12 May 2013

Silicon Carbide (SIC) Lightning Arresters

Silicon Carbide Arresters (SIC):

The Non linear lightning arrester basically consists of set of spark gaps in series with the silicon carbide non linear resistor elements. Lightning arresters are connected between the phase conductors and ground. During normal system operating voltage conditions, the spark gaps are non conducting and isolate the high tension (HT) conductors from the ground. However whenever an overvoltge of magnitude dangerous to the insulation of the apparatus protected occurs ( these over voltages or over surges may be caused due to lightning strikes on the conductors or due to Extra High Voltage (EHV) switching) the spark gap breaks down and allows the high voltage surge current to flow through the ground.

Working Principle of Silicon Carbide (SIC) Lightning Arresters:

The volt-ampere characteristics of the non linear resistor in the lignting arrester can be approximately described by expression V = KIβ. Where K and β are dependent on the composition and manufacturing process of the Non linear Resistor (NLR). The value of β lies generally in the range of 0.3 and 0.45 for modern silicon carbide (SIC) lightning arresters. If the voltage across the Non Linear Resistor (NLR) doubles, the current would increase approximately by 10 times.
Therefore, with multiple spark gaps arresters can withstand high Rate of Recovery Voltage (RRRV). The non-uniform voltage distribution between the gaps (which are in series in lightning arresters) presents a problem. To overcome this, capacitors and non-linear resistors are connected in parallel across each gap. In case of lightning arresters employed for high voltage applications, capacitors and nonlinear resistors are connected across the stock of gaps and NLRs. With the steep voltage wave surge the voltage is mainly controlled by the capacitor and at the power frequency by the non-linear resistors. It is obvious that when the over voltages cause the break down of the series gaps, the current would be very high so as to make the voltage to subside very fast. The highest voltage that appear across the lightning arrester would be either the spark over voltage of the arrester or the voltage developed across the non-linear resistor during the flow of surge current. The lowest spark over voltage of the arrester is called the hundred percent impulse spark over voltage of the arrester. The voltage developed across the non-linear resistor during the flow of surge current is called residual voltage. The lower the value of the voltage developed the better the protection of the lightning arrester.

Disadvantages of Silicon Carbide (SIC) Arresters:

Some of the disadvantages of silicon carbide arresters compared to gapless arresters are given below:
  • Silicon Carbide (SIC) arresters have inferior V-I Characteristics compared to Zno arresters (Metal oxide arresters). 
  • Decrease in energy absorption (surge wave) capability compared to Zno arresters.
  • Probability of sparking between the gaps 

Advantages of Silicon Carbide (SIC) Arrester :

  • Due to the presence of gaps the normal power frequency voltage during normal operation is negligibly less compared to gap less arresters. Hence no leakage current flow between the line and earth in SIC arresters  

11 May 2013

Conductor Size Selection in Distribution Power System

In power distribution system both aluminium and ACSR are commonly used. Mostly aluminium conductors are used in the distribution system because of cheaper in cost. Some of the factors which decides the size of the conductors designed for distribution system are given below:
  • Current carrying capacity of the conductor or distribution line
  • Allowable voltage drop or line regulation
  • Breakdown strength of the conductor


Current Carrying Capacity of Line:

The current carrying capacity of a conductor decided by the maximum conductor temperature rise or operating temperature. Operating temperature is limited by mechanical aspects such as allowable span, mid-span sag, joints, creep in conductors and long term mechanical efforts. Generally 85oC (AAAC), 75oC, 70oC, 65oC, or 60oC (ACSR) maximum operating temperature is used. Lower temperature is used for long spot lines particularly in rural distribution system. where jumpers may give trouble at higher loading.
The permissible operating temperature of an overhead conductors depends on the maintenance of adequate clearances and limitations of the loss of strength through annealing. Generally, the maximum current which an over-head conductor size designed to carry must not cause it to be heated such that it may result in the annealing of the metal of the conductor or reduction in the clearances specified. Usually for normal day loading, a maximum operating temperature 75oC is permitted which is allowed to reach up to 100oC for emergency loading.


Voltage drop and Voltage Regulation:

The allowable voltage drop is considered as critical factors in determining the  conductor size for 11kV and Low Tension (LT) distribution line with thermal loading (ampere loading) about 80 percent of the normal thermal rating based on the maximum operating temperature. Large conductor size (cross section) employed in distribution lines reduces the resistance of the line and hence the I2R losses and voltage drop in the line; and hence voltage regulation of the line improves. But using large cross section conductor size will increase the cost as the material required is more. Hence an optimum value must be chosen in between the cost and improving voltage regulation while designing the conductor size for distribution power system.

Mechanical Characteristics of Conductors:

The choice of conductor size from mechanical view point depends on the :
External Loading: Wind speed, ice loading and ambient temperature
Internal Characteristics: Stranding, modulus of electricity, thermal expansion of the creep. For example, considering the creep and economics AAC is used in LT distribution lines. The line characteristics includes voltage regulation is influenced by distribution line parameters and system frequency, current carrying capacity is assessed from the heat balance (amount of heat generated and heat dissipated).