Cathodic Protection

Electrochemical Corrosion is one of the types of corrosion in metals.  Electrochemical corrosion occurs, chiefly, in submerged metallic structures such as pipelines, storage tanks, water circulating systems, ship hulls and off shore platforms.  Electrochemical Corrosion occurs when the potential on the surface of the metal is not uniform.  

This uneven potential on the surface is caused due to impurities in the surface or uneven stress on the surface of metals.  This causes certain parts of the surface to act as an anode while other parts of the surface act like the cathode.  The current which flows between the anodic and the cathodic regions can cause corrosion.

Cathodic protection refers to a method of protection of these metallic structures from electrochemical corrosion.  This is achieved by making the metal to be protected the cathode.  When a material which has a higher electrode potential is kept in the same medium as the object to be protected, it becomes the cathode while the object to be protected becomes the cathode.  A simple electrochemical cell is created.  The electrons move from the anode to the cathode in the medium.  In the process, the anode gets corroded while the cathode is protected from corrosion.  The anode used is called the sacrificial anode. 

The sacrificial anode has to be periodically replaced as it gets eaten away.

Manufacturing Power Transformers

Distilled water in batteries

Distilled water needs to be added periodically to the Lead Acid Batteries to compensate for the water lost during electrolysis.  When a discharged battery is charged with the power from an external charger, the current causes an electrolysis of the water in the electrolyte.  The water dissociates into hydrogen and oxygen.  These gases are released through the vents.  This process results in drop in the level of the electrolyte.  To compensate for this, distilled water is added when the level of electrolyte falls below a particular level. 

Distilled water alone should be used as it is pure water without any impurity.  Tap water should never be used as the chemicals present in tap water can seriously damage the batteries.  Care should be taken that water is not added in excess known as overwatering.  Overwatering will dilute the electrolyte and affect battery performance. 

APC UPS Systems - A Guest Post

UPS or Uninterrupted Power Systems are widely used in offices, hospitals, industries, in telecommunication as a back up against sudden power cuts. Sudden Power cuts can cause disturbances and losses in the form of lost data, interruption to life support systems in hospitals and unpredictable behaviour in industrial systems. A UPS which usually consists of an battery system and an inverter steps in the moment the supply voltage dips beyond a certain value preventing the interruption of supply to critical systems.

The central part of the UPS is the battery which stores power when the supply is available and makes power available when the mains supply is switched off. A UPS system can provide power backup for a time period ranging from a few minutes to many hours. The duration of the back up supply depends on the capacity of the battery. APC is a division of Schneider Electric which deals with UPS and Power back up systems. The company provides a wide range of batteries with different capacities and ratings for the power back up solutions.

Besides their use as sources of back up power, these batteries can also be used to protect critical equipment against sudden dips in the voltage which can electronic equipment to switch off or reset causing unwanted interruption. APC provides a wide range of UPS solutions for Homes, Small Business and for Large Corporations. The UPS provides sine wave output and distinct LED indicators which indicate the state of the battery and the power conditions.

The systems have a Home away switch which charges the batteries while you are away besides a intelligent battery charging system which enables controlled charging of the batteries thus prolonging battery life. All products are backed by a warranty and the wide service network of APC.

You can check out their products in their website http://www.apc.com

About the Author 

Alex from HTBS  is a technology expert who often writes about topics related to batteries and especially regarding the APC Replacement Battery

Stuck breaker protection

Stuck breaker protection is a situation in which a circuit breaker fails to operate even after receiving a tripping signal from a relay or a switch.  Stuck breaker can undermine the protection scheme and can cause damage to machinery and is a danger to personnel.  

Common reasons for a circuit breaker not opening are a disconnection in the trip circuit or a mechanical problem with the circuit breaker.  In these conditions, there needs to be a backup protection device which can interrupt the fault and isolate the system.  In some cases, the entire section of the bus to which the breaker is connected is de-energized to interrupt power. 

A simple Stuck breaker protection schemes functions by sensing the position of the circuit breaker through the limit switches in the circuit breaker.  The protection system waits for the open status from the circuit breaker after the open signal has been given.  If the signal is not received within a preset time, the scheme assumes that the breaker is stuck and initiates backup measures. 

However, this system has its limitations.  The system cannot detect a situation where the current continues to flow despite the breaker having tripped.  This can occur due to situations where the arc has not been quenched (failure of the arc extinction system) and the current flows even though the contacts have mechanically separated. 

To ensure proper feedback of the interruption of the current, advanced stuck breaker schemes sense the current as well as the position contacts of the circuit.  This ensures that an accurate feedback of the breaker status.

Voltage Supervision Relays

The Voltage Supervision Relay is an integral part of any  protection system.  The voltage supervision relay protections systems from undervoltage and overvoltage.  Overvoltage in a system can result in serious damage to insulation and equipment while undervoltage can cause motors to draw more current and reduce the speed of the motors, disturbing the process. 

Besides protecting against overvoltage, the voltage supervision relay can also be used to detect earth faults as the phase to earth voltage is distorted when there is an earth fault in one of the phases.  Voltage supervision relays can generate alarms when the voltage is low or high in only one phase.  This is also known as phase asymmetry. 

In motor circuits, the voltage supervision relay protects against single phasing.  Single phasing can cause serious damage to motors.

A simple auxiliary relay can also be used to generate alarm for undervoltage.  When the voltage drops, the relay can drop off thus generating an alarm or a shutdown.

Trip Circuit Supervision in Circuit Breakers

Trip circuit supervision in Circuit breakers is an vital part of any protection scheme. If the trip relay fails to operate, it may result in upstream tripping or even in damage to equipment.  Trip circuit supervision makes sure that the tripping coil of a circuit breaker is always in the healthy condition.  

The Trip circuit supervision is particularly important in breakers which have only one trip coil.    The Trip circuit supervision relay continually measures the resistance of the trip coil of circuit breakers.  It also measures the control voltage of the trip coil and gives and alarm when the control voltage falls to low levels. 

The Trip circuit supervision relay injects a constant current through the trip coil of the breaker and measures the voltage drop across the coil.  Thus, the relay is able to measure the resistance of the coil.  

The Trip circuit supervision relays can also monitor more than one breaker coil. 

If the Trip circuit supervision Relay detects a fault, it activates the breaker failure logic which can activate a backup breaker if installed or cause the tripping of upstream breakers.

What is the difference between sine wave and non-sine wave ups

Square Wave Output

Inverters are circuits which convert DC into AC.  Inverters are an integral part of UPS (Uninterrupted Power Supplies).  They convert the DC stored in the batteries of the UPS and produce an AC voltage which is provided to connected load in the event of a power failure.  

The AC produced by the inverter is not always a sine wave.  Some inverters produce a square wave.  These inverters are known as square wave inverters.   Square waves can be used where the load on the UPS is going to be mostly resistive loads such as heaters, incandescent lamps, etc.

Modified Sine Wave Output

Another form of inverter output is the modified sine wave or the quasi-sine wave inverter.  These inverter produce a waveform that has an intermediate voltage level which brings it closer to a sine wave.  

Sine Wave

Sine wave inverters produce an actual sine wave. Sine wave inverters are more expensive than the square wave and the modified sine wave inverters.  However, they are ideal as electric devices such as motors, Television sets, chargers are designed to use a natural sine waveform.  Using square wave inverters on these devices can produce harmonic distortion, humming.  This leads to reduced efficiency and loss of power. 

What are Trip Free and non-Trip Free Circuit Breakers

Trip Free Circuit Breakers are circuit breakers which can trip even if they are held in the "ON" position.  Hence, it is not possible to forcibly keep them in the closed position.  Trip free circuit breakers are used in circuits with equipment which are sensitive to overload and in circuits which are not critical.

Non-Trip Free circuit breakers are used in circuits which are critical for plant operation and for safety.  In these systems, it may be economical to lose a motor or a heater than to interrupt the manufacturing process.    In these circuit breakers, the trip function can be bypassed by forcing the circuit breaker to the on position. 

Conditions for Paralleling two Transformers

Transformers may need to be paralleled with other transformers to share loads greater than the capacity of the individual transformer.  When transformers are to be connected in parallel, it is necessary for them to satisfy certain basic conditions. 

They are

The Same Voltage Ratio.

Two transformers in Parallel should have the same primary and secondary voltage ratings.  Any error in the voltage ratio would cause heavy circulating currents to flow between the transformers.   This circulating current will result in a corresponding imbalance in the primary currents, and result in overloading of one transformer.  This circulating current will result in increased copper losses. 

The Same Percentage impedance

For two transformers of different capacity to share the load proportionally, their impedances should be in the inverse ratio of their  ratings.  That is, the percentage resistance and the percentage reactance of the two transformers should be equal to ensure equal sharing of the active and reactive loads between the transformers. 

The Same Polarity

Two Transformers in parallel should have the same polarity.  Connecting transformers with wrong polarity can result in circulating currents or short circuits.  Some transformers may have inherent polarity errors which needs to be detected before connecting them.

The Same Phase Sequence

Only Transformers having the same Phase sequence can be connected in parallel.  The phase sequence can be detected using a phase sequence indicato.

The Same Phase Angle

The difference in the Phase angle between the secondary voltages should be zero.  This is ensured by checking the compatibility of the vector groups of the transformers to be paralleled.

Innovations in DC Technology by ABB

Informative video from ABB on developments in DC Power Transmission and distribution. It also touches on some of the advantages of DC over AC