electrodez

Typical Cable Selection Prodecure

Fri, 30 Aug 2013 06:40:57 +0100

Cable sizing is the process of selecting the appropriate sizes for electrical power cable so that the chosen cable works efficiently. Cable sizes are typically described in terms of cross sectional area or in terms of Standard Wire Gauge (SWG) according to the geographic region. It is important to chose the apt size of cable for all applications so that it is ensured the cable satisfies the following requirements;
a) The cable should be able to operate continuously under full load with out any damage.
b) The cable should be able to provide the load with suitable / required voltage, avoiding any excessive voltage drops
c) The cable should be able to withstand any short circuit current passing through.

GENERAL STEPS FOR CABLE SELECTION

Cable sizing methods differ with different standards adopted viz. IEC, BS etc. But the general principle that underpins the cable selection methodology shall be as mentioned below;
1) Gathering Data - Installation, load details etc.
2) Determine the minimum size of the cable based on the ampacity.
3) Determine the minimum size of the cable based on the voltage drop calculations.
4) Determine the minimum size of the cable based on the short circuit current calculations.
5) Choose the maximum cable size based on the calculations above.

GATHERING DATA

The initial step is to gather data regarding the cable viz, its construction, application and installation.

Constructional Data - Details such as the type of cable (i.e if it is an Aluminium or Copper Cable), the type of insulation of the cable (i.e PVC or XLPE etc), number of cores in the cable (i.e single core or multi core)

Application Data - Details of the type of load of the cable viz, number of phases of the supply ( Single phase or Three Phase), Full load current, Length of the cable from source to load etc.

Installation Data - Details of where and the cable is being is installed, i.e cable tray or ladder etc. the temperature at site were the cable is being installed, details of cable grouping, cable spacing etc.

AMPACITY CALCULATION

Current flowing through a cable will be generating some heat and implies resistive losses in conductor. A cable's insulation must be capable of handling the heat emanating from the cable. The ampacity of the cable is the maximum current it can carry with out damaging the insulation. Cable with larger conductor cross sectional area can carry larger current and hence have greater ampacity. Say, a 16sq.mm cable has more ampacity that that of what a 4 sq.mm cable has.

By referring to the cable manual, details regarding the ampacity can be obtained. The cable manufacturer provides with details of ampacity based on the construction of the cable. These values will be specific to ideal conditions and / or may not be suitable for all site conditions. The ampacity would differ based on the conditions at site. The manufacturer would also provide details of derating factors for a range of installation conditios such as ambient temperature, grouping of cables etc. A base derating factor is obtained by multiplying all the given derating factors.

Say, It - derating factor for ambient temperature and Ig - derating factor for grouping, and Ib - base derating factor is obtained as; Ib = It X Ig

The derated ampacity can be obtained by multiplying the ampacity with the derated ampacity.

Based on the current required by the load, a minimum size of the cable is chosen with the apt ampacity.

VOLTAGE DROP CALCULATION

A Cable can be seen as an Impedance, and hence a voltage drop will be associated with each and every type and size of cable. The impedance of the cable is dependent on the cross sectional area of the cable and the length of the cable. The voltage drop will be higher if the current flowing through the cable is high and if the cable has higher impedance.

The reactance and resistance values of a cable will be provided by the manufacturer.

The voltage drop can be calculated as;

Vr = 1.732 X If (Rf Cos Er + Xf Sin Er) / Number of Runs

Where, Vr = Voltage Drop during running condition
If = Full load current
Rf = Resistance of the cable
Xf = Reactance of the cable

Using the above equation, the voltage drop can be calculated in %. The allowable voltage drop may be considered with in 3% to 5% depending upon the specifications. The minimum size of the cable with significantly lower voltage drop shall be chosen.

SHORT CIRCUIT CURRENT CALCULATION

A short circuit can cause high amount of current to flow through the cable. This surge in current flow causes a temperature rise within the cable which can degrade the condition of the cable. The minimum cable size to withstand the short circuit current can be calculated as below;

A = Sqrt(i^2 X t) / k

where, A = minimum cross sectional area of the cable (in mm )
i = short circuit current (in A)
t = duration of short circuit current (in seconds)
k = short circuit temperature rise constant

Protective Relaying in a TSS

Thu, 26 Apr 2012 07:20:20 +0100

What could possibly prevent the mishap that would happen if the live overhead catenary - contact system comes in contact system comes into contact with earth or train roof ?? This was asked by a fellow passenger of mine whilst my recent train journey. The simplest, yet convoluted answer is "Protective Relaying".

A relay is the most important component in any protection system. During a fault or any abnormal conditions, one or more of the electrical quantities such as current, voltage, phase angle, frequency, rate of rise of current in the circuit of the relay changes. Relays can be categorized as Electro-mechanical relays, Static relays, Numeric relays and some special types of relays; namely Buchholz relay and Thermal relays.

Components of a Protection System :

1. Protective relays and its associated components in the Control Panel
2. Instrument Transformers ( CTs and PTs)
3. Trip Supply
4. Inter-tripping relays
5. Wiring

Electro-mechanical relays were used since the beginning and are now being replaced by static and numeric relays which are of high precision and requires lower maintenance.

Nature of Faults :

The faults occurring on the catenary system are due to;

Earth faults - caused by failure of overhead equipment insulators, flashover at arcing horns etc.
Overloads - caused by abnormal traffic conditions.
Incorrect Switching Operation - caused by incorrectly closing the bridging interrupter at Neutral section.

Relays used :

The Power Transformers have the following types of protection:

Over Current and Restricted Earth Fault Relay :

Over Current Protection :

The Transformer is protected for over current on both HV and LV side. Over Current Relays are set to a predetermined value such that they operate when the magnitude of current exceeds the pre-set value. The relay gets its input from the current transformers placed in the HV and LV side in the transformer circuit.

Restricted Earth Fault Protection :

The REF protection is provided as back up protection to the differential protection. Protection against earth faults is given by this relay. The term Restricted earth fault is because, the relay only protects for the earth faults in the restricted area (circuit). It works by measuring the actual current flowing to the earth and will operate if it exceeds a pre-set limit.

High Speed Differential Relay :

Differential Protection :

Differential relay is the reliable protection against internal faults for equipment like generators, transformers and busbar. Differential Protection operates for faults occurring in clearly defined region, i.e on two sides of the equipment that is to be protected. The differential protection relay is connected to bushing type CT of HV and LV sides of the transformer. The current entering and leaving the transformer is compared and the relay operates in case of any inequalities.

Buchholz Relay :

Buchholz Relay is a gas actuated relay that is used for the protection against internal faults of the transformer. It sts off an alarm in case of a slow developing fault or an incipient fault. The relay consists of two elements, a mercury switch connected to float on the upper end and at the lower end is a mercury switch hinged to a flap.

In case of a slow developing fault, the heat due to the fault causes some of the transformer oil to decompose thus producing hydrogen gas. The gas being light, tries to get into the conservator and in the process it gets blocked in the upper part of the relay chamber. When sufficient pressure if accumulated, the float tilts and closes the mercury switch attached to it. This completes the alarm circuit to sound the alarm.

In case of severe internal faults, the oil in main tank rushes towards the conservator through the lower part of Buchholz relay. By doing so, it closes the flap and hence its associated mercury switch to complete the alarm circuit.

In addition to the above mentioned protection schemes, excessive winding temperature and oil temperature alarm circuits with trip contacts are also provided.

The substation and its equipments are protected against over voltages by means of Lightning arresters, and against abnormal loads by feeder circuit breakers. The protective relays used for the protection against the faults in catenary system has its associated breakers whose functions are to detect all the short circuits on the catenary system and to operate with a minimum time delay.

The protective system for the catenary system should be such considered that it should be sufficient in case of an extension of the protection zone (if necessary the bridging interrupter shall be closed which extends the zone of protection).

Integrated Digital Traction Feeder Protection Relay

This relay is connected with the current transformer and potential transformers of type-1. It aids in the protection against catenary - earth fault, auto - reclosing of CB and wrong phase coupling and also over current.

Generally the zone of protection in case of normal scenario is from a TSS - FP to the next SP. This relay samples the current and voltage signals and when voltage drops and current raises it will calculate the values of R and X according to the RCA angle (parallelogram characteristics). When R and X are within set limit then relay will give the trip command to the connected breaker.

At a SP, there are possibilities for the bridging interrupters to be closed under normal conditions. This causes a short circuit of two different feeds from two different substations. Apart from this, wrong phase coupling can also occur when a train crossing an SP's Insulated Overlap, the pantograph is not lowered inspite of the signboards being placed. The wrong phase coupling occurs only as a result of mistakes. The integrated digital traction feeder trips the associated feeder circuit breaker in the event of wrong phase coupling.

This relay also provides protection against over current in the catenary system.

The voltage and current signals are sampled and in the event of any abnormal conditions the corresponding relay is actuated and the associated circuit breaker is tripped. Then after a pre-set time the circuit breakers are automatically closed. This is to avoid discontinuity of operation in case of any incipient or any short duration faults (which are not hazardous for the operation and/or people). If the CB is tripped again after the reclosure, it is locked indicating that the fault is persistant.

Panto - Flashover Relay :

When ever one section of the IOL is tripped on intermittent fault and the electric locomotive enters from live to dead section of FP, there will be a heavy flashover, particularlly when the panto leaves the IOL which may damage the panto. The extent of the damage is propotional to the intensity of the current drawn by the locomotive. The pantofalsh over relay is connected to the type-1 PT in the FP. It detects for any flashover and trips the CB connected to the live side of the overlap.

High Resistive Fault Relay :

The distance protective relay may fail to detect faults of high resistive nature. Protection against high resistive earth faults is provided by this type of relay which works on principle of vectorial difference between base and fault currents. If the difference is more than the pre-set limit, the trip command is initiated. This is used as a backup protection the main distance protection.

Power Factor and Capacitor Banks

Fri, 09 Mar 2012 19:36:15 +0100

As a one liner, it can be said that the Shunt Capacitor banks (used with series reactors) serves the purpose of 'power factor correction'.

What is Power Factor:

When asked what is Power Factor (hereafter PF), the most likely answer that one would get in mind is Cos Φ.

Any equipment handles a job with some degree of efficiency. The quantification of this efficiency is termed to be "Power Factor". Equipment like induction motors, Transformers etc. require two types of power. One of them is the power required to perform the useful work which is called as the Real Power, measured in kW. The second type of power is the one that is required to produce the magnetic flux which is termed as Magnetizing Power or the Reactive Power, measured in kVAR. The vectorial summation of Real and Reactive Power is termed as the Apparent Power, measured in kVA.

The PF is defined as the ratio of the Real Power to the Apparent Power. Hence, it can be inferred that, more the reactive power which results in higher values of apparent power, the PF is low. Similarly, for a system with lower reactive power results in the PF being closer to unity.

Thus, for a system to be efficient, the value of PF has to be as close to unity as possible.

Reasons for a low PF:

As mentioned earlier, the PF can be manipulated by the reactive power. The requirement of reactive power gets higher with the use of Inductive loads. Some of the examples of Inductive loads are, Transformers, Induction Motors, Relays, Reactors etc.

For instance, the transformers requires kVAR (reactive power) for producing the magnetic flux. This eventually increases the apparent power and hence a drop in PF.

Benefits of Improved Power Factor:

An improved PF; i.e a PF value closer to unity will have the following benefits:

1. Reduction in Power bills:
An increase in the reactive power causes an increase in the apparent power. The power that the state electricity board supplies is the apparent power.

So, lower the PF, lower will be the value of apparent power. Hence the power consumed is lower. This equals reduction in the power bills.

Apart from this, state electricity board charges the user with a penalty if the system runs with a lower PF.

2. Improved System capacity and Voltage level:
By improving the PF of the system, kW capacity of the system is also increased. Apart form this, a lower PF would result in power system losses in the distribution system. As power losses increase, it would result in voltage drops. Excessive voltage drop would result in premature failure of motors, overheating of cables etc.

Improving Power Factor:

1. Capacitive load in the system:
The PF is directly manipulated by the inductive loads. The inductive loads and the capacitive loads acts opposite to each other. By adding a capacitive load equivalent to the inductive load will reduce the magnitude of the reactive power and hence PF can be improved. The PF of the system is constantly changing due to the variations in the number and size of the inductive load (i.e. the number and size active transformers and/or induction motors) connected to the system varies. And hence, it is difficult to balance the inductive and capacitive loads continuously.

In addition to this, the capacitive load that is being added has to be designed for specific requirement of the system, else it would result in harmonic problems.

2. Proper use of the Equipment:
The low PF is caused by the presence of Induction motors and other inductive loads. But, more specifically low PF is caused by running the induction motors with light load. Any equipment should be operated only at its rated voltage and not above it. Even with energy efficient motors, the PF is affected with variations in load. Hence, it is advisable to replace the worn out equipment with energy efficient ones.

Shunt Capacitor Banks for PF Correction:

The most common and cheapest method for Power Factor Correction is by the use of Shunt Capacitor Banks or Capacitor units in the system. There are two times of Capacitor banks viz. Fixed Capacitor Banks and Variable Capacitor Banks.

The former method has a fixed capacitor units connected to the transformer or the switch-gear bus. The capacitors store the kVAR and release energy opposing the reactive energy caused by the inductor. The fixed capacitor bank system is sized to regulate 0.9 PF during maximum operational inductive load. This means that during the periods of operation when less than maximum inductive load is utilised, extra kVAR is fed into the system. This is one of the major draw backs of having a fixed capacitor bank unit. This system does not consider the future expansion of the loads in the system.

The latter method involves a use of variable capacitor units. This is similar to the fixed capacitor bank but the bank monitors the systems PF and varies the capacitive load to be connected automatically. The variable capacitor banks comes with internal protection. The drawback of this type of capacitor bank is that it is more likely to have harmonics problems because of the switching of capacitors.

The capacitor banks will be having a series reactor connected to it. The reactor has nothing but a wound coil of high impedance. The reactors are used in series with the capacitor units to limit the sudden inrush of starting current at the time of switching in the capacitor unit. These are also called as current-limiting reactor.

Lightning Protection

Tue, 06 Mar 2012 08:09:34 +0100

Lightning is a natural hazard, being the discharge of static electricity. Some of them cause damage to buildings or equipment and a few even kill or injure people and animals directly or indirectly by causing fire and explosions. The important part of a lightning flash, from the resulting damage point of view is the "Return Stroke". This is the part in which the a charged cell in a cloud is discharged to earth.

Effects of Lightning Stroke:

As the current is discharged through the resistance of the earth electrode of the lightning protective system, it produces a resistive voltage drop which raise the potential of the protective earth system, to a value higher than the true earth potential. It may also produce around the earth electrodes, a high potential gradient which causes damage to persons and animals.

The effects of a lightning discharge is confined to temperature rise of the conductor through which current passes. Although the current is high, the duration is short and hence the thermal effect on the protective system is usually negligible.

Zone of Protection:

The term Zone of protection can be explained simply as the area within which the lightning conductor gives protection against direct lightning strokes by directing the stroke to itself.

A Substation has to be shielded from direct lightning strokes either by using Shield wires / Earth Screen wires or by using Spikes (on masts). The procedure followed is by suitably placing the shield wires by forming an 'air termination network'. An acceptable degree of protection is defined as 45°, considering the zone protected by only one mast. For zones that are protected by more than one masts, the angle of protection between the two masts (the distance between the two masts is twice that of the height of one mast) is 60°. ( Figure - 2)

Components of the Protective System:

The components of the the lightning protective system includes the following;
1. Air terminations
2. Down Conductors
3. Joints and Bonds
4. Testing Joints
5. Earth Electrodes

The 'air terminations' include both horizontal and vertical conductors. The earth screen wires or the shield wires that are strung across the masts forming a protective zone, are the air termination network.

The purpose of a down conductor is to provide a low impedance path from the air termination to the earth electrode so that the lightning current can be safely conducted to earth. A down conductor should follow the most direct path between the air termination network and the earth termination network.

The bonds and joints in a lightning protective system has to be electrically and mechanically effective and has to be reduced to the maximum extent. Any other joints other than being welded represents a discontinuity in the current carrying system.

Each down conductor should be connected to the earth termination network through test joints. Each of the earths should have a resistance not exceeding the product obtained by  multiplying 10 ohms by the total number of earth electrodes.

Procedure for Lightning Protection:

Consider a simple area that is to be protected by two masts (Figure - 1). As stated above, the acceptable degree of protection for area outside the structure is 45° and for the area between the two masts is 60°. The height of the protective zone is to taken. In general, the height of the Circuit breaker is taken as the height of the equipment bus since the height of the circuit breaker is more than other equipment. A margin of 0.2 meters is added to the equipment bus height and is taken as the height of the zone to be protected.

The total height (h1)of the supporting tower for protection is considered including the height of the plinth. The height of the object to protected(h2) is the sum of equipment bus height and a marginal height of 0.2m. From this, the height above the equipment (h3) can be found out as (h1-h2).

The zone of protection (in distance) outside the mast can be found using the formula : [ h3 x tan(45) ]

Similarly, the zone of protection between two or more masts can be found using the formula : [ h3 x tan(60) ]

For instance;
h1 = 16.3 m
h2 = 5.70 m (5.50 + 0.2)
h3 = (h1-h2) = (16.3 - 5.70) = 10.60

As per above stated formula, the protection zone outside the mast is : 10.60 x tan(45) = 10.60
and, the protection zone between masts is : 10.60 x tan(60) = 18.36

Layout Preparation:   (Figure-3 to Figure-6)

The Lightning Mats (LM) has to be placed in suitable position maintaining the clearance required and as per site conditions. The protection zone is to be calculated as per the aforementioned procedure. From the above example, we get the protection zone outside the mast as 10.60m of radius. Considering this, a circle of 10.60m radius is to be drawn from center of the mast. (Figure - 3)

Similarly, the protection between the masts are 18.36m of radius (Figure - 4). So, circles of 18.6m radius is drawn from the center of the mast. The interference portion of the circles drawn are joined by tangent lines (Figure - 5). This forms the 'protective zone'. (Figure - 6)

If the equipment to be protected falls well within this zone, the position of the LMs are correct and if not the position and/or the number LMs needs to adjusted and checked again.

   Figure - 1 [ A simple Protective Zone of two masts ]

Figure - 2 [ Angle of protection for two or more masts ]

Figure - 3 [ Two LM with circle of 10.6m (45deg protection)

Figure - 4 [ Intersection of 60deg protection circle ]

   Figure - 5 [ Tangent lines for portion between two LM ]

Figure - 6 [ Zone of Protection between two LMs ]

Rating of a Lightning Arrestor

Sat, 10 Mar 2012 10:02:08 +0100

When you look into a substation layout, say for instance a 132kV Switchyard you would notice that all the equipment installed will be same as the nominal system voltage except for the lightning arrestor. A few days back I found that many of them still have this as unanswered question so as to why is the LA rating chosen lower than the system voltage. So I thought I could share my idea on this with everyone through this post.

When you consider a three phase system, the LA is placed between each Line and Ground and that is the reason why you have three individual LAs in the circuit where as to one three phase circuit breaker or isolator.

The basic need of having a Lightning Arrestor or Surge Arrestor installed is to protect our equipment from surges or simply saying protect it from damage caused by any rise in voltage. I came to know that a there are quite a few who are of the impression that the rating of LA / SA is chosen depending on surge voltage. That it is absolutely incorrect. The reason being that the surge voltage cannot be predetermined.

The nominal system voltage is one of the important factor to choose the LA/SA rating. As I mentioned earlier, since the LA is placed in between the Line and ground, hence we have to consider the line voltage and then the maximum voltage. The rating of the LA is chosen which is the nearest to this value.

For instance, consider a 132kV substation. The line to ground voltage will be 132/sqrt(3); i.e 76.21kV

The peak value for this line voltage will be 76.21 X sqrt(2); i.e 108kV

Consider a margin of 10%, we can say the voltage will be 119.37kV

The standard rating of the LA that is nearest to this value is 120kV and hence at a 132kV substation, we choose an LA of 120kV rating.

In case of a Traction Substation ( for information on Traction Substation, refer the previous post "Traction Power System), the input is a three phase supply, yet a double transmission line. In this case the secondary side bus voltage will be 25kV AC single phase.

The peak value will be 25 X sqrt(2); i.e 35.35kV and considering a margin of 10% to the peak value we have 38.885kV

The nearest standard rating available is that of 42kV and the same is chosen.

This is why the primary side LA rating in a TSS is less than the nominal voltage and that at the secondary side is higher than the system voltage.

Traction Power System

Sat, 19 Nov 2011 13:58:06 +0100

In an Overhead electrification systems, the electricity is supplied by means of electrified wires (live conductors) that run parallel and above the track. These wires are termed as Contact wire. Contact wires can be found attached ( or perhaps supported by ) another length of wire which is termed as Catenary wire.

The electric locomotive uses a Panto-graph, a metal structure which can be raised or lowered so as to make contact with the catenary-contact system and draw current. The return path for the electricity i.e. the return current is through the body of the locomotive and wheels to the rails (tracks) that are electrically grounded. Ground connections are provided from the rails at periodic intervals. The return current, after flowing from the wheels to the rails, flow through the rails and also partly through the earth beneath it. Earth bonding are provided periodically to keep the rails firmly connected to the earth so as to prevent form formation of a step voltage ( For information on Step Voltage, refer the post "Earthing Design")

The Power Flow to the OHE

The overhead catenary-contact system which feeds the electric locomotive is in turn electrified by the "Feeding Posts" that are frequently places along the track. The Feeding Posts (FP) are themselves are a part of the Traction Power Substation (TSS). The FP is fed by the TSS, which is placed at an interval of 50 to 70 km. The interval between two TSS shall be reduced (or suitably adjusted) considering the load and the traffic on the route.

A Remote Control Center, usually close to the Divisional Traffic Control office has the facilities of controlling the power fed to the catenay system through different TSS at a particular section.

The TSS gets the input power from the regional grid (of the state electricity board). The supply authorities supply power at 220kV, 132kV, 110kV, 66kV Extra High Voltage (EHV) at each TSS which is owned and maintained by the railways. To ensure the continuity of the supply, the high voltage feed to the TSS is arranged form a double transmission line, so that even in the event of failure of one, the other service remains. Suitable protective equipments and other required switch-gears are installed at the TSS.

Each TSS, normally has two single phase traction power transformer ( one taking 100% load and other as Standby - for more information on selection of transformer, refer to the post "Transformer Selection" posted earlier). This voltage level is then stepped down to 25kV which is the suitable voltage for railways. One lead form the secondary terminal of the transformer is solidly earthed and is connected to the rails.

The power from the TSS reaches the OHE system through the FP. The TSS has two leads (voltage bus) running into the FP with a coupling unit ( in the form of an Interrupter ) between them. Each of the two leads feeds into two different electrical sections formed by an insulated overlap (IOL) in the catenary-contact system. The coupling interrupter is installed so that the power supply can be extended from one section to the other in case of a failure in any one of the sections. An insulated overlap is to create different sections in OHE. In an IOL, the two catenary-contact wires forming different sections are kept apart 500mm away from each other and the electrical discontinuity is bridged by an interrupter or isolator.

Traction Substation

The power received form the supply authorities are from the three phase power system. The single phase traction load will cause an unbalance in the three phase power system. This unbalance will have undesirable effects on the suppliers generator. Hence, to keep the unbalance within a permissible limit, the power for traction load is tapped off form the grid system across different phases, in a cyclic manner. One of the phase is electrically grounded and connected to the rails. The other phase is supplied to the FP through the required switch-gear equipment. Apart form the single phase traction power transformers, Capacitor banks are installed in the TSS for power factor correction. For instance, consider a TSS - 1 feeding R phase supply to the OHE. In this case the TSS -2 or TSS-3 that is placed 50 to 70 km away or before (respectively) will not be feeding R phase supply to the OHE. Hence, consecutive TSS are not connected in parallel.

Thus, it becomes necessary to separate electrically the OHE systems fed by adjacent substations. This is done by the provision of a Neutral Section in the OHE, to ensure that the two different phases are not bridged. In the neutral zones, the catenary-contact system is not energized. These neutral zones are the limits till where a TSS feeds. In this zone, the two energized portion of OHE is separated by a PTFE.

Section and Paralleling Post

In order to avoid wrong phase coupling, between every two TSS, a Sectioning and Paralleling Post (SP) is placed. The SP is placed at a point where there is a neutral zone in the OHE. The portion of OHE between the feeding point and the nearest neutral is called as a section. Normally, a TSS feeds two sections.

The location on an SP is also decided such that it is far away from the signals or level crossing gates. This is done so that the locomotive could pass the neutral section with sufficiently high speed to avoid the possibility of getting stuck in the neutral section. The SP, provided at the neutral zone has paralleling interrupters to keep the two portion of OHE ( one in each direction) to be supplied in parallel. It also has a bridging interrupter that is normally open with under voltage relays, used to bridge the two different sections, extending power supply in case of emergency. This is in case of an SP operating for more than one line. In case of only one line, the SP has only a bridging interrupter, which is normally kept open.

Subsectioning and Paralleling Posts

The sections may be further sub-divided into sub-sections for the purpose of maintenance in OHE. Each section is split into sub-sections every 10 to 15km by placing a Sub-sectioning and paralleling post (SSP) at required intervals. The SSP has paralleling interrupters to parallel the Up and down tracks ( in case of more than one line ) and bridging interrupters to bridge different sub-sections when required.

The below given is a schematic sketch of a 132kV/25kV Taction Substation.

Procedure for Earthing Design

Fri, 04 Nov 2011 17:05:29 +0100

The earthing system shall consist of a network of buried conductors forming the earth grid, providing the earthing connections to equipment ground terminals, equipment housings and structures. If the calculated Mesh and Step potentials for this earthing system is less than the attainable mesh and step potentials, then the design is considered to optimum one.

The earth grid shall encompass all of the area of the sub-station within the fencing and also shall extend for approximately one meter outside the fencing.

The resistivity of the soil is then tested by any suitable method. As mentioned in the previous post titled "Earthing", the Wenner method is the most adopted.

The size of the conductor is chosen keeping in mind the fault current in the system and the entire area within the fencing and one meter outside the fencing has to be covered with crushed rock possessing a minimum resistivity of 3000ohm-m
The earth grid consists of horizontal conductors placed about 0.6m to 1.5m below the earth forming a checkerboard pattern. The depth of the earth grid is to be considered excluding the layer of the crushed rock put.

Vertical earth rods or the earth electrodes are to be buried at the grid corners and at the junction points along the perimeter of the earth grid. Electrodes (Earth Pits) may also be installed at the major equipment such as Transformers, Lightning Arresters, Towers / Structures, Lightning Masts. In the case of a Transformer, the transformer body has to earthed to a separate earth electrode and the neutral has to be connected to a separate earth electrode (Neutral Pit).

Calculation of Tolerable Mesh Voltage

The Tolerable Touch or Mesh potential can be calculated from the below mentioned formula

E touch = (1000 + 1.5 Cs * ps) * 0.157 / SQRT( t * s ) (Ref: IEEE 80-2000 section 8)

Where, Etouch = Mesh Voltage

Cs = Surface Layer Derating Factor
ps = Top layer resistivity of the soil
ts = fault clearing time

Cs = 1 - a * [ (1-p/ps) / (2*hs + a) ]; where a =0.09 (Ref: eqn.27 of IEEE 80-2000)

With the above details, the Tolerable Touch potential can be calculated.

Calculation of Tolerable Step Potential

The tolerable Step potential can be calculated form the below formula

E step = (1000 + 6 Cs * ps) * 0.157 / SQRT( t * s ) (Ref: IEEE 80-2000 section 8)

Where, Etouch = Mesh Voltage

Cs = Surface Layer Derating Factor
ps = Top layer resistivity of the soil
ts = fault clearing time

Calculation of Design Mesh Voltage (Attainable Mesh Voltage)

The attainable Mesh voltage can be calculated form the below mentioned formula

Em = (p * Km * Ki * Ig) / Lm

(Ref: IEEE 80-2000 section 16)

Where, Em = Attainable Mesh Voltage
p = Soil Resistivity ohm-meter
Km = Spacing Factor for Mesh Voltage
Ki = Correction Factor for Grid geometry
Ig = Maximum Earth Fault Current
Lm = Effective length for Lc and Lr for Mesh Voltages
Lc = Total length of grid conductors
Lr = Total length of earth rod (earth electrodes)

The geometrical factor Km can be expressed as below.

Km = (1/2*Pi) * [ ln(D^2/16.h.d + (D+2h)^2/8D*d - h/4d) + Kii/Kh * ln(8/Pi(2*(n-1)) ]

Where, D = Spacing between parallel conductors
d = Diameter of the grid conductors
h = Depth of earth grid condcutors
n = Geometric factors
Kh = Corrective weight factor that emphasizes the effects of grid depth
Kii = Corrective weight factor that emphasizes the effects of inner conductors

Most of the earthing design has earth electrodes at corners of the earth grid or along the perimeter. In such cases, the value of 'Corrective weight factor that emphasizes the effects of inner conductors' will be unity.

i.e. Kii = 1

The corrective factor that emphasizes the effects of grid depth can be expresses as below.

Kh = SQRT(1+(h/h0) ); h0 is 1m (ref depth)

Effective number of parallel conductors (n) in a grid can be made applicable to both rectangle layout and other irregular layout.

n = na*nb*nc*nd

Where, na = 2Lc/Lp
nb = 1 for square grids
nc = 1 for square and rectangular grids
nd = 1 for square, rectangular and 'L' shaped grids

Otherwise;

nb= SQRT ( Lp/(4SQRT(A) )

nc= SQRT [ Lx.Ly/A ]^(0.7*A/Lx*Ly)

nd= Dm/(SQRT(Lx^2 + Ly^2) )

Where; Lc = Total length of the conductors in horizontal grid
Lp = Peripheral Length of the grid
A = Area of the grid
Lx = Maximum length of the grid in x direction
Ly = Maximum length of the grid in y direction
Dm = Maximum distance between any two points in the grid
D = Spacing between the parallel conductors
d = Diameter of the grid conductors
h = depth of ground conductors

The irregularity factor is expressed as

Ki = 0.644 + 0.148*n

For grids with no earth rods, or with a few earth rods ( none located in corner or along the perimeter of the earth grid), the effective buried length is expressed as;

Lm = Lc + Lr

Lr is the total length of all rods

For grids with earth rods in the corners and the perimeter of the grid, the effective buried length is expressed as;

Lm = Lc + [ 1.55 + 1.22 ( Lr / SQRT (Lx^2 + Ly^2) ) ] * Lr

Calculation of Design Step Voltage ( Attainable Step Voltage )

The attainable Step Voltage is calculated form the below formula;

Es = p*Ks*Ki*Ig / Ls

Where, p = soil resistivity
Ks = Spacing factor for Step voltage
Ki = Correction factor for grid geometry
Ig = Maximum Earth fault current
Ls = Effective buried length

For grids without earth rods or few earth rods, Ls is expressed as;

Ls = 0.75 Lc + 0.85 Lr

Lc = Total length of grid conductors

Lr = Total length of ground rods

The spacing factor is expressed as;

Ks = 1/Pi ( 1/2h + 1/D+h + 1/D (1-0.5^n-2) )

Where, D = Spacing between parallel conductors

h = Depth of earth grid conductors

n = geometric factor

If the Designed Mesh and Step Potential are less than the tolerable Mesh and Step potentials, the design is an apt one. But, if the calculated mesh and step potential are more than the tolerable potentials, the grid design has to be modified.

To reduce the grid mesh and step potential, the mesh size has to be decreased, by increasing the number of parallel conductors in each direction.

Earthing

Fri, 04 Nov 2011 09:48:23 +0100

The Earthing of a system is designed with the primary focus being the safety and security of the system by ensuring that the potential on each conductor is restricted to such a value that it is consistent with the level of insulation applied. Most high voltage public supply systems are earthed. Unearthed overhead lines can be found but this may be small as 11kV derived from 33kV mains, where the capacity of earth current is of 4A.

An effective earthing system consists of earth electrodes, buried main earth grid and equipment earth mats, interconnections of the earth grid to the structures and equipment where ever necessary.

Before design process can begin, the soil resistivity has to be measured at the site. This is required when checking for the resistance of earth rod (electrodes) and earth grid (main earth mat). The resistance of one meter cube of soil measured between opposite surface is termed as soil resistivity. The 'four rod or the Wenner method' is the most followed method for measuring the soil resistivity. In this method, four rods are placed at an equal spacing (a) and equal depth, into the soil. The diameter of the rods need not be the same but the depth at which the rod is inserted into the ground must be equal. The voltage between the two inner electrodes is then measured and divided by the current between the two outer electrodes to get the value of resistance (R). The soil resistivity is then derived from the formula [ p = 2*pi*a*R ]; where p is the soil resistivity.

Two major factors considered in the design of an earthing system are the a) Touch Potential and b) Step Potential

The potential difference between a grounded metallic structure and a point on earth's surface separated by a distance equal to one meter is termed as Touch Potential or Mesh Potential.

The potential difference between two points on earth's surface, separated by a distance of one meter is termed as Step Potential.

The design procedure includes the selection of the size of the earthing conductor depending on the nominal voltage level, system fault level, the fault current and the material that is to be used as the earthing conductor. Once the material to be used as earthing conductor is finalized, the touch and step potential of the soil at the site has to be determined, using the values of soil resistivity and soil resistivity at the top layer (as explained earlier in this post). Followed by this the touch and step potential that is attainable with the earthing system is to be calculated.

The attainable value of Touch and Step potentials has to be lesser than the Tolerable Touch and Step potentials and also the combined resistance of the earth grid and the earth electrode has to be less than one ohm for a safe earthing design.

Transformer Sizing

Thu, 03 Nov 2011 09:33:36 +0100

Any transformer is rated based on their losses. The losses of the transformer i.e. the Iron loss depend on the voltage and the copper loss depends on the current. Since the loss does not depend on the power factor, a transformer is rated in terms of kVA (kilo-Volt-Ampere).

A transformer is sized based on the load connected to it. There are two types of load, viz. Continuous load and Intermittent load. As the name suggests, continuous load will always be connected with to secondary of the transformer while the intermittent loads are the one that is not intended to run at all time.

Simply saying, in a factory all the motors and other such machines are intended to run all the time except for some maintenance purpose. This is the connected load. While the lighting loads and EOT cranes are the intermittent loads, because it is not going to be used all the time.

The transformer is to be sized considering the total load that is to be connected on the transformer.

The standard ratings of the transformers available in market are 5kVA, 30kVA, 45kVA, 75kVA, 112.5kVA, 150kVA, 225kVA, 300kVA, 500kVA and 750kVA etc. The total load on the transformer is calculated and the nearest standard rating is to considered as the required transformer rating.

In order to explain this context in a more practical way, consider that we are having a panel board serving a load, say air conditioners of 1.85 kW ( a 1.5 tonne AC consumes approximately 1.85 kW/hr ) and some power receptacles with a load of 1.5 KW. Let us consider this to be the continuous load. A watering pump of 2kW as the intermittent load on our transformer. For calculating the rating of the transformer, only 50% of the intermittent load is considered.

The load in kilo-watt is converted into kilo-volt-ampere by dividing it with the product of efficiency and the power factor.

From the above example, we have a continuous load of 3.35 kW which is 4.2 kVA ( assuming and efficiency of 80 and power factor of 0.8 ). For this a transformer of rating of 5kVA can be chosen.

Similarly, in case of large demand such as in industries, a 10% to 20% margin is considered for the purpose of meeting the starting VA requirements of the motor with largest rating.

Transformer Selection

Thu, 03 Nov 2011 07:49:20 +0100

From the previous posts, it can be inferred that, ultimately at a Substation, either a Voltage level is stepped up or stepped down aiding to the transmission or distribution of power. Hence, we can say that the Transformer is the core of a substation which is supported by other equipment for protection.

Primary function of a Power Transformer is to transform voltage form a nominal level to another. In most cases, the use of two smaller size transformer, to operated in parallel in one circuit in lieu of one full size transformer is not recommended. The reason being, the two transformers would cost more than a single full transformer, their combined loss will be higher and obviously, an expensive and bigger structure of Substation is required.

The 1 x 100 % Approach:

In this approach a single transformer with a capacity of 20% more than the Maximum Demand at the Main Substation (A Main Substation is where the EHV level voltage is stepped down to 11 or 6.6 or 3.3 kV) is used. In case of an outage of the transformer, the captive site-generation is adequate to maintain in service, the critical and major portion of the plant. Since the restoration of the failed transformer back into service, takes several months, this approach is not recommended.

Even though the parallel operation has the disadvantages as mentioned earlier in this post, when situation exists for parallel operation where the continuity of at least parallel service in event of failure of one of the transformer is of greater importance, any two or more transformers with impedance in same order of magnitude can be used. The impedance has to be in proportionate order so that the load is equally shared between the transformers.

In the case of a parallel operation, in order to meet the HV bus arrangement has to match the transformer capacity provided on the basis of concepts labelled as follows:

The 2 x 100 % Approach:

In this concept, two power transformers each capable of meeting 100 % load of the project demand is used to be operated in parallel. The substations will be having four number of circuit breakers in an 'H' type bus arrangement. Two circuit breakers are for line side control and two other for transformer. A fifth circuit breaker is used to parallel the buses by connecting it between the two buses. Generally, an off load isolator is used for this purpose. If 20 MVA is the project demand, two transformers of 25 MVA is used so that, even at the outage of one of the transformer, the other will carry 100% of the 20 MVA project demand with a margin of 5 MVA left for meeting the MVA requirements while starting of the HV motor of largest rating.

The 3 x 50 % Approach:

In this approach, three power transformers each with a capacity of 50% more than maximum demand of the project is used, so that at anytime one of the three transformers can be released for maintenance purpose. With only two transformers in service out of the three, each transformer has to cater for 50% of the maximum demand. But, the capacity of the transformer has to be greater than 50% of Maximum Demand so that there is some margin for the start of largest HV motor. For instance, if 50% of MD is 15 MVA, then three transformers of 18 MVA each is to be used.

The 4 x 33 % Approach:

This arrangement calls for four power transformers each rated to more than 33% of the maximum demand and generally has a six bus arrangement. for instance, if the maximum demand of the project is 60 MVA, four transformers of 25 MVA each will be needed such that at the outage of one transformer, the other three can meet the maximum demand.