Friday 20 July 2018

Online Partial Discharge Testing


Partial discharge
Online Partial Discharge Testing:
Online Partial Discharge Testing is performed online without disrupting plant or facility operations. This nondestructive, noninvasive predictive maintenance tool assesses the condition of switchgear, feeder cables, and bus duct to provide critical information regarding levels of partial discharge and its potential impact on the health of the electrical asset. The cost to perform online testing is relatively inexpensive compared with offline testing that requires interruption of service and production. For critical facilities that operate 24x7, online testing is considered the single best option for detecting insulation condition.

 
partial discharge
Technology:
·         ACOUSTIC METHOD
·         ULTRASONIC DETECTION
·         HFCT
 Acoustic Method:Acoustic partial discharge measurement can be easily applied on AIS
without the need of interrupting the operation. Acoustic Partial discharge measurements rely
on close acoustic contact of the area producing discharge. This method is also very effective in
localization of PD.
 Ultrasonic Detection:
All electrical equipment produces a broad range of sound. The basic electrical problems that
produce distinct ultrasound waves that can be detected by Ultrasonic Testing include partial
discharge, corona and tracking. Ultrasonic measurement is most powerful on comparative basis
and will significantly increase the reliability of correct detection of partial discharge.
 HFCT
HFCT is used to detect high frequency current pulse discharging through earth from system
ground. It is also helpful in getting PD Pattern.


PD Tolerance Levels for Switchgear:
mV Level
Condition and Action
< 6
Discharge within acceptable limits*
612
Monitoring recommended
 (Recommended to conduct PD after 1 year.)

1232
Regular monitoring recommended
(Recommended to conduct the PD after 56 months. The panel having increasing trends in PD value should be monitored with help of IR thermography immediately.
> 32
Major concern, located PD with alternate technology

Benefits of On-Line Partial Discharge Field Measurements
·         It is truly a predictive test, indicating insulation degradation in advance of the failure.
·         It is a nonintrusive test, requiring no interruption of service and is performed under
normal operating voltage, load and environmental conditions.
·         It is a nondestructive test; it does not test to failure or adversely affect the equipment under test in any way.
·         It does not use any overvoltage, thereby not exposing the tested equipment to higher voltage stresses than those encountered under normal operating conditions.
·         Trending can be accomplished by storing baseline measurement results to allow comparison with future tests.
·         In many instances the site of the partial discharge occurrence can be located within
the cable/plant under test, so the localised problem can be repaired.
·         The cost to perform a PD-
survey is relatively inexpensive compared with off-line testing, allowing annual surveys to be performed economically at most facilities.

Examples of HV & MV Plant that can be tested
·         Cables and Cable Accessories (terminations and joints)
·         Switchgear (AIS, SIS and GIS)
·         Instrument transformers (voltage and current)
·         Power transformers and bushings
·         Motors and generators
·         Surge arrestors
·         Capacitors


Electrical safety v/s OISD 192



i) Only persons having valid licenses shall be allowed to work on electrical facilities as per prevailing IE Act and rules there under including CEA rules/regulations.
ii) No person should be allowed to work on live circuit. The same, if unavoidable, special care and written authorization need to be taken.
0ISD 192

iii) Treat all circuits as "LIVE" unless ensured otherwise.
 iv) Electrical " Lock Out - Tag Out (LOTO)" procedure "MUST" be followed for work on electrical system. v) Display voltage ratings prominently with "Danger" signs in local language also.
vi) Put caution/notice signs before starting the repair works.
vii) All electrical equipment operating above 250V shall have two separate and distinct connections to earth grid.
viii) Proper grounding to be ensured for all switch boards and equipment including Portable ones prior to taking into service.
 ix) Make sure that electrical switch boards, portable tools, equipment (like grinding machine etc.) don't get wet during their usage. If it happens, stop the main supply, make the tools dry, check for specified insulation value and then only use them. Check proper earthing. All temporary switch boards/ KIOSKS put up at work site should be suitably protected from rain and the level of same should be high enough to avoid contact with water due to water logging.
 x) Don't work wet on electrical system.
 xi) Don't overload the electrical system.
xii) Use only proper rated HRC fuses / ELCB / MCB.
 xiii) Only ISI marked or equivalent industrial type extension boards and Plug sockets are to be used. xiv) ELCB for all temporary connections must be provided using 3 pin plug.
 xv) All power supply cables should be laid properly and neatly so that they don't cause hindrance to persons working and no physical damage also takes place to the cables during various construction activities.
xvi) All Power cables to be properly terminated using glands and lugs of proper size, type and crimped. xvii) Use electrical fittings in Hazard zones as per area classification under OISD-STD-113.
xviii) Ensure pipe sleeve / conduit to protect underground cables at crossings.
xix) Don't lay unarmored cable directly on ground, wall, roof or trees. All temporary cables should be laid at least 750 mm below ground and cable markers should be provided. Proper sleeves should be provided at road crossings. In case temporary cables are to be laid on wooden poles/steel poles, the minimum cable heights should be 4.5 M.
xx) Maintain safe overhead distance of HT transmission lines as per latest CEA Safety Regulation. xxi) Don't use pipelines/structures for earthing.
xxii) Don't make any unsafe temporary connections, e.g. naked joints etc.
 xxiii) Ensure that temporary cables are free from cuts, damaged insulation, kinks or improper insulated joints. xxiv) Check at periodic intervals that pins of sockets and joints are not loose.
xxiv) Check at periodic intervals that pins of sockets and joints are not loose.
xxv) Protect electrical wires/equipment from water and naked flames.
 xxvi) Illuminate level in all the work areas should be in line with OISD-RP-149.
 xxvii) All switchboards should be of MS structure only and incoming/outgoing feeders should be marked.
 xxviii) Hand lamps/ Torch should not be of more than 24V rating, and type should be in line with hazardous area classification.
xxix) Fire extinguishers (DCP/CO2/Sand buckets) should be kept near temporary switch boards being used for construction purposes. Don't use water for fighting electrical fires.
 xxx) ISI marked Insulating mats shall be provided in the front and back end of switch boards.
 xxxi) All parts of electrical installations should be so constructed, installed and maintained as to prevent danger of electric shock, fire and explosion. Periodic checking of electrical safety appliances such as gloves, insulating mats, hoods etc. to be done/witnessed in line with OISD-STD-137, and records to be maintained duly endorsed by the concerned.
xxxii) A notice displaying following, should be kept exhibited at suitable places in local language also: a) prohibiting unauthorized persons from entering electrical equipment rooms or from handling or interfering with electrical apparatus; b) containing directions as to procedures in case of fire, rescue of persons in contact with live conductors and the restoration of persons suffering from electric shock; c) specifying the person to be notified in case of electrical accident or dangerous occurrence, and indicating how to communicate with him.
xxxiii) No other cables/pipes to be laid in trench used for electrical cables. xxxiv) Utmost care should be taken while excavating Earth from cable trench to avoid damage or any accident.
xxxv) Sub-station floor cut-outs meant for switch board installations to be covered wherever installation is incomplete.
 xxxvi) Flameproofness integrity of all flameproof equipment / fittings/fixtures to be ensured at all times.
NOTE: A Residual Current Operated Circuit Breaker (RCCB) or Earth Leakage Circuit Breaker (ELCB), when installed, protects a human being to the widest extent. RCCB or ELCB should be provided as per latest CEA Safety

Typical fault level



The following earthing systems and design fault currents (3-phase symmetrical) are in use throughout the distribution network.
Voltage
Type of earthing
Fault current
Fault level
132KV
Slid earthing
21.9 KA
5000 MVA
33 KV
Impedance earthed
17.5 KA
1000 MVA
11 KV
Impedance earthed
13.1 KA
250 MVA
6.6 KV
Impedance earthed
21.9 KA
19.4 MVA
< 650V
Slid earthing
27 KA

Where the network is impedance earthed the typical neutral earth resistor will limit the earth fault current to 1000A per transformer, i.e. giving a maximum earth fault level of up to 3kA.
Design earth fault clearance times are:
VOLTAGE    CLEARANCE TIME
132 KV           200ms ( mili second)
33 KV                         500ms
6.6/11 KV       1s
Short circuit current, Isc (kA) at the LV terminals of a 3-phase HV/LV transformer.
Transformer rated power (kVA)  
100
315
500
800
1000
1250
1600
2000
2500
Transformer rated (LV) current (A)
137
433
687
1100
1375
1718
2199
2749
3437
Psc = 250MVA
33.1
8.38
13.2
17.4
20.4
21.5
26.4
40.4
49.1











                                                                                           

Cost of electricity


Cost of electricity
The utility electricity sector in India has one National Grid with an installed capacity of 344.00 GW as on 30 June 2018. Renewable power plants constituted 33.23% of total installed capacity.
The gross electricity consumption was 1,122 kWh per capita in the year 2016-17. India is the world's third largest producer and third largest consumer of electricity. The per capita electricity consumption is low compared to many countries despite cheaper electricity tariff in India.
 
power plant model
India has surplus power generation capacity but lacks adequate infrastructure for supplying electricity to all needy people. In order to address the lack of adequate electricity supply to all the people in the country by March 2019, the Government of India launched a scheme called "Power for All. This scheme will ensure continuous and uninterrupted electricity supply to all households, industries and commercial establishments by creating and improving necessary infrastructure. It's a joint collaboration of the Government of India with states to share funding and create overall economic growth.

India's electricity sector is dominated by fossil fuels, and in particular coal, which in 2017-18 produced about three fourths of all electricity. However, the government is pushing for an increased investment in renewable energy. The National Electricity Plan of 2018 prepared by the Government of India states that the country does not need additional non-renewable power plants in the utility sector until 2027, with the commissioning of 50,025 MW coal-based power plants under construction and achieving 275,000 MW total installed renewable power capacity.
The cost of electricity can be divided into
a) Plant-level costs,
·         Capital cost
·         Operation and maintenance  cost
·         Fueling cost
b) Grid-level costs,
c) Other costs.
a) Plant-level costs consist of capital, operation and maintenance, and fuelling cost. Capital cost is reflected in the cost of generation by way of interest on debt and return on equity. For nuclear power plants, capital cost is high, but fuelling cost is low. For coal-fired power plants, capital cost is low, but fuelling cost is high. The capital cost of solar and wind is continuously decreasing; fuelling cost is nil.
Electricity reaches a consumer through the grid. Laying a grid needs significant investment. A distributor buys electricity from a generator, adds transmission and distribution charges, a charge to recover technical losses, operating expenses, and his profit to determine the tariff to be charged from a consumer. Since several generators are connected to the grid, interaction with the grid and grid-management policies influence the working of a generator. At present, electricity markets do not assign any price to system effects, that is, to the complex interactions among various generators connected to the grid.

In recent years, a large capacity based on variable renewable energy (VRE) sources has been connected to the grid. These sources are intermittent, but get priority feed-in due to nil fuelling cost. A grid manager may ensure that adequate dispatchable generation capacity is connected to the grid to meet the peak load in the evening when solar power is not available. Dispatchable generation is provided by base load technologies like coal and nuclear, and by large hydropower.

b) Grid-level costs have several components: grid connection, grid extension and reinforcement, short-term balancing costs, and long-term costs for maintaining adequate back-up supply. VRE sources demand much higher back-up, grid connection and reinforcement costs. This aspect needs attention during policy formulation.


The emphasis on VRE sources without any investment in energy storage has converted daily load profile for dispatchable generating stations into a “duck curve”, that is, with a reduced electricity load during the day when solar is available and a rapid ramp up in the evening. This lowers the capacity factor of dispatchable generators.

c) Other costs
Other costs include those arising from the influence of electricity generation on health, influence on existing generation capacity due to adding new capacity, cost of accidents, security of supplies and net energy gain for society.
It adds health costs, costs of intermittency, opportunity cost of land, cost of government incentives and cost arising from stranded assets.
Conventional metrics like levellised cost of electricity generation cannot be relied on to compare intermittent and dispatchable electric supply options. India’s electricity requirements are enormous. It doesn’t need a ‘technology versus technology’ debate, but a policy framework that integrates all low-carbon energy technologies with coal in a manner that ensures reliability and security of electric supply along with affordability and climate-resilient development.
The average unit cost of energy during 1999-2000 in the country as below:
Item                                        % of cost/KWH
Fuel                                         17.2%
Power purchase                       44.5
Estt/admn                                13.6
Miscellaneous                         1.8
Depreciation                            6.6
Interest                                    12.9
Types: There are two tariff systems, one for the consumer which they pay to the DISCOMS and the other one is for the DISCOMS which they pay to the generating stations. Let us first discuss the tariff of electricity for the consumer i.e the cost consumer pay to the DISCOMS. The total cost levied on the consumer is divided into three parts usually referred as 3 part tariff system. Here, a = fixed cost independent of the maximum demand and energy consumed. This cost takes into account the cost of land, labor, interest on capital cost, depreciation, etc. b = constant which when multiplied by maximum KW demand gives the semi-fixed cost. This takes into account the size of power plant as maximum demand determines the size of power plant. c = a constant which when multiplied by actual energy consumed KW-h gives the running cost which8m takes into account the cost of fuel consumed in producing power.
Ref:
Electric power distribution, A.S Pabla

Central Electricity Regulatory Commission




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