STARTING MY LECTURE

A power system, containing a vast spread out area, is left to the vagaries of Mother Nature.Thunderstorm cyclone snowstormcausing interruption in power transfer and deliveryThunderstorm Problemscan cause short circuit between two linesthe magnitude of a short circuit current is several times the normal operating current.

Power apparatus must be protected from large currentElectrical systems can fail - short circuit -overheating The aim of the protection system is to protect the remaining equipment, consumers and life and property

The protection system should prevent the flow of continuous fault current

To detect unwanted & intolerable conditions faults conditions.To isolate the fault conditions:- Automatically & in short time possibleAt the smallest portion of the system (minimizing the power cut off).

To prevent

-Personnel injury- Damage to the equipment

Simple Fuse (actually as a combined CT, protection relay and CB).MCB with overcurrent or undervoltage tripping arrangements. At Distribution LevelFuse for overcurrent protection of individual feeders, rings and etc by means of fuse switch (A combination of mechanical load breaking switch and a series of fuses to interrupt short circuit currents).

Automatic reclosure schemes controlled by simple protection systems based on a current measurement (GIS / GCB, VCB, ACB, MCCB).

At Transmission LevelTechnique of protection is prevailed the economics considerations.The protection must: Fast to maintain system stability (this problem increases with system voltage and line MVA capacity).

Posses good discrimination to minimize supply disruptionEmployed one or more main protection systems with one or more additional back up protection systems.

Instrument Transformer: CT - Current Transformer (50:5, 100:5, etc)VT Voltage Transformer (100:1, 1000:1, etc)

Protection Relay:

Decision maker

Is there any fault?Should I react to it?Send trip signal to CB

Isolate the healthy system from the fault.

Communication Systems (additional component)Alarm indication, data measurement, block/accelerate commands, fault record, etc.

Degree of Protection Scheme

Discrimination / Selectivity- An ability of the protection system to select whether to operate or not & isolate the fault part only (the nearest circuit breaker should trip).This can be done by discriminating the protection system using time delay.

2. Stability- An ability of the protection system to maintain not operates under certain faults conditions since another protection system will be triggered.- This is because the fault is out of its zone.

Some of the main reasons for protection are listed below.To prevent voltage dips for customers (quality of supply).To minimize loss of revenue for supply industry and customers.To prevent and/or minimize plant damage.To maintain system stability (for transmission networks).To maintain public and personnel safety.To ensure discrimination and maximize reliability.

Figure . Currents flowing through the breakers for a 2LG fault at Bus 3.

Currents flowing through the breakers when the 2LG fault is isolated by BRK1.

The specifications of a fuse are normally based on the following four factors

Voltage Rating: Continuous Current RatingInterrupting Current RatingTime Response

They are typically mounted at the top of the pole for 11-33kV systems

CIRCUIT BREAKERS The circuit breakers are the main protection element of the power system Open when fault current flows through the circuit.

Isolate the high primary voltage of the system (main system) from the protection and measuring equipmentTransform the high primary current I the circuit to a small secondary current in the 1 5 Amp range Example of CT ratio: 100/1, 200/1 100/5, 200/5, etc

If the primary current changes the secondary current output will change accordingly. For example, if 150 amps flow through the 300 amp rated primary (300:5), the secondary current output will be 2.5 amps.

Protection CTMonitor operation of power gridnot as accurate as Measuring CTsfor supplying current to protective relays. The wider range of current allows the protective relay to operate at different fault levels.

*