Relays--Distribution systems, future of protection [Industrial Electrical Power Systems]

Distribution systems and future of protection

With the second generation of microprocessor relays now available, the emphasis is on the broader use of the protection relays as data acquisition units and for the remote control of the primary switchgear. Protection relays continuously monitor the primary system parameters such as current, voltage, frequency, etc. as part of the protection function of detecting faults. Since faults seldom occur, protection relays are expected to fulfill the protection requirements for a very small portion of their lifetime.

By utilizing the protection relays for other duties during the periods when the power system is normal, it permits integration of the various systems such as protection, supervisory control and data acquisition and results in savings on other interface components such as measuring transducers for current and voltage, meters, circuit breaker control interface, etc.

Improvements in digital communications by means of optical fibers allow the information available at the relay to be transferred without interference to the substation control level for information or event recording.

The following information is typically available from the relay:

• • Measurement data of current and voltage
• • Information stored by the relay after a fault situation
• • Relay setting values
• • Status information on the circuit breakers and isolators
• • Event information

The communication link to the relay can also be used for control purposes:

• • Circuit breaker open/close commands
• • Remote reset of the relay or auto-reclose module
• • Changes to the protective relay settings

++++ shows the components of an integrated protection and control system that could be implemented in distribution substations.

++++ Components of an integrated protection and control system -- PC in the station M SPA bus PC in the office BUS connection module

Def.: Kilovolt-amperes reactive, thousand Volt-ampere reactive, a unit of reactive power

IED (intelligent electronic device)

As already discussed earlier, protection relays became more advanced, versatile and flexible with the introduction of microprocessor-based relays. The initial communication capabilities of relays were intended mainly to facilitate commissioning. Protection engineers realized the advantages of remotely programming relays, the need developed for data retrieval, and so the communication aspects of relays became steadily more advanced.

PLC functionality became incorporated into relays, and with the development of small RTUs, it was soon realized that relays could be much more than only protection devices.

Why not equip protection relays with advanced control functions? Why shouldn't protection functions be added to a bay controller? Both of these approaches have been followed, with different manufacturers (and sometimes different divisions within the same manufacturing group) adopting different approaches to the question of protection, control and communications. This resulted in an extensive range of devices on the market, some stronger on protection, some stronger on control, and the term protection relay became too restrictive to describe these devices. This resulted in the term intelligent electronic device (IED).

Definition

The term 'intelligent electronic device' (IED) is not a clear-cut definition, as For example the term 'protection relay' is. Broadly speaking, any electronic device that possesses some kind of local intelligence can be called an IED. However, concerning specifically the protection and electrical industry, the term really came into existence to describe a device that has versatile electrical protection functions, advanced local control intelligence, monitoring abilities and the capability of extensive communications directly to a SCADA system.

Functions of an IED

The functions of a typical IED can be classified into five main areas, namely protection, control, monitoring, metering and communications. Some IEDs may be more advanced than the 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 because a feeder protection relay is used on almost all cubicles of a typical distribution switchboard, and that more demanding protection functions are not required to 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-reclosure function

• Under frequency protection

• Over frequency protection

• Synchro-check function

• Three-phase thermal overload protection.

Control

Control function includes local and remote control, and are fully programmable.

• Local and remote control of up to twelve 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, relay temperature, etc.

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 12 analog channels.

Communications

Communication capability of an IED is one of the most important aspects of modern electrical and protection systems, and is the one aspect that clearly separates the different manufacturers' devices from one another regarding their level of functionality.

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Ethernet LAN Hardware Firmware Programmable Auxiliary outputs Breaker Digital inputs Analog inputs Computed parameters A/D DSP Protection Control Flexlogic Virtual outputs Physical outputs Mechanical--Soild state

CTs VTs Analog Digital inputs Virtual inputs RS 485; RS 485 Power supply 48- 250VDC UR L90 Line UR F30 Feeder UR T60 Transformer

++++ Typical IED internal configuration (source: GE universal relay)

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Substation automation

Substation automation is not an easy thing to be achieved in existing substations.

Automation in a substation is considered as provision of new generation intelligent electronic devices (IEDs), programmable logic controllers (PLCs) and computers to monitor and communicate. It’s always simple to incorporate these components in new substations at the design stage itself. But in an existing substation, which is already using some old types relays, automation is a question of what can be done to reduce the operating expenses and improve customer service, from practical and economical viewpoint. One way to improve customer service is giving the control over the feeder breakers to the operator of distribution substations. This effectively reduces the number of trips that have to be made to substations and allows service to be restored more rapidly after an outage. The question is also, how the operator is able to decide the tripping or otherwise of certain feeders, which are achieved by the capability of the substation to record the data over a period.

Existing substations

A typical scheme for providing just those functions needed to reduce expense and improve customer service. It helps planners to reduce capital expenditures and helps operators to minimize trips to substations and reduce out-age time. Existing relays are retained and feeder automation units are added on each feeder panel ---- the diagram below. This is referred to as a distributed architecture whereby the feeder automation units are installed close to the input/output wiring sources.

++++ Typical incorporation of feeder automation units in a substation

The feeder automation units are available as add-on units to the existing feeders. Feeder current and bus voltage are given as inputs for each feeder automation unit, in addition to the status inputs like reclose status, breaker status and output from trip current relays.

Some outputs are needed from the automation unit for trip, close, and enable/disable reclose. The feeder automation units generate relay target information from the trip current relay inputs. Trip current relays are available that can be mounted directly on the studs of relay cases. This allows them to be installed without changing the trip circuit wiring.

The provision of simple feeder automation units provides the following capabilities in automation of existing substations.

Control functions

• Trip/close for each feeder breaker

• Disable/enable reclosing relay on each feeder.

Status information

• Breaker position for each feeder (open/closed)

• Reclose status of reclosing relay on each feeder (enabled/disabled).

Target data

• Sequential listing of all time-overcurrent trips on each feeder

• Sequential listing of all instantaneous-overcurrent trips on each feeder.

Revenue accuracy real-time metering

• Bus voltage (average three-phase measurement)

• Current on each feeder

• Kilowatts and kVars on each feeder.

Planning data on 30 day long at 15/30 min intervals

• Bus voltage (average three-phase measurement)

• Current on each feeder

• Kilowatts and kVars on each feeder

• Accumulated kWh and kVar on each feeder.

Power quality data once a minute

• Bus voltage harmonics through the 31st order on each phase

• Total harmonic distortion in percent.

Adding new multi-function feeder relays New feeder relays can be added along with the feeder automation units described above to get more detailed fault data and to implement improvements in feeder protection.

Outputs from the overcurrent relay are for breaker trip, breaker close, and breaker fail.

The relay requires an input for breaker status. The additional capabilities provided over and above the previous scheme are as follows:

Fault data

• Fault summary reports

• Oscillo-graphic records of fault clearings

• Sequence of events data.

Additional protective functions

• Negative sequence overcurrent for more sensitive phase-to-phase protection

• Four relay setting groups for adapting protection to system conditions

• Coordination with bus relay to achieve feeder relay backup

• Automatic reclosing

• Breaker failure detection.

Maintenance data

• Breaker contact wear data

• Breaker operations counter

• Breaker trip speed monitoring.

Operating information

• Yesterday's peak demand current

• Today's peak demand current

• Peak demand current since last reset.

Use of computer in the substation

Ultimately a computer is the most useful addition to record all the information about the various systems and feeders. A substation computer is a key system element in that it allows intelligence to be moved downward to the substation. That intelligence reduces the amount of data that must be communicated between substations and the master station. For example, information can be retrieved as and when needed from databases maintained at the substation. It also provides a way to overcome the need for a standard protocol.

If the optional monitor is employed along with a keyboard, the computer can also serve as the human-machine interface for information and control in the substation.

The additional capabilities added by including a computer in the substation are as follows:

Maintains databases at substation

• One or more years of demand data

• One or more years of chart data.

Mathematical operations on the data

• Watts and VARs can be derived from the three-phase current and voltage data.

Flexibility

• Easy to add new functions and for expansions.

Human-machine interface in the substation

• Elimination of manual control switches

• Displays reports

• Alarm panel

• Event log

• Load control schedules

• Daily summary

• Volts and amps

• Monthly peaks report.

General system requirements

There are some basic requirements that should be considered when selecting an automation system for a substation. These are:

• An open system

• Distributed architecture to minimize wiring

• Flexible and easily set up by the user

• Inclusion of a computer to store data and pre-process information

• Setting up the computer software without requiring a programmer

• Ability to communicate with the existing SCADA master

• Ability to handle various communications protocols simultaneously.

Communication capability

IEDs are able to communicate directly to a SCADA system. ++++ shows the present day concept of SCADA using IEDs connected through LAN and other network configurations.

++++ Typical structure of IED communication