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58 Cards in this Set
- Front
- Back
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According to CEC, HV is any voltage above how many volts? |
750v |
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WCB specifications state that only people who are ___________ and ___________ May be allowed to work on HV |
Trained and qualified |
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What is medium voltage as defined by the IEEE standards? |
2.4Kv-69Kv |
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What effect does electric stress have on insulation? |
It weakens the insulation |
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What is creepage distance and why is it kept relatively long? |
creepage distance is the act of current tracking across the insulation to ground. It is kept long to minimize leakage current over insulation |
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What is BIL and what significance does it have for the insulation if HV equipment? |
Basic Impulse insulation level. Requires higher levels of insulation than rated voltages would indicate. |
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What are the 3 broad classes of HV distribution systems? Give a brief description of each |
-radial: cheapest, but least reliable because only one overhead line -loop or ring: plant or locality is supplied from two directions (alternative supply incase one goes down) -network (grid): power can be delivered by several different routes. Most efficient system but also most expensive. |
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CT ratios are typically selected on a basis what percentages? If a circuit current was in the 400-450A range, what would be a suitable CT? Can CT’s have more than one ratio? |
115%-125% 500:5 Yes- usually only have one, but some are available with two or more ratios |
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Which of the three classes of HV distribution systems is the cheapest and simplest? Which requires the most comprehensive protection system? |
Radial- simple Network- expensive and complex |
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Which HV distribution system would a rural overhead be? |
Radial. |
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is high voltage equipment always installed in vaults? |
No |
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How can the use of a series reactor reduce the cost of a substation instal? |
The series reactor allows circuit breaker to be of lower current interrupting capacity, therefore cheaper breakers. |
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What are two main parts of a lightning arrester? |
Airgap and thyrite material |
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What are 2 functions of an instrument transformer? What are the typical secondary voltage and secondary current ratings of VT’s and CT’s? |
Monitor Voltage and current, and isolate the monitoring devices from the HV system. VT- 120V CT- 5a |
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What is the purpose of key interlocking? How does it work? |
Protection of personnel and equipment. There’s a single key for two locks. You must open the disconnect with the key, and then you remove the key and use it to unlock the cabinet where the fuses are located so that power can never be on when working in the fuse cabinet. |
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What are 5 steps to safe switching procedure? |
1: disconnect 2: lockout 3: test for absence of voltage 4: ground and short circuit 5: screen off neighboring live parts where required. |
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When work is done in proximity of HV lines, according to WCB how far must you be from them if the phase to phase voltage is: 751v-75KV Over 75kv-250kv Over 250kv-550kv |
1: 10 feet (3m) 2: 15 feet (4.5m) 3: 20 feet (6m) |
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What is the most common insulating material used in medium voltage cables? |
XLPE (cross linked polyethylene) |
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What determines the physical size of VT’s (voltage transformers)? How many HV bushing are needed if the primary side is connected between line and ground? What about if the primary is connected across two phases? |
The primary voltage. One high voltage bushing for line-ground, two high voltage bushings if across two phases. |
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What is a dielectric more so known as? |
An insulator. |
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What is dielectric strength and what units is it typically stated? |
Ability to withstand electrical break down- kv/mm |
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What are two types of shielding used on a cable? |
Strand and insulation |
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Why are lightning arresters used on overhead power lines? |
Lightning can create an over-voltage which may cause the dielectric strength of surrounding air to be exceeded which will result in a flash over between line and the closest ground point. |
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What are lightning arresters also called, and how are they connected? |
Also called surge arresters. One must be connected to each phase, and they are connected from the phase through the arrester to the ground rod. |
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What is an electrical equipment vault? |
A room in a building specially constructed to house electrical service equipment. (Transformers, etc.). |
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What are four common types of HV cables? |
Teck, shielded power cable, concentric neutral cable, mining cable |
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What equipment is contained in a substation? List 4 |
Terminals, disconnects, circuit breakers, power transformers, lightning arresters, instrument transformers and metering equipment. |
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Where are 2 places that HV equipment may be located? |
In an out door switching compound Within a building on the premises, in the case of consumers using 13.8KV or less. |
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What are two types of shielding used on a cable? Which one must be grounded? |
Strand insulation (grounded) |
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What is the primary purpose of lead sheathing in a cable? |
Keep moisture out |
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What are two classes of insulation level found on medium voltage cables? |
100% and 133% |
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What are two functions of the outer concentric conductor on a URC cable? |
Current carrying neutral, sheilding |
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What factor determines the minimum available AWG size of a HV cable? |
Voltage rating |
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The ampacity of a non-shielded cable is based on an ambient temperature of what? What about a shielded cable? |
Non shielded- 30 degrees Shielded- 40 degrees |
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What are two most common voltage ratings of medium voltage cables for power applications? |
15000 v and 25000 v |
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What’s one main precaution that must be considered when handling and pulling cables? |
Keep both ends sealed at all times. |
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What are two common types of stress relief used in medium voltage cables? |
Geometric stress relief: stress cone is used to build up insulation in the region of high electric stress Capacitive stress relief: prefabricated stress termination that uses a capacitive method of overcoming the problem of high electric stress in insulation. |
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According to IEEE what are 3 classes of HV termination? Briefly describe. |
Class 3: only requires stress relief. Used for indoor termination (clean and dry environment) Class 2: requires stress relief and anti tracking. Used where there’s dust, chemicals, salt etc. in the air. Class 2 is mainly obsolete now Class 1: used outdoors, has a skirted silicone or porcelain finish. Must be sealed again rain, fog, etc. must have stress relief, anti tracking, and seal to the environment |
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All cables must have an insulation shield if rated for over how many KV? |
5kv |
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What are 3 forms of HV terminations? |
Pothead: geometric stress relief, metal enclosure and porcelain bushings filled with liquid pitch to cool and solidify. Stress cone: friction fit, increases thickness of insulation, must lubricate before attempting to instal HI-K termination |
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What 3 reasons may cause a high voltage splice to fail prematurely? |
Poor install and prep, moisture, tracking due to contaminants, temperature fluctuations. |
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HV switches are rated for which three primary electrical quantities? |
Current, voltage, frequency. |
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What type of fuse has a replaceable link? Which has a replaceable cartridge? |
Distribution cutout- replaceable link Solid material- replaceable cartridge |
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What must be used on a solid materials fuse when used indoors? |
Snuffer or condenser to collect boric acid when fuse blows |
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What are 5 common types of HV circuit breakers? |
Air magnetic, vacuum, oil, air-blast, sulphur hexafluoride. |
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What provides the force that moves the contacts in most circuit breakers? |
Motor charged spring |
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What is the purpose of a recloser and where would it be used? |
Automatically recloses after interrupting an over current. Used for overhead, medium voltage rural distribution lines where faults are transient. |
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What is the difference between expulsion and non expulsion fuses? |
Expulsion means the gases and fuse material are expelled from the fuse when it blows |
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Where is switch gear used for industrial plants normally located? |
Indoors |
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How is a metal clad switch gear disconnected? |
It’s on rollers and is withdrawn horizontally from the bus bars after it is tripped. |
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What type of switch might be operated using a hot stick? |
Isolating switch |
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Load break switches are normally used on what level of high voltage? |
Medium voltage |
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What material are hot sticks made of? |
Fiber glass. |
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What 3 electrical quantities are fuses rated for? |
Voltage: determines length of fuse current: magnitude of current which fuse link can carry interrupting current: highest magnitude of current the link can handle |
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What are 4 most common HV fuses? |
Current limiting: one time fuse. Used for protection of equipment (transformers, motors, etc.) Solid materials: fuse link surrounded by boric acid powder. When link opens heat reacts to the powder which produces de-ionizing vapor which quenched the arc. Either disposable or have replaceable cartridges. Liquid: old. Not used anymore. Distribution cut outs: drop out type fuse used by power utilities on overhead lines. Consists of epoxy fuse tube containing replaceable fuse link and a porcelain support attached to the pole. |
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What are two useful features of the distribution cut outs fuse? |
Provides visible evidence that the fuse has blown, provides adequate separation to prevent restrike |
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What’s the main physical difference between HV And LV fuses? |
Hv are longer. |
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What is the term “voltage gradient”? |
Flux concentrations decrease with distance, therefore the stress on insulation decreases with distance. |
