PSCAD

PSCAD (Power Systems Computer Aided Design) •

A graphical User Interface to EMTDC

EMTDC (Electro Magnetic Transients for DC) •

Developed in 1975 by Manitoba Hydro to study the Nelson River HVDC power system

•

Used extensively for many types of power simulation studies,

STUDY: •

Find over voltages in a power system due to a fault or breaker operation. Transformer

non-linearities (ie saturation) are a critical factor and are represented. Multiple run facilities are often used to run hundreds of simulations to find the worst case when varying the point on wave of the fault, type of fault, or location of the fault. •

Find overvoltages in a power system due to a lightning strike. This simulation would be

performed with a very small time step (nano-seconds). •

Find maximum energy in a surge arrester for a given disturbance.

Internal Insulation

 Is the internal solid, liquid, or gas elements of the insulation of the equipment, which are protected from the effects of atmospheric and other external conditions such as contamination, humidity, and animals.  Transformer insulation, cable insulation, gas insulated substation, dielectric fluid in capacitors, oil, etc.

External Insulation  Air insulation and the exposed surfaces of solid insulation equipment, which are both subjected to dielectric stresses and to the effects of atmospheric and other external conditions such as contamination, humidity, and animals.  Can be affected by the environment by such things as rain, altitude, winds, dirt, etc.  Examples are Bushings, bus support insulators, switches, air, etc.

Insulation Strength

 Conventional- The strength of the insulation described in terms of the voltage it is able to withstand without failure or disruptive discharge under specified test conditions.

 Statistical- The strength of the insulation described in terms of the voltage it is able to withstand with a given probability of failure or disruptive discharge under specified test conditions

BIL Most electrical equipment is rated for traveling wave voltage surge capability by the Impulse Test. The Impulse Test is most common and consists of applying a full-wave voltage surge of a specified crest value to the insulation of the equipment involved. The crest value of the wave is called the Basic Impulse Insulation Level (BIL) of the equipment.

Back Flash Over

If a transmission line tower is struck by lightning and the potential of the tower is raised above the voltage impulse strength of the insulator string, a flashover will occur from the tower to a phase conductor which may lead to serious outages of the system. This type of flashover is called backflashover. The question arises as to why we should have a low value of tower footing resistance. It is clear

that, whenever a lightning strikes a power line, a current is injected into the power system. The voltage to which the system will be raised depends upon what impedances the current encounters. Say if the lightning stroke strikes a tower, the potential of the tower will depend upon the impedance of the tower. If it is high, the potential of the tower will also be high which will result in flashover of the insulator discs and result in a line-to-ground fault. The flashover will take place from the tower structure to the power conductor and, therefore, it is known as back flashover,

Switching surge Switching surges assume great importance for designing insulation of overhead lines operating at voltages more than 345 kV. 

Must remain as a non-conducting path during the normal operating voltage &ensure only over-voltages are conducted to earth. •Minimum leakage current & have good heatdissipation. •Quick response -absorb incoming surgeswithout time delay. •Maintain good thermal stability for long life.

Surge Arrester Selection

The objective of arrester application is to select the lowest rated surge arrester which will provide adequate overall protection of the equipment insulation and have a satisfactory service life when connected to the power system. The arrester with the minimum rating is preferred because it provides the greatest margin of protection for the insulation. Both arrester survival and equipment protection must be considered in arrester selection.

The proper selection and application of lightning arresters in a system involve decisions in three areas: 1. Selecting the arrester voltage rating. This decision is based on whether or not the system is grounded and the method of system grounding. 2. Selecting the class of arrester. In general there are three classes of arresters. In order of protection, capability and cost, the classes are: Station class Station—Available in ratings up to 466kV, station class arresters offer the best performance among the four different types. They are typically used to protect substation equipment, rotating machines or other applications where premium protection is required.

3. Intermediate class Intermediate—Available in ratings up to 144kV, intermediate type arresters offer improved protective characteristics and durability. They are generally used for protection of smaller substations, or medium class power equipment.

Distribution class Distribution—Typically used for the protection of equipment on power distribution circuits. They are available in ratings up to 42kV. This type of arrester is further defined by Normal Duty and Heavy Duty and includes the special application arresters such as riser pole or dead front elbow type.

The station class arrester has the best protection capability and is the most expensive. Solid Grounding: solidly grounded” classification is usually found in Electric Utility distribution systems where the system is usually only grounded at the point of supply. These systems can

exhibit a wide range of grounding coefficients depending upon the system or location in the system. Accordingly, these systems may require a study to ensure the most economical, secure, arrester rating selection. If this information is not known or available, the ungrounded classification should be used. Un Grounded: The “ungrounded” classification includes resistance grounded systems, ungrounded systems and temporarily ungrounded systems. Both high resistance and low resistance systems are considered ungrounded for the selection of the proper surge arrester since during a line-to-ground fault the unfaulted phases and their arresters experience essentially line-to-line voltage. The same is true for the infrequently used

Continuous System Voltage When arresters are connected to the power system they continually monitor the system operating voltage. For each arrester rating, there is a limit to the magnitude of voltage that may be continuously applied to the arrester. This is referred to as the Maximum Continuous Operating Voltage (MCOV). The MCOV values for each arrester rating are shown, for one manufacturer, in Table 2a below. These values meet or exceed those values contained in the referenced standard. Note that the MCOV is less that the arrester voltage The arrester selected must have a MCOV rating greater than or equal to the maximum continuous system voltage. Attention must be given to the circuit configuration (single phase, wye, or delta) as well as the arrester connection (line-to-ground or line-to-line). Most arresters are connected line-to-ground, in which case, the grounding methods discussed above must be considered. If the arrester is connected line-to-line, then the phase-to-phase voltage must be considered. There are also special applications that require additional consideration such as the use of a delta tertiary winding of a transformer where one corner of the delta is permanently grounded. In this case the normal voltage applied to the arrester will be full phase-to-phase voltage even though the arresters are connected line-to-ground.

Arrester Voltage Rating: The challenge of selecting and arrester voltage rating is primarily one of determining the maximum sustained line-to-ground voltage that can occur at a given system location and then choosing the closest rating that is not exceeded by it. This maximum sustained voltage to ground is usually considered to be the maximum voltage on the unfaulted phases during a single line-to-ground fault. Hence, the appropriate arrester ratings are dependent upon the manner of system grounding. Location of lightning arresters This reduces the chances of surges entering the circuit between the protective equipment and the equipment to be protected. If there is a distance between the two, a steep fronted wave after being incident on the lightning arrester, which sparks over corresponding to its spark over voltage, enters the transformer after travelling over the lead between the two.

Overvoltage - Any time-dependent voltage between line and ground having a peak value exceeding the corresponding peak value of the nominal system voltage. Switching surge - A phase to ground overvoltage at a given location on a system due to any of the switching events such as energization, de-energization, fault clearing or line reclosing. Temporary overvoltage - A weakly damped phase to ground voltage of relatively long duration. Usually, a temporary overvoltage originates from switching operations, faults or load rejection.

Temporary Overvoltages Temporary overvoltages (TOV) may be caused by a number of events such as line-to-ground faults, circuit back feeding, load rejection, and ferro- resonance. The system configuration and operating practices will identify the most probable forms of temporary overvoltage that may occur at the arrester location. The temporary overvoltage capability must meet or exceed the expected temporary overvoltages. Table 3 identifies the temporary overvoltage capability of one manufacture's arresters in per unit of MCOV. It also defines the time duration that the overvoltages may be applied before the arrester voltage must be reduced to the arrester’s continuous operating voltage capability. These capabilities are independent of system impedance and are, therefore, valid for voltages applied at the arrester location. If detailed transient system studies or calculations are not available, the overvoltages due to a single line-to-ground fault should be considered as a minimum. The arrester application standard ANSI C62.22 gives some guidance in determining the magnitude of single line-toground fault overvoltages. The effects of TOV on metal oxide arresters are increased current and power dissipation and a rising arrester temperature. Temporary power frequency overvoltages (TOV) can occur due to the voltage rise on unfaulted phases during a line-toground fault, the loss of the neutral ground of an effectively grounded system, sudden loss of load, generator over speed and other conditions. Transient Recovery Voltage (TRV): The TRVs are the voltages measured across the circuit breaker poles during opening. The severity of a TRV depends on both the magnitude and the rate of rise of the voltage across the opening circuit breaker poles.