. according to WO 86/03350 or US 5 559 439 work according to egg nem reduced circuit diagram, so that the coordination of the earth fault canceller in the healthy Network for small displacement voltages is successful. In the process according to the patent US 5 559 439 the zero impedance of the network including the earth fault quenching coil in ge healthy condition measured. The asymmetry of the net is due to the differential measurement zes connected in parallel to the resonance circuit. As long as the asymmetry is very high, the error due to the asymmetry is negligible. A determination of the vote is however, this only applies to a healthy network and only up to a displacement voltage of approx. 10% feasible.

The same applies to the method according to patent WO 86/03350, in which it started is that the greatest displacement voltage is at the resonance point. Both  a common unbalanced load as well as the internal impedance of the transformer neglected. Because of this neglect, voting is only possible in a healthy network, as long as the impedance of the asymmetry is significantly higher than the transformer impedance Lich. In the case of a low-resistance earth fault, these simplifications are not more permissible.

In the patent DE 195 25 417 C2 is a combination of a stepped Earth fault quenching coil listed in connection with an auxiliary transformer. This combination he by generating a "compensation current" allows both compensation of the Reactive component as well as the usually remaining ohmic active current over the Fault location. The setting of the "compensation current" is done using known ter procedure for calculating the necessary parameters in the "healthy network". It will only described the possibility of complete compensation at the fault location, but not like the "compensation current" from the available measurements in the substation in the case an earth fault is set. A necessary readjustment of the "compensation stromes "due to the shutdown of power supplies during an earth fault is not be wrote.

US Pat. No. 5,699,219 A describes an earth-fault cancellation coil that can be switched in stages with thyristors or GTOs. In the described method, it is assumed that in the case of a low-resistance earth fault, the zero sequence voltage U ₀ and the zero current I ₀ in the substation are in phase. The longitudinal impedance of the line up to the fault point and the load current are neglected. It can be easily demonstrated that this method only works for low-resistance earth faults on short lines with no load current. With longer lines and large load currents via this line, the load current is coupled to the zero voltage. Is regulated to a phase minimum, an additional current is generated across the fault location in the size of the load current, which compensates for this coupling. This means that the current through the fault point is increased with this line for normal cable lengths and load currents. Also for the assumption that the zero sequence voltage is the greatest for low-resistance errors with ideal compensation, the influence of the longitudinal line impedance and the voltage component coupled into the zero-sequence system from the load current are neglected. This procedure also leads to an increase in the current at the fault location with the usual cable lengths and load currents.

In the patent specification DE 195 14 698 C1, the establishment of a circuit diagram for symmetrical components for the fault location for the purpose of estimating the fault distance for a rigidly grounded network with power from two points was shown. However, it was assumed for the calculation that the current across the fault location is divided in phase into two currents that flow over the respective zero impedance of the feed points. Regarding the faulty line, this is only true for one point between the two feeding points. The situation is exacerbated for deleted networks with single-phase supply, as shown in Figure 2.

Today's practice for limiting a single-pole earth fault depends on the Network structure. Can cable outlets be interconnected to form rings or if if they are already connected to rings, the fault is usually narrowed down by separating them of the ring at different points on the ring and observation on which exit the display of the earth fault drifts. Through this simple but time consuming Search method, the line segment affected by ground faults can be narrowed down.

However, are the line outlets switched as "stub lines", or if this Lei can no longer be switched to a ring, this is how it works above described narrowing process no longer. In this case, the lines must be removed be switched off during the stitch. By switching off line segments however, the line capacity to be deleted changed. This will flow over the Feh changed and can exceed specified limit values. Usually, among other things rem exceeded the deletion limit according to VDE 228. In addition, the increased currents exceeding the permissible step and touch voltages in the vicinity of the Feh effect.

For these reasons, the length of the line should be changed, especially during one standing earth fault, recognized as quickly as possible and the earth fault quenching coil so be re-tuned as quickly as possible.

This object is achieved by a method according to claim 1 solved.  

The known methods can only tune the earth fault cancellation coil during of healthy network operation. A determination of the necessary coordination points during an earth fault are excluded. The adjustment of the earth The closing coil is blocked during the earth fault.

With the new method, a calculation of the current detuning possible, which also causes an adjustment of the earth fault quenching coil to the desired tuning point during the earth fault. As a side Effectively you get an estimate of the fault resistance at the fault location, the Stro with the fault location and a rough estimate of the fault distance.

According to the invention, a method for determining the current "compensation" of a deleted network and readjusting the earth-fault cancellation coil, even during a single-pole, low-resistance earth fault, is characterized by the following steps:

• - Measuring the three phase-to-earth voltages U _1E , U _2E , U _3E and the three phase currents I _L1 , I _L2 and I _L3 at a measuring point A ₁ at a first point in time t ₁ .
• - Calculation of the symmetrical components U ₁ , U ₂ , U ₀ , I ₁ , I ₂ , I ₀ for the first time t ₁ according to known methods from the phase-to-earth voltages U _1E , U _2E , U _3E and the conductors I _L1 , I _L2 , I _L3
• - Change the zero impedance Z _P / 3 by d Z P / 3.
• - Measuring the three phase voltages and the three phase currents again at the measuring point A ₁ at a second point in time t ₂ .
• - Calculation of the symmetrical components U ₁ , U ₂ , U ₀ , I ₁ , I ₂ , I ₀ for the second point in time t ₂ according to known methods.
• - Establishing the equations for the time t ₁ , the unknowns being broken down into real and imaginary parts.
• - Establishing the equations for the time t ₂
• - Solving equations (1) to (6) separated by real and imaginary part with the result of the current through the fault location I _F and the contact resistance R _F
• - Calculation of the capacitive zero current of the network I _CE from I ₀ and the current via the fault location I _F.
• - Determination of the necessary setting of the earth fault quenching coil around this capacitive To compensate for electricity
• - Adjust the earth fault cancellation coil to the desired position

In the following, the invention will be exemplified using the following circuit diagrams wrote:

Figure 1 shows a quenched three-phase network with two outlets and a controller with measuring point A1 for tuning the earth fault quenching coil;

Figure 2 Representation of the above network using the symmetrical components.

Figure 3 reduction of the image 2 to the essential control components for the

Figure 4 reduction of the frame 3 by considering the remaining network.

Figure 5 Equivalent circuit for determining the current current via the fault location I _F and the value of the fault resistance R _F.

Figure 1 shows a three-phase network with two outlets. The feed takes place via a transformer. The zero impedance d Z _P / 3 can be changed via the star point of the supply transformer or with the help of a star point generator. Likewise, a change in the zero impedance can be made by another known three-phase or single-phase circuit. The change in zero impedance can also be single-phase z. B. by switching a resistor or a capacitor directly at the star point or with the help of a transformer. A change in the zero impedance is also possible by a small detuning of the earth fault quenching coil or with the help of a "current feed".

The representation was made for a deleted network, but the procedure is also for others Star point treatments are valid, such as B. high-resistance grounded networks, low-resistance grounded nets, BUD.  

The method is used to determine the current over the fault location and thus to determine the necessary adjustment of the earth fault quenching coil during the earth statements. The method is also suitable for very high-resistance errors. The measurement can can be repeated at any time.

Figure 2 shows the result of reshaping the network described above using the "symmetrical components". The forming takes place according to the known rules and is not described here. The coupling of the systems through the earth fault and the contact resistance R _{F is} already shown in the picture.

For further considerations in Figure 3, the residual network is reduced to the essential components. In order to illustrate the relevant current curve more clearly, the component systems are additionally nested.

For further considerations, the following simplifications are made for Figure 4

• - The capacities in the outgoing earth connection in the secondary and negative system are assigned to the load. The rest of the network is combined into C _1RL and Z _1RL .
• - During the transition from Figure 3 to Figure 4, the residual network in the co- and counter system is assigned to the load.
• - The factor x is the distance from the controller to the earth fault. The longitudinal impedances are taken into account using the line amounts Z ' _L. Since there are several outlets on the busbar, an average value of the lines used for estimating the error distance is assumed.
• - Due to the usual symmetry of the cables, the longitudinal impedances of the cables are sluggishly assumed in the co-system and counter system.
• - The distributed zero capacities of the earth leakage and the rest of the network are combined in the transition from Figure 3 to Figure 4 to a capacity C ₀ .
• - The load impedance is usually higher than the short-circuit impedance of the transformer. Usually, only a voltage drop of the order of 10% is permitted from generation E ₁ to the consumer. The impedance of the load is then at least 10 times greater than the short-circuit impedance of the transformer.

The three voltages U _1E , U _2E and U _3E to earth and the three conductor currents I _L1 , I _L2 and I _{L3 are} measured in the controller. From these, the symmetrical components U ₁ , U ₂ , U ₀ and I ₁ I ₂ and I ₀ can be calculated using the known methods. The zero voltage U ₀ and the zero current I ₀ can alternatively also be measured directly using the known methods.

Figure 5 shows the equivalent circuit for setting up the equations for the two times t ₁ and t ₂ . The times t ₁ and t ₂ are only a few seconds apart since the settling processes have subsided in the range of a few periods.

The advantages of the procedure are:

• - The measurement can be repeated at any time and checked for plausibility.
• - The measurement can also be used to check the detuning after switching networks be performed.
• - Statistical averages can also be carried out over several measurements, to reduce small disturbances caused by load changes.
• - The not negligible influence of the impedance of the transformer with low impedance Earth faults are taken into account. Only then will there be a regulation during the earth possible.
• - The disturbing influence of the counter voltage E _g with asymmetrical loads is compensated.

The normally unknown size of the source impedance of the transformer Z _1Tr can be determined either from the differential measurement of the co-system or from the differential measurement of the counter system using the following formula.

With the differential values for the co-system:

or for the opposite system:

According to Figure 5, the following three equations can be set up for the time t _1, the components to be calculated being divided into real and imaginary parts:

or for time t ₂ :

Legend

U

₁

, U ₂

, U ₀

, I ₁

, I ₂

, I ₀

: Symmetrical components
E _g

: Counter voltage due to asymmetrical loading
I _F

: Current over the fault location
_{_R}

: Real part
_{_I}

: Imaginary part
x: error distance measured from the controller installation location
k: ratio of line impedance to zero impedance
R _F

: Resistance of the fault location
Z

' _1L

: Average impedance covering of the line in the co-system
Z ₃

: Residual network parallel to the line impedance from the fault location to the load + load impedance
Z _1Tr

: Substitute impedance of the transformer in the component system

It is essential that the contact resistance R _F at the fault location can be regarded as an ohmic resistance.

If equations (9) to (14) are considered separately according to real part and imaginary part, then you get twelve equations for twelve unknowns. These can now be used with the known Algebra methods can be solved symbolically or numerically.

The result of the calculation is:
x: Distance to the point of failure
R _F : resistance of the fault location
E _{g_R +} iE _{g_I} : counter voltage
Z _{3_R +} iZ _{3_I} : Impedance of the line from the fault _location to the load + impedance of the load
I _{F_R_ (t1) +} iI _{F_I_ (t1)} : Current over the fault _location at time t ₁
I _{F_R_ (t2) +} iI _{F_I_ (t2)} : Current over the fault _location at time t ₂
Z _{1Tr_R +} iZ _{1Tr_I} : Substitute impedance of the transformer

The capacitive component of the zero current can now be calculated from these values:

I _CE = I ₀ - I _F (14)

The earth fault cancellation coil can now be set according to this current I _CE to be compensated and the desired compensation.

Claims (4)

1. Method for determining the current "compensation" of a deleted network and readjusting the earth-fault cancellation coil even during a single-pole low-resistance earth fault, characterized by the following steps:

• - Measuring the three phase-to-earth voltages U _1E , U _2E , U _3E and the three phase currents I _L1 , I _L2 and I _L3 at a measuring point A ₁ at a first point in time t ₁ .
• - Calculation of the symmetrical components U ₁ , U ₂ , U ₀ , I ₁ , I ₂ , I ₀ for the first time t ₁ from the phase-to-earth voltages U _1E , U _2E , U _3E and the phase currents I _L1 , I _L2 , I _L3
• - Change the zero impedance Z _P / 3 by d Z P / 3.
• - Measuring the three phase voltages and the three phase currents again at the measuring point A ₁ at a second point in time t ₂ .
• - Calculation of the symmetrical components U ₁ , U ₂ , U ₀ , I ₁ , I ₂ , I ₀ for the second point in time t ₂ from the phase-to-earth voltages U _1E , U _2E , U _3E and the phase currents I _L1 , I _L2 , I _L3 .
• - Establishing the equations for the time t ₁ , the unknowns being broken down into real and imaginary parts.
• - Establishing the equations for the time t ₂
  in which
  E _g the counter voltage due to asymmetrical loading,
  I _F the current over the fault location,
  _{_R} the real part,
  _{_I} the imaginary part
  x the error distance measured from the controller installation location,
  k the ratio of the line's co-impedance to zero impedance,
  R _F the resistance of the fault location,
  Z ' _1L the average impedance covering of the line in the co-system,
  Z ₃ the residual network parallel to the line impedance from the fault location to the load + load impedance,
  Z _1Tr the equivalent impedance of the transformer in the component system,

are.

• - Solve these equations (1) to (6) separately according to the real and imaginary part with the result of the current through the fault location I _F and the contact resistance R _F calculation of the capacitive zero-sequence current of the network I _CE from I ₀ and the current through the fault location I _F according to the formula:
  I _CE = I ₀ - I _F (7)
• - Determination of the necessary setting of the earth fault quenching coil to compensate for this capacitive current
• - Adjust the earth fault cancellation coil to the desired position

2. The method according to claim 1, characterized, that the change in the zero impedance takes place in the star point of the feed transformer. 3. The method according to claim 1, characterized, that the change in zero impedance is caused by detuning an earth fault canceller follows. 4. A method according to one of the preceding claims, characterized in that with the data obtained, the calculation of the contact resistance R _F, the loading calculation is carried out up to the fault point of current through the fault point I _F and an estimation of the fault distance from the controller.