Thursday, 8 December 2016

Class-A, Class-B and Class-C Tripping Classification of Generator

Generator, Generator Transformer and Unit Transformer protections have been classified into Class-A, Class-B and Class-C. Class-A tripping is further classified into Class-A1 and Class-A2. In this post we will discuss each type of tripping classes and their significance.

Picture taken from http://universalerectors.com/steam_turbine_generator.html


Basis of Tripping Classification: The tipping classification of Generator is based on the need of isolation of Generator on the basis of type of fault. For example, there are some faults like Generator Differential Protection which calls for immediate tripping of Generator Breaker without delay whereas there are some fault like Loss of Excitation, Rotor Earth Fault etc. which do not call for immediate tripping of Generator.

Class-A1 Trip: The protections for the faults in the Generator which need immediate isolation are grouped under this Class-A1.  There are a list of faults which are kept under this class. They are as follows:
a)    Generator Differential Protection
b)    100% Stator Earth Fault Protection
c)    Generator Over Voltage Protection
d)    Dead Machine Protection
e)    95% Stator Earth Fault Protection
f)     Starting Over Current Protection
In case of actuation of Class-A1 protection, Generator Circuit Breaker and Filed Circuit Breaker are opened along with turbine tripping.
Class-A2 Trip: The protections for the faults in Generator Transformer (GT), Isolated Phase Bus Duct (IPBD), and Unit Transformer (UT) which need immediate isolation are grouped under this Class-A2. Normally following protections are kept under Class-A2:
a)   Over fluxing Protection of Generator
b)   Back up Impedance Protection of Generator
c)   Differential Protection of GT
d)   Buchcholz Relay of GT
e)   PRD of GT
f)    Trip from OTI & WTI of GT
g)   Fire protection of GT
h)  Differential Protection of UT
i)    Buchcholz Relay & PRD of Main Tank of UT
j)     Trip from OTI & WTI of UT
k)   Fire protection of UT
These protection when operated initiate tripping of Generator Circuit Breaker, Field Circuit Breaker, Generator Transformer Circuit Breakers & Unit Transformer LV Circuit Breakers and turbine.



Class-B Trip: The protections for the faults in the Generator which do not need immediate isolation are grouped under this Class-B. The turbine is tripped first and Generator is allowed to run utilizing trapped steam in turbine. Let us suppose that there is some fault in the process side i.e. in steam cycle, under that condition also turbine will be tripped first while Generator will continue to run utilizing trapped steam till reverse power relay operates. Generator Circuit Breaker is tripped on initiation of reverse power. Normally, Loss of Excitation and Rotor Earth Fault of Generator are kept under this class. These protection when operated initiate tripping of Generator Circuit Breaker, Field Circuit Breaker and turbine.
Class-C Trip: The protections for the faults / abnormal condition in the Grid which call for disconnection of the Generator from the Grid are grouped under this Class-C. In this case, Generator is isolated from the Grid by opening the suitable breaker i.e. Generator Transformer HV side Breaker. Mind that in this case only Generator is isolated from the Grid. Thus Generator continues to feed Station loads (also known as house load). Such scheme where generator is operated on house load at reduced power is known as Generator Islanding. Normally following protections of Generator are kept under this class:
a)  Unbalance or Negative Sequence Protection
b)  Back up Impedance Protection
c)   Under Frequency
d)  Over Frequency
e)  Pole Slipping Protection

Wednesday, 7 December 2016

Reluctance Torque – Why Reluctance Torque exists in Salient Pole Machine?

Lets us consider a doubly excited magnetic system as shown in figure below. In this model of machine, both stator as well as rotor is assumed to be salient pole type. Let’s assume that stator is energized from source Vs whereas rotor is energized with source Vr.



Total torque developed in such doubly excited magnetic system is given as

Te = (Is2/2)(dLs/dƟr) + (Ir2/2)(dLr/dƟr) + IsIr(dMsr/dƟr)

Case1: If rotor current is made zero i.e. Ir = 0

Torque Te = (Is2/2)(dLs/dƟr)

Thus we see that even though rotor current is zero, an electromagnetic torque is developed as the Reluctance seen by the stator produced flux changes with the position of rotor Ɵr. A change of reluctance seen by stator produced flux varies the self inductance of stator winding Ls with Ɵr.

Case2: If stator current is made zero i.e. Is = 0

Torque Te = (Ir2/2)(dLs/dƟr)

Thus we see that even though stator current is zero, an electromagnetic torque is developed as the Reluctance seen by the rotor produced flux changes with the position of rotor Ɵr

In view of the above two cases, we can say that the torque expressions as obtained in two cases are Reluctance Torque as this torque is not produced by the interaction of two fluxes rather this torque is produced just because of change of reluctance and hence a change in self-inductance.

To have a physical understanding of the Reluctance Torque let s consider the doubly excited magnetic system as shown in figure above. If only stator of the system is excited, then the stator flux would have a tendency to follow less reluctance path for which the rotor turns anticlockwise direction. Now if only rotor is excited, rotor produced flux would have a tendency to follow minimum reluctance path and for achieving this rotor will again turn anticlockwise.

Why Reluctance Torque is not present in Cylindrical Rotor?

In the figure shown above in the discussion, suppose the salient rotor is replaced with cylindrical rotor and is excited by current Ir. As the rotor is cylindrical, the reluctance en by the stator produced flux will be same irrespective of the rotor position Ɵr. Thus the self-inductance of stator winding Ls will be a constant quantity and hence dLs/dƟr = 0 which in turn means that no reluctance torque will be present. Therefore the equation of torque becomes,

Te = (Ir2/2)(dLr/dƟr) + IsIr(dMsr/dƟr)

Note that as the stator poles are salient in the figure, the reluctance seen by the rotor produced flux will vary with the position Ɵr and hence there will a change in self-inductance of rotor Lr, this why reluctance torque term (Ir2/2)(dLr/dƟr) is appearing in the above torque equation.

To summarize, we can say that the first two terms in the generalized torque equation of doubly excited magnetic system, are Reluctance Torque.

Te = (Is2/2)(dLs/dƟr) + (Ir2/2)(dLr/dƟr) + IsIr(dMsr/dƟr)

It shall also be noted from the reluctance torque term that Reluctance Torque, is independent of direction of current in stator and rotor windings.

Last term i.e. IsIr(dMsr/dƟr) represents that component of total torque which depend on both rotor and stator current and also on the angular rate of change of mutual inductance Msr. This component of Torque is called Electromagnetic Torque. From the electromagnetic torque term IsIr(dMsr/dƟr), following conclusion can be drawn:

“Electromagnetic torque can only exist if the two winding i.e. stator and rotor are mutually coupled and both winding carry current.”

The physical concept of electromagnetic torque in figure shown previously in the discussion is as follows:

The North, South poles produced on stator by current Is and South and North poles produced on rotor by current Ir attract each other tending to align their fields. The torque so developed by the interaction of stator and rotor magnetic field is the electromagnetic or interaction torque. Electromagnetic torque or interaction torque depends on the direction of current in stator and rotor windings.




In the figure, the direction of electromagnetic torque for current stator and rotor current is anticlockwise. If the direction of rotor current Ir is reversed, the interaction or electromagnetic torque will be reversed i.e. in clockwise direction but the direction of Reluctance Torque would still remain in the same direction as before i.e. anticlockwise.

Tuesday, 6 December 2016

df/dt Relay - Rate of Change of Frequency (ROCOF) Relay

Rate of Change of Frequency (ROCOF or df/dt) relay is used for fast load shedding, to speed up operation time in over- and under-frequency situations and to detect loss of grid. For better understanding of role and operation of df/dt relay, let us first study the variation of frequency with load for a Grid. Graphical relationship between power and frequency of a Grid is shown in figure below.



We can have two things to be noted from the above graph:

a)    If the power available in the Grid increases i.e. in other words generation is more than the load, frequency will go up.

b)    If the generation is less than load i.e. power deficient in the Grid then frequency will decreases.

Large power grids (here large power grid means having large installed capacity in MW) are characterized by a very high stiffness constant which means that a large perturbation (load generation mismatch) is required to cause the grid frequency to change by 1 Hz. Thus we can say that, for the same magnitude of perturbation, the rate of change of frequency (df/dt) will be much smaller for a very large power grid as compared to a medium sized grid. Therefore, large sized grids are inherently resilient to rapid frequency fluctuations.

If it happens that due to increase in load, frequency dips below a certain threshold the Generators connected to the Grid will trip which will result into further dip in frequency and such a cumulative dip in frequency result into complete failure of Grid.  Thus there is a need of Rate of Change of Frequency or df/dt relay which can detect the dip in frequency earlier and initiate load shedding to resume the normal frequency of the Grid.

ROCOF or df/dt relays are particularly effective in arresting the frequency collapse of a grid in the event of sudden loss of major generation. This is because by measuring the frequency decay rate, the corrective action can be initiated much ahead of the time when frequency of the synchronous interconnection would have actually dipped to a point at which generator under-frequency relays or unit auxiliaries would trip / operate leading to a complete system shutdown. The df/dt is used for load shedding in situations where sudden loss of generating capacity on a system will be accompanied by a decrease in system frequency. In such a situation of load Generation mismatch, the system frequency tends to fall. The df/dt relay can control the circuit breakers and allow feeders to be disconnected from the network, one by one. Figure below shows a typical df/dt Relay.

Picture taken from L&T Products & Services


Operating principle and Setting of df/dt Relay:

The Rate of Change of Frequency (ROCOF) Relay operation is based on the measurement of two successive frequency and the time difference between the frequency measurements. The setting of the df/dt relay is in Frequency/Time e.g. 0.3Hz/second or 0.4 Hz/.5 seconds. The minimum df/dt relay setting available is 0.1 Hz/sec. However, some df/dt relays have a minimum setting of 0.2 Hz/sec only.

Monday, 5 December 2016

Transformer Oil Filling- Why Vacuuming Required?

The oil can only be filled in the transformer which satisfies the standard specifications mentioned by the manufacturer of transformer. Normally Transformer is supplied with gas filled i.e. the Main Tank of Transformer is filled with Nitrogen gas under some specified pressure to prevent any degradation of the insulation of core and winding. But we cannot keep Transformer Main Tank filled with nitrogen for a period more than three months and hence oil filling becomes important. Oil filling in Transformer also becomes important if we want to store the Transformer for more than six months.

The oil filling is done in the main tank under vacuum. The large Transformers are generally designed to withstand the full vacuum for long periods. Oil filling and filtration of oil is carried out simultaneously. Oil sampling is done during the process of oil filtration and if sample result for Dielectric Strength and moisture comes under limit then filtration of oil is stopped. Figure below shows Transformer Oil Filtration unit.



During oil filling in the main tank of the transformer, it is preferable to connect the inlet hose pipe at the bottom of the tank. After filling the oil up to the top of the transformer core, maximum suitable vacuum is maintained above the oil level in the tank to minimize the hazard of bubbles lodging in the inaccessible corners of winding.

It should be noted that condenser bushing, diaphragm, Buchholz relay, tap changer board, conservator tank, radiators units etc are not designed for withstanding vacuum hence they should not be subjected to vacuum. That means vacuum processes only to be done in main tank without connecting or with blocking all the above-mentioned components of the transformer.

You may like to read


Why Vacuuming Required?

If we not doing vacuuming then problem is that, it will create bubble in oil which is very dangerous for Transformer operation at high voltage. If there is bubble in oil then at the time of application of high voltage it offers less dielectric strength than oil. It results in partial breakdown of an insulating material which further result in breakdown of whole oil insulation.

Concept of Magnetization or Saturation Curve: B-H Curve

Magnetization curve or saturation curve is basically the graphical relationship between the magnetic flux density B and magnetic filed intensity H for a given magnetic material. As we know that,

B = μ0μrH
where  μ0 = Permeability of free space = 4πx10-7 Tm/A

 μr = Relative Permeability of material

Thus by using the above equation, we can find the relative permeability of the material. Unfortunately relative permeability do not remain constant for a given material rather it varies with the value of magnetic flux density B and magnetic field intensity H. This variation in the value of relative permeability μr is due to saturation of the material which can be better understood by domain theory. However we will concentrate here on the magnetization curve.

Figure below shows a typical B-H curve for a ferromagnetic material. As can be seen from the figure below, it consists of three zones i.e. OA, AC and beyond point C.



Initial zone OA is non linear while the zone AC is linear and beyond point C, it is again non linear. This non linear zone beyond point C is called Saturation region. It shall be noted here that tough OA is also non linear but in this zone the rate of change of B with respect to increase in H is increasing which means that increasing the value of H increases the magnetic flux density B rapidly (How am I saying this? Just draw tangent at different points between OA. You will notice that slope of tangent is increasing). But thing is not so in Saturation Region. In saturation region, the rate of increase of B with respect to increase in H is decreasing (Just draw tangent at different point beyond C. You will notice that slope of tangent is decreasing) which simply means that flux density B in the Saturation Region increases less rapidly with H as compared to its change in linear zone AC.

It shall also be noticed that in saturation region,

Relative permeability μr = B/ μ0H = (1/ μ0)xSlope

Therefore relative permeability decreases in saturation region as the slope of curve is decreasing. Thus during magnetic circuit calculations, the value of μr and H should correspond to the flux density under consideration. For free space or non magnetic material as μ0 is constant, hence magnetization curve shall be a straight line (B = μ0H).

You may like to read Basics of Magnetic Circuit

From economic consideration, the design of rotating electrical machines and Transformer are based on the flux density somewhere in the saturation region. Therefore for analysis of magnetic circuit of electrical machinery or transformer, real B-H curve of the material used shall be considered.

Sunday, 4 December 2016

Variable Frequency Transformer (VFT) – Construction and Working Principle

Variable Frequency Transformer or simply VFT is quite a new technology to connect two asynchronous Grids. Asynchronous Grids mean two power systems operating at two different frequencies. The world’s first VFT was installed and commissioned in the year of 2004 at Hydro- Quebec’s Langlois substation, where it is used to exchange up to 100 MW of power between the asynchronous power grids of Quebec (Canada) and New York (USA). 

First thing to note here that though the name suggests it to be Transformer but construction wise it is like a Slip Ring Induction Motor. The basic concept behind the VFT is a rotary transformer with three phase windings on both rotor and stator. A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT. VFT is a controlled bi-directional device allowing flow of power in both directions i.e. from one Grid to another and vice versa. Mind that VFT is serving the same purpose as a Back to Back HVDC system.

Construction of VFT:

The Variable Frequency Transformer consists of a stator which is much like the stator of a hydro generator. There are laminations of steel stacked inside a stator frame. Windings are configured into a three phase four pole arrangement. Stator is connected to one power system via a Step-up Transformer. The rotor is constructed in the same manner as the stator. The rotor also contains three phase four pole windings. One grid is connected to the stator windings, while the other grid is connected to the rotor. The net effect is that a circular transformer has been produced, with the windings separated by an air gap.

In order to make connection to the rotor and still allow the rotor to turn freely, a slip ring arrangement is necessary. The VFT contains a device known as the collector. The collector consists of three phases of brushes and large copper slip rings. The number and size of the elements of the collector are such that the full rating of the machine current can be transferred continuously through the full range of speeds in either direction, including zero speed. Also on the shaft of the VFT a DC drive motor is connected. This motor is used to align the rotor with respect to the stator and maintain the rotation necessary to bridge the difference in the frequency of the two grids. Through the slip ring arrangement, rotor of VFT is connected to another power system through a Step-up Transformer.




Working Principle of Variable Frequency Transformer:

As discussed earlier in the post Stator of VFT is connected to one Grid through Step-up Transformer and Rotor is connected to another Grid through Step-up Transformer. Now we will see how Power Transfer take place in such doubly fed machine i.e. VFT. For any AC system the equation of power flow

P = V1V2Sinδ / X

Where V1 and V2 are voltages and δ is the angle between them.

Here in case of VFT, the voltage V1 is voltage of one Grid (say connected to Stator) and V2 is voltage of another Grid (say connected to Rotor). As it is expected that both the Grid are operating at same voltage level and turn ratio of Step-up Transformer are also same, that means V1 = V2 = V (say). Thus the power flow equation for VFT will reduce to

P = V2Sinδ / X

Thus δ is here variable quantity which we can change. Here comes the role of Drive Motor. The drive motor connected to the shaft of rotor just changes the angle δ by rotating the rotor. The angle introduced in the rotor with respect to the stator, by the torque motor, is proportional to the amount of torque (T) applied to the shaft. Therefore we can say that, power flow through a Variable Frequency Transformer i.e. VFT is directly proportional to torque applied by the drive motor.

A closed loop power regulator maintains power transfer equal to an operator set point. The regulator compares measured power with the set point, and adjusts motor torque as a function of power error. The power regulator is fast enough to respond to network disturbances and maintain stable power transfer.



In the figure above, suffix R stands for Rotor while suffix S for Stator. Phase angle difference between Stator voltage and Rotor voltage can easily be seen in the figure. The ability of the machine to rotate continuously is how the machine can bridge an asynchronous boundary. For example, assume a VFT connecting two asynchronous grids. For this example we shall call the Grids connected to VFT as Grid A and Grid B. When both grids are operating at exactly the same frequency, say 50.0 Hz, the VFT rotor will be stopped. If Grid A’s frequency increases slightly to 50.1 Hz, while Grid B’s frequency remains constant at 50.0 Hz, the rotor of the VFT will turn to allow for this difference. Because VFT is a four pole machine, in this case it will be rotating at 3 rpm [120 (50.1-50) / 4 = 3 ]. Consider now the condition where Grid A slows down to 49.9 Hz and Grid B is still at 50.0 Hz. The rotor will now be turning in the opposite direction at 3 rpm. The existing VFTs have a maximum operating speed of 90 rpm yielding a three hertz maximum frequency delta.

Like an ordinary transformer, the VFT has also magnetizing currents. Therefore we can classify the VFT as an induction machine. From a system level perspective, this means that the VFT consumes reactive power. In order to remain neutral to the grid from a reactive power point of view, shunt capacitors are supplied with a VFT to satisfy the machines own requirements. These are switched in and out by the VFT controls as needed. Additional shunt capacitors can be added to supply reactive power for either or both grids, if the application dictates that this is helpful to the systems. 

Must watch this video taken from GE Reports. This video shows the use of VFT connecting New York (NY) and New Jersy (NJ) grids.



This is the basic concept of Variable Frequency Transformer. Hope you enjoyed the post. Thank you!

Saturday, 3 December 2016

Low Forward Power Protection of Generator

Low Forward Power means the output of Generator is sufficiently reduced. This is basically a check not a protection i.e. Generator is tripped after having a check that its forward power has reduced. When the machine is generating and the CB connecting the generator to the system is tripped, the electrical load on the generator is cut. This could lead to generator over-speed if the mechanical input power is not reduced quickly. Large turbo-alternators, with low-inertia rotor designs, do not have a high over speed tolerance. Trapped steam in the turbine, downstream of a valve that has just closed, can rapidly lead to over speed.

To reduce the risk of over speed damage to Turbine, it is sometimes chosen to interlock non-urgent tripping of the generator breaker and the excitation system with a low forward power check.


Note:

“There are two types of tripping of Generator in broad sense, one is Non-urgent where immediate tripping of Generator is no required and another is Urgent tripping where immediate tripping is required. To be more accurate, if there is a fault in the Generator then it needs immediate tripping while if there is some problem in process side i.e. steam cycle then immediate tripping of Generator is not required. Here in this case it is better to utilize the trapped steam in between the Steam Stop Valve and Turbine. There is classification of Generator Protection as Class A, Class B and Class C which we will discuss latter.”


This ensures that the generator set circuit breaker is opened only when the output power is sufficiently low that over speeding is unlikely. The delay in electrical tripping, until prime mover input power has been removed is acceptable for non-urgent protection trips.



For urgent trips for example, stator current differential protection the low forward power interlock should not be used.

To prevent unwanted relay alarms and flags, a low forward power protection element can be disabled when the circuit breaker is opened.


The threshold setting of the low forward power protection function should be less than 50% of the power level that could result in a dangerous over speed transient on loss of electrical loading. Normally no time delay is provided for Low Forward Power Protection.

Reverse Power Protection of Generator


A synchronous Generator / Alternator is expected to supply active power to the system in normal operating condition. If the turbine i.e. prime mover fails the Generator / Alternator connected to the system will continue to operate as synchronous Motor drawing active power from the system. This reversal of power flow due to loss of prime mover can be detected by reverse power relay.

The  consequences  of  generator  motoring  and  the  level  of  power  drawn  from  the  power system will be dependent on the type of prime mover as under this condition prime mover acts as a load for synchronous Motor. For steam turbines, the motoring power is around 0.5-3 % of rated power of Generator. Under the failure of prime mover, due to motoring of turbine windage loss will be more in turbine blades as there is no steam to cool it down. Thus it will lead to damage of turbine.

Reverse Power element of Numerical Relay calculates the three phase active power using its current and voltage input based on the following formula,

P = VaIaCosØa + VbIbCosØb + VcIcCosØc

The Numerical Relay is connected with the convention that the forward current is the current flowing from the generator to the busbar. This corresponds to positive values of the active power flowing in the forward direction. When a generator is operating in the motoring mode, the machine is consuming active power from the power system and if this active power crosses the set value, then after the set time delay the relay will operate to trip the Breaker.



Normally reverse power setting is kept less than 50% of the motoring power. For example if the motoring power of steam turbine is 4% then reverse power setting shall be kept less than 2%. It shall also be noted that reverse power protection is provided with a time delay of around 5 s to prevent spurious operation due to disturbances or following synchronization.

You may like to read,



Causes of Reverse Power in Generator:

As discussed earlier, one cause of reverse power flow in a generator is failure of prime mover. Now failure of prime mover may be because of failure of Governor or failure of Governor Valve or maloperation of Boiler Pressure Control System.

Another cause of reverse power flow occurs during synchronization of Generator. Let us assume that Generator is to be synchronized with the Grid. The general practice for synchronizing a Generator is to close the breaker when the needle on the synchroscope is moving clockwise and crossing 11 O’clock position as shown in figure below.

Picture taken from Yokogawa Meters and Instruments Corporation


As shown in figure, the position of needle is at 12 O’clock. Suppose the frequency of Generator is less than the frequency of Grid, in this case the needle of synchroscope won’t move in clockwise i.e. fast direction rather it will move anticlockwise direction i.e. toward slow direction. If we close Generator circuit breaker in such condition, then the Grid will try to take the Generator to synchronous speed by feeding power to the Generator and motoring of Generator will take place. Thus in such case also reverse power will flow.