(Parte 2 de 4)

8.4.1 Direct Tripping

In direct tripping applications, intertrip signals are sent directly to the master trip relay. Receipt of the command causes circuit breaker operation. The method of communication must be reliable and secure because any signal detected at the receiving end will cause a trip of the circuit at that end. The communications system design must be such that interference on the communication circuit does not cause spurious trips. Should a spurious trip occur, considerable unnecessary isolation of the primary system might result, which is at best undesirable and at worst quite unacceptable.

8.4.2 Permissive Tripping

Permissive trip commands are always monitored by a protection relay. The circuit breaker is tripped when receipt of the command coincides with operation of the protection relay at the receiving end responding to a system fault. Requirements for the communications channel are less onerous than for direct tripping schemes, since receipt of an incorrect signal must coincide with operation of the receiving end protection for a trip operation to take place. The intention of these schemes is to speed up tripping for faults occurring within the protected zone.

8.4.3 Blocking Scheme

Blocking commands are initiated by a protection element that detects faults external to the protected zone. Detection of an external fault at the local end of a protected circuit results in a blocking signal being transmitted to the remote end. At the remote end, receipt of the blocking signal prevents the remote end protection operating if it had detected the external fault. Loss of the communications channel is less serious for this scheme than in others as loss of the channel does not result in a failure to trip when required. However, the risk of a spurious trip is higher.

Figure 8.1 shows the typical applications of protection signalling and their relationship to other signalling systems commonly required for control and management of a power system. Of course, not all of the protection signals shown will be required in any particular scheme.

8.5 PERFORMANCE REQUIREMENTS Overall fault clearance time is the sum of: a.signalling time b.protection relay operating time c.trip relay operating time d.circuit breaker operating time

The overall time must be less than the maximum time for which a fault can remain on the system for minimum plant damage, loss of stability, etc. Fast operation is therefore a pre-requisite of most signalling systems.

Typically the time allowed for the transfer of a command is of the same order as the operating time of the associated protection relays. Nominal operating times range from 5 to 40ms dependent on the mode of operation of the teleprotection system.

Protection signals are subjected to the noise and interference associated with each communication medium. If noise reproduces the signal used to convey the command, unwanted commands may be produced, whilst if noise occurs when a command signal is being transmitted, the command may be retarded or missed completely. Performance is expressed in terms of security and dependability. Security is assessed by the probability of an unwanted command occurring, and dependability is assessed by the probability of missing a command. The required degree of security and dependability is related to the mode of operation, the characteristics of the communication medium and the operating standards of the particular power authority.

Typical design objectives for teleprotection systems are not more than one incorrect trip per 500 equipment years and less than one failure to trip in every 1000 attempts, or a delay of more than 50msec should not occur more than once per 10 equipment years. To achieve these objectives, special emphasis may be attached to the security and dependability of the teleprotection command for each mode of operation in the system, as follows.

8.5.1 Performance Requirements – Intertripping

Since any unwanted command causes incorrect tripping, very high security is required at all noise levels up to the maximum that might ever be encountered.

8.5.2 Performance Requirements – Permissive Tripping

Security somewhat lower than that required for intertripping is usually satisfactory, since incorrect tripping can occur only if an unwanted command happens to coincide with operation of the protection relay for an out-of-zone fault.

Chap8-112-121 21/06/02 9:03 Page 115

For permissive over-reach schemes, resetting after a command should be highly dependable to avoid any chance of maloperations during current reversals.

8.5.3 Performance Requirements – Blocking Schemes

Low security is usually adequate since an unwanted command can never cause an incorrect trip. High dependability is required since absence of the command could cause incorrect tripping if the protection relay operates for an out-of-zone fault.

Typical performance requirements are shown in Figure 8.2.

8.6 TRANSMISSION MEDIA INTERFERENCE AND NOISE

The transmission media that provide the communication links involved in protection signalling are:

a.private pilots b.rented pilots or channels c.power line carrier d. radio e.optical fibres

Historically, pilot wires and channels (discontinuous pilot wires with isolation transformers or repeaters along the route between signalling points) have been the most widely used due to their availability, followed by Power Line Carrier Communications (PLCC) techniques and radio. In recent years, fibre-optic systems have become the usual choice for new installations, primarily due to their complete immunity from electrical interference. The use of fibre-optic cables also greatly increases the number of communication channels available for each physical fibre connection and thus enables more comprehensive monitoring of the power system to be achieved by the provision of a large number of communication channels.

8.6.1 Private Pilot Wires and Channels

Pilot wires are continuous copper connections between signalling stations, while pilot channels are discontinuous pilot wires with isolation transformers or repeaters along the route between signalling stations. They may be laid in a trench with high voltage cables, laid by a separate route or strung as an open wire on a separate wood pole route.

Distances over which signalling is required vary considerably. At one end of the scale, the distance may be only a few tens of metres, where the devices concerned are located in the same substation. For applications on EHV lines, the distance between devices may be between 10- 100km or more. For short distances, no special measures are required against interference, but over longer distances, special send and receive relays may be required to boost signal levels and provide immunity against induced voltages from power circuits, lightning strikes to ground adjacent to the route, etc. Isolation transformers may also have to be provided to guard against rises in substation ground potential due to earth faults.

The capacity of a link can be increased if frequency division multiplexing techniques are used to run parallel signalling systems, but some Utilities prefer the link to be used only for protection signalling.

Private pilot wires or channels can be attractive to an Utility running a very dense power system with short distances between stations.

8.6.2 Rented Pilot Wires and Channels

These are rented from national communication authorities and, apart from the connection from the relaying point to the nearest telephone exchange, the routing will be through cables forming part of the national communication network.

changed without warningThis may be a problem in

An economic decision has to be made between the use of private or rented pilots. If private pilots are used, the owner has complete control, but bears the cost of installation and maintenance. If rented pilots are used, most of these costs are eliminated, but fees must be paid to the owner of the pilots and the signal path may be protection applications where signal transmission times are critical.

The chance of voltages being induced in rented pilots is smaller than for private pilots, as the pilot route is normally not related to the route of the power line with which it is associated. However, some degree of security

P r ot ect ion:

Sig nalling and Int er t r ipping

Network Protection & Automation Guide• 116•

Sec

Intertrip Blocking

Analogue Digital

Digital Analogue

Intertrip T- 0.04sec

Intertrip T - 0.04sec P P

T- 0.015sec P

T- Maximum operating time

Dependabilityª100(1-PMCP)%

Figure 8.2: Typical performance requirements for protection signalling when the communication link is subjected to noise

Chap8-112-121 21/06/02 9:03 Page 116

Network Protection & Automation Guide• 117• and protection against induced voltages must be built into signalling systems. Electrical interference from other signalling systems, particularly 17, 25 and 50Hz ringing tones up to 150V peak, and from noise generated within the equipment used in the communication network, is a common hazard. Similarly, the signalling system must also be proof against intermittent short and open circuits on the pilot link, incorrect connection of 50 volts d.c. across the pilot link and other similar faults.

Station earth potential rise is a significant factor to be taken into account and isolation must be provided to protect both the personnel and equipment of the communication authority.

The most significant hazard to be withstood by a protection signalling system using this medium arises when a linesman inadvertently connects a low impedance test oscillator across the pilot link that can generate signalling tones. Transmissions by such an oscillator may simulate the operating code or tone sequence that, in the case of direct intertripping schemes, would result in incorrect operation of the circuit breaker.

(Parte 2 de 4)

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