DEVELOPMENT OF HYDRO TURBINE CONTROL SYSTEM FOR RETROFITTING EXISTING TRUBINE GOVERNORS AT KOTMALE

HYDRO POWER STATION T.D.K. Dharmapriya, Ceylon Electricity Board, Sri Lanka, +94 716846013, ee3ciamhe.gen@ceb.lk

D.P.Ariyasighe, Ceylon Electricity Board, Sri Lanka, +94 714938499, eeciamhe.gen@ceb.lk

ABSTRACT Kotmale hydropower station was commissioned during 1985-1987 period. The power station has three 90MVA generators which coupled to vertical Francis turbines. The associated turbine governors are mainly consisting with discreet analog electronic based control systems and hydraulic amplifiers. Operation of hydraulic amplifiers and actuators of turbine governors are still in adequate condition. But due to aging of electronic components the electronic controllers are showing poor performance. Unavailability of those electronic spare parts would lead the power plant at high risk situation and ultimately reduce the plant reliability. This paper presents the research conducted at Ceylon Electricity Board (CEB) to develop a programmable logic controller (PLC) based generic turbine governor controller to mitigate the performance issues in existing older governor control systems at Kotmale hydro power station Sri Lanka. Comprehensive study on existing governor system carried out to identify the characteristic of existing governor control system. The PLC based control system was designed as a combination of feedforward controller and feedback controller with PID regulation to obtain faster response to user commands. This system is implemented by using a suitable programmable logic controller and installed for testing at one of the 90MVA hydro generator at Kotmale power station, Sri Lanka. Main objective of this paper is to discuss the possibility of developing the replacement system, for existing electronic turbine governor control systems at Kotmale Hydropower Station by using a programmable logic controller based control system and discuss the success of approached method and conclusion of results obtained. Key Word: Turbine governor, Retrofitting, PID control, Feed forward, Programmable logic controllers. 1. Introduction The economic development in Sri Lanka results an increasing demand in electricity supply due to infrastructure and commercial development [1]. CEB has a moderate generation mix to dispatch this electricity demand [1]. As the second largest hydro power station in CEB, Kotmale power station has a vital role in dispatching energy as well as frequency controlling of Sri Lankan power system [2]. Kotmale PowerStation has the capacity of delivering 201MW (3X67MW) active power under 201.5m nominal water head. The power station is consisting with three units of 90MVA hydro generators. Each generator couples with Francis turbine which is capable of delivering 68,100 kW at 201.5m nominal head. Condition monitoring tests indicated that the hydraulic system and actuators associated are in adequate condition but electronic controller associated with the turbine governor system has shown poor performances due to ageing. To mitigate the performance issue in electronic controller a Programmable Logic Controller based (PLC) control system was developed as CEB in-house development. Proper starting, stopping, quick synchronizing, and speed droop operation are some of the vital functions of turbine governor controller. Hydro generators are capable of varying the loads rapidly due their nature of energy transformation method. Hence the faster response to the operator active power set-point is essential for a hydro turbine controller, where traditional cascade feedback control structure has insufficient quicker response. Therefore it is necessary to modify the cascade feedback control structure to gain this quicker response. Feed forward control architecture is a well known method for achieving quicker response from controlled system [3]. The speed droop operation of hydro turbine governor system is the key process which determines load sharing upon system frequency changes. Generic turbine controllers used PID feedback control architecture for achieving speed droop operation [4]. The governor control system describe in this paper was implemented by using both feedback and feed forward architectures to achieve faster response to the user set-point changes while keeping better regulation and speed droop operation through feedback control loop. A programmable logic controller with suitable input, output modules is used to implement the control structure and control logic of the governor control system. Existing turbine governor system was used electromagnetic

relays to implement the control logics and operational amplifiers based analog electronic controller for implementing the control loops. The newer development replaces both relays and the electronic control system by the PLC and associated input output devices. The system installed in 90MVA hydro generator at Kotmale hydro power station which is having Francis turbine runs at 375 rpm. The offline simulated dry test results provided evidence of the successful operation. 2. Methodology Functional block diagram of implemented governor control system is shown in figure 1. Scope of the developed PLC based control system is demarcated in dotted lines. Transfer function represent the governor hydraulic actuator and represent the generator and turbine system. The governor control strategy at Kotmale hydropower station can identified as power feedback speed regulation governor [5]. Hence governor control system takes generator active power measurement instead of gate position feedback denoted as . Generator frequency (Speed) is mentioned as and ∆ denoted as the frequency deviation. For Sri Lankan power system is 50Hz. is the user power set-point of the generator and is the speed regulation coefficient. For Kotmale power station two speed regulation modes are defined as Ep1 (2.3%) which used during frequency controlling and Ep2 (9.2%) which used during power production mode. Feed forwarded power set-point is denoted as FF and Y0 represents the gate position reference which is the output of the designed governor control system. The characteristic of hydro turbine generator assembly is different when it is connected to grid than no load spinning. Hence control parameters and control functions need to be switched depending on the generator is gird connected or spinning no load specially during starting the generator. Such control decisions take place by the control logic block in figure 1. User power set-point fed through a ramp block to achieve the power set rate of 2MWs-1 which is a design parameter of existing governor system. The user power set-point is feed forwarded to the final summer to achieve faster response to operator command. The PID regulator output added to final summer to obtain the feedback response of governor system.

When considering power systems without ALFC loop, generator active power output is maintained by the turbine governor system by considering the user desired set-point (Pset) and turbine governor speed droop regulation [6]. Mathematical model of position reference is constructed by using equation (1),(2),(3)&(4). ∆ = − (1)

∆ = − (2) = 50

= ∆ + ∆ . (3)

-

∆ .

+

+ +

∆

∆

∆ .

+ -

-

+

+ +

-

Figure 1- Block diagram of governor control system

= − . ( ) − . ∆ − . (4) ,

, , ℎ ,

During generator start the control logic block switch the PID parameters to offload settings and the is modified accordingly as shown in equation (5) and (6).

= ∆ + . ℎ (5)

= − . ( ) − . ∆ − . (6)

During synchronizing the additional parameter ∆ introduce by the control logic block. Value of the ∆ is determine by the synchronizer speed matching command. The modified equation for deviation during synchronizing is shown in equation (7).

= ∆ + . + ∆ (7)

Gate opening limit is set by the user as preferred. When the unit is online is set to 95% or 100% depending on the available water head. During starting the gate limiter is set to gate start opening limit (20% for Kotmale generators) by control logic block as ad additional safety action. Allowable operational span for the integral controller needs to be limit to overcome the endless integration behaviour of typical numerical integration. The upper and lower limit of numerical integrator is calculated and control by the limiter block. Equation (8) and (9) show the way of calculating upper and lower limits for the integrator.

= (8)

= − (9)

3. Tuning of PID regulator

There are several methods accepted methods for tuning a PID controller. Some methods required close loop system for initial estimation of parameters and some methods do the calculation by using open loop performances of the system and then do the fine tuning. Trial and error method, Ziegler-Nichols, Modified Ziegler-Nichols, Damped oscillation and Cohen-Coon are some of the popular methods that practice in control Engineering [7]. Method chosen for tune a respective PID control system depends on the availability of the closed loop system for testing. Since provision for dry testing the system with closed loop operation available on this governor development Ziegler-Nichols method was adopted to find the initial PID parameters and fine tuning was done in trial and error method with comparing the step responses of existing governor control system. All the tests results included in this paper were conducted as dry tests.

In Ziegler-Nichols method proportional gain is increased until desired a steady oscillation appear in the system output. The respective proportional gain at this time is the “Ultimate gain” and the period of oscillation is named as “Ultimate period” shown in figure 2. . During this time derivative and integral gains set to zero.

Figure 2- Sustained system oscillation during PID tuning

Accordingly to the Ziegler-Nichols method approximate values for the initial gain coefficient of PID controller is shown in equation 10(a), 10(b) and 10(c).

( ) = 0.6 × (10 a)

( ) = (10 b)

( ) = (10 c)

Finding the initial coefficient for PID parameters are don by the Ziegler-Nichols method and fine tuning was carried out in trial and error method.

4. Implementation

Implementing PID regulation with integrator anti-windup was one of the major challenges. It is necessary to execute the PID regulation algorithm recursively in precise time intervals. Sequential Execution behaviour of conventional PLC program would result the scan cycles deviate one another due to prioritized runtime burdens on CPU. Because of that implanting PID loop inside PLC main scan cycle results the execution of PID algorithm in irregular time intervals affecting results of integration and derivation functions. As a countermeasure the PID loop is implemented as a time driven interrupt routine. This interrupt routine is invoked exactly in 10ms time interval which allowing to execute the PID algorithm accurately in 10ms intervals. Other functional blocks shown in figure 1 implemented as sequential set of subprograms under PLC main scan cycle. Figure 3 explains the program execution of governor controller.

Time driven interrupt routine executes at the frequency of 100Hz, hence it is necessary to optimise the PID calculations to several milliseconds in order to avoid overlapping interrupt invokes and to provide sufficient time to execute the main cyclic program. Since the PID algorithm runs in a very smaller time intervals compared to the time constant of controlled process a simple instantaneous local tangent calculation and trapezoidal numerical integration is effective in calculating derivative and integral components. Equation (10 a), (10 b) and (10 c) explain the PID algorithm implemented.

Time invoked interrupt

Event driven interrupt

Start

Diagnostic

Error Error handling

Control logic Reading Input Calculate FF, ∆ , ∆ Calculate ( ) Limiters Final summer

PID algorithm Integrator anti-windup

Output (Y0)

Sequential operation Interrupt operation

Figure 3- Program flowchart of governor controller

Considering the instantaneous samples taken in deviation at time interval tn and tn-1,

= . 10(a)

= . 10(b)

= . ∑ . ( − ) , −4095 ≤ ≤ +4095 10(c)

As shown in the equation (8) and (9), the cumulated value of needs to be bound in the range of and to prevent the integrator windup during large step changes in the system. The

implemented anti-windup algorithm is shown in figure 4.

Consider ( ) as the cumulated integral value of at time tn and ( ) as the value of instantaneous area of the trapezium calculated in time interval tn.

< ( ) < ℎ , ( ) = ( ) +

( ) ≤ & ( ) > 0 ℎ ( ) = ( ) + ( ) = ( )

( ) ≥ & ( ) < 0 ℎ ( ) = ( ) + ( ) = ( )

In order to optimise the CPU usage by recursive PID loop operation and anti-windup program the algorithms are implemented in PLC by using Instruction List (IL) language. Instruction list language let the programmer to access the CPU accumulator registry and more primitive operations as desired which eventually eliminate the code burden created by IEC 61131-3 compliance higher level graphical languages during compiling. This approach results faster mathematical operations in floating point variables. Other functions of the control system is implemented by using IEC 61131-3 compliance functional block diagram (FBD) language and all those sub programs are linked in sequential manner in PLC main scan cycle.

The designed control system is fabricated and installed on a free space available on unit no 02 existing governor control panel at Kotmale hydro PowerStation, Sri Lanka. Plant signal interface to the PLC control panel is shown in figure 5. Since the controller is installed for the testing in a commercial running generator, plant interfacing was done with special attention to signal isolation.

Figure 4- Integrator anti-windup algorithm

Figure 5- Pant interface diagram

5. Results

The primary purpose of a control system must be foremost in all situations and ultimately keep the system stability under controllers ceiling limits. This can be sensitivity to measurements, attenuation of load disturbances, ability to follow the set-point as well as reliable operation under model uncertainty. The test proceedings specially setup to cover all above aspects. Primary objective of the conducted tests was to ensure the PLC based turbine governor controller has acceptable operation characteristic for controlling a Francis type hydro turbine. In addition to conventional control system test proceedings, the performance of new governor system was compared with the performance signature of existing governor control system.

Accuracy and scale of analog measurements was tested before examine the system dynamic performances. Span of numerical values for both integer and floating point numbers in control program is limited to the range of +4095 and -4096 to fit with 12bit resolution in input and output modules. Test results of measurement of generator frequency figure 6(a) and active power output are shown in figure 6(b). The mean value of error in frequency measurement 0.79% and that of the active power measurement is 0.9%.

Ability to follow the set-point was tested by adjusting the feed-forwarded active power set-point of the control system. During this test the PID control loop was disabled to avoid the PID reaction on system output. Gate opening against feed-forward set-point is plotted in figure 7(a) and actuator step response to a step input of set point is indicated in figure 7(b). Results on figure 7(a) confirms that the actuator opening is linearly following the set-point command and figure 7(b) shows the acceptable stable dynamic response to the given step inputs of power setter. Actuator

Figure 6(a) - Governor frequency measurement Figure 6(b) - Governor active power measurement

Figure 7(a) – Actuator position vs set-point Figure 7(b) – Actuator step response

For the purpose of initial dry tuning of PID parameters the system is modified as shown in figure 8. The gate position feedback connects to the controllers actual power measurement input to simulate the active power signal. This approach provides the opportunity to simulate the governor system without rotating the generator.

Tuning of PID parameters done by following the Ziegler-Nichols closed loop tuning method as described in methodology on this paper. Configured system as shown in figure 8 was simulated by injecting frequency steps of 1% and the power set-point was fixed as 45MW. Figure 9 shows the steady oscillation when proportional gain reaches to the ultimate gain of the system and the ultimate period was found by analysing the figure 9.

According to the Ziegler-Nichols closed loop ultimate gain method, suitable initial values for PID parameters found the values are tabulate in table 1.

Ultimate gain (Ku) = 0.53 Ultimate time (Tu) = 0.40 PID parameters P=0.6 Ku Tu/2 Tu/8

Pgain =0.32 Igain=0.20 Dgain=0.05

Controller response for 1% frequency step with initial PID parameters is shown in figure 10(a) and 10(b). Results obtained by proceeding the same test to the existing turbine governor control system is shown in figure 10(c) and 10(d). When comparing the figure 10(a) with figure 10(c) it is clear that gate opening time with initial PID parameters (310s) is much larger than that of existing governor which is 185s for the given step input. Figure 10(c). Proportional and deviation outputs of figure 10(b) are much lower than that of the existing

-

∆ .

+

+ +

∆

∆

∆ .

+ -

-

+

+ +

-

Figure 8 – Modified system for PID tuning

Figure 9 – Sustained steady oscillation of the system during tuning

Table 1 – Initial PID parameters from Ziegler –Nichols method

governor response figure 10(d) resulting a longer response time to settle to the newer state. The result implies that the initial PID parameters should fine tune to match the PLC controller output with existing governor output. Necessary fine tuning was carried out on trial and error method to achieve a similar response from PLC governor controller. Final fine tuned values of PID parameters listed in table 2. The performances of PLC governor controller after fine tune is shown in figure 10(e) and 10(f). The output of the system after fine tuned is approximately similar to the existing governor system.

Ultimate gain (Ku) = 0.53 Ultimate time (Tu) = 0.40 Initial PID parameters Pgain =0.32 Igain=0.20 Dgain=0.05 Fine tuned PID parameters Pgain =0.42 Igain=0.28 Dgain=0.12

Table 2 – Fine tuned PID parameters

Figure 10(a) - Step response with initial PID parameters Figure 10(b) – Magnified view of figure 10(a)

Figure 10(c) - Step response of existing governor system Figure 10(d) – Magnified view of figure 10(c)

Frequency step (1%)

Deviation (dev)

Gate opening

PD output

Position reference Y0

Integrator output (I)

Frequency step (1%)

Deviation (dev)

Gate opening

PD output

Position reference Y0

Integrator output (I)

Integrator output (I)

Deviation (dev)

Frequency step (1%)

Integrator output (I)

Gate opening

PD output

Deviation (dev)

Frequency step (1%)

PD output

Integrator output (I)

Gate opening

Governor speed droop operation tested by using the same test configuration illustrated in figure 8. The governor system response for both speed regulation settings Ep1 and Ep2 tested and results are shown in figure 11(a) and 11(b). Existing governor system has speed droop of 2.4% for Ep1 and 9.2% for Ep2. The new controller under test was shown speed droop operation of Ep1 as 2% and 7.1% as Ep2.

050

100150

49.2 49.4 49.6 49.8 50 50.2 50.4

Frequency (Hz) Vs Gate position (%) Ep1-Speed regulation

Frequency (Hz) Vs Gate position (%)

0

50

100

47.5 48 48.5 49 49.5 50 50.5 51 51.5 52

Frequency (Hz) Vs Gate position (%) Ep2-Speed regulation

Frequency (Hz) Vs Gate position (%)

Figure 10(e)- Step response after fine tuned Figure 10(f)- Magnified view of figure 10(e)

Figure 11(a) - Speed droop of Ep1 (2%)

Figure 11(b) - Speed droop of Ep2 (7.1%)

Deviation (dev)

Frequency step (1%)

PD output

Gate opening

Integrator output (I)

Deviation (dev)

Frequency step (1%)

PD output

Gate opening

Integrator output (I)

6. Conclusion

The paper has aim to investigate the possibility of developing a turbine governor controller for Francis type hydro turbine by using a programmable logic controller to retrofit the outdated governor control systems at hydro power plants. Methodology of developing a governor control system and techniques followed during implementing a PID regulator using a PLC is discussed. Although the work is targeted a governor system for Kotmale hydropower plant, Sri Lanka the generalized methodology and implementation techniques are possible to use with any hydro governor control system implementation. Practical method of tuning a PID regulator is discussed and results confirmed the successful approach followed for PID tuning. Dry test results conclude that the designed control system is viable for controlling a Francis hydro turbine but investigating the dynamic response during actual generator rotation is essential and yet to be complete. Implementing redundancy algorithms, sensor fault tolerance methods are some of the areas left for further studies.

7. References

[1] Ceylon Electricity Board. 2014 CEB Statistical Digest Report, http://www.ceb.lk/publications, www.ceb.lk, Ceylon Electricity Board, Colombo 002000, Sri Lanka.

[2] Ceylon Electricity Board. 2013, Mahaweli Complex Annual Report, http://www.mahawelicomplex.lk/Reportann.aspx

[3] Michal Malek, Pavol Makys, Marek Stulrajter, 2011 Feedforward Control of Electrical Drives Rules and Limits, 2011 advanced in Electrical and Electronic Engineering, Volume:9, Number 1, 2011 March.

[4] J.Culberg, M. Negnevitsky, M.A. Kashem, (2006) Hydro-turbine governor control: theory techniques and limitations Research Online, University of Wollongong, Faculty of Engineering and Information Science, http://ro.uow.edu.au/eispapers/4405.

[5] IEEE std 125-2007 IEEE Recommended Practice for Preparation of Equipment Specifications for Speed-Governing of Hydraulic Turbine intended to Drive Electrical Generators.

[6] Abhijit Chakrabarti, Sunita Halder, Power System Analysis Operation and Control, Third Edition 2010, Published by PHI Learning Private Limited, New Delhi, India. ISBN-978-81-203-4015-2.

[7] K.J. Astrom, T.Hagglund, 2004 Revisiting the Ziegler-Nichols step response method for PID control. Journal of process control 14(2004) 635-650, https://www.sciencedirect.com