Why Fluorescent Light Bulbs Flickering?

Have you seen fluorescent light bulb flickering? Sure does. But have you ever wondering what the cause of this flickering are?

There are several factors that contribute to flickering in these light bulbs which are:

a)    Frequency

In reality, the lighting bulb is always flickering. This is due to the frequency of the power supply itself. Let say, for 50Hz of power supply, the bulb will be in OFF for 100 times in every second due to the a.c signal reaching zero. This flickering however is non-identifiable by using naked eye.

b)    Ballast

Ballast that is equipped in the light fitting is to control the flow of electricity from a power source to the light source. This ballast purposely to deliver a steady flow of electricity to the light bulb and when this steady flow is disrupted, the bulb will flicker.

c)    Improper bulb and improper installation

Mismatch of voltage might occur once improper bulb being use which results flickering. Bulb that not tightly fitted can cause this flickering problem.

d)    Wear & Tare

As the years goes by, the phosphor coating inside the bulb that reflect ultraviolet light will deteriorate thus flickering occur.

e)    Weather Effect

Fluorescent Bulb is filled with gas. This gas basically requires enough temperature (higher than 50 Fahrenheit) in order to fully light up the bulb.

f)     On certain cases, you might influence flickering when the bulbs are operated on a dimmer switch.

What is Earthing Transformer?

Have you heard about Earthing Transformer? Some says YES, but most will say NO. It is not surprise since most of electrical engineers are being exposed with normal types of transformer such as power transformer (132/33kV and etc), distribution transformer (33/11kV or 11/0.433kV) and instrument transformer (VT and CT).

Earthing transformer is usually being used in electrical substation. The main purpose is to create a neutral point in areas where earth point is not available. Earthing transformer having  the zig-zag (interstar) winding. This winding arrangement is to achieve the required zero phase impendence stage which provides the possibility of neutral earthing condition. In addition auxiliary windings can also be provided to meet the requirement of an auxiliary power supply.

As for connection, the earthing can be connected directly, through an arc-suppression reactor or through a Neutral Earthing Resistor (NER). The excess current from power transformer is being fed to the NER then to the HV part of earthing transformer. This earthing transformer will convert the incoming power to 415V of power supply. This power supply eventually will be supplied to the LV Board which is being used for the power consumption of the substation itself. In the nutshell, the leakage current from power transformer is being used as a source of power for the whole substation rather than it’s going directly to the ground as wastage.

Generally, there are 4 inspection/tests need to be done to ensure good functionality of the earthing transformer:

i)              Physical Check

ii)             Electrical Test

iii)            Insulating Oil Test and

iv)           Functionality check.

Physical Check

Below is the itemized that need to be check;

i)              Check for any corrosion, damage or defect

ii)             Coordinating gaps (alignment and gap where applicable)

iii)            Name plate

iv)           Earth connection

v)            Pressure Relief Device (PRD)

vi)           Bucholz relay

vii)          Breather (leaks inspection, silica gel and etc)

viii)         Valves in operational position

ix)           Oil level in conservator tank

x)            Missing components

xi)           Ensure all terminal blocks, gauges and other accessories are fully sealed and moisture proof.

Electrical Test
Below is the testing that needs to be carried out:

No.	Test	Description	Remarks
1	Insulation Resistance	It shall be perfomed using a 5kV insulation tester. The following measurement must be recorded: a) Between Primary and Earth b) Between Secondary and Earth c) Between Primary and Secondary	All value must acceed 200MΩ
2	Polirization Index	It shall be perfomed using a 5kV insulation tester. The following measurement must be recorded: a) Between Primary and Earth b) Between Secondary and Earth c) Between Primary and Secondary	value shall be > 1.2 (R10/R1min)
3	Turn Ratio	Ration of HV over the LV winding	Within 0.5% of the nameplate ration of the rated voltage.
4	Impedance Voltage	Percentage measurement of zero and positive sequence impedance	As declared on nameplate
5	Winding Power Factor	Measurement of power factor of all windings	shall be < 0.5% at 20˚C
6	Vector Group	Checking the vector group of the transformer	measured vector diagram shall be equal with manufacturer's vector diagram

Insulating Oil Test

Below is the testing that needs to be carried out:

No.	Test	Description Highest Voltage for Equipment (≤72.5kV)	Remarks
1	Moisture Content (ppm)	≤ 15	IEC 814
2	Neutralization Value (ppm)	≤ 0.03	IEC 296
3	Interfacial Tension (mN/m)	> 35	IEC 6295
4	Colour	≤ 2.0	IEC 296
5	Dielectric Breakdown Voltage (kV) Test Gap (2.5mm)	> 50	IEC 156
6	Oil Power Factor	< 0.1% at 20°C
No.	Test	Test Method	Remarks
7	Dissolved Gas Analysis (DGA)	Using Full Gas Extraction and Headspace Methods Standard ASTM Standard D 3612-01 Analysis od Gasses Dissolved in Electrical Insulationg Oil by Gas Chromotophraphy Method A & C IEC567 Standard Section 7.3 Vacuum Extraction by Partian Degassing Method.	Hydrogen gas levels should not exceed 20ppm No detactable amount of Methane, Ethylene and Acetylene gases. This test dhall be carried out one to two eweek after continuous energisation of transformer.

Functional Check

To ensure that the transformer is functionally well, the itemized below need to be inspected:

i)               Transformer guard check: alarms, indications, flags, facia and relay operation at transformer, relay panel and control panel shall be recorded clearly. Actual operation for each signal of all earthing transformer guards shall also be check and mentioned except for PRD.

ii)             Oil level indicator shall be functional.

iii)            All temperature indicators shall be operational.

TNB's Term For Newbies

For those who are newbies with working related with Tenaga Nasional Berhad, this are few technical term that commonly used.

No.	Term	Definition
1	Transmission Facility	The transmision substation and/or line cable equipment (66kV and above) or part thereof. It shall also include the distribution equipment in the transmission substation.
2	Site Acceptance Test (SAT)	Pre-commissioning test performed on the transmission facility before PIAT
3	Pre-commissioning Inspection and Testing (PIAT)	Quality Assessment activity performed by PCU after the successful completion of SAT to determine the readiness of the Transmission Facility to be energised.
4	Primary Injection	Testing by means of injecting current into the primary circuit to verify the CT Ratio, CT Polarity and the CT secondary circuits. It is not being considered as a stability test.
5	Stability Test	It is the test to asess unit protection as a complete scheme thatremain unstable for all out-zone faults and operate only for in-zone faults.
6	Unit Protection	Protection system with a clearly defined zone of protection and it is usually demarcated betwenn 2 or more sets of CT's.
7	On-Load Test	It is the test  or measurement for by means of using the energised secondary voltage and secondary load current after a circuit has been energised.
8	Live Plant	A fenced area with energised high voltage equipments.
9	Live Bay	A transformer, line/cable, bus coupler or bus section bay that being energised at high voltage. Since the word is "LIVE", those equipments basically is not isolated or earthed.
10	Electrical Service Engineer	A person who being certified by Energy Commission to carry-out the work.
11	PTT	Protection, Telecommunication & Telecontrol Department in TNB Transmission Division.

The Chronicle of The Lightning Protection System.

BS CP326 was first published in 1965. Then, the BS 6651:1999 Protection of structure against lightning has been adopted as guidance on the design and installation of lightning protection since 1985.

Since that 1985 publication, the UK has become more involved with the European Union (EU), particularly relating with the standardization. As a result, any British Standard now published has to be in common with its European equivalent.

By publishing the BS EN 62305, there will be a finite period of time (this period will be within 2 years) when both BS 6651 and the new BS EN 62305 series will run in parallel. Eventually, the British Standard Institution has to withdraw all their conflicting National Standard (i.e. BS 6651) in favour of the EN Standard. By the end of August 2008, the withdrawal of BS 6651 will take place.

British Standard European Norm (BS EN) 62305 is a standard which describing the “Protection Against Lightning’. It is itself consists of four distinct parts:

a)    Part 1: General Principle

It is an introduction to the other part of the standard and essentially describes on how to design the Lightning Protection System in accordance to the standard.

b)    Part 2: Risk management

The risk management is more concentrate on the risk of human life, loss of service to the public, economic losses as well as loss of cultural heritage.

c)    Part 3: Physical damage to structures and life hazard

It relates directly to the major part of BS 6651. It differs from BS 6651 in as much that this new part has four Classes or protection levels of Lighting Protection System.

d)    Part 4: Electrical and electronic system within structures

This part covers the protection of electrical and electronic systems housed within structures. With the new zonal approach called Lightning Protection Zones (LPZs), it provide information for the design, installation, maintenance and testing of a Lightning Electromagnetic Impulse (LEMP) protection system for electrical / electronic system within structure.

As the above is a general content of my posting and I will elaborate more on each of every part in BS EN 62305 in my next post.

How To Test A Power Transformer.

Based on IEC-76, there are few testing need to be carried out to ensure the power transformer are technically sound to be used. 

In general, testing of power transformer are divided into three namely

a) Routine Test

b) Type Test and

c) Special Test

Other than the above testing, Transformer Oil Test needs to be done prior to the energizing of power transformer.

Routine Test consists of a few tests as below:

a) Ratio Test

b) Winding Resistance Test

c) Load Loss Test

d) No Load Loss Test

e) Induced Over-voltage Withstand Test

f) Separate Source Withstand Test

g) Test On-Load Tap Changer Type Test however consists of:

h) Lightning Impulse for Line Terminal

i) Temperature Rise Test

And for Special Test will be:

a)    Insulation Resistance Test

b) Sound Level Test

c)    Lightning Impulse Chopped On The Tail

d)    Tan Delta Test

Okay, we narrow down into individual test.

Routine Test

a)    Ratio Test

This testing is to check that the deviation of the voltage ratio does not exceed the limit of the transformer standard used (generally 0.5%). It is also to ensure that the turn ratio of the winding is correct and that any tapping on the windings have been made at the correct position.

At the same time, the vector relationships are also being checked.

A ratiometer will be used during the testing and the result shall be compared with those calculated.

b)    Winding Resistance Test

The resistance between all pairs of the phase terminal of each transformer winding are measured using Direct Current (DC). The measurement is performed foe each connection of windings and for each tapping connection. Furthermore, the corresponding winding temperature is measured.

The measured resistance are needed in connection with the load loss measurement when the load loss are corrected to correspond to the reference temperature. The resistance measurement will also show whether the winding joints are in order and the winding are correctly connected.

c)    Load Loss Test

This test is to determine the load losses of the transformer and the impedance voltage at the rated frequency and the rated current. The measurements are made separately for each winding pair. (e.g. pair 1-2, 1-3 and 2-3).

In the testing, current is generally supplied to the HV winding and the LV winding is short-circuited.

d)    No Load Loss Test

This test is to fine out the electrical losses due to the core of the transformer. In the no-load measurement, the no-load losses and the no-load current of the transformer are determined at rated voltage and rated frequency.

e)    Induced Over-voltage Withstand Test

The objective of the test is to ensure that the insulation between the phase windings, turns, coils, tapping leads and terminal can withstand the temporary over-voltage.

Double rated voltage (e.g. 11kV becoming 22kV) and double rated frequency (e.g. 50Hz becoming 100Hz) is applied to the terminal of the LV windings. The HV windings are left open-circuited.

The test considered successful if no collapse of the test voltage occurs.

f)     Separate Source Withstand Test

This test is to ensure that the insulation between the windings and the insulation between windings and earth parts can withstand the temporary overvoltage and switching overvoltage which may occur in service.

The test is made with single phase voltage of rated frequency. The test voltage is applied for 60 seconds between the winding under test and all terminals of the remaining windings, core and tank of the transformer connected together to earth.

g)    Test On-Load Tap Changer (OLTC)

i.        Once the OLTC is fully assembled on the transformer, the following tests are to be performed at 100% of the rated auxiliary supply voltage with the exception of Test No.2.

ii.        Test No. 1

8 complete operating cycle with the transformer not energized.

iii.        Test No. 2

1 complete operating cycle with the transformer not energized with 85% of the rated auxiliary

supply voltage.

iv.        Test No. 3

1 complete operating cycle with the transformer energized at rated voltage and frequency at no 

load.

 v.        Test No. 4

10 tap change operation with ± 2 steps on the other side of the principle tapping with as far as 

possible the rated current of the transformer with one winding short circuited.

Type Test

a)    Lightning Impulse For Line Terminal

This test is to ensure the transformer insulation can withstand the lightning over voltage which may occur in service.

This test basically performed with the standard lightning impulse of negative polarity. Conforming to the IEC 76-3 Clause 12.1 Standard, the Front Time (T1) and the Time To Half Value (T2) are defines as below:

Front Time (T1)                       = 1.2µs ± 30%

Time To Half Value (T2)         = 50µs ± 20%

The test will be conducted based on the below sequence:

                      i.        One (1) impulse of a voltage between 50% and 75% of the full test voltage.

                     ii.        Three (3) subsequent impulse at full voltage.

Oscilloscope is to be used during the testing and it shall clearly shows the applied voltage impulse shape for Front Time and the Time To Half Value.

b) Temperature Rise Test

The objective of this test is to check the temperature rise of the oil and the windings do not exceed the limit agreed on or specified by the standard.

The test is performed by using the short circuit method. The temperature rise of the windings is determined by the resistance method.

Special Test

a)    Insulation Resistance Test

The measurement is to determine the leakage current resistance of the insulation. This measurement given the informations about the condition of the insulation and the leakage current of the transformer.

As a rule of thumb, the insulation resistance is inversely proportional with the impurity contents and the moisture of the insulation itself.

b)    Sound Level Test

The objective of this testing is to ensure the sound produced by the transformer comply with specification such as IEC 551and meets the transformer guarantee value given by the transformer manufacturer.

The customer are advised to insist the sound spectrum analysis

c)    Lightning Impulse Chopped On The Tail

The purpose of the chopped lightning impulse test is to ensure that transformer insulation can withstand the voltage stresses caused lightning impulse which may occur during service.

The test is the special test line terminal of a winding. When it has been agreed to make this test, it shall be combine with the lightning impulse test. The peak value of the impulse shall be same as for the full impulse.

The impulse is chopped by means of a triggered-type chopping gap connected to the terminal where the impulse is applied. The delay of the chopping gap ignition impulse in relation to the ignition impulse generated is adjustable, thus the time Tc from the start of the impulse to the chopping can be adjusted.

The test is performed with the impulse of negative polarity. The duration Tc from the beginning of the impulse to be chopping can vary within the range of 2 to 6µs.

Front Time (T1)                       = 1.2µs ± 30%

Time To Half Value (T2)         = 50µs ± 20%

Time To Chopped (Tc)           =  2 to 6µs

The test is combine with the full impulse test in a single sequence. The recommended order of the difference impulse application is:

                      i.        One (1) reduced full impulse.

                     ii.        One (1) 100% full impulse.

                    iii.        One (1) or more reduced chopped impulse.

                   iv.        Two (2) 100% chopped impulse.

                    v.        Two (2) 100% full impulse.