Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
133
Transmission Lines and Feeders Protection
Pilot wire differential relays (Device
87L)
Distance protection
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Power System Protection: Transmission Lines and Feeders Protection - Part VIII, Monografías, Ensayos de Ingenieria Eléctrica
An in-depth analysis of power system protection, focusing on transmission lines and feeders protection. It covers topics such as pilot wire differential relays (device 87l), distance protection, and power line carrier protection schemes. The operating principles of pilot wire differential relays, the use of summing transformers, and the characteristics of distance relays, including plain impedance relays, bridge comparator static impedance type distance relays, and their tripping characteristics. It also discusses the reach, underreach, and overreach of distance relays, and the advantages of distance protection. Examples and calculations for setting zones in distance relaying systems.
Tipo: Monografías, Ensayos
2023/2024
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Transmission Lines and Feeders Protection
Pilot wire differential relays (Device
87L)
Distance protection
1. Pilot wire differential relays (Device 87L)
The pilot wire differential relay is a high-speed relay designed for protection of transmission and distribution lines. They are generally applied on short lines, normally less than 40 km long.
The scheme requires communication channel (link) to carry system voltage and current information to the control location. The main objective of using pilot relaying is to remote control of the circuit breakers.
Four basic communication channels are used:
- Separate telephone circuit (telephone wire or cable) this is called pilot wire carrier.
- Microwave system using directional dishes.
- Fibre optic cable.
- Power line carrier.
(a) Operating principles of a current pilot wire relay
Pilot wire differential relaying is a relay system consisting of two identical relays located at each end of a line (see Figure 1). The relays are connected together with a two-conductor pilot wire. The output from three individual phase CTs is applied to a summing transformer that produces a composite current which is proportional to the line current and has a polarity related to line current flow direction.
The circuit is basically that of the percentage (restraint) differential relay with the operating circuit broken into parallel circuits separated by pilot wires. This relay is available in both electromechanical and static designs.
When the fault is external to the relay’s protective zone, current flows in the pilot wire through each relay’s restraint coils, but not through the relay’s operating coil. If the fault is within the relay’s protective zone and current is flowing into the fault from both directions, the direction of pilot wire current IPA remains the same; but the direction of current IPB reverses and forces current to flow into each relay’s operating coil. If the fault current flows through circuit breaker A only, the relay at A still passes sufficient current through the pilot wire to operate the relay at circuit breaker B.
Fig.2 Power line carrier system.
Fig.3 A 500kV Line Trap.
2.Transmission Line Protection: Distance Relay
Transmission line protection by pilot wires (pilot relaying) is limited to 30 to 40 km in rout length. For longer transmission lines and subtransmission lines or even distribution feeders, distance protection is used.
Principle of Distance Protection
The term distance is used for a family of relays that respond to a ratio of voltage to current and therefore to impedance or component of impedance .Distance relay may have the following features:
A distance relay has the ability to detect a fault within a pre-set distance along a transmission line 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 (see Figure 1). Consider the simple radial line with distance protection system installed at the end A (the local end) while end B is called the remote end. These relays sense local voltage and current and calculate the effective impedance at that point. This means that the relay requires voltage and current information.
Fig.
Plain impedance relays:
1 - Balanced beam relay
The concept can best be appreciated by looking at the pioneer-type balanced beam relay (see Figure 2). The voltage is fed onto one coil to provide restraining torque Tr , whilst the current is fed to the other coil to provide the operating torque To. 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.
Voltage coil Current coil Fig.2 Balanced beam relay used as a distance relay. For voltage coil : Tr = K 1 V^2
For current coil : To = K 2 I^2 Where K 1 and K 2 are constants.
For balance case: Tr = To or K 1 V^2 = K 2 I^2 For the contact of relay to close: Tr ˂ To or K 1 V^2 ˂ K 2 I^2
or
Hence, the relay will operate when the impedance it ‘sees’ is less than a predetermined value.
Tripping characteristics of distance relay
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 in ohms (Figure 3).
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 is 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.
Fig.3 Plain impedance characteristic.
2. Bridge comparator static impedance type distance relay
A more modern technique for achieving the same result is to use a bridge comparator distance relay (see Figure 4).
Referring to Fig.4:
VT output is converted to a current IR which is the restrain current due to ZR. CT provide the operating current Io. IR and Io are converted to scalar values by two rectifiers , and the relay current is the numerical difference of the two currents.
Relay Reach, underreach and overreach
The reach of the distance relay is that distance from the relaying point to the point of fault. The reach is usually refers as the relay setting and can be as a distance (m), or as a primary or secondary impedance.
Relay reach adjustment
Referred to Fig.2, by changing the ampere-turns relationship of the current coil to the voltage coil, the ohmic reach of the balanced - beam relay can be adjusted. A more modern technique for achieving the same result is to use a bridge comparator (see Figure 4).
Advantage of distance relay:
- Provide backup protection easily.
- Eliminates the pilot channel.
Features
Distance protection is available for both phase and ground faults. Step distance protection combines instantaneous and time delay tripping.
Zones of protection
Due to the tolerance in the circuit components, the measuring accuracy cannot be perfect so it is usual to set the relay at the local point A at 80% of the secondary impedance of AB. This is referred as zone 1 or stage A1 setting (see figure 5). The remaining 20% of AB is protected by by changing the setting of the relay to reach 50% into zone BC (zone 2 or stage A2). Stage A2 is usually set at 0.3 s time. For system reliability (failure of relay A will cause failure of stage A1 and stageA2), another distance relay is added for backup protection. This separate relay should have a reach of 20% into CD and called zone 3 or stage A3 which has a time delay of 0.6 s.
Fig.5 A three- sage distance protection system.
Notes:
Zone 1 is an underreaching element, any fault within Zone 1 is known to be on the protected line. When Zone 1 operates, the line is tripped instantaneously. Zone 2, however, will operate for some external faults.
Summary:
Discrimination zone (or setting zone) by:
3
2
1
Z protection
Z protection
Z protection
Fig.
Zone 2 setting : 120 % -150% Choose 140% ;
Zone 2 setting _=1.40 x 80 = 112 ohm, primary ohm setting (Zfp)
Relay setting for zone 2 = Zfp. CT ratio/VT ratio
=112 (400)/(2900)
= 15.44 relay ohms
Example-3 Consider the 230-kV transmission system shown in Fig.8. Assume that the positive-sequence impedances of the lines L1 and L are 2 + j 20 Ω and 2.5 + j 25 Ω, respectively. If the maximum peak load supplied by the line L1 is 100 MVA with a lagging power factor of 0.9, design a three-zone distance-relaying system for the R12 impedance relay by determining the following:
(a) Maximum load current ) b) CT ratio and VT ratio
(c) Impedances measured by relay ) d) Zone 1 setting of relay R
) e) Zone 2 setting of relay R (f) Zone 3 setting of relay R
(g) Draw the zones of protection for R12 and suggest time settings for the three zones.
Fig.
Solution :
(a) Max.load current = = 251.02 A
(b) Choose: CT ratio = 250 / 5
VT ratio =
(c) Impedances measured by relay is
Impedance of line L1and line L2 as seen by the relay (Line impedances based on secondary ohms) are:
Zsec - L1 = 0.026 (2+j20) = 0.52 + j 0.5196 Ω
Zsec – L2 = 0.026 (2.5+j25) = 0.65 + j 0.6495 Ω
(d) Zone 1 setting of relay R 12 is
Z1 = 0.8 (0.52 + j 0.5196 )= 0.0416 + j 0.4157 sec. Ω
(e) Zone 2 setting of relay R 12 is
Z2 setting is = 120% - 150% , Choose 140% Z2 = 1.4 (0.52 + j 0.5196 )= 0.0728 + j 0.727 sec. Ω
(f) Zone 3 setting of relay R12 : Since the zone 3 setting must reach beyond the longest line connected to bus 2 ,thus
Z3 = 120% (Zsec - L1 + Zsec – L2) = 1.20 (0.52 + j 0.5196 + 0.65 + j 0.6495) = 0.1400 +j 1.402 sec. Ω
Fig.11 Typical per-phase arrangement for a three – zone distance relay with directional unit .The directional unit may be a wattmetric relay.