Neutral Break to Distribtion System

 

Study of Neutral Break Condition

 Preamble

In case of transmission system, there is situation when load (received) voltage is higher than supply (sending) voltage. This phenomenon is Ferranti Effect. This happens on Unloaded Long EHV line. But similar situation occurs in distribution system on Loaded Short LV line under neutral break condition. 

There are two conditions of neutral break as under

 A.    Conditions when neutral connection to ground is open as shown in the figure due to break between transformers neutral to earth connection.                                                                

     

Consumers continue to get normal phase and line voltage as usual. Effect of such isolation is on its ground fault protection and insulation system. 

A.    B.    Conditions when neutral connection to distribution system is open as shown in the figure due to break between transformer neutral to distribution system.

    


This condition of neutral break has serious impact on single phase power users in distribution system.

 Importance

This fact is important for engineer who has to work in power distribution companies or industries having distribution network. Ignorance of the fact may lead to havoc due to severe damage to single phase equipments.

Theory

Each phase voltage is equal to line voltage divided by √3. When line voltage is 433V, phase voltage is 250V. Normal system neutral point is at the centre N of the vector triangle as in the diagram. Phase voltage Vrn, Vyn and Vbn are equal ( = Line voltage / √3) as appear in the diagram. Condition when transformer neutral connection toward distribution side is open is known as Floating Neutral. If the load is balanced on three phases than floating neutral continue to be at center and still all the three phase voltage will be normal. But if the load is unbalanced on three phases than neutral gets shifted from N to N’ as sown in the vector diagram and phase voltages are also unequal. Phase voltage with respect to shifted neutral may be higher on one or two phase and lower on other phases. Shifting of neutral and deviation in phase voltage depends on pattern of unbalance of single phase load. Ultimately single phase consumers on particular phase may have low voltage and face associated problems like reduced lamp brightness, starting trouble of tube light and also high current, overheating and occasional failure of motors of various gadgets like water pump, fridge, air conditioner etc. Whereas single phase consumers on other phase have high voltage and face associated problem like fusing of lamp, damage to electronic circuits like that of TV, Audio system, Desktop and also failure of gadgets due to breakdown of insulation. 

Instrument/equipment Required

Transformer

It should be delta / star of any vector group so as to have three phases with neutral on load side.

Secondary phase voltage should be less than 200V.

Can be adjusted by variac on primary side.

For neutral       AC Ampere meter 0-10A  1 No.

  AC Voltmeter 0-500V        1 No.

   Single pole switch       1 No.

For Three phase           AC Ampere meter 0-10A   3 No.   (OR Single pole switch    3 No.)

Three phase variac

Three phase lamp bank

Connecting wires

Connections

Connections made as per diagram.

Ampere meters are connected in each phase to monitor loads. Single pole switch can be connected in place if required meters not available. This will facilitate to utilize one ampere meter for all.

Measure phase voltage at load and adjust to less than 200V by variac on supply side.

This is to protect lamps when high voltage occurs.

Load can be three phase lamp bank OR three single phase lamp banks connected in star.

Procidure

Four conditions to be studied

A.    Balananced Load & Continuous Neutral

B.     Balananced Load & Broken Neutral

C.     Unbalananced Load & Continuous Neutral

D.    Unbalananced Load & Broken Neutral

A.    Data to be recorded

Each phase load current. Ir, Iy, Ib

Read ampere directly if ammeter is provided. Else follow as under

First connect ammeter across switch and then toggle switch to OFF and read the current.

Than first toggle switch to ON and disconnect meter. Repeat this for others current readings.

Each Phase Voltage (phase to neutral) r2-n2, y2-n2, b2-n2

Neutral Current  n1-n2 (with switch OFF)

Open Neutral Voltage  n1-n2  (with switch OFF)

Steps

Keep all switches ON

Switch ON supply, measure phase voltage and adjust to less than 200V by variac.

Switch ON lamp bank and adjust balance load and take meter readings as per table.

With same load, neutral switch made OFF and take meter readings as per table.

Neutral switch made ON and adjust unbalance load and take meter readings as per table.

With same load, neutral switch made OFF and take meter readings as per table.

Step D can be repeated with different combination of unbalance load.

Observation Table

Case

Ir

Iy

Ib

Vr2n2

Vy2n2

Vb2n2

In1n2

Vn1n2

Remark

Case 1A

 

 

 

 

 

 

 X

X 

 

Case 1B

 

 

 

 

 

 

 

 

 

Case 2A

 

 

 

 

 

 

X 

 X

 

Case 2B

 

 

 

 

 

 

 

 

 

Extra

 

 

 

 

 

 

 

 

 

Extra

 

 

 

 

 

 

 

 

 

 Exercise

Use reading of case 2B

 Voltage Vector Diagram

Draw equilateral triangle with line voltage  =  normal phase voltage × √ 3

Centre of triangle n1 is transformer neutral

Draw three arches of measured phase voltage from respective node.

Common intersection of arches n2 is the load side displaced neutral.

Measure n1n2 . It should tally with observed voltage between transformer and load neutral.

Current Vector Diagram.

Draw phase current end to end to add up. (This is equilateral triangle for balanced load. )

Here this may be open triangle due to unbalanced load.

Measured open side that represent neutral current. It should tally with observed neutral current.

 Note : This can also be checked analytically using trigonometric functions.

 Conclusion

Phase voltage in distribution system increase/decrease when neutral break in unbalance load condition. 

Field Application

The effect of unequal phase voltage may not be severe where load is almost balanced as in industrial zone where most of the load is three phase.

But effect is severe in residential and commercial zone where most of the load is single phase.

It is important to take extra care for continuity of neutral line.

Break in phase line is failure of supply to consumers on that phase. Impact is no supply for some time.

But impact of break in neutral line is damage to numbers of gadgets of many consumers.