Power System Stabilty : Basic Concepts and Important Terms

This article deals with the basic concept of power system stabilty and types of power system stability like steady state stability, transient state stability and transient stability limit etc.

Power System Stability

The term stability is closely related with synchronism. Synchronous generators or alternators and synchronous motors have a tendency to remain in synchronism or in step with each other. During system disturbance such as sudden increase in load, sudden switching, power swings etc. The synchronous machines experience oscillations of torque angle about the mean position. However, the synchronous machines have inherent tendency or maintaining synchronism. Loss of synchronism is called loss of stability.

What is stability ?

Ability of synchronous machine or part of a system to develop restoring forces equal to more than disturbing forces so as to remain in synchronism is called stability.
The disturbance may be sudden or the change in load may be very gradual. Accordingly, there are two distinct terms called transient stability and steady state stability.
The term steady state stability refers to ability of a system or its part to respond to small, gradual change in power at a given point of the system. Steady state stability limit is the maximum possible power that can be transferred at a given point of the system without loss of synchronism, with very gradual increase in power.
The term transient stability limit refers to the maximum possible power that can be transferred at a given point of the system without loss of synchronism for given sudden large change in power.
The concept of stability can well be explained by means of two machine system. The system is used as a conceptual aid.

Losses in DC Machines

This article discuss various types of losses that occurs in DC Machines. It include Copper losses, iron losses, brush losses, mechanical losses, stray-load losses.

Hello Friends,
We know that no system is perfect in this world. Every system has some kind of unwanted loss of input. The form of this drop may vary from system to system but in every system it is called a loss.
Similar is the case with the DC Machines either motor or generator.
The whole input power is never converted into the output power. The difference between input power and output power is called loss.
In a DC Machine losses are classified into five main categories

• Copper Losses or Electrical Losses
  
• Core Losses or Iron Losses
  
• Brush Losses
  
• Mechanical Losses
  
• Stray-Load Losses
  

Copper Losses or Electrical Losses

These losses are the winding losses because these occurs in the winding of the machine. The copper or electrical are present because of the resistance of the winding. Currents flowing through these windings produce ohmic losses (i.e. I²R losses). The windings that may be present addition to Armature winding are the field windings, interpole and compensating windings.

• Armature current losses = I_a²R_a, where I_a is Armature current and R_a is Armature Resistance. These losses are about 30% of total full-load losses.
  
• Copper losses in the shunt field of a shunt machine =I_sh²R_sh where I_sh is the current in the shunt field and R_sh is the resistance of the shunt field winding. The shunt regulating resistance is included in R_sh.
  
• Copper loss in the series field of a series machine = I_se²R_se where I_se is the current through the series field winding and R_se is the resistance of the series field winding.
  
• In a compound machine, both shunt and series field losses occur. There losses are about 20% of full load losses.
  
• Copper loss in interpole windings =I_a²R_i where R_i is resistance of interpole winding.
  
• Copper loss in compensating windings =I_a²R_c where R_c is resistance of compensating winding.
  

Core Losses or Iron Losses

These losses also called magnetic losses. These are of two types viz. Hysteresis and Eddy-current losses. Since DC machines are usually operated at constant speed and constant flux density, these losses are almost constant. These are about 20% of full-load losses.

Brush Losses

There is a power loss at the brush contacts between the copper commutator and the carbon brushes. In practice, thin loss depends upon the brush contact voltage drop and the Armature current I_a. It is given by
P_BD=V_BDI_a
The voltage drop across a set of brushes is approximately constant over a large range of Armature currents. Unless stated otherwise, the brush voltage drop is usually assumed to be about 2V. The brush droploss is, therefore, taken as 2I_a.

Mechanical Losses

The losses associated with mechanical effects are called mechanical losses. They consist of bearing friction loss and windage loss. Windage losses are those associated with overcoming are friction between the moving parts of the machine and the air inside the machine for cooling purposes. These losses are usually very small.

Stray-Load Losses

Stray-load loss consists of all losses not covering above. These are the miscellaneous losses that result from such factors as (i) the distortion of flux because of Armature reaction, (ii) short circuit currents in the coil, undergoing commutation etc. These losses are very difficult to determine. The indeterminate nature of the stray-load loss makes it necessary to assign reasonable value. For most machines stray losses are taken by convention to be 1% of full load output power.
So friends this was all about various losses occurs in a DC Machine and here I think every kind of loss was covered. If any loss is still left, feel free to add a comment.