Analysis of Power Oscillations and Their Treatment in Hydro Power Plant with Large Bulb Turbines

Miljenko Brezovec HEP-Proizvodnja d.o.o.

Varazdin, Croatia miljenko.brezovec@hep.hr

Igor Kuzle Faculty of Electrical Engineering and Computing

Zagreb, Croatia igor.kuzle@fer.hr

Abstract—The paper describes a specific case of power oscillations problem of hydroelectric power units with double regulated bulb turbine. Results of previous research are summarized and discussed. Measurements of characteristic quantities of generator and turbine are presented and analyzed in detail for few cases. Different aspects of the phenomenon are discussed and explained, and finally treatment of oscillations during operation is defined.

Keywords— electromechanical oscillations; hydroelectric unit; bulb turbine

I. INTRODUCTION Because of many different instability reasons, power unit

local stability problems should be analyzed in detail for each specific case. Power oscillations of hydro units have been present on the HPP Dubrava since the first commissioning of hydropower plant (1989). There are two identical hydroelectric units (rated power 39 MW) with bulb turbines (runner diameter 5.5 m, speed 125 rpm, rated flow 250 m3/s, head 16-20m).

Power oscillations in normal undisturbed operation are negligible in all operating conditions, and disturbances which result with increased power oscillations appear occasionally on both machines [1]. Amplitude of oscillations depends on active power and reaches its maximum near nominal power. Frequency of oscillations corresponds to rotor speed regardless of the operating conditions.

II. DESCRIPTION OF THE PHENOMENON Power swings in hydroelectric power plants have been

analyzed since 1930s [2]. Except power swings, many different types of vibration and oscillations appear in operation of turbine-generator units in hydroelectric power plants, with different causes and consequences [3]. Flow-induced pulsation and vibration in hydroelectric machinery and related oscillations are analyzed in [4] and [5].

Different researches of the HPP Dubrava power oscillations were carried out and two main theories have been discussed, but certain cause of such disturbances is still not proven.

One approach to the explanation of this phenomenon is that units loaded near maximum power could enter resonance mode [6], [7]. Natural frequency of electromechanical oscillations strongly depends on working conditions and near full active power natural frequency is close to the rotor speed frequency. In such conditions, disturbance of mechanical torque produce power oscillations which could be significantly amplified due to electromechanical resonance.

Second approach starts from mechanical torque as the main cause of oscillations because in same operating conditions power oscillations are negligible until step disturbance appears. There are indications that mechanical torque disturbances are connected with the turbine cavitation [8].

It should be emphasized that plant operates with tailwater level lower than designed [9] what increases cavitation risk.

Peak-to-peak values of power oscillations are usually up to 3% of nominal power, but few cases with 10% and more are also recorded. To decrease big power oscillations when it appears, power of the unit should be lowered and the only solution to eliminate power oscillations in most cases is stopping the unit. Such cases appear occasionally and each unit should be stopped because of such behavior for more than 10 times in one year.

Probability for increased power oscillations appearance is bigger if second unit is not in operation and specially during trash rack cleaning, what indicates that mechanical torque disturbance could be a trigger for such behavior.

III. CHARACTERISTIC CASES To illustrate the nature of the phenomenon few

characteristic cases will be presented and shortly described.

Machine condition monitoring system is extended and upgraded with additional functionality developed specially for analysis of power oscillations. With the help of the monitoring system, large number of measurements could be analyzed for each case. The basic insight in power oscillation appearance is obtained when specially designed transient recorder is implemented. This enables recording high resolution measurements and post-processing of the waveforms. Furthermore, tools for signal processing (filters, averaging) are implemented in the monitoring system what enables fast and simple analysis.

Another disturbance recorder is implemented within excitation system, which is triggered when power system stabilizer (PSS) is switched off because of high power oscillations. In this way, many different power oscillations were recorded for last few years.

Fig. 1 and Fig. 2 show typical power oscillation which appears during normal steady state operation near rated power. In this case, amplitude of sudden oscillations was 1.7 MW (1st harmonic peak-to-peak value).

Because of such high oscillations active power had to be decreased immediately and unit had to be stopped to eliminate the cause of instability. After starting and loading on the same power as before, power oscillations were negligible.

On Fig. 3 there are detailed measurements and waveforms of electrical quantities recorded with excitation control system. Those records show detail of active power oscillations sudden increase and excitation system control quantities during the disturbance. Active power oscillations correspond to generator terminal current oscillations, and reactive power oscillations correspond to generator terminal voltage oscillations. First harmonic of active and reactive power oscillations has opposite phase.

Fig. 1. Power oscillations appearance (unit B, 18.02.2018.) - upper diagram (D1): active power (MW); lower diagram (D2): power oscillations (MW)

Fig. 2. Power oscillations (MW) waveform recorded with monitoring system

Fig. 3. Waveforms of electrical quantities recorded with excitation system (terminal voltage Ug, active power P, reactive power Q, exciter excitation voltage Uff and exciter excitation current Iff )

IV. POWER OSCILLATIONS ANALYSIS

A. Active Power Waveforms In all cases, power oscillations frequency is equal to rotor

speed frequency with dominant first harmonic, and in previous research, oscillations were considered as sinusoidal disturbance. After the waveform recording was enabled, more attention is paid to waveform differences when power oscillation appears depending on operating point. Analysis of many waveform records has shown that waveforms deviate from assumed sinusoidal form depending strongly on operating point. Although there are many different waveforms, basic samples are repeated in most cases.

Regardless of sinusoidal form deviations, first harmonic (2.083 Hz) remains dominant. For active power between 33 and 37 MW second harmonic could be important. Typical results of active power signal spectral analysis is showed on Fig. 4. Except changes of first harmonic amplitude, there is a signal phase change what could be seen on Fig. 5 which shows power waveforms and trigger (marker signifying one revolution of the turbine-generator). First harmonic amplitude and phase as a function of active power are shown on Fig. 6.

Fig. 4. Spectral analysis of power oscillations

Fig. 5. Waveforms for three operating points (38, 35 and 33 MW)

Fig. 6. First harmonic amplitude (upper diagram) and phase (lower diagram) depending on active power

B. Influence on Turbine Efficiency At the moment of power oscillations beginning, a power

failure occurs what is evident as an average value decrease (Fig. 7). The same phenomenon of power failure is apparent on trend diagrams (1 point for each 2 seconds) as on Fig. 8.

Fig. 7. Power failure at the moment of power oscillations (red – basic signal, blue – averaged signal)

Fig. 8. Correlation of active power failure (upper diagram) and power oscillations (lower diagram)

Power value difference depends on the size of the disturbance, and in this specific case equals 0.35 MW. When analyzing many cases, it was established that the ratio between first harmonic amplitude and value of power failure is between 1.5 and 2.5. It should be emphasized that turbine opening and flow in analyzed cases remained unchanged.

C. Comparison with Oscillations During Power System Disturbances When power unit operates connected to the power system,

transients due to transmission system disturbances could appear. Typical responses of the electrical quantities of the unit with the action of AVR on disturbance in power system are shown in Fig. 9. In this specific case, one of two 110 kV transmission lines connecting the plant was switched off.

Analysis of power signal waveform during the transmission network disturbance (Fig. 10) show that dominant frequency is bigger than rotor speed frequency, and spectral analysis (Fig. 11) shows that frequency is 2.84 Hz what correspond to calculated natural frequency of electromechanical oscillations.

Fig. 9. Response on transmission line switching off (active power - red, reactive power - blue, exciter excitation voltage – green, exciter excitation current – magenta)

Fig. 10. Waveform od the power signal during the transmission network disturbance correlated with trigger (marker position)

Fig. 11. Spectral analysis of the power signal during the transmission network disturbance

D. Effects of Automatic Voltage Regulator There was a lot of discussion about possible electric causes

of power oscillations and about automatic voltage regulation influence. Many specialized measurements and testing of excitation regulation system were performed. Among other, special test was made using backup regulation system with constant exciter excitation voltage (Fig. 12). Such operation mode is provided for the case of voltage regulation fault and in such case there is a backup excitation current regulation (sometimes is called manual regulation).

Fig. 12. Waveforms in manual and automatic voltage regulation (generator current Ig, terminal voltage Ug, active power Pg, reactive power Qg, exciter excitation voltage Ufu)

Transition from backup (manual) to automatic regulation is recorded during first commissioning, and it is obvious that there were power oscillations in both modes, even more that oscillations don’t depend on voltage regulation mode.

E. Effects of Power System Stabilizer Power System Stabilizer (PSS) is implemented as

additional part of microprocessor based digital voltage regulator. Stabilizer has a structure of IEEE type PSS2B, and parameters are defined according to recorded phase characteristic of the brushless excitation system [10]. The main goal was to compensate the phase as much as possible for dominant frequencies of power oscillations (in range from 1 to 3 Hz).

PSS acts on exciter excitation voltage, and in this way oscillations are partially transferred from active power signal to the voltage regulation signals (generator terminal voltage and reactive power).

Because of this, it was necessary to make a compromise between achieved damping of active power oscillations and introduced exciter excitation voltage oscillations.

Partial damping of permanent electromechanical oscillations is accomplished, and for bigger damping higher gain should be set, but then negative impact on oscillations of reactive power and terminal voltage would be too big.

Effects of PSS action are most obvious when switching on (Fig. 13) or switching off the stabilizer (Fig. 14) when power oscillations are present.

Additional improvements in PSS action by parameter settings tuning are probably possible, but with existing algorithm PSS2B based on calculated speed signal these possibilities are limited. Improvements are possible by using advanced algorithm for which advanced high-quality speed measurement is needed, and existing measurement doesn’t have sufficient accuracy.

Regardless of these additional features, the conclusion remains that with the presence of forced oscillations, active power oscillations could be reduced by PSS action but with increase of reactive power oscillations.

Fig. 13. PSS switching on (31 MW)

Fig. 14. PSS switching off (35 MW)

F. Draft Tube Pressure Pulsation To identify phenomena in water flowing through the

turbine, continuous measurement of pressure pulsations in draft tube is implemented. Pressure pulsations were measured during turbine commissioning, but as for other measured quantities it is important to have continuous measurement and automatic transient records including the waveforms. Pressure sensor is mounted on conical metal part of the draft tube, downstream of the runner in upper position. In normal conditions pressure pulsates in regular rhythm 4 times per turn, what corresponds to passing of the runner blades. When disturbance appears, uniform pulsation rhythm 4 times per turn is changed and additional component appears with frequency 1 time per turn (Fig. 15).

From pressure pulsations, waveform of the mechanical moment could be approximately identified.

Fig. 15. Draft tunbe pressure pulsation change at the moment of distrubance (active power - red, draft tube pressure - blue, draft tube pressured averaged signal – magenta, trigger – green)

V. FLOW-INDUCED PULSATION AND VIBRATION IN BULB TURBINE

Because of their construction and horizontal arrangement, there are different causes of pulsation and vibration in bulb turbines. The entire unit is usually fixed by upper and lower stays and lateral supports and axial asymmetry of inner flow field could occur. The pressures pulsation produced by blade undergoes cyclic process what results in unit vibration.

Because of horizontal position and large runner diameter, the water gravity has a significant influence on the internal flow of the turbine. There is vertical pressure gradient caused by gravity which must be taken in analyses [11].

The pressure above the free surface is constant atmospheric pressure, and the pressure increases with the water depth. The hydrostatic pressure difference caused by the gravity of water gives hydraulic imbalance of flow. The inhomogeneous pressure distribution results in pressure variation on the blade during rotation.

Fig. 16. Pressure distribution in the bulb turbine [11]

Due to hydrostatic pressure a cavitation on upper blades is

much more intensive than at the bottom ones. During rotation cavitation intensity at each blade increases when the blade rotates up and decreases when it rotates down [12], [13].

The asymmetric locality and periodicity of cavitation lead to the hydraulic imbalance and oscillation.

Fig. 17. Pressure distribution and attached cavitation at suction side of bulb turbine runner blades [13]

In this regard, assumption about disturbed flow on one runner blade and cavitation connected phenomena whose intensity is changing with rotation of the blade become completely realistic. All aspects of analyzed cases correspond to such interpretation of mechanical torque oscillations.

VI. CONCLUSION Power oscillations phenomenon on HPP Dubrava are

described and analyzed using measured data. Treatment of those specific disturbances during plant operation is presented.

Different disturbance cases are presented and analyzed from aspects of mechanical torque disturbance as a cause, and electromechanical oscillations as an outcome.

Waveforms for different disturbance cases are analyzed based on spectral analysis of measured signals. Comparison with transients caused by electrical faults is described.

Flow-induced pulsation and vibration in bulb turbine as one of possible causes is analyzed according to available data. Cavitation characteristics of low head large bulb turbine and correlation with oscillations are discussed.

According to analysis of many different disturbance occurrences, power oscillations are identified with clearly defined assumptions on physical phenomena which cause the mechanical torque disturbance. Basic assumptions are proven to be true, and other could be proved with additional research of cavitation during operation of the units, and this is complicated because of characteristic of the phenomenon.

Periodical disturbance of mechanical torque is defined as main cause of power oscillations, and the key proofs are:

power failure at the moment of oscillations occurrence indicates flow disturbance which causes turbine efficiency decrease in the same operating point,

natural frequency of electromechanical oscillations and rotor speed frequency are sufficiently different what is obvious during transmission network disturbance, and

additional confirmation is that in operation of almost identical units on HPP Cakovec there is no power oscillations,

waveforms of the active power deviate from sinusoidal form, and changes of the waveforms at different power indicate connection with the cavitation,

change of draft tube pressure pulsations at the moment of power oscillations occurrence correspond to change of mechanical torque,

effect of PSS operation is manifested as transferring a part of oscillations from active to reactive power – this is diminishing the consequences of forced oscillations caused by mechanical torque disturbance.

Future research will be therefore focused on flow-induced pulsation and cavitation advanced measurements and analyses.

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