Process Controllers Functions In Hydro Power Plant

PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT

d c

Designed by

B. Horvat

KON^AR

b

Checked by

B. Premer

POWER PLANT AND ELECTRIC

Approved by

T. Duzevic

TRACTION ENGINEERING Co.

a

.

Rev

. Description

Date

Sign.

Project:

TD No: Title:

PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT File: KET-201

Name

Sign.

ZAGREB, C R O A T I A

8345-62-98-000X

Dep.No:

Drawing No:

KET-201

BOOK:

=

KON^AR

POWER PLANT AND ELECTRIC ZAGREB, C R O A T I A Format:

A4

Project: Title:

- PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT Book: No. TD: 8345-62-98-000X

CONTENTS 1 Introduction 2 Data acquisition 3 Start/stop sequences 4 Active and reactive power setpoint control 5 Stator current limitation 6 Thermal supervision 7 Spillway gate position adjustment 8 Auxiliary services power supply control 9 Flow calculation 10 Water level control 11 Joint active power control 12 Joint reactive power control 13 Reports

File:KET-201

KET-201 Sheet

2 of 6

KON^AR

POWER PLANT AND ELECTRIC ZAGREB, C R O A T I A Format:

A4

Project: Title:

- PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT Book: No. TD: 8345-62-98-000X

KET-201 Sheet

3 of 6

1 INTRODUCTION The purpose of the following description is to give to the reader a quick general insight into the way Kon>ar uses DCS technology (ABB and Siemens) in hydro power plants control. It doesn't refer to control scheme in any specific hydro power plant. Some typical functions realized in hydro power plant control system are given. The final scope of these functions is in any particular case tailored according to customer needs as expressed in tender documents and agreed upon during the project design phase. 2 DATA ACQUISITION Digital data are red out either cyclically or by change of state, and time tagged for event handling in operator station (1 ms time resolution in controllers). Analog data reading takes place only cyclically, with selectable scanning time. Analog signals are prefiltered, converted from raw to engineering units, and checked against the limits. Passing of any of limit values (2 low and 2 high limits) is treated as the event. For living zero signals additional validity check is done, and the alarm is given in the case measured value is out of expected range. The alarm signal is also given in the case of faulty channel on input module. 3 START/STOP SEQUENCES Each start up / shut down sequence is divided into a number of steps. Start/stop orders come either from the operator, via Operator Station, or in some cases from water level control program. Before the commencement of the start sequence, fulfilment of start conditions is checked; the same is case with each step of the sequence. The whole sequence and the steps in it are time monitored and the fault signal is given with proper fault information in the case of malfunctioning. 4 ACTIVE AND REACTIVE POWER SETPOINT CONTROL Depending on the type of turbine governor (voltage regulator) setting device, setpoint is simply transmitted in the form of BCD or mA signal, or adjusted by sending the train of raise/lower pulses of variable length. Controlled value is monitored during adjustement, and the fault signal is given when it doesn't behave in accordance with the sent pulses. Setpoints come from different places within control system: start/stop sequences, joint or water level control, limitation functions or directly from the operator. Selection of valid one is done by program, depending on the control status of generating unit and the priority level of the function setting the request.

File:KET-201

KON^AR

POWER PLANT AND ELECTRIC ZAGREB, C R O A T I A Format:

A4

Project: Title:

- PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT Book: No. TD: 8345-62-98-000X

KET-201 Sheet

4 of 6

5 STATOR CURRENT LIMITATION When the unit is in automatic mode of operation, stator current is monitored, and corrective action is taken when it exceeds the limit longer than allowed. Reactive power is reduced first, if necessary to zero. If this reduction doesn't bring stator current back into the allowed range, the active power is reduced. Very often, the supplier of the excitation system realizes the same function within this equipment. In this case these limit values should be co-ordinated. However, the implementation of this function within the integrated plant control system has several advantages. The first of them is the ease of changing of limit value, via the keyboard and not with a potentiometer, especially if it is to be done temporarily, e.g. due to some troubles in the cooling system. Also, the excitation system can act only on excitation current, which might be unsatisfying in the cases, actually very seldom, of too low network voltage. Generally, supervision and limitation functions in such a system are more flexible in comparison with those realized with conventional circuitry. All measurements of interest are accessible (active and reactive power, current, voltage, excitation current, guide vane opening, temperatures, vibrations intensity ...), limitations can be done on any one or combinations of them, obeying the priority, and by acting either on excitation system or turbine governor, or both, even in some cases on auxiliaries (cooling water system or fans). 6 THERMAL SUPERVISION Temperatures of all points of interest are continuously measured and shown on process displays and trend curves. They can be also used for tripping of generating unit in case of high temperature or for reduction of active or reactive power, rarely directly, but more often using the results of calculation of dependency between stator current and winding temperature for setting of stator current limit. By means of local display all temperatures can be selected and displayed. An alarm is activated in the case of adjustable limit crossing of each and every measured chanel. 7 SPILLWAY GATE POSITION ADJUSTMENT This adjustment takes place in form of continuous raise/lower orders. Measured value is monitored during adjusting, and fault signal is given in the case of malfunctioning. A setpoint comes either from water level control program or is issued directly by the operator. 8 AUXILIARY SERVICES POWER SUPPLY CONTROL Auxiliary services are usually supplied from two or more sources, e.g. auxiliary power transformers connected to transmission or distribution network and diesel-generator set, or other auxiliary generating unit. Sometimes the power of diesel-generator set is not enough for the supply of all auxiliaries and care must be taken of its load. As the philosophy of auxiliary power supply varies from plant to plant, control program is adapted to each particular case.

File:KET-201

KON^AR

POWER PLANT AND ELECTRIC ZAGREB, C R O A T I A Format:

A4

Project: Title:

- PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT Book: No. TD: 8345-62-98-000X

KET-201 Sheet

5 of 6

9 FLOW CALCULATION Turbine flow is usually measured using flow transducers, but it can be also calculated from the measured guide vane opening and the difference between head - and tailwater. Spillway gates flow is normally calculated on the basis of their openings and water level, using spillway discharge curves. Measured and calculated flow values are shown on process and trend displays, and are included in daily and monthly reports. Turbine flows can be used for unit efficiency calculations. 10 WATER LEVEL CONTROL There exist in fact two water level control programs. First of them tries to keep water level at a desired value adjusting the active power and therefore turbine flow. Here it should be emphasized that the water level control makes sense only in plants with rather small reservoirs, i.e. where utilizable volume can be emptied in several hours. The second program is protective one and it regulates the level in the reservoir by spillway gates, doing this of course only when generating units work at their maximum or they are out of operation. These two programs are therefore coordinated by setting the level reference value for turbines little below the setpoint for spillway gates. For rather short and narrow reservoirs, a statism cm/MW is very often included in the calculation of error signal. In addition, these reservoirs have strong dynamics, and more sophisticated control methods (like feed-forward, variable control parameters etc.) must be used. The output from this controller is active power which is distributed among the generating units taking into account their efficiency curves, limit values and possible forbidden operation areas, for instance due to oscillations, vibrations and cavitations. The program takes care of the number of units in operation, with respect to efficiency. Besides, water level measurement, as the most critical one, is continuously checked, and if it fails the units are switched off from this control. This program package can be simply adapted to the case when there exist more users of water resources: irrigation, water supply of big cities etc. 11 JOINT ACTIVE POWER CONTROL Active power is distributed among the generating units following the same rules as in water level control. As the applied regulator is of PI-type, the statism of joint controller is valid for the behaviour of the plant active production in the cases of frequency disturbances and not the statisms of individual governors. Of course, this is an important issue only in isolated networks. 12 JOINT REACTIVE POWER CONTROL Total reactive production is also controlled in closed loop. Depending on the plant size and its importance in power system, busbar voltage deviation is included in error calculation with greater or lower weighing coefficient. Required reactive power at the regulator output is distributed among the generating units proportionally to their maximum reactive powers, taking into account the limit values. File:KET-201

KON^AR

POWER PLANT AND ELECTRIC ZAGREB, C R O A T I A Format:

A4

Project: Title:

- PROCESS CONTROLLERS’ FUNCTIONS IN HYDRO POWER PLANT Book: No. TD: 8345-62-98-000X

KET-201 Sheet

6 of 6

13 REPORTS Energy meter reports and flow reports are commonly used in hydro power plants. Energy counters are software realized if using ordinary digital inputs. Daily and monthly reports are printed out automatically or upon operator's request. Reports can be also shown on process displays on VDUs and copied on hardcopy. Beside flow and energy, other reports are also possible, including both measured and calculated values: electrical measurements, temperatures, levels, and so on.

File:KET-201