Transmission lines are a vital part of the electri cal distribution system, as they provide the path to transfer power between generation and load. Transmission lines operate at voltage levels from 69kV to 765kV, and are ideally tightly interconnected for reliable operation.
Transmission protection systems are designed to identify the location of faults and isolate only the faulted section. The key challenge to the transmission line protection lies in reliably detecting and isolating faults compromising the security of the system.
The advents of satellite-based time-keeping systems and advances in computer technology have made possible protective relay sampling synchronization within 1µs. These relays can now provide synchronized phasor measurements that eliminate the need to have different devices for p rotection, control, and electric power system analysis for system-wide applications and traditional protection applications.
In this report, we have suggested a protection scheme which uses the concept of phase comparison protection using synchrophasors. This scheme effectively detects faults inside the zone of protection and is unaffected by any mal-operations which affect the distance protection schemes currently being employed.
2. PRESENT DISTANCE PROTECTION SCHEME
Fig. 2.1 Trip characteristics of MHO rela y
If the impedance seen at the end of the line falls inside the trip region then the relay will trip.
Fig. 2.1.1 Three Stepped Protection Scheme
In present distance protection scheme we use Stepped Zones of P rotection as shown in fig.2.1.1. Faults in 85-90% of the line fall in zone 1 and are cleared instantaneously. The faults beyond this fall in zone 2 and are cleared after a certain time delay (due to uncertainty in location). The delay insures proper coordination, and helps to avoid shutting down of a longer section of line than are necessary to clear the fault. Stepped Distance Protection Schemes:
– Trips with no intentional time delay
– Under reaches to avoid unnecessary operation for faults beyond remote terminal
– Typical reach setting range 80-90% of ZL
– Set to protect remainder of line
– Overreaches into adjacent line/equipment
– Minimum reach setting 120% of ZL
– Typically time delayed by 15-30 cycles
– Remote backup for relay/station failures at remote terminal – Reaches beyond Z2, load encroachment a consideration
1. Low cost
2. Design simplicity
3. Reliability of operation.
1. In case of ground faults the arc resistance is measured along with the line impedance. This leads to mal-operation known as “underreach”. Due to under- reach the faults at the far end of zones of protection remain undetected.
2. Due to dc offset during fault, the current value can jump or drop to a certain value and hence the impedance seen will be such that the relaying scheme will give trip command leading to “over-reach” maloperation. Due to this mal- operation even if fault occurs outside zone of protection, for instance in the adjoining transmission line then also this relaying scheme acts as if fault is in its zone of protection and trips. 3. During power swings in case of double ended transmission lines the impedance seen by relays enters in tripping region of relay. Thus leading to a mal -operation.
Also, this present scheme provides relatively low accuracy in fault location. For example if there is an error of 5% in the estimate of fault location in a 100 km line means a potential error of 5 km. This 5km region will have to be visually inspected to find the location of fault for repair or maintenance purposes.
The development of communications and the introduction and availability of a standardized time reference over wide geographic areas by Global Positioning System (GPS), laid the...
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