DE19621567A1 - Controlled loading device for earth current generation in AC network - uses transformer with secondary winding in open delta configuration closed by controlled resistance provided by combination of partial resistances - Google Patents

Abstract

The loading device (B) has a transformer (Tr) coupled to the AC network (N) with the transformer secondary winding (S1) connected in an open delta configuration, closed by a controlled resistance (W) for earth current generation. The resistance is provided by a number of partial resistances with a resistance value of between 0.75 and 0.05 Ohm, coupled together via switch elements, controlled by an automisation device (A).

Description

The invention relates to a controllable loading device for generating electricity in a three-phase network.

With an earth current directional relay in a three-phase system (especially near generators in busbars circuit) there is often the problem that the existing earth currents are not sufficient to excite or trigger the relay chen.

It was therefore in practice in the past of the fene delta winding of a transformer a motor controllable resistor with tap for generating an earth current in the three-phase network. When a Displacement voltage, the resistance value was then so far continuously decreased until the desired earth current to Triggering of the relay was given. This arrangement is very elaborate and expensive, the resistance is not for everyone application cases can be used.

The invention has for its object a device specify, in which the disadvantages mentioned above are eliminated are.

According to the invention, the solution is a controllable loading Load device for generating earth current in a three-phase current network, the one connected to the three-phase network, Trans Formator, which has a secondary winding in open three  has a corner circuit to which a controllable resistance to earth power generation is connected, the resistance one Combination of resistors includes that of switching elements can be combined with each other, and wherein the switching elements are controlled by an automation device.

This setup is simple with standard components feasible, with a good time ver hold is given. The decor is flexible from small up to large currents, in particular from 0.1 A to 400 A, especially 1 A to 300 A can be used in the secondary winding. Be A standard is preferred for the automation device dard product in modular technology, e.g. B. a memory programmer bare device used.

The resistors are advantageously dual or binary switchable. This makes the change in resistance very easy designed. For this, simple output relays of the car Automation device can be used.

It is beneficial if the resistances depend on the displacement voltage occurring at the secondary winding controls are. This is a good adaptation of the earth current to the input sensitivity of the earth current directional relay possible, the security against the state of the release Technology is improved. For measuring the zero sequence voltage can analog input modules or modules of automation tion device are used.

The earth current is preferably used to excite an earth current converter relay, which ensures safe tripping and safe  Error detection is given. The selectivity of the earth fault Recording is fully guaranteed. A preferred type Use of the device is the earth fault direction detection for generators in busbar connection (as in the Drawing shown).

Embodiments of the invention, further details and before Parts are exemplified with reference to the drawing explained in more detail. Show it:

Fig. 1 A three-phase mains with connected load device,

Fig. 2 and 3 connecting images of the loading device with switchable resistors,

Fig. 4 is a characteristic diagram for switching behavior of the loading device,

Fig. Stungseinrichtung Tables 5 and 6 for logical switching behavior of the loading and

Fig. 7 is a block diagram for the interconnection of the logical modules (software or network blocks) of the loading device.

In Fig. 1 a three-phase network arrangement is shown in which two generators G1, G2 are electrically connected to a network N. In the corresponding connection lines, switching elements for separating and connecting the electrical connections, in particular circuit breakers LS, are switched between.

The respective generators G1, G2 are earth currents Directional relay R1 or R2 assigned. These get over  each assigned current transformer SW current signals of the resp generators G1 and G2 for earth current detection.

With particularly low earth currents it is often a safe one Tripping by the earth current directional relays R1, R2 not always possible. Therefore, in the present case there is a loading device B provided that serves to increase the earth current.

The loading device B first includes one connected to the network N connected transformer Tr, which is at least one second the winding S1. The secondary winding S1 is of fene triangular winding to which a controllable Wi the combination W can be created. This will follow also referred to as resistor W.

The resistor combination W comprises a predetermined number of partial resistances not shown in this figure, which of switching elements can be combined. The switching links are controlled by an automation device A. The partial resistors have resistance values of playful under one ohm, especially between 0.75 and 0.05 ohms.

The partial resistors are preferably switched in two stages, the respectively generated resistance combination depending of the ver occurring at the secondary winding S1 storage voltage is controlled. The automation can do this tion device A, if necessary, measured value inputs for detection the zero sequence voltage at the secondary winding S1 point.  

A programmable automation device with modular input and output modules is preferably used as the automation device A. Such a device is generally known to the person skilled in the art under the name "Simatic". Such an automation device can be programmed or parameterized in a simple manner, so that the function of increasing the earth current is easily possible. See also FIGS. 2 to 7.

The operation of the entire device is explained in more detail below with reference to a technical functional description with reference to FIGS . 2 to 7. The figures and the functional description complement each other.

Variable Load Resistor Unit (VLRU)

The specification / task is:
Detection of earth faults that cause displacement voltages in the three-phase network and between 20-100% of the conductor / earth voltage. A zero current is required for selective earth fault detection, which exceeds at least the smallest setting value of the earth fault directional relay of 0.002 xIn (corresponds to 190 A on the secondary transformer side).

When a displacement voltage occurs in an isolated This zero current must be generated in the grid. That happens with an ohmic resistance depending on the occur the displacement voltage. But since with a Erdstromerzeu supply device only limited currents for the primary side  to be produced is an adjustable load resistance required.

In the present circuit, a binary staggered Be load resistance W, consisting of four elements or Partial resistors T, used by powerful Switching elements, especially SG contactors, as required be short-circuited. The control of the SG contactors takes place depending on the level of the displacement voltage an automation known as "Simatic S5" rungseinrichtung A. By using three SG shooters there is a graduation of the load resistance W in 2³ = 6 levels, which - depending on the occurring Displacement voltage (U₀) - can be short-circuited. The generated earth current is kept almost constant.

The inputs of the Simatic control A are connected as follows:
The digital inputs DE are controlled via auxiliary relays 125 V ∼ with an excitation signal from the 7RE26 (F421 + F422) of the transformer protection cabinet and from the generator switch position of the generator branches GS. These DE inputs are indicated by LEDs on the module. Analog inputs AE are controlled by the transducers for U₀ and I₀. The U₀ transducer is connected to a 5: 1 voltage divider (3PP13). The I₀ transducer is connected to a small current transformer (4NC12).

The blocking or reset input on the digital input construction group can be activated by pressing a light button and right turn can be controlled. A white signal lamp signals blocking the VLRU.

The outputs of the Simatic A control the three auxiliary contactors H, the contacts of which actuate the powerful SG contactors (460 V, 60 Hz). Because of the high current load, three contacts (90 A each) are connected in parallel and short the load resistance elements or partial resistors T in a predetermined manner (see also FIGS. 4 to 7).

In addition, the following signals are wired to the terminal strip.

• - Failure not in protection zone Protection zone)
• - Pick-up + U₀ too low (= protective relay activates but loads does not measure U₀)
• - I₀ too low (earth current too low)

All of the above output signals are on the relay module signaled via LED.

Description of the signal flow

The entire control or loading device B is started via the protective excitation of the respective 7RE26 (Earth fault or earth current direction relay not closer shown) and the activation message of the concerned Generator switch (GS) (network N1). If the GS is not is switched on does not need earth current for an excitation  be generated. Earth fault is then measured via U₀ measurement, a trigger signal is then sent with a time delay.

If these conditions are met, a timer t₁ ge starts (network N2).

In addition to the earth fault relay, the load ap parature B the zero sequence voltage at the transformer Tr measured a transducer (network N1). The time step t₁ (network N2) covers the settling time of the U₀ measured value converter from approx. 0.4 s. After t 1 has expired whether U₀ exceeds the minimum value (network N2).

T₁ is a cycle time, after which it is checked every time whether the condition U <Umin (<100V) is fulfilled.

The excitation signal is sent to an AND gate via network N4 leads, which also evaluates the negated U <Umin signal and possibly to the message: "Excitation and U <" leads.

If the condition: "Excitation and U <" is met, starts a flip-flop and thus the time stage t₂ (network N3). This is used to monitor whether an external fault after Ab time is present or not (t₂ ≈ 4 s). At the same time becomes the condition for the control during this time the shooter SG maintained. The Sagittarius SG are only stimulated during t₂!

In parallel to t₂ a pulse is generated and the time step t₃ (Network N6) started. The impulse gives approval for the  Voltage evaluation of the CPU continues. Only during the one time against the impulse, the applied voltage is evaluated and ge saves. Changes in the U₀ voltage will not be continued considered. The pulse sets the signal "I₀ via an OR too low "back so that the current during the measuring process is evaluated and reported. As soon as the starting conditions the 7RE26 or the generator switch position are eliminated the memory modules N3 and N4 reset.

The time stage t₃ (network N6) covers the settling time of the current transducer of approx. 0.4 s. About AND Time step and the current query linked. You are forming with it the message: "I₀ too low" (I <190 A).

Reference list

N network
LS circuit breaker
G1, G2 generator
R1, R2 earth current directional relay
Tr transformer
B loading device
A automation device
S1, S2 secondary winding
SW current transformer
W resistance
I₀ earth current
U₀ zero sequence voltage
SG contactors or switching elements
T partial resistors
DE digital inputs
AE analog inputs

Claims (4)

1. Controllable load device (B) for generating electricity from the earth in a three-phase network (N), with one connected to the three-phase network (N) connectable transformer (Tr), which is a secondary winding (S1) in an open delta connection to which a tax erable resistor (W) is connected, the opp stand (W) is a combination of partial resistances (T) that of switching elements (SG) can be combined with each other, and wherein the switching elements (SG) from an automation device (A) are controlled. 2. Controllable loading device according to claim 1, wherein the partial resistors (T) are switched in two stages. 3. Controllable loading device according to claim 1 or 2, the partial resistances (T) depending on the at the Secondary winding (S1) occurring displacement voltage (U₀) are controlled. 4. Controllable loading device according to claim 1, 2 or 3, the earth current (I₀) to excite an earth current tion relay (R1, R2) is used.