TRANSLATION

SPECIFICATIONS FOR THE OPERATIONAL PER-FORMANCE OF POWER PLANTS

Introduction

Power system security requires that the integrated opera-tion of power plants and network comply with the plans in all operational circumstances, including disturbances.

It is important that power plants remain synchronised in the network and behave as planned in case disturbances in the network occur. Otherwise the disturbance may lead to total blackout, and the restoration after the blackout will be more difficult and may take a considerably longer time.

Specifications for the operational performance contribute to the system operational security so that

power plant withstands the variations in voltage and frequency caused by the power system

power plant does not cause any inconvenience to other power plants synchronised to the electrical power system.

In order to maintain security during the normal operation of the power system, it is also important that power plants have adequate and compatible capabilities concerning load following, power control rate and starting times.

These specifications follow in applicable parts the Nordel specification "Operational Performance Specifications for Thermal power Units larger than 100 MW", 1995

Application

Specifications are applied to new power units to be con-nected to the Finnish power system. Specifications shall also be applied to the existing units, if new installations or modifications e.g. in the excitation system is introduced. Otherwise the existing units are supposed to fulfil the re-quirements, which were valid when connecting the units to the power system.

These minimum specifications apply to all units1 larger than 50 MW, if the text does not mention any other power limit. It is recommended, however, that the requirements are also applied to power plants smaller than 50 MW.

All the units shall fulfil the requirements specified in sec-tions 1 - 5. In section 6 there are also recommendations concerning the power control characteristics of the power units. In case the power unit is utilised as frequency con-trol reserve and/or as instantaneous disturbance reserve, a separate contract is concluded concerning the power control properties and power reserve of the unit.

In addition to these specifications there can also be addi-tional requirements for a unit caused by the local grid. These shall be agreed separately with the local grid owner.

1 GENERATOR AND VOLTAGE REGULATOR CHARACTERISTICS

1.1 Generators

The reactance of the generator as well as its step-up transformer shall be as low as technically and economic-ally possible in order to support the system stability and reactive power control.

Generators with outputs below 500 MVA shall have

no-load short circuit ratio Kc (saturated) of at least 0.5,

direct axis transient reactance Xd' (saturated) of less than 0.35.

Generators with outputs above 500 MVA may deviate from these values, the allowable limits being Kc 0.43 and Xd' 0.42.

Each generator shall be capable of operating on the rated active power continuously at power factor down to at least 0.95 underexcited, and 0.9 overexcited. This shall be possible in connection with voltage and frequency con-ditions described in section 3.1, however, at underexcited conditions the normal grid voltage is applied instead of the 90% voltage.

1.2 Voltage Regulation

The preferred dynamic characteristics of a steady state are defined in a measurable way as follows:

The 10% step response of generator voltage is recorded in no-load conditions, the unit disconnected from the grid. The set value of the voltage is changed with plus and minus stepwise changes causing change of generator terminal voltage from 95% to 105%, and from 105% to 95%. In both cases, the step response of the generator terminal voltage shall be as follows:

response is non-oscillating

rise time from 0 to 90% of the change is 0.2 ... 0.3 sec in case of static exciter, or in case of brushless exciter: 0.2 ... 0.5 sec at a step upwards, 0.2 ... 0.8 sec at a step downwards.

overshoot is less than 15% of the change.

The excitation system shall be designed so that its ceiling voltage is in case of static exciter at least twice and in case of brushless exciter at least 1.6 times the magnet-ising voltage of the generator at its rated values. How-ever, the other requirements for voltage control have to be taken into account. The excitation system shall be capable of supplying its ceiling voltage for 10 seconds.

To secure the operation of the power system, the excita-tion system shall be implemented so that automatic con-

1 'Unit' refers to one or more generation plants combined in a way that one internal fault in the unit will cause the disconnection of the whole generation plant.

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trol system has at least a manual control of the excitation current as a reserve.

1.3 PSS, Power System Stabiliser

PSS shall normally be included in each generator. It shall be tuned to improve the damping of the oscillations of the generator and the power system, in particular the low fre-quency (0.2 ... 1.0 Hz) interarea oscillations. It shall be possible to disconnect the stabiliser. The stabiliser output shall also have limiters with adjustable settings.

1.4 Additional Voltage Control Equipment

In addition of the excitation system and the power system stabiliser (PSS), the control system shall also include pro-tective limiters and reactive current statics (providing droop characteristic) equipment.

Current limiters for generator rotor and stator shall have inverse time characteristics to utilise the generator over-loading capability in various network conditions.

1.5 Voltage Control Mode

The normal way of operation is automatic control of the generator voltage.

In case a different type of control is needed, such as con-trol of reactive output or power factor, the control shall also be able to react to the voltage variation. The applica-tion of these control systems shall be agreed with the grid company.

2 TOLERANCE TO FREQUENCY VARIATIONS

2.1 Frequency Range 49 Hz to 51 Hz

It shall be possible to operate the unit continuously at the

full output power2 within the grid voltage range of 90-

105% of the normal3 voltage, and at any frequency between 49 and 51 Hz. A maximum operating time of 10 h/year and maximum duration of 30 minutes per case can be assumed within the frequency range of 50.3-51 Hz. At a frequency above 50.3 Hz, a small power reduction is accepted, if stable operation at full power can be re-es-tablished when the frequency again drops below this value (see Figure 1).

2.2 Frequency Range 49 Hz to 47.5 Hz

It shall be possible to operate the unit in disturbance con-ditions for 30 minutes within the grid voltage range of 95-105% of the normal voltage, at any frequency down to 47.5 Hz. The output power may then be reduced by 0% at 49 Hz and a maximum of 15% at 47.5 Hz, and by a value found with linear interpolation at frequencies between these two limits. Efforts should be made to lower this reduction in output power, if this can be achieved without high additional costs.

2.3 Transitory Frequency Variations 51 Hz to 52 Hz

It shall be possible to operate the unit for 5 seconds in transitory conditions of the network during exceptional disturbances within the grid voltage range of 95-105% of the normal voltage at any frequency between 51 and 52

Hz. During such transients, the power may be reduced, if stable operation at full power can be re-established when the frequency again drops below 50.3 Hz.

2 In the following, the term power means active power supplied to the grid.

3 During normal operation of the power system, the variations in 400 kV: 380 - 420 kV, in 220 kV: 215 - 245 kV and in 110 kV: 105 - 123 kV (in a radially operated network 100 - 123 kV)

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Figure 1. Performance requirements for power production in relation to voltage (requirements in section 2) and fre-quency (requirements in section 3).

2.4 Frequency Range 51 Hz to 53 Hz

When feeding a separate electrical network (island opera-tion) it shall be possible to operate the unit at a strongly reduced output power within the grid voltage range of 95-105% of the normal voltage, at any frequency between 51 and 53 Hz for 3 min.

2.5 Frequency Below 47.5 Hz

The unit may be tripped from the network at frequencies below 47.5 Hz. The unit shall then be capable of chan-ging over to house load operation. However, this should not take place instantaneously, the time delay being de-termined by the design limits of the unit and so that reli-able changeover to house load operation can be ob-tained.

2.6 Frequency Gradients

The control system shall be designed so that the unit will not trip because of the transient frequency gradients oc-curring in case of short-circuits or switchings in the high-voltage network to which the unit is connected.

3 TOLERANCE TO VOLTAGE VARIATIONS

3.1 Grid Voltage Range 90% to 105% of the Normal Voltage

It shall be possible to operate the unit continuously at the full load within the frequency range of 49 - 51 Hz and at a grid voltage between 90 and 105% of the normal voltage. At a frequency above 50.3 Hz, a small power reduction is accepted, if stable operation at the full power can be re-established when the frequency again drops below this value. A maximum operating time of 10 h/year and a maximum duration of 30 minutes per case can be as-sumed within the frequency range of 50.3 - 51 Hz (Same requirements as in section 2.1). (Figure 1).

3.2 Grid Voltage Range 85% to 90% of the Normal Voltage

It shall be possible to operate the unit for 1 hour within the frequency range of 49.7 - 50.3 Hz at a grid voltage between 85% and 90% of the normal voltage, and an out-put power reduction of up to some 10% of the full output power may then be acceptable.

3.3 Grid Voltage Range 105% to 110% of the Normal Voltage

It shall be possible to operate the unit for 1 hour at a fre-quency within the range of 49.7 - 50.3 Hz and at a grid voltage between 105% and 110% of the normal voltage. A slight reduction in output power is acceptable.

3.4 Consequences of Nearby Grid Faults

3.4.1 Ability to Withstand Mechanical Stresses Due to Line Side Faults

Power units shall be designed so that the turbine gener-ator set can withstand the mechanical stresses associ-ated with any kind of single-, two- and three-phase earth or short circuit faults occurring in the grid on the high voltage side of the step-up transformer. The fault can be assumed to be cleared within 0.25 seconds. Neither dam-age nor any need for immediate stoppage for studying possible consequences is allowed.

3.4.2 Maintaining Stability During Line Side Faults

The faults occurring beyond the line breaker of an outgo-ing line will normally be isolated from the 400 kV grid within 0.1 seconds. In exceptional cases the clearing time may, however, extend to 0.25 seconds. In case of 220 kV and 110 kV line faults the normal clearing time is within 0.5 seconds and in exceptional cases 1 second.

The unit shall be designed so that it remains connected to the grid and continues its stable operation after isolation of a line side fault within 0.25 seconds.

A unit equipped with a large single-shaft turbine generator may be disconnected from the grid at a shorter time limit, if it is obvious that it will be impossible to maintain stability

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85 90 95 100 105 11047

48

49

50

51

52

53

Grid voltage (%)

Frequency (Hz)

50.3

49.7

30 minpower reduction

0 % at 49 Hz15 % at 47.5 Hz

30 min. 10 hours/yearsmall power reduction

3 min island operationlarge power reduction

3 min island operationlarge power reduction

or 5 sec transientsat reduced power

1 hour10 % reduc.

1 hoursmall

reductionContinuous operation

anyway. In this case a solution must be agreed upon with the grid operator.

3.4.3 Deep Voltage Transient

The units with their auxiliary systems shall be designed so that they can withstand the following generator voltage variations resulting from faults in the grid, without discon-nection from the grid (Figure 2):

step reduction to 25% of the rated generator voltage lasting for 0.25 sec,

followed by linear increase to 95% in 0.5 seconds,

followed by constant generator voltage of 95%.

Consequently, only a small power reduction can be ac-cepted.

Generator voltage is used in this section to emphasise the effect of grid faults on the unit as a whole. It shall also be noted that the design criteria for the voltage protection may vary, as the unit must manage several kinds of other faults in various operating conditions.

Figure 2. Deep voltage transient in generator voltage caused by a network fault.

3.5 Large Voltage Disturbances

The unit may be disconnected from the power system, if there occur larger voltage variations or longer durations than those for which the unit has been designed, and shall, in each case, be disconnected if the unit falls out-of-step.

The unit and its auxiliary power system shall be designed for such voltage variations as to enable safe changeover to house load operation after disconnection from the grid.

3.6 Reactive Power Output at Low Voltages

Power unit shall be equipped with such excitation system and shall be designed for such power factor as to enable the generator to be capable of providing a reactive power output of about the same magnitude as the rated active power output for 10 seconds, in conjunction with a net-work disturbance where the generator busbar voltage has declined to 70% of its rated value.

3.7 Reactive Power Capability

Transformation ratio and tap-changers of the step-up transformer and the auxiliary service transformer of the unit, as well as voltage range and excitation system of the generator shall be designed and adjusted so that the gen-erator is at normal operating voltage of the grid capable of generating continuously the reactive power defined by the capability curve of the generator (at generator voltage level) and, on the other hand, is at normal operating voltage of the grid capable of consuming continuously the reactive power defined by the capability curve of the gen-erator (at the generator voltage level).

The generator shall be capable of consuming continu-ously the reactive power (underexcited operation) defined by the capability curve of the generator also at the max-imum voltages (420, 245 and 123 kV) of the grid, assum-ing that the stability is maintained.

4 REQUIREMENTS CONCERNING POWER CON-TROL CHARACTERISTICS

4.1 Turbine Governor

All power plants shall be equipped with a turbine governor

and associated speed control systems.

4.2 House Load Operation

All power plants shall be designed to safely change over to house load operation from conditions specified in sec-tions 2.4, 2.5 and 3.5 or when the grid frequency and/or the grid voltage variation exceed the requirements.

House load operation is a unit operating with its own aux-iliary supply as the only load.

Power plants shall be designed so that they can operate in house load operation for at least 1 hour. Nuclear power units shall be capable of operating in house load opera-tion for the duration determined by the nuclear safety conditions.

4.3 Island Operation

In island operation the power system is separated into smaller systems, which operate asynchronously. In order to be able to resynchronise the power system, power plants connected to islands shall be able to control their power according to instructions given by the grid oper-ator.

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Generatorvoltage (pu)

1.00

0.95

0.25

0 0.25 0.75

Time (seconds)

5 VERIFICATION

5.1 Tests

The specifications shall be verified with full-scale tests, if possible.

These tests shall be made at the commissioning and by the owner at regular intervals throughout the entire life-time of the power plant. Recordings of data from the ac-tual operation shall be reviewed regularly in order to prove compliance with the specification.

5.2 Verification during Commissioning

5.2.1 Tests Concerning the Grid

The tests concerning the grid must be separately agreed with the grid operator. Following tests and measurements shall be done:

Deep voltage transient by short circuit Changeover to house load operation House load operation for 1 hour

5.2.2 Tests Concerning the Power Unit

Following tests and measurements shall be done:

Full output power and capability curve Step response of generator voltage Power system stabiliser (PSS) test Overload capacity.

6 RECOMMENDATIONS CONCERNING POWER CONTROL CHARACTERISTICS

In this section some recommendations are given, that aim to maintain adequate power control characteristics in power units connected to the Nordic grid. These charac-teristics may be needed for instance in exceptional situ-ations, where normal frequency control and instantan-eous disturbance reserves are not enough to return the power system to its normal condition.

6.1 Power Control - General

6.1.1 Starting Time

Units for load following should be designed so that the starting time from standstill to full load operation is as short as possible.

The following guidelines apply to gas turbines for emer-gency and peak load generation, from rolling-up to full power output:

gas turbines of the jet engine type 3 to 3.5 minutes

industrial gas turbines 10 to 15 minutes.

6.1.2 Minimum Output

The minimum continuous output power should be as low as possible. The following practical guidelines for the min-imum output power of power units can be used:

Coal-fired units: 40% of full output power

Oil and gas-fired units: 20% of full output power

Nuclear units: 20% of full output power

Hydro units (Francis): 20% of full output power

Hydro units (Kaplan): 10% of full output power

For different kinds of power plants the above guidelines can be used when applicable.

6.1.3 Overload Capacity

It is recommended to utilise the overload capacity only to the extent that is intrinsically available in the unit.

6.1.4 Operational Modes

The change of output power of a power unit, during nor-mal control and during disturbance control, is normally activated either with manual operation or with the unit controller.

The set value of the unit controller is 50 Hz. The set point resolution or the adjustable frequency dead band of the controller should be 0.05 Hz or less. It shall also be pos-sible to disengage the dead band.

The statics set point (providing droop characteristic) is re-commended to be adjustable within the range from 2% to 8%. The normal operation is generally within the setting range of 4% to 6%.

6.1.5 Power Step Change Limiter

The units are recommended to be equipped with ad-justable devices for limiting the magnitude and rate of the power change.

6.1.6 Power Control - Normal Operation and Disturb-ances

The required power output during normal operation is the manually pre-set power output, modified by a frequency-sensing unit controller (or turbine governor). Power con-trol during normal operation is recommended to meet the specifications of section 6.2. When using the power unit to frequency control, the control characteristics must be specified more accurately in a separate contract.

The normal operation turns into disturbed operation when the rate of change in frequency exceeds ± 0.5 Hz/s, or the error in frequency set point is more than 0.5 Hz. The need for disturbance control shall be governed by the fre-quency-sensing equipment (e.g. consisting of frequency relay set at a certain value below normal frequency). Power control during disturbances is recommended to meet the specifications of section 6.3. If the power unit is utilised as instantaneous disturbance reserve, the control characteristics must be specified more accurately in a separate contract.

6.2 Power Response Capability during Normal Oper-ation of the Power System

6.2.1 Load Following

All condensing units are recommended to be designed so that they can be used for load following using the load change rates specified in sections 6.2.2 - 6.2.5. In case of other type of units than mentioned in these sections, the recommendations given in section 6.2.3 can be used when applicable.

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6.2.2 Power Response Rate and Range - Oil and Gas

Oil-fired and gas-fired units are recommended to be de-signed for a power response rate of at least plus or minus 8% of the full power per minute.

The above rate of change is recommended to be applic-able to any 30% change between 40% and 100% of the full power according to the load schedule. The power re-sponse rate may be limited to the maximum power re-sponse rate permissible for the turbines or the steam boil-ers in the range below 40% and above 90%.

6.2.3 Power Response Rate and Range - Coal

Coal-fired units are recommended to be designed for a power response rate of at least plus or minus 4% of the full power per minute.

The above rate of change should be applicable to any 30% change between 60% and 100% of the full power according to the load schedule. This range may be re-stricted to 20% in certain cases. The power response rate may be limited to the maximum power response rate per-missible for the turbines or steam boilers in the range be-low 60% and above 90%

6.2.4 Power Response Rate and Range - PWR Nuclear

PWR nuclear power units are recommended to be de-signed for a power response rate of at least plus or minus 5% of the full power per minute within the output range of 60% to 100% of the full power. At outputs below 60%, the power response rate may be limited to the maximum power response rate permissible for the turbines.

6.2.5 Power Response Rate and Range - BWR Nuclear

BWR nuclear power units are recommended to be de-signed for a power response rate per minute of at least plus or minus 10% of the initial output value. This is re-commended to be maintained throughout the output range within which the power can be controlled with the speed of the main circulation pumps. This output range is recommended to be at least 30% of the initial output power. In the remainder of the power range between the minimum load and the full load, the power response rate is recommended to be at least 1% of the full power per minute.

Comment on items 6.2.4 and 6.2.5: The power response rates of the units equipped with standard versions of light water reactors are usually sufficient. However, it should be noted that the power response rate is subject to cer-tain restrictions, due to the structure of fuel elements. It is expected that these problems will be solved, and the units should therefore be designed to conform to the re-commended power response rates. However, in order to limit the stresses imposed, the power changes during normal daily and weekly load following should be carried out gradually over a period of about two hours.

6.3 Power Response Capability during Power System Disturbances4

6.3.1 Power Step Change - Fossil Fuel

Fossil-fired thermal units are recommended to be de-signed with an operating mode allowing an instantaneous step change in output power of at least 5% of the full out-put within the range of 50-90% when requested.

Backpressure units with condensing turbines are recom-mended to be designed so that during network disturb-ances the maximum condensed power is available for 15 minutes by steering the heating power to the condenser.

6.3.2 Power Step Change - Nuclear

PWR nuclear power units to which the power change sig-nal is applied directly to adjust the turbine control valve are recommended to be designed so that a power step of 10% of the full power can be accommodated within 30% of the power range. BWR nuclear power units operating on pressure control are recommended to be designed so that, within the range of the pump control, they will be capable of accommodating a power step change of 10% of the initial value.

6.3.3 Subsequent Power Response Rate

After the power step changes specified above, thermal power units are also recommended to be capable of ac-commodating a load change at the rates specified in sec-tion 6.2. However, the total load change may then be lim-ited to the values specified in sections 6.2.2 - 6.2.5 above.

6.3.4 Spinning Disturbance Reserve

Power units are recommended to be designed so that they can be used as spinning disturbance reserves. In case the power system can not be restored to its normal state by utilising the separately contracted reserve, the grid operator has the right to demand units to adjust their power in the region specified above. This kind of excep-tional situation can be caused by a disturbance more severe than the main grid dimensioning criteria specify, or by some other unexpected reason.

6.3.5 Island Operation

In case of very serious (and exceptional) disturbances, in which the power system is separated into smaller islands, the units are also recommended to be capable of per-forming the above power changes (upwards or down-wards), and then achieving stable operation and normal power control capability according to section 6.2. The ad-ditional requirements for the units caused by the local grid shall be taken into account separately.

14.4.2000 / JYJU

4 Power system disturbances are frequency or voltage disturbances and include in severe cases stability disturbances as well.

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