Differential protection of a transmission line with digital communication via pilot wires

The choice of pilot wires

Wire connections have a limited transmission bandwidth (3.4 kHz in the case of telephone twisted pairs). In the past they were used for differential protection with analog (50 Hz) measured value comparison and for phase comparison protection by implementation of modulated voice frequency signal transmission.

Only a few Kbit/s can be transmitted digitally in the base-band. Modern modulation techniques (multiple level instead of simple binary coding, phase modulation) allow for higher transmission rates of above 100 Kbit/s.

Contents:

1. Pilot wire application
   1. Network protection cable
   2. Leased telephone lines
2. Interference induced by the earth short circuit current
   1. 1. Ohmic voltage coupling
      2. Inductive voltage coupling
3. Measures against voltage interference
4. Monitoring of the pilot cores

1. Pilot wire application

The application of differential protection with digital communication via pilot wires (cables) of up to approx. 20 km is therefore now possible.

The choice of pilot wires (cables) depends on the type of protection, the distance that must be spanned and the level of interference voltage that is expected. On short links up to approx. 2 km, where the level of system frequency (50 Hz) interference voltage is not expected to be very large, normal control cables (e.g. NYY) with 2 kV rated voltage are suitable.

They are often used with current differential protection (3 core differential protection).

Wherever possible, high voltage isolated communication cables (special protection class cables) should be applied. This is particularly so when the cable runs parallel to a high voltage cable or overhead line.

Additional isolation is also required at the termination of the pilot wire cable in the high voltage substation to prevent breakdown due to the transverse voltage induced in the pilot wire pair by earth short circuit currents.

The pilot wires are usually isolated against ground (“floating”). The terminal devices (protection devices or communication equipment) are thereby symmetrically connected.

In Figure 1 an example of a differential protection for two pilot wire cores (telephone twisted pair) is shown. The cable screen is earthed on both sides so that it effectively eliminates inductive interference.

The induced transverse voltage drives a charging current via the earth capacitance of the cable cores so that the transverse voltage to earth, shown in Figure 2, arises. The areas F1 and F2 above and below a potential of zero correspond to the product of line length and voltage to earth at the corresponding location.

On non-earthed cables they are always the same.

Assuming a constant level of interference along the entire pilot wire the potential to ground is distributed symmetrically at both cable ends. Therefore only half the induced voltage between the terminals of the cable appears as shunt voltage to the screen or earth.

For an uneven distribution, for example if at one end the pilot wire continues, a non-symmetrical distribution of the potential will result.

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1.1 Network protection cable

Special network protection cables with different numbers of twisted pairs and triplets can be supplied (Figure 3).

Example:

The pairs and triplets are separately twisted and in PE insulation. The pilot cores are enclosed with a screen consisting of plastic coated aluminium tape and an external PE sleeve.

Table 1 shows the typical parameters.

Table 1 – Network protection cable, PE-insulated with copper core triplets and pairs for direct installation in the ground or pulling into pipes.

The permissible temporary induced voltage according to VDE 0228, Part 11 is 60% of the rated AC voltage between core and screen, i.e. for the cable described above, a voltage of 4.8 kV for the triplet cores and 1.2 kV for the twisted pairs.

If the voltage between the terminals is larger, the cable must be split and barrier transformers have to be applied.

In the example in question (Table 1) one of the triplet cores with increased isolation is designated for the current differential protection. Other core combinations, for example with high voltage isolated twisted pairs can however also be provided by the manufacturer.

The protection devices themselves are usually designed with an AC rated voltage of 2 kV so that barrier transformers must in any event be applied at the cable terminals if the induced voltage across the cable exceeds 1.2 kV. Common isolation voltages of the barrier transformers for this purpose are 5 or 15 kV.

In the event of non-symmetry, the voltage induced on the pilot wire core differs, whereby the voltage difference appears directly in the pilot core loop and thus influences the protection measurement. Relay specific limits may not be exceeded here.

The screen consisting of plastic coated aluminium tape however only has limited effectiveness.

The pilot wire pairs (telephone twisted pair) are suitable for the transmission of analog signals with frequencies ranging from low frequency up to approximately 4 kHz (differential protection, phase comparison protection, voice frequency inter trip devices).

They can only carry limited current levels (some tens of mA), so that differential protection can only be implemented using the voltage comparison protection (two core pilot wire differential protection) technique.

For digital signal transmission, modems must be applied. With a newly developed technique it is even possible to transmit 128 Kbit/s up to a distance of approximately 20 km.

The triplet (three core pilot cable) with increased cross-section is intended for the current differential protection (three core pilot wire differential protection). The current in the pilot wire cores in this case is approximately 100 mA at CT nominal current.

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1.2 Leased telephone lines

Twisted pairs can also be leased from telephone companies. In some countries this has been common practice. This may have the advantage that the pilot wire connection is not in the vicinity of the high voltage overhead lines or power cables and is therefore not significantly influenced by power system interference.

On the other hand reduced security against manipulation of the pilot wires must be accepted.

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2. Interference induced by the earth short circuit current

The pilot wire circuits are mainly affected by the large short circuit currents flowing via earth. In this context there are two methods by which the overvoltages are induced:

1. By the potential difference (ohmic coupling) via the station earthing
2. By magnetically induced lateral voltages

2.1 Ohmic voltage coupling

Figure4 shows a “voltage funnel” that arises in the vicinity of a substation during an earth short circuit.

Where:

• R_G – Station grounding resistance
• STP – Station potential
• PGA – Potential gradient area
• RGP – Remote ground potential
• E_Ω – Station potential against remote ground

Due to the voltage drop across the station earth a difference between the earth potential in the station (device earth, screen) and the remote end (core potential) arises.

For example, with a station earth resistance of R_G = 0.5 Ω and an earth short circuit current of I_F-G = 10 kA a potential difference E_Ω = 10000 A  × 0.5 Ω = 5000 V would arise.

The station earthing would have to be improved, or a barrier transformer would have to be applied for isolation of the high voltage.

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2.2 Inductive voltage coupling

If the pilot cable is close to a high voltage overhead line or a high voltage cable, lateral voltages are induced in the cores by earth short-circuit currents. These can also be several kV (see Figure 2).

The influencing voltage is proportional to the earth fault current, the coupling inductance (proximity of the pilot cable to the HV-feeder) and the length over which this proximity of pilot cable and HV-feeder exists.

The level of reduction introduced by overhead line earth wires and the conducting screens (armouring) of high voltage cables depends on their ohmic resistance (material, cross section) and lies in the range from 0.9 (steel earth wire) to 0.2 (lead cable screen with steel armouring).

Railway tracks can introduce an additional reduction factor of between 0.8 and 0.2.
On the communication cable which is the influenced circuit, the influencing voltage may be further reduced by earthing the cable screen at both ends.

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3. Measures against voltage interference

The following influencing factors with positive effect must be included in the planning:

1. Attempt to achieve the lowest possible earthing resistance in the HV substations.
2. Ensure that the pilot cables in the proximity of HV substations are fitted with a metal sheath having very good conducting capabilities. The field of the current flowing in the sheath induces a voltage on the core that reduces the potential difference between the core and the sheath in the “voltage funnel”.
3. Maintain as large a separation as possible between the HV equipment and the pilot cables.
4. Try to make the reduction factors as small as possible on the influencing side (cable metal armouring, cable screens, earth wires).
5. Small reduction factors on the influenced side are obtained by metal armouring and conducting screens on pilot cables, earthing of unused cores at both ends, additional reduction conductors.
6. Use pilot cables with high insulation levels (insulation between protection core-screen and protection core-parallel core).
7. Ensure that the pilot cores are symmetrical (twisted).
8. Apply barrier transformers if needed.

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4. Monitoring of the pilot cores

Pilot cores can be damaged for example by earth moving. In particular on long distances a continuous monitoring is therefore important.

In the case of phase comparison protection or with the signal communication device that operates in the voice frequency spectrum, the keyed measuring signal or the quiescent frequency may be directly used for the monitoring.

For signal transfer with DC voltage, a circuit with current flow in the quiescent state should be applied.
With numerical signal transfer, the monitoring is contained in the protection devices.

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Source: Numerical Differential Protection by Gerhard Ziegler (Siemens)