Friday 10 February 2012

Pole Foundation

Foundation:
Foundation is required to support structure for transmission tower, lighting poles, and transformer erection etc in the field of electrical engineering.
For foundation of single wooden, concrete or steel pole, the pit is excavated to a required depth. Precast concrete poles or steel poles are inserted in the pit. Concrete mixture is poured layer by layer and is rammed every time. Mixture ratio is normally 1:2:4 of cement: san: stone.
Depth for foundation is generally 1/6 Th of the length of pole.

Typical depth of foundation pits are as follows:
Pole size (meter):                                          9 10        12        15        18
Depth of foundation:                                  1.5         1.7          1.8        1.8          2
Location of poles:
Poles are located at least 1.5meter away from fire hydrant, underground cable trench, and 5meter away from road boarder and railway

Height of the pole in distribution system-
10M, 12M, 14M
Span of the pole:
Residential area: 30 to 50m
Rural areas        : 50 to 60m

Foundation for 9 meter street pole                                                                       
a)      Lean concrete ratio-1:3:6 (cement: sand: stone)
Volume:  0.7X 0.7X0.15 = 0.0735M3
Cement required per M3= 220kg
Cement for above volume= 16.17 kg
b)     Pedestal PCC at ratio 1:2:4
Outer dia =0.540M
Inner dia= 0.15M
Height = 1.5M
Volume = π/4 x (D2-d2) x H
             = π/4 x (0.542-0.142) x 1.5
             = 0.321M3
Cement required for ratio 1:2:4 @ =320kg per M3
For 0.321 M3 volume cement required = 320 x 0.312 = 102.72kg
c)      Copping ratio 1:2:4
Outer dia =0.30M
Inner dia= 0.14M
Height = 0.3M
Volume = 0.021M3- 0.004M3)
             = 0.017M3

Total cement consumption per pole =a+b+c = 16.17 +102.72+ 5.44 = 124.33kg



Earth pit chamber:
Plaster 1:4 cement =5.47kg/M3
Brick masonry 2x0.6x0.45x0.115=0.062M3
                       2x0.3x0.45x0.115=0.031M3
                                      -------------
                                          0.093M3

Cement consumption: 0.093x95=8.835kgs
Plaster 4x0.3x0.45 = 0.54M3
            4x0.6x0.1 = 0.24M3
            2x0.3x0.15 = 0.09M3
                                ------------
                                1.05x5.47 = 5.749kg
Total cement consumption
 For one earth pit chamber = 14.58kg

FOUNDATION OF FEEDER PILLAR & TELEPHONE DB    
1. Lean concrete: 1:3:6
Cement 0.11x 220kg/M3
2. Brick work = 2x1.0x0.5x0.075= 0.075M3
                        2x0.5 x0.35x0.6= 0.21M3
                             0.9 x0.5x0.3 = 0.135
-------------------------------------------------
Total                                          =0.42M3

Cement = 0.42x95kg/M3 (1:4 mortar)
                 =39.9 kg
3. Plaster 12 mm thick
0.9 x0.3x2 = 0.54M2
    Total  = 0.84M2
Cement: =0.84x547kg/100M2
                =4.6kg
4. Neat coat =1.7kg@1.9kg/M2 for area of 0.84M2
Total cement for one foundation = 70kg

Foundation of post top fixture
a)      Lean concrete ratio-1:3:6 (cement: sand: stone)
Volume:  0.7X 0.7X0.15 = 0.0735M3
Cement required per M3= 220kg
Cement for above volume= 16.17 kg
b)      Pedestal PCC at ratio 1:2:4
0.5x0.5x0.1 = 0.25M3x320 =80kg
Height = 1.5M
Cement required for ratio 1:2:4 @ =320kg per M3
c)      Copping ratio 1:2:4
 0.3x0.3x0.045x320=14.40kg

Total cement consumption per pole
=a+b+c = 16.17 +80+ 14.4 = 110.57kg

Friday 3 February 2012

Instrument transformers

Instrument transformers
Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. Their role in electrical systems is of primary importance as they are a means of "stepping down" the current or voltage of a system to measurable values, such as 5A or 1A in the case of a current transformers or 110V or 100V in the case of a voltage transformer.
·         Voltage transformers
·         Current transformers
Voltage transformers
  • Principle of operation
  • Definitions
  • Standards
  • Tests
  • Typical Specifications
Principle of operation
The standards define a voltage transformer as one in which "the secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections."
In a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance.
 The best grades of cold rolled grain oriented electrical steels which enables operation at optimum levels of magnetic induction.

Terminology  
voltage transformer (VT) or potential transformer (PT)
  1. Rated primary voltage: This is the rated voltage of the system whose voltage is required to be stepped down for measurement and protective purposes.
  2. Rated secondary voltage: This is the voltage at which the meters and protective devices connected to the secondary circuit of the voltage transformer operate.
  3. Rated burden: This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on the VT. This includes the burden imposed by the connecting leads. The VT is required to be accurate at both the rated burden and 25% of the rated burden.
  4. Accuracy class required: The transformation errors that are permissible, including voltage (ratio) error and phase angle error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and Class 3. Both metering and protection classes of accuracy are specified. In a metering VT, the VT is required to be within the specified errors from 80% to 120% of the rated voltage. In a protection VT, the VT is required to be accurate from 5% upto the rated voltage factor times the rated voltage.
  5. Rated voltage factor: Depending on the system in which the VT is to be used, the rated voltage factors to be specified are different. The table below is adopted from Indian and International standards.
Rated voltage factor
Rated time
Method of connecting primary
winding in system
1.2
Continuous
Between phases in any network
Between transformer star-point and earth in any network
1.2
1.5
Continuous
for 30 seconds
Between phase and earth in an effectively earthed neutral system
1.2
1.9
Continuous
for 30 seconds
Between phase and earth in a non-effectively earthed neutral system with automatic fault tripping
1.2
1.9
Continuous
for 8 hours
Between phase and earth in an isolated neutral system
without automatic fault tripping or in a resonant earthed
system without automatic fault tripping
  1. Temperature class of insulation: The permissible temperature rise over the specified ambient temperature. Typically, classes E, B and F.
  2. Residual voltage transformer (RVT): RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is no voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. When using a three phase RVT, the primary neutral should be earthed, as otherwise third harmonic voltages will appear across the residual winding. 3 phase RVTs typically have 5 limb constructions.
  3. Metering Units: 11kV metering units consist of one 3 phases VT and 2 CT's connected together in a single housing. This can be used for three phase monitoring of energy parameters. It is used with trivector meters and energy meters.
Standards
Standard
Standard Number
Year
Indian
3156
1992
British
BS EN 60044-2
1997
British
BS 7729
1994
International
Electro technical
Commission (IEC)
IEC 60044-2
1997
A number of routine and type tests have to be conducted on VT s before they can meet the standards specified above. The tests can be classified as:
  1. Accuracy tests to determine whether the errors of the VT are within specified limits
  2. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, induced over-voltage test , impuse tests with 1.2u/50u wave, and partial discharge tests (for voltage>=6.6 kV) to determine whether the discharge is below the specified limits.
  3. Temperature rise tests
  4. Short circuit tests
  5. Verification of terminal markings and polarity
Typical specification for a 11 kV VT
System voltage: 11 kV
Insulation level voltage (ILV) : 12 /28/75 kV
Number of phases: Three
Vector Group: Star / Star
Ratio: 11 kV/ 110 V
Burden: 100 VA
Accuracy: Class 0.5
Voltage Factor: 1.2 continuous and 1.5 for 30 seconds
With provision for fuse


Current transformers
A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns is utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be "transformed" into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into a) the current or ratio error and b) the phase error.

Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current.
Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current a)  are 1 A for metering and 5 A for protection. In the case of transformer differential protection, secondary currents of 1/ 3 A and 5/ 3 A are also specified.
Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)
Accuracy class: In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.
Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions.
Accuracy limit factor: The value of primary current upto which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.
Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.
Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished.
Interposing CT's (ICT's) : Interposing CT's are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformers.
Standard
Standard Number
Year
Indian
2705
1992
British
BS EN 60044-1
1999
International
Electro technical
Commission (IEC)
IEC 60044-1
2000
  1. Accuracy tests to determine whether the errors of the CT are within specified limits.
  2. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, inter-turn insulation test at power frequency voltage, impulse tests with 1.2u/50 wave, and partial discharge tests (for voltage >=6.6kv) to determine whether the discharge is below the specified limits.
  3. Temperature rise tests.
  4. Short time current tests.
  5. Verification of terminal markings and polarity.
System voltage:11 kV
Insulation level voltage (ILV) : 12/28/75 kV
Ratio: 200/1 - 1 - 0.577 A
Core 1: 1A, metering, 15 VA/class 1, ISF<10
Core 2: 5A, protection, 15 VA/5P10
Short time rating:20 kA for 1 second

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