Industrial Power Distribution: Medium-voltage Switchgear -- part 4

Internal arc proofing

Internal arc proofing refers to the certified ability of switchgear to withstand an internal short circuit, typically a busbar fault, for a specified period of time without causing injury to persons standing directly in front of the switchgear panel. Internal arc proofing need to be certified by an authorized testing authority, and will state a current and time rating, e.g. 25 kA, 1 s. The following will typically be features of an internal arc proof panel for 200 ms:

• Mild steel of at least 3 mm thickness for panel construction

• Secured, bolted front doors. The door to the CB enclosure cannot be opened while the CB is racked in. CB needs to be racked out with the door closed

• No easy access to busbar chambers, only via bolted covers

• Arc energy will be directed to the top of the panel away from the front doors and 'blown out' at the top via false covers.

It’s theoretically safe to stand in front of a panel that is being faulted.

Because the internal arc proofing is short time rated, bus zone protection is required to reduce the fault clearing time. Examples of bus zone protection include:

• High- and low-impedance differential protection schemes (operates in less than 10 ms)

• Busbar blocking schemes (operates in 100 ms or more)

• Arc detection schemes (operates in less than 20 ms).

Modern protection relays used with switchgear

Although this is not a text on electrical protection, a short discussion on modern protection relays are justified, as modern relays are changing the way electrical engineers think, not only about protection, but regarding a much wider scope, including switchgear applications and maintenance issues.

The most advanced protection relays available today are termed IEDs (intelligent electronic devices), as they offer much more than only the protection functions of the traditional relay.

Functions of an IED:

The functions of a typical protection IED can be classified into five main areas, namely protection, control, monitoring, metering and communications. Some IEDs may be more advanced than others, and some may emphasize certain functional aspects over others, but these main functionalities should be incorporated to a greater or lesser degree.

Protection:

The protection functions of the IED evolved from the basic overcurrent and ground fault protection functions of the feeder protection relay (hence certain manufacturers named their IEDs 'feeder terminals'). This is due to the fact that a feeder protection relay is used on almost all cubicles of a typical distribution switchboard, and the fact that more demanding protection functions are not required enable the relay's microprocessor to be used for control functions. The IED is also meant to be as versatile as possible, and is not intended to be a specialized protection relay. For example generator protection. This also makes the IED affordable.

The following is a guideline of protection-related functions that may be expected from the most advanced IEDs (the list is not all-inclusive, and some IEDs may not have all the functions). The protection functions are typically provided in discrete function blocks, which are activated and programmed independently.

• Non-directional three-phase overcurrent [low-set, high-set and instantaneous function blocks, with low-set selectable as long time-, normal-, very-, or extremely inverse, or definite time]

• Non-directional ground fault protection [low-set, high-set and instantaneous function blocks]

• Directional three-phase overcurrent [low-set, high-set and instantaneous function blocks, with low-set selectable as long time-, normal-, very-, or extremely inverse, or definite time]

• Directional ground fault protection [low-set, high-set and instantaneous function blocks]

• Phase discontinuity protection

• Three-phase overvoltage protection

• Residual overvoltage protection

• Three-phase undervoltage protection

• Three-phase transformer inrush/motor start-up current detector

• Auto-re-closure function

• Under-frequency protection

• Over-frequency protection

• Synchro-check function

• Three-phase thermal overload protection.

Control:

Control functions include local and remote control, and are fully programmable.

• Local and remote control of up to six switching objects (open/close commands for circuit-breakers, isolators, etc.)

• Control sequencing

• Bay level interlocking of the controlled devices

• Status information

• Information of alarm channels

• HMI panel on device.

Monitoring:

Monitoring includes the following functions:

• Circuit-breaker condition monitoring, including operation time counter, electric wear, breaker travel time, scheduled maintenance

• Trip circuit supervision

• Internal self-supervision

• Gas density monitoring (for SF6 switchgear)

• Event recording

• Other monitoring functions, like auxiliary power and relay temperature.

Metering:

Metering functions include:

• Three-phase currents

• Neutral current

• Three-phase voltages

• Residual voltage

• Frequency

• Active power

• Reactive power

• Power factor

• Energy

• Harmonics

• Transient disturbance recorder (up to 16 analog channels)

• Up to twelve analog channels

Communications:

Communication capability of an IED is one of the most important aspects of the device today and is the one aspect that clearly separates the different manufacturers' devices from one another regarding their level of functionality.

IEDs are normally able to communicate directly to a SCADA system, i.e. upper level communications. Different manufacturers use different communication protocols, and this will determine certain critical aspects, e.g. response times, possibility of direct relay to-relay communications, etc.

An IED will, in addition to upper level communications, also have a serial port or optical interface to communicate directly to substation PC or laptop for configuration and data downloading purposes, should the SCADA link not be available or desirable in that instance.

Comparison of electromechanical relays and digital relays

Note 1: The cost will depend on the application. If a multi-function relay, like an intelligent feeder terminal, is used for only overcurrent protection, the cost for that function will be high. However, if a digital relay is fully utilized, for example a generator protection relay, the cost per function will be low.

Note 2: Not all digital relays will comply with the features as in the table, depending on the type of relay. However, the table is an overview of what is possible with digital relays in general, compared to what could be achieved with electromechanical relays.