Operational Beneeits of Integrated Digital Devices for Power Plant Protection

OPERATIONAL BENEEITS OF INTEGRATED DIGITAL DEVICES FOR POWER PLANT PROTECTION

C.R.Hodgson

ALSTOM T&D protection & Control limited Stafford. United kingdoin

The protection of a power plant has previously involved using number of a discrete protective relays. Advances in the use of digital technology enable a number of protective functions to be integrated into a single unit. In addition to a direct equipment cost saving, savings in both panel space and scheme engineering are also achieved.

Digital relays also provide increased functionality. Power system measurements can be continuously displayed and recorded to provide instrumentation , fault records and disturbance records. A communication facility provides remote access to these and other parameters, including settings and control functions. The availability of a digital device is monitored by continuous self checking of its own operation.

INTRODUCTION

The major items of electrical plant in a power station are generators and transformers. Generators can be affected by a number of faults and a

range of abnormal operating conditions. Each generator requires a number of protection functions to be employed to afford comprehensive protection. Previously generator protection schemes have used separate protective relay to fulfill each protective function. Common trip and alarm outputs are achieved through the use of dedicated logic controllers or auxiliary relays and scheme wring.

A digital integrated generator protection package offers a number of protection functions within a single relay (see figure 1. the protection functions control trip and alarm outputs via user configurable scheme logic which is internal to relay. This results in large savings in terms of panel requirements and external wiring.

A number of functions associated with transformer protection can be also be integrated into a single device (see figure2).

Traditionally, transformer differential protection required the use of external interposing current transformers. The interposing current transformers were compensate for phase and amplitude differences between the secondary currents either side of the transformer.

A digital relay uses software to provide both phase and amplitude compensation, eliminating the need for external devices.

There are some protective devices which are applied to transformers which cannot be integrated into a digital device. Buchholz protection and temperature monitoring devices are examples. These devices are situated at the transformer but trip indication is required on the protection panel. Isolated inputs to the digital transformer protection relay allow the relay to monitor, and log, the operation of such devices. Operation of external devices can be annunciated by the digital relay rather than employing separate flagging relays.

Some control features, such as tap up and tap down control of a tap changing transformer, are also offered in a digital transformer protection relay.

Digital relays provide additional benefits in terms of non-protection features. Analogue inputs to digital devices are processed and can be displayed as measurements. If a fault inputs occurs then, at the moment that the trip signal is sent to the breaker, the measurements can be stored in the relays memoery creating a detailed fault record. Sufficient memory is allocated for storing samples of the analogue inputs both during and after the fault to allocated for storing samples of the analogue inputs both during and after the fault to provide waveform disturbance records.

Communications facilities can be provided to allow remote interrogation of digital protective relays. Connecting a personal computer to the relays via the communications system allows the user to remotely view measurements and fault records as well as being able to change settings. The waveform disturbance data can also be extracted from the relay in comtrade format and displayed graphically, using suitable viewing software.

RELIABILITY AND AVAILABILITY

Whilst integration of protection functions in a single package offers many benefits, the integration does result in a finite decrease in protection scheme reliability. The provision of discrete relays afford a certain amount of duality one relay fail. However, failure of more than one discrete relay would not be known until the proactive scheme is tested, during routine maintenance, or until the relays are required to operate.

A digital protection package ensures maximum availability by monitoring its own operation. Should the protection fail the user is alerted immediately in order that corrective action can be taken.

For larger power stations, the cost of any down time could be high in relation to the cost of providing protection. In such applications it would prudent and economically viable to duplicate the protection. Depending on the importance of the station full redundancy could be provided by a separate protection device powered from an independent supply, of by a spare relay which could in if the main device develops a fault.

GENERATOR PROTECTON FUNCTIONS

The generator protection functions shown in figure 1 are those which have been identified as the most common for a wide range of applications. Some protection functions may be extraneous to requirements for smaller generators. Other generators may require additional protection devices to be applied, for example rotor earth fault protection.

Superfluous protection functions are permissible as the protection functions can be enabled and disabled as necessary and their provision does not significantly increase the cost of the protection package. Isolated inputs allow the operation of any additional external protection to be monitored, logged included within the tripping logic of the integrated generator protection device.

The protection functions provided in an example package are:

*Generator differential (87G) – to detect stator winding multi-phase faults.

*Stator earth fault (51N) – a current operated earth element which will provide earth fault protection for 65% of the stator winding.

This is usually applied where the generator is connected directly to earth or through a neutral ear thing resistor.

*Neutral displacement (59N)-a voltage operated element which will provide 95% earth fault protection of the stator. This elements is commonly used where a distribution transformer ear thing arrangement is applied.

*Sensitive directional earth fault (67N)- where generators are operating in parallel sensitive directional earth protection is required. This protection will determine if an earth fault has occurred on the associated generator or on a parallel generator.

*voltage dependent over current or,(51V) – provides system back-up protection. The protection can be simple over current or, where the armature reaction of the generator significantly reduces the fault current, it can be voltage controlled (switched current setting)or voltage restrained (continuously varying current setting).

*Reverse power (32R)/ LOW forward power (32L) – both reverse power and low forward power protection can be used to detect failure of the prime mover. LOW forward power protection is more commonly utilized to control shut down of the generator under non-urgent trip conditions.

*Negative phase sequence (46)-models the thermal condition of the generator when exposed to negative phase sequence currents. If the thermal withstand of the generator is approached the generator is removed from the system.

*Field failure (40) – detects failure of the excitation system. Controlled shut down prevents damage to the generator, and bus voltage depression, due to the high levels of reactive power which will be drawn during this condition.

*Under voltage (27)/over voltage (59) –three phase protection to detect under and over voltage conditions is provided.

*Under frequency (81>)/over frequency (81<)- under and over frequency protection is provided to ensure that the generator operates within a controlled frequency range.

*Voltage balance protection-the VTs supplying the generator protection are monitored by the voltage balance protection.

Failure of a VT is immediately alarmed. Those protection functions which will be affected by failure of VT are bloked.

SCHEMELOGIC PROTECTION

Generator protection comprises of a number of protection which are combined through scheme logic to control a few trip and alarm outputs. Traditionally, such scheme logic has been provided by a dedicated logic controller or combinations of auxiliary relays. A digital relay uses an internal programmable logic to obtain the AND-OR logic shown in figure3.

FIGUR 3) SCHEME LOGIC

Whit the example relay there are 32 inputs to

the scheme logic nineteen from the protection functions, eight from the digital status inputs and the remainder from internal inverted inputs, which allow blocking logic to be created. The AND function combines two or more inputs to provide interlocking or blocking logic, or to simply act as a through connection for a single inputs. The OR function allows each line of AND logic outputs to control one or more output relays. Up to thirty- two input combinations can be set to control up to fifteen output relays.

Figure 4 illustrates the use of the scheme logic. During an urgent trip condition, such as differential protection operation, both the steam valve and the circuit breaker are operated simultaneously. A non-urgent trip condition, such as the operation of the field failure protection, results in a controlled shut down of the generator. Here the steam valve is immediately closed to remove the motive power to the generator. Without generator combination of the operation of the filed failure protection and low forward power protection which operates the circuit breaker. This prevents overspeeding problems when the breaker is opened.

FIGUR 4)SCHEME LOGIC EXAMPLE

GENERATORPROECTION ENHANCEMENTS

Additional beneficial features are provided within the digital device. Example package offers:

*Dual setting-an independent group of protection and scheme logic setting that can be selected either locally or remotely.

Applications in which this facility is useful include pumped storage schemes and dead machine protection.

* Blocking inputs-isolated inputs to the relay can be used to block certain protection functions. Some protection functions must be blocked at certain, for example, the under voltage and under frequency protection should be blocked during run up and run down of the generator.

* Frequency tracking- the sampling rate of analogue to digital converters is locked to the power system frequency. This allows protection operate down to low frequencies (where adequate CTs and VTs are provide). Protection can be afforded to a generator where variable frequency starting is employed.

TRANSFORMER PROTECTION FUNCTONS

The protection functions provided by the example device are:

*differential protection-to detect multiphase and earth fault within the transformer.

*Restricted earth fault protection-for each transformer winding. This protection offers increased earth fault coverage for the transformer windings.

*Over fluxing protection- of particular importance for generator transformers. Over fluxing causes the transformer to overheat, this can threaten the integrity of the insulation leading to permanent faults. Generator transformers are sometimes subjected to over fluxing during the run up and down period or during load rejection if problems occur with the AVR system.

*Ancillary protection monitoring- eight isolated inputs are provided on the example package to monitor ancillary protection. Operation of any of the ancillary devices will be flagged at the protection panel by the digital relay.

TRANSFORMER PROTECTION ENHANCEMENTS

The digital transformer protection relay includes the following

enhancements:

*Internal phase and amplitude correction-previous transformer protection devices have required the use of external interposing transformers. The purpose of the interposing transformers was to provide vector and ratio correction as well as forming a zero sequence trap. A digital relay utilizes internal software routines to compensate for phase and amplitude differences and to simulate a zero sequence trap.

*Magnetizing inrush restraint high set operation- the example package implements a unique gap detection technique to detect magnetizing inrush waveforms. During magnetizing inrush the differential element is established. The magnetizing inrush detector circuit requires one cycle of waveform to ascertain whether differential current is magnetizing inrush current or fault current. Faster operation of the protection can be achieved for heavy faults using a high set element. A current setting higher than the maximum inrush current would be applied.

*Transient over fluxing restraint-transient over fluxing occurs during loss of load and on generator transformers during the run up and run down periods. over fluxing causes saturation of the transformer core, resulting in an increase in the core magnetizing current. This current may be sufficient to unbalance the differential protection. The differential element must therefore be restrained from operating during transformer over fluxing. The transformer is protected from the effects of prolonged over fluxing by the over fluxing protection incorporated within the digital relay.

NON- PROTECTION FUNCTIONS

Numeric design of protection allows several non-protection functions to be provided functions common to both the example devices include:

*Local and remote instrumentation

*Event, fault and disturbance recording

*Test aids for commissioning and maintenance

*Self monitoring to improve availability

Instrumentation: All the electrical measurements and derivatives used are available for display. The measurements can be display locally on the front panel of the protective relays or remotely, using the communications facility.

Event, fault and disturbance recording: the last 100 transformer events or the last 100 events involving the generator are recorded. An event is typically the operation of a protection function or the energisation of an input or output of the relay, but also a host of the changes, such as a setting group change.

At the instant that the output relay responsible for tripping the breaker issues the system measurements, and derivatives, are recorded to produce a fault. The 50 most recent transformer faults and 50 most recent generator faults are recorded by the associated protection relays.

Disturbance by the records are also available. The disturbance records must be downloaded from the relays to a personal computer using the communications system. Samples of the analogue inputs and also the operation of the digital inputs and relay outputs are recorded. A graphical representation of the records can then be displayed using suitable viewing software or a spreadsheet software package, such as Microsoft Excel.

Test facilities: facilities are provided to enable digital relays to be thoroughly tested during commissioning and routing maintenance. The instrumentation facility allows the External AC circuitry to be checked. A digital input monitor allows the status of the isolated inputs to the relays to be checked. A relay trip test option allows the relay contacts and operation of any connected plant to be checked. In addition the scheme logic settings contained in the generator protection device can be proved using a scheme verifier function.