Digital (Logic) Input Modules

More often, you can meet with the structures where output relays are installed on the same common board with other electronic parts of the DPR, such as, for example, logic (digital) input assembly; therefore, such combined assem-blies are often called input-output modules (I/O modules); see Figure 2.21.

Usually, the DPR logic (digital) input assembly is a combination of a cer-tain number (5–40) of identical input channels. In the design, each input channel is generated by a high-power resistor (R1) quenching the principal component of the 100–250 V input voltage, an optocoupler (Opt2) providing galvanic isolation of the DPR’s internal circuits from the 100–250 V input voltage, an overvoltage protection element (RU) at the input, and several aux-iliary elements (see Figure 2.22).

Miniature electromechanical relays IGBT transistors

1000 V, 20 A

Varistors

FIGURE 2.19

Detail showing the hybrid output relay module.

In this diagram, the logic element disable is realized on the basis of the VT transistor. The optocoupler-based (Opt1) inhibit input of the element allows blocking the logic input upon the microprocessor command activating the Opt1 optocoupler.

The digital input units in various DPRs, manufactured by Areva (see Figure 2.23), also have a similar set of elements. However, unlike those described above, in these units there is a possibility of a change in voltage threshold level to activate a logical input. It can be realized by changing the biasing potential on the base of the switching transistor.

IGBT transistor 900 V, 50 A

Varistor

Connector

Diode bridge

Electromechanical relays

Driver

VT1 + VD1–

Ru

~

FIGURE 2.20

Detail showing the diagram of the DPR hybrid output relay module type SEL-487 designed for AC and DC switching.

FIGURE 2.21

Combined input-output module of DPR type REC-316. 1: input elements assembly; and 2: out-put relay assembly.

R1

R1 VD1

VD3

VT

VT

C

C

RU

VD2 RU Opt1 Z

Opt1

Opt2 R2 Opt2

R2

R3

R4 Prohibit

input

Input+ 220 V

FIGURE 2.22

Schematic circuit diagram of a logic input single channel of relay series 316 (REL, REC, RET, REG, etc.), manufactured by ABB.

Sometimes logic input channels have less sophisticated designs. They con-tain one quenching resistor, an optocoupler, and a small ceramic capacitor with the input of some hundreds of picofarads. It should be noted that such a capacitor cannot absorb the energy of the impulses coming to the input of the logical channel under the transition processes followed by surge over-voltages within a powerful and very long DC network at the substations.

Also, an important element of such a channel is a diode connected in paral-lel to the input in the opposite direction (or in series in the forward direction) and preventing voltage reverse polarity at the DPR input (see Figure 2.24).

This prevents false activation of the logic inputs due to the voltage impulse reverse polarity occurring at the DPR inputs upon the switching of inductive loads (switched trip coils, and intermediate relay windings). Unfortunately, not all DPR types are equipped with such diodes and bidirectional input photocouplers (see Figure 2.25), leading to the activation of DPR logic inputs under the voltage of any polarity.

In one of the new DPR models (SIPROTEC 7U6125; see Figure 2.26), all electronic parts except the input voltage and current transformers block are

Adjusted logic input modules

FIGURE 2.23

Adjusted logic input modules in AREVA-manufactured DPRs, and enlarged detail showing the single module.

installed on a common PCB in such a way that the switching power supply is located, literally, end to end with the central processing unit (CPU) arranged under the communication module (see Figure 2.26). This is the only arrange-ment of this sort that I have ever seen. The effectiveness of this arrangearrange-ment in relation to the electromagnetic compatibility raises doubts, considering the high-frequency electromagnetic emissions at the switching power sup-ply. The method employed for improving the switching capacity of the min-iature relay contacts also looks strange. The relays are shunted with 4.7 nF, 250 V capacitors. This raises two issues: first of all, one capacitor is not able to improve the switching capacity of the contacts, because you also need a resis-tor connected in series; and, second, a capaciresis-tor with a rated voltage of 250 V is not sufficient for 220–250 V mains with significant switching overvoltage.

Optocoupler Capacitor Diode

Power resistor Supressor

SMD phototransistor optocoupler TCLT1002 type

SOP-4L E

C

4 3

2 1

A C

FIGURE 2.24

Logic input module of DPR type SEL-787.

Resistors quenching most of the input voltage at the logic input assembly are located on the back side of the PCB. Voltage-regulator diodes and transis-tors constitute the input voltage stabilizer (its operating principle is similar to the one discussed above and implemented in DPR series REC, REL, and RET from ABB), providing an operation over a wide range of input voltages (24–250 V), while diode bridges of the logical inputs make them insensitive to the polarity of the input voltage. Both of these features of the arrangement are more likely disadvantages than advantages (see above). Besides, the complete absence of high-voltage surge protection (such as varistors or suppressors) at the inputs of this assembly is a significant design flaw, in our opinion.

The current protection relay, type 7SJ8032, has a similar design (see Figure 2.27). Unlike the previous design, the analog input assembly is installed on the common PCB, while the central processor is installed on a separate board, placed in line to the main board. The logic inputs’ design concept, output relays, and protective element at the relay contacts are the same as those detailed above.

Power resistor

Supressor

Connector

Varistor

Additional resistors Bidirectional input optocoupler

Bidirectional input optocoupler IL252 type A/C

C/A NC 3

2

1 6

5 4 E

C B

FIGURE 2.25

Detail showing the logic input assembly of a SEL-487 relay.