Feeder protection cable feeders and overhead lines [Industrial Electrical Power Systems]

. Introduction

Two commercially available systems have been chosen as typical examples to illustrate the concepts. However there are other similar products on the market using essentially the same technology.


Current transformer Current transformer; Pilot wires

++++ Simplified connections illustrating principles of operation

Translay is a voltage balance system

Whilst the feeder is healthy, the line CTs at each end carry equal currents. Equal and opposite voltages are induced in the secondary windings 12 and 12a and no current flows in the pilots. No magnetic flux is set up in the bottom magnets 16 and 16a so the relays don’t operate. Under heavy through-fault conditions there may be a small circulating current due to line CT mismatch. A restraint torque is produced by bias loop 18, which also stabilizes the relay against pilot capacitance currents. A fault fed from one end causes current to circulate in the pilots and the relay at that end will operate to trip. A fault fed from both ends will cause a current reversal in the remote CTs, making the circulating current additive so that both ends operate to trip.

Solkor protection

Solkor unit protection is used where solid metallic pilot wires are available. The system is a differential protection system and is available as Solkor R/Rf. Optional equipment includes pilot wire supervision and injection intertripping systems.

Solkor protection can be con figured in two modes. The R mode caters for systems where pilot/ ground insulation levels are 5 kV or less. The Rf mode is used in newer systems with either 5 or 15 kV insulation. Solkor Rf gives faster clearance times for internal faults whilst its stability for through-faults is the same high value as Solkor R. The standard relay has a pilot circuit to ground withstand of 5 kV, and interposing transformers are available to cater for circumstances when a 15 kV pilot insulation level is required. Solkor equipment will trip both ends of a faulty feeder even if current is fed from one end only. Solkor R/Rf relays are designed to use telephone type pilot cables having a loop resistance of up to 2000 ohm and a maximum capacitance between cores of 2.5 uF. The relays incorporate diodes, which act as switches and permit the use of a single pilot loop for two-way signaling using a system of time sharing switched at the power frequency (). Normally, only two pilot wires are used for interconnecting the relays at the two ends of the feeder. For this reason, summation transformers are incorporated in the protection (). Because of the summation transformer the sensitivity of the protection is dependent upon phases that are involved in short circuit. Typical figures for sensitivity are as follows:

++++ Basic circuit of Solkor R protection system -- Operating coil

5 kV Insulation, Non-linear resistor, Selenium rectifier, Summation transformer

Fixed resistor, Pilot padding resistor

Test link --Links --Operating element; Pilots

++++ Schematic diagram of complete, 5 kV Solkor R protective system, Point X Point Y; Ra Rp Y X M2

++++ Behavior of basic circuit under external fault conditions when Ra = Rp

(a) and (b) show the effective circuit during successive half-cycles; (c) indicates the voltages across relaying points X and Y during one cycle

XY Ra M2 Rp

XY Ra M2 Rp Point X Point Y

++++ Behavior of basic circuit under external fault conditions when Ra = Rp ; (b) show the effective circuit during successive half-cycles; (c) indicates the voltages across relaying points X and Y during one cycle.

++++ Behavior of basic circuit under internal-fault conditions (fault fed from both ends (a) and (b) show effective circuits during successive half cycle).

++++ Schematic diagram, table of characteristics and vectoral demonstration of summation transformer Type of Fault Sensitivity (%) Red to ground 25% White to ground 32% Blue to ground 42% Red to white 125% White to blue 125%; Blue to red 62% Three phase 72% Primary Secondary Summation transformer

Fault Settings

5 kV Equipment; 15 kV Equipment

Type of Fault

N Tap N1 Tap N Tap N1 Tap Red- ground 25 18 33 23.5 Yellow- ground 32 21 41 27.5 Blue- ground 42 25 55 33 Red-yellow 125 165.0 Yellow-blue 125 165.0 Red-blue 62 82.5 Three-phase 72 95.0

Recommended-- Protection of Cables and Short Lines; Pilot wire differential protection; Solkor and Translay, etc.

Back-up--Inverse definite minimum time lag: IDMTL


Cables selected for volt drop and fault level

Distance protection

Basic principle

A distance relay, as its name implies, has the ability to detect a fault within a pre-set distance along a transmission line or power cable from its location. Every power line has a resistance and reactive per kilometer related to its design and construction so its total impedance will be a function of its length or distance. A distance relay therefore looks at current and voltage and compares these two quantities on the basis of Ohm's law.

Distance relay: ++++ Basic principle of operation

The concept can best be appreciated by looking at the pioneer-type balanced beam relay. The voltage is fed onto one coil to provide restraining torque, whilst the current is fed to the other coil to provide the operating torque. Under healthy conditions, the voltage will be high (i.e. at full-rated level), whilst the current will be low (at normal load value), thereby balancing the beam, and restraining it so that the contacts remain open. Under fault conditions, the voltage collapses and the current increase dramatically, causing the beam to unbalance and close the contacts.

++++ Balanced beam principle

By changing the ampere-turns relationship of the current coil to the voltage coil, the ohmic reach of the relay can be adjusted. A more modern technique for achieving the same result is to use a bridge comparator.

Restrain; Operate: ++++ Bridge comparator

Tripping characteristics

If the relay's operating boundary is plotted, on an R/X diagram, its impedance characteristic is a circle with its center at the origin of the coordinates and its radius will be the setting (reach) in ohms.

Restrains, Operates: ++++ Plain impedance characteristic

The relay will operate for all values less than its setting i.e. is for all points within the circle.

This is known as a plain impedance relay and it will be noted that it’s non-directional, in that it can operate for faults behind the relaying point. It takes no account of the phase angle between voltage and current.

This limitation can be overcome by a technique known as self-polarization. Additional voltages are fed into the comparator in order to compare the relative phase angles of voltage and current, so providing a directional feature. This has the effect of moving the circle such that the circumference of the circle now passes through the origin. Angle ? is known as the relay's characteristic angle.

++++ MHO characteristic

This is known as the MHO relay, so called as it appears as a straight line on an admittance diagram. By the use of a further technique of feeding in voltages from the healthy phases into the comparator (known as cross polarization) a reverse movement or offset of the characteristic can be obtained. This is called the offset MHO characteristic.

++++ Offset MHO characteristic

Application onto a power line

Correct coordination of the distance relays is achieved by having an instantaneous directional zone 1 protection and one or two more time-delayed zones. A transmission line has a resistance and reactance proportional to its length, which also defines its own characteristic angle. It can therefore be represented on an R/X diagram as shown below.

Zone 1: The relay characteristic has also been added, from which it will be noted that the reach of the measuring element has been set at approximately 80% of the line length. This 'under-reach' setting has been purposely chosen to avoid over-reaching into the next line section to ensure sound selectivity, for the following reasons:

• It’s not practical to accurately measure the impedance of a transmission line, which could be very long (say 100 km). Survey lengths are normally used and these could have errors up to 10%.

• Errors are also present in the current and voltage transformers, not to mention the possible transient performance of these items.

• Manufacturing tolerances on the relay's ability to measure accurately, etc.

Zone 1: Busbar A Busbar B Line

++++ Zone 1-- MHO characteristic

This measuring element in known as zone 1 of the distance relay and is instantaneous in operation.

Zone 2: To cover the remaining 20% of the line length, a second measuring element can be fitted, set to over-reach the line, but it must be time delayed by 0.5 s to provide the necessary coordination with the downstream relay. This measuring element is known as zone 2. It not only covers the remaining 20% of the line, but also provides backup for the next line section should this fail to trip for whatever reason.

Zone 3: A third zone is invariably added as a starter element and this takes the form of an offset mho characteristic. This offset provides a closing-onto-fault feature, as the mho elements may not operate for this condition due to the complete collapse of voltage for the nearby fault. The short backward reach also provides local backup for a busbar fault. This element can also be used for starting a carrier signal to the other end of the line - see later section.

The zone 3 element also has another very useful function. As a starter it can be used to switch the zone 1 element to zone 2, reach after say 0.5s, thereby saving the installation of a second independent zone 2 measuring element so reducing its cost.

++++ Three zone MHO characteristics

Effect of load current

Load current can also be expressed as impedance, again by the simple application of Ohm's law. This can be shown on the R/X diagram as depicted by the shaded area, the angular limits being governed by the power factor of the load.

It’s important that when setting a distance relays, especially zone 3, which has the longest reach, that its characteristic does not encroach on the load area, as unnecessary tripping will undoubtedly occur.

Zone 3-- Load area-- ++++ Load encroachment

Effect of arc resistance

Resistance of the fault arc can also have an impact on the performance of a distance relay, as can be seen on the following R/X diagram. It will be noted that the resistance of the fault arc takes the fault impedance outside the relay's tripping characteristic, so that it does not detect this condition. Alternatively, it’s only picked up by either zone 2 or zone 3 in which case tripping will be unacceptably delayed. The effect of arc resistance is most significant on short lines where the reach of the relay setting is small. It can be a problem if the fault occurs near the end of the reach.

High fault-arc resistances tend to occur during midspan flashovers to ground during a veldt fire or on transmission lines carried on wood poles without ground wires. These problems can usually be overcome by using relays having different shaped characteristics such as described below.

Arc resistance-- Fault impedance

++++ Effect of arc resistance

Different shaped characteristics

To overcome the problems of load encroachment and arc resistance, distance relays have been developed having different-shaped tripping characteristics, some examples of which are as follows:

  1. Circular
  2. Lenticular
  3. Figure eight
  4. Trapezoidal

With the advent of modern digital technology, many shapes are now possible to suit a variety of applications.

++++ Lenticular characteristic

++++ Figure-of-eight characteristic

++++ Trapezoidal characteristic

Distance protection schemes

Because of its various zones, distance protection is strictly speaking not a pure form of unit protection. However, with the addition of an information link between the two ends of the line, it can be made into a very effective unit protection system.

The normal method of achieving this is to install a power line carrier signaling channel between the two ends. The signal is injected into the power line conductors at one end via a capacitor voltage transformer and picked off the other end by a similar device. Line traps are installed at either end to prevent the signal dispersing through all other lines, etc. in the network. Other types of communication medium can be used such as copper or fiber-optic pilots or microwave radio could be considered if line-of site is available.

Conventional distance scheme:

When carrier or signaling equipment is not available, the conventional distance scheme illustrated; however, faults in the end 20% of the line will only be cleared in zone 2 time, namely 0.5 s.

++++ Stepping time/distance characteristics

++++ Trip circuit (contact logic)

++++ Trip circuit (solid-state logic)

Zone 1 extension or overlap: Fast tripping for these portions can be achieved by extending the reach of zone 1 to 120% of the line and cutting it back to 80% reach after tripping before auto-closing the breaker.

The logic is shown...

++++ Distance/time characteristics Range change relay (Z1 ext reach, when energized)

++++ Trip circuit (contact logic) -- Zone 1 ext Auto-reclose Reset zone 1 ext

++++ Simplified solid-state logic

Direct transfer trip (under-reaching scheme)

When carrier/signaling is available, the simplest way to speed up fault clearance at the terminal which clears an end-zone fault in zone 2 time is to adopt a direct transfer trip or intertrip technique ---- +++. A zone 1 contact is used to send a carrier signal to the remote end to directly trip that breaker via a receive relay.

The disadvantage of this scheme is the possibility of undesired tripping by accidental or mal-operation of the signaling equipment.

Receive relay contact

++++ Trip circuit contact logic Z1 Send circuit To remote terminal

++++ Signaling channel send arrangement (contact logic) OR Trip Signal send, Signal receive

++++ Simplified solid-state logic

Permissive under-reach scheme

The direct transfer trip scheme is made more secure by monitoring the received signal with an instantaneous zone 2 operation before allowing tripping.

Receive relay contact--Time delay on reset

Trip circuit (contact logic)

Z1 Send circuit --To remote terminal

++++ Signaling send arrangement (contact logic)

Trip Signal send-- Signal receive

++++ Simplified solid-state logic

Time-delayed resetting of the 'signal received' element is required to ensure that the relays at both ends have time to trip when the fault is close to one end. When the breaker at one end is open, instantaneous clearance cannot be achieved for end-zone faults near the breaker open terminal.

Acceleration scheme This scheme is similar to the permissive under-reach scheme in its principle of operation, but is applicable only to zone-switched distance relays, which share the same measuring elements for zones 1 and 2.

In this scheme, the incoming carrier signal switches the zone 1 reach to zone 2 immediately without waiting for the zone 2 timer (0.5 s) to switch the reach. This accelerates the fault clearance at the remote end. The scheme is shown. The longer reach of the measuring elements gives better arc resistance coverage so it’s better suited to short lines. This scheme is sometimes referred to as a 'directional comparison' scheme.

++++ Distance/time characteristics

Receive relay contact --Trip circuit (contact logic)

++++ Signaling channel send arrangement (contact logic) -- Send circuit To remote terminal

Range change auxiliary relay Signal send Signal receive

++++ Simplified solid-state logic

Permissive over-reach scheme:

In this scheme a measuring element at end A, is set to over-reach beyond the far end of the protected line, typically 120% or more. The same scheme is installed at the other end B of the line, looking towards end A. This element sends an intertrip signal to the remote end. It’s arranged to trip its own breaker immediately after a signal is received. If no signal is received, it will trip its own breaker after zone 2 time (0.5 s). The longer reach of the measuring elements gives better arc resistance coverage so it’s better suited to short lines.

This scheme is sometimes referred to as a 'directional comparison' scheme.

Receive relay contact:

++++ Trip circuit (contact logic)

Z2 Send circuit--To remote terminal

++++ Signaling send arrangement (contact logic)

++++ Simplified solid-state logic

Blocking scheme:

The schemes described above have used a signaling channel to transmit a tripping instruction, normally through the fault.

A blocking scheme uses inverse logic, the signal preventing tripping. Signaling is initiated only for external faults and takes place over healthy line sections. Fast fault clearance occurs when no signal is received and the over-reaching zone 2 measuring elements looking into the line operate.

The signaling channel is keyed, by a reverse-looking distance element (zone 3). An ideal blocking scheme is shown.

++++ Distance/time characteristics

++++ Trip circuit (contact logic) -- Receive relay contact

++++ Signaling channel send arrangement (contact logic) --Send circuit To remote terminal

++++ Simplified solid-state logic – Signal receive; Trip

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Monday, April 20, 2015 21:07