Review Operating time of an overcurrent relay Data required required are: are: 1. TimeTime- PSM curve 2. Current setting 3. Current setting and time setting 4. fault current and CT ratio Directional Over current relays - Operation Operation in one direction direction of power flow - Consists of two unitsunits- Directional Directional unit and an overcurre overcurrent nt unit General equations of comparators
A
K3 A
K1 S1
B
S2
K2
Duality between an amplitude and a phase comparator co mparator
The output device works based on the overage area over one cycle of the output waveform
2. Instantaneous comparators Rectifier circuit
Averaging circuit
Rectifier circuit
Level detector
Output
Restraining signal
Operating signal
Operating signal rectified Restraining signal Rectified and smoothened
Output
3. Sampling comparators-One or both the signals are sampled at the same instant or at the different instants and are compared Eg. In the case of reactance relay, The relay operates when the reactance seen by the relay, X< K Let J be the power factor of the circuit. When current passes through Zero, the voltage will be V sin * The reactance relay operates when,
V
Z sinJ < K,
sin * K
I m
2
I
m
Rectifier
Averaging
Iav
sin J
sin J
I
av
2v
v 1.11 I
av
v 1.11
' I
av
Vm sin*-k¶Iav Zero crossing detector V
Sampling circuit
Vm sin*
Difference
Polarity detector
Output
Phase comparatorThe operation of the relay takes place when the phase relation Between two inputs S1 and S2 varies within specified limits. Two types- Coincidence type and vector product type
Static overcurrent relays 1.Instantaneous over current relays 2.Time over current relays 1. Instantaneous over current relays
Auxiliary CT with filter
Bridge rectifier
Zero level detector
From summation circuit
I input < I threshold : I output = 0 I input >= I threshold : I output = Present
Advantages of static relays 1. Low power consumption. 2. Quick response, long life, high reliability and high degree of accuracy. 3. Absence of moving contacts 4. By combining various functional circuits, several conventional relays can be substituted by a single static relay. 5. Very compact. 6. The risk of unwanted tripping is less with static relays. 7. The characteristic of static relay is accurate and superior. They can be altered within certain range as per protection needs. 8. Static relays are provided with integrated features for self monitoring, easy testing and servicing.
Micro processor based digital relays
Inputs
Analog Input system
A/D converter
Digital processor
To tripping device
Control and Data outputs Advantages 1. 2. 3. 4.
Very small burden on the CTs and PTs. Saturation can be avoided by using air gap CTs. They can process and display signals very efficiently. One microprocessor unit may be able to perform the relaying function of several systems. 5. More reliable. Disadvatages 1. The processor must be properly sheilded from external influences. 2. The system eathing must be very good.
