In case of multi storied building, what are the limits insisting for provision of transformer?

In case of multi storied building consumers, what is the load limit for HT-LT tariff?

Conditions to avail HT common supply in a high rise apartment building may be explained.

Explain AVR (Automatic Voltage Regulation)

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(i) When ACB is provided as the incomer of a switch board, the outlet switch (not for breaker) rating shall not be less than 1/3rd of the incomer current setting. [If the outlet device is a breaker, this grading need not be applied, since the breaker is having breaking capacity, where as switch doesn't have it.]

(ii) When MCCB is provided as the incomer of a switch board, the outlet switch (not for breaker) rating shall not be less than 1/5th of the incomer current setting.

(iii) When ACB/MCCB is provided as the incomer of a switch board, the outgoing fuse rating shall not exceed 1/3rd of the incomer current setting.

(iv) When ACB is provided as the incomer of a switch board, the outlet MCCB can be set up to 80% of the incomer current setting.

(v) When MCCB is provided as the incomer of a switch board, the outlet MCCB can be set up to 50% of the incomer current setting.

 (vi) When switch fuse act as the incomer of a switch board, the outlet switch rating shall not be less than 1/10th of the incomer fuse rating. Also, the outlet fuse shall not be greater than 1/2 of the incomer fuse rating.

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Fault clearing time

Cu

Al

Steel (GI)

1 Second

205

126

80

3 Second

118

73

46

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The ratio of neutral CT for earth fault protection is selected based on the sensitivity we need.

For example consider a 5 A relay with plug set at 2A ( say ). ie. the relay will operate when 2A current flow through the relay coil. Let us consider different CT ratios for this relay. The minimum primary current in CT for operating the relay will be as follows:

 It can be seen that , as the CT ratio becomes lower , the relay operates at lesser current. ie. the sensitivity of the earth fault system is improved when the CT ratio is lowered. Hence depending up on the sensitivity we need we may select the corresponding CT ratio. ( This is similar to using 30 mA ELCB in light circuits and 100/300 mA ELCB in power circuits )

If no specific sensitivity is required we take the CT ratio as 20 % of the rated current of the equipment protected. It is an empirical value based on practical experiences. It is recommended that the CT ratio shall not be higher than this value.

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The relationship between current and tripping time is usually shown as a curve, known as the MCB's trip characteristic. The most important curves are B, C and D.

Type B MCBs react quickly to overloads, and are set to trip when the current passing through them is between 3 and 4.5 times the normal full load current. They are suitable for protecting incandescent lighting and socket-outlet circuits in domestic and commercial environments (resistive loads), where there is little risk of surges that could cause the MCB to trip.

Type C MCBs react more slowly, and are recommended for applications involving inductive loads with high inrush currents, such as fluorescent lighting installations. Type C MCBs are set to trip at between 5 and 10 times the normal full load current.

Type D MCBs are slower still, and are set to trip at between 10 and 20 times the normal full load current. They are recommended only for circuits with very high inrush currents, such as those feeding transformers and welding machines.

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'A' - Alluminium

'2X' - XLPE Insulation

'F' - Steel Strip Armour

'y' - PVC Insulation

'W' - Steel Round Wire Armour

'y' - PVC Outer Sheeth

'ww' - Steel Double round wire armour

'yy' - Steel Double Strip Armour

'Wa' - Non Magnetic round wire armour

'Fa' - Non Magnetic flat strip armour

'FF' -Double Steel; Round Wire Armoured

Note - No code letter for conductor is required when the conductor material is copper.

 eg:- AYFY - Aluminum PVC Insulated Flat strip armour and PVC outer sheeth cable

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V3f (Variable Voltage Variable Frequency) drive consists of 3 blocks. A diode bridge rectifier gives DC supply to a Dynamic Braking Unit. Now this DC supply is inverted to AC and fed to the Lift Motor. The power factor to the AC line at the input side of the bridge rectifier is near to unity. The output may have an inductive, or lagging, PF due to the motor's inductive reactance. However, the motor's reactive current is circulated between the motor and the inverter but not to the input line. Hence, capacitor is not necessary for such variable speed drives for power factor improvement. More over, the power factor improving capacitors connected to the motor terminals may become overheated due to the potential high frequency noise transmitted from inverter. 

The V3F drive results in good ride comfort with 50% energy savings compared to single-speed elevators. The V3F drive will adjust and maintain the rotational speed of the motor in accordance with the load and position of the car relative to floor level. A motor mounted encoder can provide velocity feedback to the V3F drive.  Information from the encoder is used with exact car position information from the leveling system to provide accurate floor leveling.

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Circuit Breakers or Fuses in a circuit will detect an unusually high current load and shut off the electric current when the current exceeds a certain level. Today, circuit breakers are commonly used compared with fuses, being simple to reset after they trip from a high electrical load. But, circuit breakers can malfunction and do not always shut off a high current. On the other hand, fuses almost always shut off the current. The element inside them literally melts when the designated current is exceeded. If you want to protect sensitive equipment, fuses are better than circuit breakers. Earth Leakege Circuit Breakers (ELCB) function differently. Their purpose is to detect a leakage current that can allow electricity to pass through your body to ground.

The other type of common electric problem that causes fires is an arc fault. This may be caused by a loose wire at a switch, a break in the wire insulation where a nail in the wall hit it, just old kinked wire insulation, etc. This arcing or spark can ignite any combustible materials inside the wall. When arcing occurs, the electrical current is not excessively high, so a standard circuit breaker or fuse will not detect any problems. An ELCB will not detect this problem either because the incoming and exiting currents are the same. There are new types of circuit breakers that incorporate an additional sensor to detect arcing. These are called arc fault circuit interrupters or breakers (AFCI). Special electronic control circuitry inside it detects the changes in the current flow that are specific to arcing and it shuts off the current.

The AFCI circuitry continuously monitors current flow through the AFCI. AFCIs use unique current sensing circuitry to discriminate between normal and unwanted arcing conditions. Once an unwanted arcing condition is detected, the control circuitry in the AFCI trips the internal contacts, thus de-energizing the power circuit and reducing the potential for a fire to occur. An AFCI should not trip during normal arcing conditions, which can occur when a switch is opened or a plug is pulled from a receptacle.

Presently, AFCIs are designed into conventional circuit breakers combining traditional overload and short-circuit protection with arc fault protection. AFCI circuit breakers (AFCIs) have a test button and look similar to ELCBs. Some designs combine ELCB and AFCI protection. Additional AFCI design configurations are anticipated in the near future.

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Generators are inherently capacitive. Let the generator starts, ofcourse at no load.  At this stage, if some additional leading current is injected (by the provision of an external capacitor), it will add voltage and hence the voltage control of the generator may get damaged. Normally as per generator capability curve, the margin for leading PF will be much less, when compared to lagging PF. So, we need to ascertain that at the start or running stages of the generator, causes for a leading PF should not be encouraged.

Generally speaking, if you introduce a capacitance manually without considering the actual requirement, it may cause over compensation, and lead to an arcing between the terminals of the equipment, control device, or the source of power.  This is a dangerous situation.  Even shunt capacitor across motors should be avoided if there are more number of motors, and the system is on captive generation. In such cases APFC (Automatic Power Factor Control) relay is more reliable. Even with APFC, suppose the design is in such a way that, it can be activated with generator source also. APFC must be be very reliable and intelligent and capacitor banks should have fine steps to accurately obtain the desired accurate correction. Otherwise, similar situation of leading PF may be created and the generator performance may be affected. So, I conclude that even with APFC, try to include it in grid path only.

Please go through the paper available from here.

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For new consumers connected load permitted under LT may be limited to 100 kVA. But consumers existing as on 2nd March-05 may be permitted to operate in LT up to a load of 150 kVA.

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In case of multistoried buildings having connected load below 50 kVA, Licensee shall provide service connection from the LT line. For loads of 50 kVA and above, connection shall be affected only after installation of separate transformer of adequate capacity by the owner/occupier.

In case of Licensees other than KSEB (eg:- Kinfra, Technopark, etc.), for multistoried buildings having several service connections with more than 100 kVA connected load, connections can be extended at Medium Voltage by providing bus ducts of adequate current carrying capacity and complying with Indian Electricity Rules 1956 or new rules framed as per provisions of the Act after installation of a single transformer of adequate capacity by the Licensee. Tariff applicable to such consumers shall be HT Tariff with appropriate modifications regarding Transformer losses, after obtaining approval of Kerala State Electricity Regulatory Commission.

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Even in case of multi storied building consumers, the load limit for HT-LT tariff is the same as explained above.

As per KSEB Terms and Conditions of Supply, 2005, Clause 13(2)(b), "If common and essential loads in the building is 100 kVA and above, separate transformer with HT metering arrangement shall be installed exclusively for such loads".

Consider there are two buildings in the same premises. Let the buildings are having the common and essential loads of say, 70kVA each (So, total of 140kVA). The two buildings are physically segregated but comes under a single builder. So, they may approach with a single application (being the same premises and in future there may be a single residence association to manage) to Electrical Inspectorate and KSEB.

(a) Consider, the common and essential loads are shared by both buildings and there is only one transformer connection. If the total common load exceeds 100kVA, then H T connection shall be insisted for the common loads.

(b) Consider two independent transformers are fed from a common 11kV panel (1+2) and the common amenities of the two buildings are not at all shared and each of them comes around say, 70kVA (so the total >100kVA). In such a case, HT tariff common supply need not be insisted.

(c) Consider, the common and essential loads are not shared by the buildings and there is only one transformer connection. In such a case, I suggest you to care with the electrical design that two exclusive different feeders with independent metering shall be taken from the main switch board to feed the two sections. Otherwise, how they can give you non-HT connection with a single metering arrangement but connected load >100kVA?

AVR (Automatic Voltage Regulation) is an electronic circuit which will watch out the electricity output power from the generator supplied to the load(s). Now if the load increases (for example) then there will be more current to be drawn which will affect the RPM of the generator then the frequency of the generator (RPM) will drop down. The AVR in this case will increase the fuel (for example) to increase the RPM of the generator so the frequency will stay fixed (50 Hz for example). Also another possibility is to increase the armature voltage and current to it will able to compensate the voltage drop over the output lines.

In which cases the outdoor transformer installations are restricted? Also, in which cases the oil type transformer installations are restricted?

Out door transformer installations are not permitted in corporation areas and in the premises of residential buildings. In case of commercial buildings in areas other than corporation areas, out door transformer installations shall be permitted, keeping a minimum distance of 6m from the building. Otherwise, in such a case there shall be a fire resistant wall in between the transformer and the building keeping a minimum distance of 1.5m both from the building and the transformer.

Oil filled transformers are not permitted inside residential/commercial buildings. But, outside such buildings oil filled transformers shall be permitted subject to certain conditions.

Why transformers are rated in kVA?

A Transformer itself will not make any change to the system power factor, since both inherent losses viz copper loss (depends on current) and iron loss (depends on voltage) are independent of power factor. So, transformers are rated in kVA.

What is the condition to have  light circuit segregation in factory premises?

Generally for factories with medium voltage connection, the lighting circuit need to be segregated and metered separately, the tariff being different.

Where as, factories  with EHT/HT tariff connections, when the total connected lighting load is less than or equal to 5% of the connected load for power, it can be tapped off from the power mains without segregation. When the above lighting load exceeds this limit, the whole lighting load should be segregated and metered by a sub-meter and lighting consumption in excess over 10% of the bulk supply consumption for power shall be charged at 7 paise extra per kWh for EHT and 25 paise per kWh for HT consumers. But, industries engaged in software development technology and tissue culture units need not segregate industrial load, lighting load and load for air conditioners. No Penalty shall be levied by the Board for non-segregation of the load by these units. However, such consumer shall install static capacitors having ISI certification to improve the power factor of the load of air conditioners if any.

How to calculate the voltage drop of a 3 run cable, say 3Rx3.5x185 Sq.mm cable?

A 3 run cable represent 3 resistors in parallel, so its effective resistance will be one third. Hence, the effective voltage drop also will be one third. So, when you consider the 3 run cable as a single unit, the voltage drop shall be one third of the usual value of a single run cable. (say 0.37/3mV=0.123mV for 3x3.5x185 Sq. mm cable). Otherwise, you may calculate the adequacy of a single run cable, by taking the current as one third of the proposed feeder capacity. Both methods must give you same value.

What is RTCC (Remote Tap Change Circuit)?

RTCC is Remote Tap Change Circuit connected to OLTC (Over Load Tap Changer) of Transformer through control cables. It raise & lower the voltage as accordingly specified by controlling the motor drive in OLTC electrically (and manually through Push Buttons) In RTCC an AVR (Automatic Voltage Regulator) is fixed to maintain the output voltage level (raise & lower) by controlling the motor.

"On Load Tap Changer (OLTC)" means a device provided on high voltage side of transformer, which is used for variation of voltage during charged condition of the transformer.

"Off Circuit Tap Changer (OCTC)" means a device provided on high voltage side of transformer, which is used for variation of voltage during OFF condition of the transformer.

Percentage Impedance - basics?

A transformer constitutes a self impedance to current flow from source to load. Higher the impedance, the more voltage drop will occur for a given load. If a fault occurs, its effect on the circuit is lessened to the extend that transformer impedance increases. This self impedance can be evaluated by a simple test.

The secondary terminals are short circuited. A low voltage is then applied across the primary terminals, and increased until the secondary current reaches the rated current. The impedance is then the ratio of that primary voltage to the rated voltage, multiplied by 100, which is called as the percentage impedance of the transformer.

A transformer's percent impedance is evaluated by a simple test. The secondary terminals are short-circuited. A low voltage is then applied to the primary terminals, and increased until the current measured in the short-circuited secondary reaches the rated ampere value. The impedance is then the ratio of that primary voltage to the rated voltage; multiplying that by 100 gives the impedance in percent.

The impedance of the transformer is assumed as 5 %. Generally this is for transformers up to 500 KVA. Above 500 KVA the impedance is higher.

For generators the impedance is much higher than that of the transformers.

Why voltage multiples of 11 is preferred? (eg:-  11kV, 110kV, 220kV, etc.)?

With AC power systems, a factor was arrived at � relating the RMS value and the average value, called Form Factor, which is the ratio of RMS value to the Average value, which for a sinusoidal wave form was about 1.1. When the voltage was to be transformed, it is easy to have a whole number for the turns ratio of the transformer and hence all subsequent AC voltages became multiples of 11.

List of materials - descending order of Conductivity

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