Over/Under Voltage Protection

::MiCOM P921 Relay::

In GEPCO GSO the MiCom P921 relay is normally used on capacitor panels for over voltage protection and disconnection of capacitor bank.

The Functions which it can perform are:

1.?????(27) Under voltage protection

2.?????(59) Over voltage protection

3.?????(59N) Zero sequence over voltage

MiCom P921 relay has

·????????2 numbers of inputs?

. 4 numbers of output relays

·????????Setting group 1 (P922 & P923 has group 2)

·????????Circuit breaker state supervision feature.

·????????Logic equation AND/OR and NOT operations.

·????????4 numbers of fixed and programmable LED’s each.

(27) Under voltage protection:

When voltage of grid/phases goes down below a certain limit then the capacitor bank will be energized automatically. In GEPCO capacitor banks are energized manually therefore this feature is disabled.

(59) Over voltage protection:

When voltage of grid/phases goes up and crosses a certain pre-set limit then the capacitor bank is automatically disconnected. It works with phase to phase and phase to neutral voltage and each voltage is monitored separately.

(59N) Zero sequence over voltage:

It detects the insulation faults by measuring residual voltage in isolated neutral systems. In GEPCO this feature is also disabled.

Connection Diagram:

??????Cantt. Grid GRW Connection Details

Pin Supply Function

18 DC Output 4 ?relay

20 DC Output 4 ?relay

33 DC + Vaux

34 DC Vaux

R Phase PT Input connections 41

Y Phase PT Input connections 43

B Phase PT Input connections 45

R,Y,B Phase Star Point 42,44,46

?Residual voltage:(Vr)

When capacitor is disconnected then it will have charge on it, this voltage is called residual voltage.

Vr = 3 x Zero sequence voltage (Vo)

It is calculated by relay program when there is an unbalancing condition between the phase voltages.

?After disconnection it will discharge completely in five time constants. Its voltage must be 50 V or less within five minutes.

?????Time constant= τ = RC

?where R is connected with in the cell.

Hystresis:

Energy needed to align/randomize the molecular dipoles of dielectric material within the capacitor plates is called hysteresis loss. ?It does not follow the electric field but depends upon the material only therefore it has a limited range or a band.

Hysteresis is required for the relay for continuous trip and reset of relay. If there is no hysteresis settings then relay will trip at 110 V and will reset at 109 V. It means small fluctuation in voltage will create nuisance trapping.

Hystresis = Reset value/Pick up value

In MiCom P921 relay Hysteresis limits are when voltage range (A) 5V to 130 V is selected

·?????????(27) Under voltage protection??????????????????????????????????(1.02-1.05)

·????????(59) Over voltage protection??????????????????????????????????????(0.95 – 0.98)

·????????(59N) Residual Over voltage protection????????????????????(0.95 Fixed)

Example:

If the over voltage setting of a relay is 110 V and the hysteresis is 0.98 (98%) then relay will reset only when the voltage is (110 -0.98*110 = 107.8 V)

Relay Settings:

In GEPCO GSO normally two voltage thresholds are used one as low level and other as high level.

Low Level Setting= 1.1* (110/√3) = 69 V????High Level Setting= 1.15*(110/√3) = 73 V

Relay Operating Time:

Cantt. S/S GRW Settings

TMS 10

V Depends upon condition

Vs 110/√3 =?63 V

t X sec

Relay has three options first of which is IDMT and remaining two are DMT. (Blocking logic1/2 parameter)

??????????? ?t = [(TMS)/{(V/Vs)-1}]???? ??(for 59 valid if?? V/Vs ?>1.1)

Where

TMS = Time multiplier setting???????????????????????

V = Applied input voltage

Vs = Relay setting voltage

Mode of operation:

There are four modes of operation. In GEPCO no residual VT is installed.

1.?????3Vp-n + Vr (3 P-N VTs & 1 residual VT)???2.?????3Vp-n???(3 P-N VTs )??

3. ??2Vp-p + Vr (2 P-P VTs & 1 residual VT)??????4. ??3Vp-p + Vr (3 P-P VTs & 1 residual VT)

Logical Operations:

In MiCOM P921 the following logical operations are available for programming:

AND, OR ,NOT, NAND and NOR .AND logic has precedence over OR logic.

Here all three voltage values are in AND mode means when all the three values will be greater than the set value of voltage then it will initiate trip command only. Similarly the the OR operation can be used for tripping when any one of the voltage will exceed the pre set value.?

Note: This is just an educational post not for implementations in real system.