TEXT 18. High Voltage Switching Equipment

The design of the high voltage substation must include consideration for the safe operation and maintenance of the equipment. Switching equipment is used to provide isolation, no load switching, load switching, and/or interruption of fault currents. The magnitude and duration of the load and fault currents will be significant in the selection of the equipment used.

System operations and maintenance must also be considered when equipment is selected. One significant choice is the decision of single-phase or three-phase operation. High voltage power systems are generally operated as a three-phase system, and the imbalance that occur when operating equipment in a single-phase mode, must be considered.

Air-insulated high voltage electrical equipment is generally covered by standards based on assumed ambient temperatures and altitudes. Ambient temperatures are generally rated over a range from -40°C to +40°C for equipment that is air-insulated and dependent on ambient cooling. Altitudes above 1000 meters (3300 feet) may require derating. At higher altitudes, air density decreases, hence the dielectric strength is also reduced and derating of the equipment is recommended.

Disconnect Switches. A disconnect switch is a mechanical device used to change connections within a circuit or isolate a circuit from its power source, and are normally used to provide isolation of the substation equipment for maintenance. Typically a disconnect switch would be installed on each side of a piece of equipment to provide a visible confirmation that the power conductors have been opened for personnel safety. Disconnect switches are relatively slow-speed operating devices and therefore are not designed for arc interruption. They are also installed to bypass breakers or other equipment for maintenance and can also be used for bus sectionalizing.

Load Break Switches. A load break switch is a disconnect switch that has been designed to provide making or breaking of specified currents. This is accomplished by addition of equipment that increases the operating speed of the disconnect switch blade and the addition of some type of equipment to alter the arcing phenomena and allow the safe interruption of the arc resulting when switching load currents.

High Speed Grounding Switches. Automatic high-speed grounding switches are applied for protection of transformer banks when the cost of supplying other protective equipment is too costly. The switches are generally activated by discharging a spring mechanism to provide the “high-speed” operation. The grounding switch operates to provide a deliberate ground on the high voltage bus supplying the equipment (generally a transformer bank), which is detected by protective relaying equipment remotely, and operates the transmission line breakers at the remote end of the line supplying the transformer. This scheme also imposes a voltage interruption to all other loads connected between the same remote breakers.

Power Fuses. Power fuses are generally accepted means of protecting power transformers in distribution substations. The primary purpose of a power fuse is to provide interruption of permanent faults. Fusing is an economical alternative to circuit switcher or circuit breaker protection. Fuse protection is generally limited to voltages from 34.5 kV through 69 kV, but has been applied for protection of 115-kV and 138-kV transformers. To provide the greatest protective margin, it is necessary to use the smallest fuse rating possible. The advantage of close fusing is the ability of the fuse unit to provide backup protection for some secondary faults. For the common delta-wye connected transformers, a fusing ratio of 1.0 would provide backup protection for a phase-to-ground fault as low as 230% of the secondary full-load rating. Fusing ratio is defined as the ratio of the fuse rating to the transformer full load current rating. With low fusing ratios, the fuse may also provide backup protection for line-to-ground faults remote to the substation on the distribution network.

Circuit Switches. Circuit switches have been developed to overcome some of the limitations of fusing for substation transformers. They are designed to provide three-phase interruption and provide protection for transient overvoltages and overloads at a competitive cost between the cost of fuses and the circuit breakers. Additionally, they can provide protection from transformer faults based on differential, sudden pressure, and overcurrent relay schemes as well as critical operating constraints such as low oil level, high oil or winding temperature, pressure relief device operation, and others. Circuit switches originally were intended to be used for transformer primary protection. Advancements in the interrupter design have resulted in additional circuit switcher applications, including: line and switching protection, cable switching and protection, single shunt capacitor bank switching and protection, shunt reactor switching and protection (line connected or tertiary connected reactors).

Circuit breakers. A circuit breaker is defined as a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying and breaking for a specific time, and breaking currents under specified abnormal circuit conditions such as a short circuit. Circuit breakers are generally classified according to the interrupting medium used to cool and elongate the electrical arc permitting interruption. The types are: air magnetic, oil, air blast, vacuum, SF6 gas.

1. Read and translate the text.

2. Read and decide if the following statements are true (T) or false (F).

1). Selecting high voltage switching equipment one should consider system operations and maintenance.

2). High voltage power systems are operated as a single-phase system.

3). The higher the altitudes, the denser the air.

4). Disconnect switches has low operating speed.

5). Fuses are an alternative to circuit switches and circuit breaker protection.

6). Fusing ratio is the ratio of the transformer full load current rating to the fuse rating.

7). Additional circuit switcher applications became impossible because of interrupter design.

8). There are different types of circuit breakers according to the interrupting medium.

3.  Put the following words in the correct order to form a sentence.

1). breakers/ typically/ between/ operate/ six and seven/ Gas/ atmospheres/ at pressures/ circuit.

2). of the density/ gas/ Monitoring/ critical/ of the SF6/ is.

3). noise/ and other/ emit/ Transformers/ equipment/ continuous/ magnetic.

4). carry/ load currents/ designed/ switches/ are/ Disconnect/ to.

5). systems/ power/ are/as a three-phase/ High/ generally/ voltage/ operated/ system.

4. Write down the differences.

5. Match the beginning of each sentence with the most appropriate ending.

6. Give a short summary of the text