Sohail Ansari

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HVDC (to improve elecrical power transmission)


Even though today most of the electricity transmission lines are alternating current ones, in some cases direct current lines are also used. Scientists have been working to improve the technology needed for this conversion. The aim has been that this transmission should be done in a more straightforward, smoother and consequently less expensive way.

21173_originalElectricity is normally transmitted by means of alternating current, but it is not the only way and not always the best one.In some cases, high voltage direct current (HVDC) is used. In Spain, for example, there is only one direct current line, the one that connects mainland Spain with the Balearic Islands; all the remaining ones transmit electricity by means of alternating current. Direct current continues to be highly suitable for underwater and underground lines.
However, as most of the lines in the power grid are alternating current ones, converters are needed to change the type of electricity transmission, and they are very costly.That is why direct current lines are appropriate beyond a certain length.

ABB_HVDC_Converter_StationBearing in mind that right now most of the transmission of electricity is done by means of alternating current lines, “our aim is in no way to replace these lines by direct current ones. Our proposal is based on using direct current as a solution in cases where there are problems with the alternating current lines..Renewable energies are produced in a very irregular way; the wind, for example, could blow very strongly at some moments and very lightly in others.And the output may not coincide with moments of peak energy consumption.”One way of solving the problem caused by this situation for the electricity supply could be to connect the farms or parks of various countries where renewables are produced. That way, if at one moment one region has a high consumption of energy but is not producing renewable energy, its demand could be met by using renewable energy which is being produced somewhere else,” explained the researcher.

The AIM, to assist transmission :
The GISEL group is working to improve the energy exchange converters between direct current lines and alternating current ones.Specifically, they are working on new technology for converters known as VSCs (Voltage Source Converters).Compared with conventional technology, “the VSC has many advantages; among others, it is easier to control the power that is transmitted, and that is very important on wind farms, for example. At the same time, given the fact that direct current has great economic advantages in underwater lines, it is very appropriate for them.

In the future HVDC grids are expected to coexist with alternating current grids, which are in the majority nowadays.Transmitting direct current via the currently existing lines could be a first step towards building HVDC grids.

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Causes of POWER GRID Failure ….???


powerhungrytIt is important to know about the possible causes of power failure in order to better protect ourselves from its devastating effects. Once we have identified all that can possibly go wrong, it is easier to ensure that adequate safeguards are put into place. Here are a few of some of the obvious, and some not so obvious, causes of power failure:

  1. Natural Causes :natural-phenomena-catatumbo-2
    Various power failures are caused by the natural things like lightening, rain, snow, ice, wind, and even dust. The damage caused due to water (flood) in electrical circuits can be very expensive so it makes sense to ensure that you are well protected from it. Electrical switchboards, wires, and circuits should be protected from exposure to water
  2. Frequency Mismatch:octaves
    we know that all the synchronous generator which is used to generate power runs at synchronous speed that is they all have the same frequency. Due to some faults occur in the transmission line , the frequency mismatches and the system will go in to the dynamic i.e. it will either accelerate or decelerate. If fault is not clear in permissible time then the system will go in to the unstable state which leads to failure of power.
  3. Power mismatch:
    sometimes there is mismatch of generating power and the power taken by the load center which again results in to the dynamic . you can understand it by studying about swing equation.
  4. Three Phase Fault:01
    Generally the faults are divided in to two categories: one is symmetrical and another is unsymmetrical fault (L-L,L-G, L-L-G). Due to breaking of the wire or due to short circuit between these wire the fault occurs which leads to sudden increase in current and therefore POWER SURGES (Power surges are the bane of any electrical system which can lead to rapid overheating and loss of critical and expensive equipment). so due to over current in the line speed of the synchronous machine decreases and system will go in to the dynamic again. If we cannot clear this fault in less than 1000 milliseconds then it leads to GRID FAILURE.

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Power System Stability & its Types


At present the demand for electricity is rising phenomenally especially in developing country like India. This persistent demand is leading to operation of the power system at its limit. On top of this the need for reliable, stable and quality power is also on the rise due to electric power sensitive industries like information technology, communication, electronics etc. In this scenario, meeting the electric power demand is not the only criteria but also it is the responsibility of the power system engineers to provide a stable and quality power to the consumers. These issues highlight the necessity of understanding the power system stability. In this course we will try to understand how to asses the stability of a power system, how to improve the stability and finally how to prevent system becoming unstable.

Power system stability is the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most of the system variables bounded so that practically the entire system remains intact . The disturbances mentioned in the definition could be faults, load changes, generator outages, line outages, voltage collapse or some combination of these. Power system stability can be broadly classified into rotor angle, voltage and frequency stability. Each of these three stabilities can be further classified into large disturbance or small disturbance, short term or long term. The classification is depicted in Fig.

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1.Rotor Angle Stability:
It is the ability of the system to remain in synchronism when subjected to a disturbance. The rotor angle of a generator depends on the balance between the electromagnetic torque due to the generator electrical power output and mechanical torque due to the input mechanical power through a prime mover. Remaining in synchronism means that all the generators electromagnetic torque is exactly balanced by the mechanical torque. If in some generator the balance between electromagnetic and mechanical torque is disturbed, due to disturbances in the system, then this will lead to oscillations in the rotor angle. Rotor angle stability is further classified into small disturbance angle stability and large disturbance angle stability.

 A. Small-disturbance or small-signal angle stability
It is the ability of the system to remain in synchronism when subjected to small disturbances. If a disturbance is small enough so that the nonlinear power system can be approximated as a linear system, then the study of rotor angle stability of that particular system is called as small-disturbance angle stability analysis. Small disturbances can be small load changes like switching on or off of small loads, line tripping, small generators tripping etc. Due to small disturbances there can be two types of instability: non-oscillatory instability and oscillatory instability. In non-oscillatory instability the rotor angle of a generator keeps on increasing due to a small disturbance and in case of oscillatory instability the rotor angle oscillates with increasing magnitude.

B.Large-disturbance or transient angle stability
It is the ability of the system to remain in synchronism when subjected to large disturbances. Large disturbances can be faults, switching on or off of large loads, large generators tripping etc. When a power system is subjected to large disturbances they will lead to large excursions of generator rotor angles. Since there are large rotor angle changes the power system cannot be approximated by a linear representation like in the case of small-disturbance stability. The time domain of interest in case of large-disturbance as well as small-disturbance angle stability is any where between 0.1- 10 s. Due to this reason small and large-disturbance angle stability are considered to be short term phenomenon.

2.Voltage stability
It is the ability of the system to maintain steady state voltages at all the system buses when subjected to a disturbance. If the disturbance is large then it is called as large-disturbance voltage stability and if the disturbance is small it is called as small-disturbance voltage stability. Unlike angle stability, voltage stability can also be a long term phenomenon. In case voltage fluctuations occur due to fast acting devices like induction motors, power electronic drive, HVDC etc then the time frame for understanding the stability is in the range of 10-20 s and hence can be treated as short term phenomenon. On the other hand if voltage variations are due to slow change in load, over loading of lines, generators hitting reactive power limits, tap changing transformers etc then time frame for voltage stability can stretch from 1 minute to several minutes.
The main difference between voltage stability and angle stability is that voltage stability depends on the balance of reactive power demand and generation in the system where as the angle stability mainly depends on the balance between real power generation and demand.

3.Frequency stability
It refers to the ability of a power system to maintain steady frequency following a severe disturbance between generation and load. It depends on the ability to restore equilibrium between system generation and load, with minimum loss of load. Frequency instability may lead to sustained frequency swings leading to tripping of generating units or loads. During frequency excursions, the characteristic times of the processes and devices that are activated will range from fraction of seconds like under frequency control to several minutes, corresponding to the response of devices such as prime mover and hence frequency stability may be a short-term phenomenon or a long-term phenomenon.

Swing Equation


Let us consider a three-phase synchronous alternator that is driven by a prime mover. The equation of motion of the machine rotor is given by

 1

where

   J is the total moment of inertia of the rotor mass in kgm2
  Tm
is the mechanical torque supplied by the prime mover in N-m
  Te is the electrical torque output of the alternator in N-m
  θ is the angular position of the rotor in rad

Neglecting the losses, the difference between the mechanical and electrical torque gives the net accelerating torque Ta . In the steady state, the electrical torque is equal to the mechanical torque, and hence the accelerating power will be zero. During this period the rotor will move at synchronous speed ωs in rad/s.The angular position θ is measured with a stationary reference frame. To represent it with respect to the synchronously rotating frame, we define

 2

where δ is the angular position in rad with respect to the synchronously rotating reference frame.

Defining the angular speed of the rotor as we can write as

 3

 Differentiating the equation 2 w.r.t. time & using the equation 3, we can write :

We can therefore conclude that the rotor angular speed is equal to the synchronous speed only when dδ / dt is equal to zero. We can therefore term dδ / dt as the error in speed. Taking derivative of above equation , we can then rewrite as

Multiplying both side by ωm we get

 4

where Pm Pe and Pa respectively are the mechanical, electrical and accelerating power in MW.

We now define a normalized inertia constant as

Substituting the value of H in above equation 4, we get

In steady state, the machine angular speed is equal to the synchronous speed and hence we can replace ωr in the above equation by ωs. Note that  Pm Pe and Pa are given in MW. Therefore dividing them by the generator MVA rating Srated we can get these quantities in per unit. Hence dividing both sides  by Srated in above equation we get

  per unit

The above equation is known as Swing Equation in P.U.

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