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A reverse power relay is a directional power relay that is used to monitor the power from a generator running in parallel with another generator or the utility. The function of the reverse power relay is to prevent a reverse power condition in which power flows from the bus bar into the generator. This condition can occur when there is a failure in the prime mover such as an engine or a turbine which drives the generator.

Causes of Reverse Power

The failure can be caused to a starvation of fuel in the prime mover, a problem with the speed controller or an other breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives the prime mover. This happens as in a synchronized condition all the generators will have the same frequency. Any drop in frequency in one generator will cause the other power sources to pump power into the generator. The flow of power in the reverse direction is known as the reverse power relay.

Another cause of reverse power can occur during synchronization. If the frequency of the machine to be synchronized is slightly lesser than the bus bar frequency and the breaker is closed, power will flow from the bus bar to the machine. Hence, during synchronization(forward), frequency of the incoming machine is kept slight higher than that of the bus bar i.e. the synchroscope is made to rotate in the "Too fast" direction. This ensures that the machine takes on load as soon as the breaker is closed.

Setting the Reverse Power Relay
The reverse power relay is usually set to 20% to 50% of the motoring power required by prime mover.  By motoring power we mean the power required by the generator to drive the prime mover at the rated rpm.  This is usually obtained from the manufacturer of the prime mover (turbine or engine).

The contacts in the circuit breaker need to checked periodically to ensure that the breaker is healthy and functinal. Poorly maintained or damaged contacts can cause arcing, single phasing, and even fire.

The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact measurement check.

The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.

The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 300 A through the contacts and measuring the voltage drop across it.  This test is done with a special contact resistance measuring Kit

Then, using Ohm's law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.

Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions.

Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.

Synchronization is a process of matching the voltage, frequency, phase angle and sequence of two AC power sources and running them in parallel. Depending on the direction, the process of synchronization can be divided into two types, viz. forward and reverse Synchronization.




Forward Synchronization

In Forward Synchronization, the voltage, frequency and phase angle of the incoming generator is synchronized to match the values of the bus bar. This is generally used when a generator needs to be synchronized with an already charged bus bar. as shown in the diagram.


Reverse Synchronization

Reverse Synchronization or backward synchronization is generally done when a the supply from a grid utility is needed to be synchronized with a bus bar in the factory. Since, it is not possible to alter the voltage, frequency, etc. of the incomer, in this case, the grid. The voltage, frequency, etc. of the bus bar are adjusted to match the incomer. (See drawing).


The instruments used for synchronization are

Double Voltmeter
The double voltmeter is a voltmeter with two dials which can simultaneously display two voltage values of the sources to be synchronized.

Double Frequency Meter
The double frequency Meter shows two frequencies, that of the two sources

The Synchroscope
The synchroscope is a device that shows the difference in phase angle between two sources. It consists of a dial with a rotating pointer. The pointer takes up a position depending on the difference in the phase angle between the two sources. The When the pointer is at the vertical position i.e 12 'O' clock position, the difference in the phase angles is zero. The two sources are now said to be "in phase" with each other.

The Phase Sequence Indicator
The phase sequence indicator is similar in functioning to a miniaturized three phase induction motor. The indicator has a rotating disc which rotates in the clockwise sequence when the phase sequence is positive i.e. R-Y-B. When the phase sequence is negative i.e R-B-Y, the disc rotates in the opposite direction.

See Also:

Synchronization of AC Sources

The Synchroscope

Forward and Reverse Synchronization

Synchronization is a process involved in connecting two AC power supplies. AC power sources
such as transformers and generators need to be operated in parallel. This may be required
for greater reliability and capacity sharing.

In the picture, the incoming power source, a generator(#1) is being synchronized with a busbar which is already to a generator(#2) which is running.

For synchronizing three conditions need to be satisfied
1. Equal Voltages
By Equal voltages, we mean that the rms value of the voltages should be equal.

2. Equal frequencies
Equal frequencies implies that the two sources should have equal frequencies. A 50Hz source
can only be synchronized with another source whose frequency is also 50 Hz.

3. Same Phase Sequence
The sources should have similar phase sequence i.e. RYB and RYB or RBY and RBY.

4. Same Phase angle
The vectors of the two sources should have the same phase angle.

It is possible to bring the frequency, phase angle and voltage within synchronizing limits by adjusting the speed and voltage of the generator.

The circuit breaker connecting the two power sources can be closed when all these conditions are satisfied. The drawing shows vector diagrams of two three phase voltages whose phase sequences, phase angles, voltages and frequencies are equal

Special equipment and checks are used to ensure that the above conditions are satisfied.


Insulations used in Electrical machines have been classified into 4 classes. They are A, B, F, H. The classification has been made by NEMA( the National Electrical Manufacturers Association).

All insulation are standardized to be operated at 40 ºC. However, the operation temperatures of a motor or a generator will be higher than this. The maximum operating temperature for the different classes of insulation are

Class

Max. operating temp(ºC )

Max. Permissible temp rise ºC

Allowance for Hot spot

ºC

A

105

60

5

B

130

80

10

F

155

105

10

H

180

125

15


The Maximum operating temperature is the maximum temperature the insulation can reach during operation. It is calculated as
Max. operating temperature = standardized operating temperature(i.e. 40 ºC) + maximum permissible temperature rise + Allowance for hot spot in winding.

The Maximum permissible temperature rise is the temperature by which the operating temperature temperature can exceed the standardized temperature i.e. 40 ºC.

The allowance for hot spot refers to the allowance in temperature allowed for hot spots which may form in the centre of the winding.

Therefore, a class F insulation will have the maximum value calculated as 40+105+10 = 155 ºC

Exceeding the maximum operating temperature will affect the life of the insulation. As a rule of thumb, the lifetime of the winding insulation will be reduced by half for every 10 ºC rise in temperature

Harmonics Resonance is a phenomenon which can occur in a power system. It can cause system instability or damage to electrical components such as capacitors and transformers. Harmonic resonance occurs when the inductive reactance and the capacitive reactance of the power system become equal.

However, as the order of the harmonics (frequency) increases, the inductive reactance increases while the capacitive reactance decreases.

At a particular frequency of harmonics, the inductive and capacitive reactances become equal and resonance sets in. Resonance can cause erratic conditions in the power systems such as transient high or low voltages, unexplained breakdown of equipment such as failure of transformer windings or failure of capacitor. Transient voltages generated by such harmonics resonance can also result in unexpected operation of relays and breakers.

The phenomenon of Harmonic Resonance should be borne in mind when modifying the system to add capacitors to improve the power factor or when adding new inductive loads such as motors, reactors, or transformers to existing systems containing capacitors.

The formula to determine the order of harmonic which may cause resonance is





where MVAsc is the impedance of the source and MVARcap is the reactive power drawn by the capacitors.Thus when a 20 MVAr capacitance is connected across a source of 1000 MVA, there will be a condition of resonance at the 7th Harmonic.

The possibility of harmonic resonance should be explored and eliminated during any modification/addition of loads in the power system.


Class X CTs are current transformers which are used for special applications. The "X" indicates that they do not belong to any standardized class. They are customized transformers therefore it is necessary to specify the individual characteristics of the current transformers such as the turns ratio, rated primary and secondary current, Voltage knee point, the maximum magnetizing current at the knee point and the Maximum resistance of the secondary winding.

Class X CTs are generally used where high knee points are require to prevent operation at higher currents without saturation.

Class X CTs are further divided into Class A and Class B CTs.

Class A CTs are more expensive and are designed to operate even at maximum fault current without saturating. Class B CTs cost less and are used in high-impedance applications. They tend to saturate during transient conditions.

In common discussion about Harmonics, the difference between current and voltage harmonics is rarely addressed. While current and voltage harmonics are related. The effects are different.

Current Harmonics are caused by non-linear loads such as thyristor drives, induction furnaces, etc. The effect of these loads is the distortion of the fundamental sinusoidal current waveform alternating at 50Hz. Current Harmonics affect the system by loading the distribution system as the waveforms of the other frequencies use up capacity without contributing any power to the load. They also contribute the Copper losses I2Z losses in the system.

Besides, Harmonic currents load the power sources such as transformers and alternators. However, current harmonics do not affect the remainder of the loads in the system which are linear. They only impact the loads which are causing them i.e non-linear loads.

Voltage Harmonics are caused by the current harmonics which distort the voltage waveform. These voltage harmonics affect the entire system not just the loads which are causing them. Their impact depends on the distance of the load causing the harmonics from the power source. If other harmless loads are connected between the source and harmonics causing loads, these innocent loads will also be affected by the harmonics.

Hence, one way of mitigating the effect of harmonics is by connecting the harmonics-causing loads as close to the source as possible in a separate feeder. Another method is by using an isolating transformers between the problem loads and the rest of the distribution system



See also:

Basics of Harmonics

Harmonic Resonance

Power outages are classified as Brownouts, Blackouts and Dropouts.
Brownouts are conditions where there is a sustained low voltage conditions. Low voltages affects the functioning of electric equipment. Lamps may glow dimmer, induction motors may draw more current to compensate for the drop in voltage and heat up. The loss of supply in one phase in three phase systems also falls under this category.

Blackouts are conditions in which there a loss of supply to a wide area lasting a few hours to many days. Blackouts are generally caused by a tripping in power stations or a malfunction in the distribution systems.

Dropouts have the shortest duration of all the three phenomena. Dropouts are condition where there is a dip in the voltage lasting a few cycles. It may manifest itself in the form of flickers in the lamps.

The Synchroscope is a device to check the phase angles of the two sources during the process of synchronization. It plays a vital role in ensuring that the two power supplies which are being synchronized are "in phase" with each other. The Synchroscope has a dial with a pointer which can occupy different positions according to the difference in the phase angle.

The positions are usually compared with the markings on the clock. Thus a 3 'O'clock position would indicate that the voltages are apart by an angle of 30 degrees. The 6 'O'clock position would indicate that the sources are apart by 180 degrees. When the pointer is at the 12'O'clock position, it indicates that the difference in phase angle between the two sources is zero. The breaker connecting the two sources can now be closed.

The dial of the synchroscope is marked with two arrows indicating the direction of rotation of the pointer. These arrows indicate the clockwise and the anti-clockwise direction. The clockwise indicating arrow is marked "Too Fast" while the anti-clockwise indicating arrow is marked "Too Slow".

These arrows indicate the speed of the incoming source as compared to the bus bar. If the incoming generator's frequency is more than that of the bus bar, the pointer rotates in the "Too fast" clockwise direction. The machine then needs to be slowed down. If the frequency of the incoming machine is less that that of the bus bar, the rotation of the pointer is in the opposite "Too Slow" direction.

During forward synchronization when the incomer is intended to supply power to the grid, the pointer of the synchroscope is allowed to rotate in the clockwise direction before it stabilizes at the 12'O'clock position after which the breaker can be closed. This is essential to prevent the machine from tripping on reverse power should power flow from the bus bar to the grid.

In the case of reverse synchronization, the direction of the rotation depends on whether power needs to be exported from the bus bar to the grid or imported from the grid to the bus bar. In the former case, the direction has to be clockwise in the latter case it is to be anticlockwise.

In newer models of the synchroscope, the pointer is replaced by LEDS which blink depending on the phase angle and give the appearance of "running lights".


The Oil inside power transformers have a vital role to play in the transformer's functioning. The function of the transformer oil is two-fold, to provide cooling to the transformer windings and to provide insulation. However, over a period of many years, the transformer oil deteriorate owing to many factors. This deterioration causes a change in the physical and chemical properties of the oil.

Some of the reasons for transformer oil deterioration are

Oxidation of the oil.

The transformer breather permits the entry of air into the transformer, although it filters the moisture. The air which flows inside the transformer oxidizes the oil and forms a sludge of hydrocarbons. This process, though, usually occurs gradually over a period of many years. The sludge thus formed hinders the cooling of the transformer and causes heating. The sludge, sometimes, blocks the cooling ducts of the transformer. Higher temperatures inside the transformers, in turn, cause further sludge formation.

Thermal Decomposition

At high temperatures, the organic compounds in the transformer oil break down due to a phenomenon known as pyrolysis. This results in the formation of unwanted carbon compounds, sludge, etc.

Moisture contamination

Under ideal conditions, the oil in a transformer is protected against the entry of moisture by means of the silica gel filter in the breather. The silica gel changes color from blue to pink when it gets saturated with moisture. If the silica gel is not renewed in time, moisture may pass through the filter contaminating the oil.

The anti-pumping relay is a device in circuit-breaker whose function is to prevent multiple breaker closures. For instance, if the operator gives the closing command to the breaker by pressing the close button and the breaker closes. However, a fault in the system causes the breaker to trip. Since the close command is still in the pressed condition, there is a chance of the breaker closing again and being tripped by the relay multiple times. This can damage the closing mechanism of the breaker. The anti-pumping relay prevents this by ensuring that the breaker closes only once for one close command from the control panel.

Agilent Technologies have launched their new DSO1000A series of oscilloscope. Weighing within 3 kgs, the unit comes with a 2 or 3 channel facility. 

The instrument has a bandwidth between 60 MHz to 200 MHz. It has a sampling rate of 2 GSa/s. It has a "True Zoom" mode that enables the user to zoom into a particular part of the waveform and display the entire waveform at the same time.The device can display about 21 measurements simultaneously.

The unit comes with a bright display which is visible from a wide range of angles. The measured signals can also be filtered with special DSP filters which can eliminate unwanted interferences.

It also has a built in counter. Measured and recorded waveforms can be transferred on to pen drives. It also has USB connections and a user-friendly software that comes bundled with a short training kit for first time oscilloscope users.

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