Power Factor Capacitor Discharge Resistor Design

August 27, 2009

When a Power Factor Capacitor (PFC) step, a single unit in a bank of several capacitors, is disconnected or switched off, the discharging resistor connected across the capacitor will discharge it to designed retained voltage V value in discharge time t seconds. Typical discharge resistor ratings for a given power factor capacitor C  in μF or MVAR include: maximum normal operation system voltage V_0  in kV and required retained discharge voltage V  in Volts (around 50V) at discharge time t  in seconds (usually <60s). The discharge resistor R  in kΩ  is given as,

R=\left | \frac{-t}{ln(\frac{V}{V_0})C} \right |          (1)

Depending whether the PFC bank step is star or delta connected (usually star connected), for given system frequency f  and capacitor step’s rated reactive power output Q in MVAR, the capacitor step’s capacitance C  in μF is given as:

  • If star connected, C=\frac{Q}{2 \pi f V_0^{2}}
  • If delta connected, C=\frac{3Q}{2 \pi f V_0^{2}}


Capacitor voltage decay across the resistor is given as, V=V_{0}e^{\frac{-t}{RC}}. Rearrange this for Discharge Resistor ‘R’ and you get (1). Simple!


H-Type Wood Pole Transmission Tower Transient Model

August 26, 2009

H-Wood Pole 3Ph 2 Shield Wire Transient Model

Protection System Block in PSCAD

August 19, 2009

When conducting voltage transient studies, equipment trip investigation studies etc, I typically need a protection  block in PSCAD that  does: Flag a parameter if it exceeds specific limits (say transient over voltage limit and tolerance limit).

Here’s a PSCAD block to flag if measured voltage peak (+/-) exceeds specified limit and tolerance time. Download.

The illustrated example could be used for both current and voltage, instantaneous and RMS values.

Three Phase Voltage Transient or Current Inrush Peak Finder

August 18, 2009

Have you ever come across a situation where you need to find the maximum  voltage/current peak of all three phases, irrespective of whether it’s a -ve or +ve peak, after say time ‘Ti’? Here’s PSCAD block to do just that. Click here to Download!

Typical use of this block include: Identifying maximum transient voltage, transformer current inrush peak etc.

When using this block, don’t forget to include time ‘Ti’ in the block parameters, i.e. time after which the maximum value will be identified. Enter zero if you don’t want this time delay.

Hope you will find it useful.

Have fun! ^__^

Lightning and Switching Surge Model

July 29, 2009

Power Systems equipment, directly or in-directly connected, is expensive and needs protection against both external factors and normal every day power system operation. Protection assessment of these equipment, especially outdoor, against voltage surges is almost mandatory due to their exposure to lightning and general switching operations. Insulation material’s strenght is usually expressed using Basic Insulation Level (BIL) in kV that indicates voltage withstand capability of the material before it breaks down, providing a path for fault current. In order to assess equipment performance to a real environment, actual equipment or prototypes are tested to Lightning and switching surge. This is where surge generators come handy that generator desired surge wave fronts and tails.

lightning-surge-type-1Various standards (IEEE, IEC, EN etc.) provide rules and guidelines on how testing is to be performed. Typical surge-wave shapes recommended by standards is a double exponential curve functions with rise time to reach the peak and then gradual decay fall time. Irrespective of the equations that define this curve, what’s of real importance is the voltage peak rise reached at the end of rise time and the total time (also the fall time) it takes to reach half the peak value during its decay. There are two variations in surge reference curve depending on how the rise time is defined, while the fall time remains the same.

  • Type 1, where the rise time is the time reached by the surge peak to 90% of its value. Also shown in Figure.
  • Type 2, where the rise time is the time reached by the surge peak to 100% of its value.

As I understand, Type 1 is used for fast-front surge waveshapes (e.g. Lightning), while Type 2 is used for slow-front surge waveshapes (e.g. Switching).

Now you know the basics, here’s a simple Linear Lightning Surge Generator Model in PSCAD based on [1]. Download. If you are looking for a double-exponential surge generator model, there’s one in PSCAD examples folder in c:\program files for MS Windows.

Connect the current source output to the point of surge injection in the test system. It’s simple to use: Enter the current peak (say 50kA), surge front time (say 20μs) and tail time (say 80μs), initial time (say 0.1s) when you want the surge in the simulation and you are done.

An opportunity to test equipment to lightning and switching surges without frying ourselves up.

Have fun! ^__^


  1. TVS Diode Application Note , Semtech Corp., 2001. Link: http://www.semtech.com/pc/downloadDocument.do?id=535. Accessed: Aug 2009.
  2. P. Chowdhuri, Electromagnetic Transients in Power Systems, Edition 1. Exeter: Research Studies Press LTD, 1996, pp 4-6.