Proceedings of 2010 IEEE Student Conference on Research and Development (SCOReD 2010), 13 - 14 Dec 2010, Putrajaya, Malaysia

EFFECTS OF MECHANICAL POWER FLUCTUATION ON POWER SYSTEM

STABILITY

*W S Kim', H Hashim', Y R Omar', I Z Izham' 'College of Electrical Power Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, 43000, Selangor,

Malaysia malcom _ wee87@hotmail. com, {halimatun, yaakob, izham}@uniten.edu.my

Abstract - Fluctuation of mechanical power is

known to be one of many major factors affecting

stability of a power plant and the power system

network connected to it. The occurrence of this

fluctuation is due to the inconsistent input supply

to the prime mover, mechanical failure and

control system itself such as the process of quick

operation of steam turbine governing, which

would cause the pressure oscillation in turbine

steam inlet pipeline. Consequences of this

fluctuation may trigger situations such as power

system low frequency oscillation and loss of

synchronism, which can occur if the speed

oscillation experienced by the machine exceeds its

allowable limits. As a result, protection system will

react, which causes the generator to be taken off

line from the system. This paper presents the

evaluation on the effects of mechanical power

fluctuation on the power system stability based on

turbine gain in static and dynamic load

environment.

Keywords - mechanical power, turbine gain, dynamic load, system stability.

I. INTRODUCTION

Stable operation of a power system requires a continuous match between energy input to the prime movers and the electrical load of the system. Any change of load demand must be accompanied by a corresponding change in terms of the amount of input power given to the turbine system. If the mechanical input power does not rapidly match the electrical power load demand, the turbine speed of the generator will not be at its optimal speed hence frequency and voltage will deviate from nominal values [1, 2, 3]. In addition, when a power system is subjected to significant transient disturbances, the machine may lose synchronism to the system if its

angle exceeds ± 1800• Voltage violation may occur if the value is out of limit, which is 0.95 to 1.05per unit. The system frequency may deviate widely from

978-1-4244-8648-9/10/$26.00 ©2010 IEEE 297

normal value of 50Hz. In cases like this, system stability would be dependent upon various factors such as fault clearing time, type and location of faults, and the stability countermeasures [3, 4, 5].

This is made possible with the use of control equipments, such as governor, generator voltage regulator (excitation system) and the tie-line load­frequency-control equipment, which senses deviations and act to restore frequency and voltage to acceptable values. Control equipment plays a vital role to ensure mitigation of any possible system collapse scenarios. Unfortunately, the control device has its limitations as such that it allows oscillations (periodic variations) around the desired conditions. In most cases these oscillations diminishes; hence, the system will remain stable [3].

II. CONTROL SYSTEM

The response of each power plant to abnormal conditions in terms of energy input to the prime mover is depending very much on the type of plant such as thermal, gas turbine, and hydro. Thermal is considered to be the fastest to reach steady state condition, followed by gas and hydro power plant as shown in Figure 1. Hence, thermal and gas turbines can be quickly synchronized to the grid system compared to the hydro turbine [3].

Speed

�� HYdrO Gas Therma' -

..... Time

Figure 1: Turbine Speed on Hydro, Thermal and Gas Power Plant

Figure 2 shows a typical generator control system. When load is lost and the energy input to the system exceeds the electrical load, the generator will start to speed up. Governor provides control of mechanical power injected to the machine by sensing shaft speed. The amount of fuel required is adjusted in proportion to the equivalent amount of output power to be generated [6, 7].

Mechanical (tu rbine) powe Angle, {j

Terminal voltage, VT Generator Speed, W

Terminal voltage, V

Current injection, C I�us terminal Volta.e. V

I excitation

EFD Controller .- Vs (other signals)

I Governor I"

Figure 2: Relationship between Machine States and

Controller Quantities [8]

When sufficient time has elapsed after a disturbance, the governors of the prime movers will react to increase or decrease energy input accordingly; thus, reestablishing a balance between energy input and the existing electrical load. This usually occurs in about 1 s to l.5s after the disturbance. The period between the time when the governors begin to react and the time when steady­state equilibrium is reestablished is the period during which dynamic stability characteristic of a system is effective [3, 4].

III. METHODOLOGY

In this paper, the effect of mechanical power fluctuation on power system stability based on the IEEE 118 Bus Test System was analyzed using PSSE™31 software. The network system consists of:

• Three different types of power plant: hydro, coal and gas.

• Four different types of loads namely static load, composite load, and synchronous and induction motors simulated to run at steady state and starting conditions.

298

To replicate the fluctuation of mechanical power, hydro governor (HYGOV) model provides the variation in turbine gain, At where the normal rating condition is set to 1.2 pu. The values used for the simulation are within the range suggested by PSSE (HYGOV, 0.8 < At < 1.5). The adjustment of the turbine gain will result in an instant decrease or increase in mechanical power depending on the value of gain adjusted before it gradually tries to regain stability. However, the fmal value of the machine mechanical power will settle slightly lower than the initial state due to the different set point of turbine gain. To comprehend the impact of fluctuation of mechanical power, two conditions are monitored, during sudden drop and sudden increase of mechanical power.

Simulations are carried out on the system at steady state and abnormal conditions, and the parameters are observed at selected buses as shown in Table 1. The performance of synchronous motor and induction motor is also analyzed by monitoring selected parameters as shown in Table 2.

Table 1: Monitored buses

Bus Number Connected To

1 Hydro Plant

80 Static load

1001 Synchronous motor

1003 Induction motor

Table 2: Types of load and monitored parameters

Type of Load Monitored Parameter

Machine speed

Synchronous Machine mechanical power

motor Machine angle

Machine speed Induction motor

Machine angle

IV. RESULTS AND DISCUSSIONS

Referring to Figure 3a and 3b, a decreased or increased in mechanical input power through increasing the turbine gain (At) causes the speed of the generator to decrease and increase respectively.

They have a direct proportional relationship based on the following equation:

Pm = T * W (1)

- Pmech - Speed

Figure 3a: Turbine gain, At = 1.1 pu

r�--", i i

! --------------/ ____________ J

/ / j

-- Pmech - Speed

Figure 3b: Turbine gain, At = l.3pu

Figure 3: Mechanical Power and Speed at different turbine gain

Figure 4a and 4b represent the behavior of the electrical and machine parameters at Bus 1 due to the introduction of fluctuation of mechanical power. Bus 1 is connected to a hydro power plant that generates an output power of 80MW.

As governor tries to stabilize the machine speed, the machine rotor angle will usually oscillate with damped oscillations until it settles down to a new angle corresponding to the new requirements. For a turbine gain of 1.1 pu, the angle continues to decline, similarly with the bus angle. This influences the bus frequency to reduce and oscillate for a short period before it resolves at lower than 50 Hz. However, when the turbine gain is increased to 1.3pu, the behavior is opposite. For both cases, the system might be transiently stable but dynamically unstable.

299

Bus Angle - Voltage Speed - Pmech

Frequency Machine Angle

Figure 4a: Turbine gain, At = 1.1 pu

.... -.... . --- ... - .... -.-- .. . . - .......... ! �--------I . : :-: ==-:=::-: :::-:=-=:.:.:�: �r·

Bus Angle - Voltage Speed _ Pmech

Frequency Machine Angle

Figure 4b: Turbine gain, At = l.3pu

Figure 4: The behavior of selected parameters at Bus 1, generator bus

Figure 5a and 5b indicate the behavior of the angle, voltage and frequency at Bus 80, which is connected to the largest static load in IEEE 118 Bus Test System.

. --... __ .... -co-o ••

-----�.

- Voltage - Angle - Frequency

Figure 5a: Turbine gain, At = 1.1 pu

Voltage _ Angle _ Frequency

Figure 5b: Turbine gain, At = l.3pu

Figure 5: System behavior at Bus 80 with static load

Voltage magnitude for both conditions in Figure 5a and 5b is not affected by the changes; however, the frequency oscillates and the angle is no longer stable. A sudden decreased in mechanical power causes the frequency to deviate for 15s before it settles at a slightly lower value due to the drop in machine speed. In contrast, the frequency settles at higher value for a turbine gain of l.3pu.

Bus 1001 and 1003 are connected to dynamic loads, namely synchronous motor and induction motor respectively. As in Figure 6 the voltage at Bus 1001 experiences minor ripples for almost lOs when subjected to a sudden change in mechanical power input to the generator. The frequency settles at lower value due to the drop in machine speed as shown in Figure 6a with bus angle continuously decreasing. During a sudden increased in mechanical power, the machine speed increases instantly causes the frequency to rise from rated value and the voltage to drop as illustrated in Figure 6b. In order to compensate the loss of frequency and to maintain a stable voltage, the bus angle has to continuously rise up.

------------------ �-----..

. _-----.-.. ....

Voltage - Angle - Frequency

Figure 6a: Turbine gain, At = 1.1 pu

300

--------------- � ___ -�_1Iiiii ,-. - " ' -

---_ .. _-- _ . . . _ ._ ----- --_._ .. __ ..•.. __ ._ ..•.• /-------

- Voltage - Angle - Frequency

Figure 6b: Turbine gain, At = l.3pu

Figure 6: System behavior at Bus 1001 with dynamic load - synchronous motor

Bus 1003 is connected to an induction motor. Referring to Figure 7, when there is a sudden variation in mechanical power, which is introduced at t = 20s, voltage angle starts to decrease or increase accordingly, while voltage and frequency experience slight oscillation but well damped at t = 36s. Similarly, the fluctuation of mechanical power gives an impact to the performance of the motor at starting condition for both types of motors, synchronous and induction motors.

- Voltage - Angle - Frequency

Figure 7a: Turbine gain, At = 1.1 pu

.,,/'/� ------------------ ��·-.. � ............ Ii : ..... • ��..o;> ...

Voltage - Angle - Frequency

Figure 7b: Turbine gain, At = l.3pu

Figure 7: System behavior at Bus 1003 with dynamic load - induction motor

Figure 8a and 8b show the behavior of synchronous motor at starting condition when there is a change in mechanical input power to the generator. The machine speed is oscillating finally stabilizing at a lower value once the mechanical input power to generator is reduced but the machine angle continues to decrease with time as shown in Figure 8a. However, in Figure 8b, when the mechanical input power is increased, the machine speed oscillates for 16s and stays slightly higher than the initial speed, while the machine angle continues to increase. In both conditions, the machine mechanical power is not affected by the presence of fluctuation of mechanical power.

Pmech - Speed - Angle

Figure 8a: Turbine gain, At = 1.1 pu

Pmech - Speed - Angle

Figure 8b: Turbine gain, At= l.3pu

Figure 8: Synchronous machine behavior at Bus IDO l

Similar behavior is observed on an induction machine when there is a fluctuation of mechanical input power to the generator as illustrated in Figure 9. The speed will stabilize to a new value, while the machine angle will either decrease or increase accordingly.

301

Angle - Speed

Figure 9a; Turbine gain, At =1.1 pu

-------------------------- ------------

Angle - Speed

Figure 9b: Turbine gain, At = l.3pu

Figure 9: Induction machine behavior at Bus 1003

V. CONCLUSIONS

Prime mover is the main source to provide the mechanical input power to tum the generator but the match of the energy input and the electrical load determine the stable operation of a power system. A well-tuned governor should give a well-damped response of both mechanical power and speed of the generator.

When there is a fluctuation of mechanical input power to the generator, the machines and system parameters are affected and the impact is propagated in the system network. Hence, it is very critical for the mechanical power to be able to meet any changes in the electrical power output as demand by the network and the control system to react rapidly, but smoothly and accurately to ensure that the electrical parameters such as frequency, angle and voltage do not deviate from the acceptable range in order to maintain system stability: transient and dynamic.

ACKNOWLEDGMENT The research team would like to acknowledge the

Ministry of Science and Technology Malaysia for research funding (03-02-03-SFO 141 and 03-02-03-SFOI87), Universiti Tenaga Nasional Malaysia, and Advanced Power Solution for the support given to this research.

REFERENCES

[1] Han Zhiyong, He Renmu and Xu Yanhui, April 2008. Effect of Steam Pressure Fluctuation in Turbine Steam Pipe on Stability of Power System. DRPT 2008 6-9 April 2008 Nanjing China.

[2] Anderson, P. M., Fouad A.A., October 2002. Power System Control and Stability: Power System Stability. Chapter 1: 1 1 1. http://ieeexp lore. ieee.org/book!

[3] Kundur, P. 1994. Introduction to the Power System Stability Problem - Basic Concept and

302

Definitions, Mid-term and Long-Term Stability. Power System Stability and Control: 33-34, McGraw-Hill.

[4] Hadi Saadat 2004. Power System Analysis, 2nd

Ed. Mc Graw-Hill Kuala Lumpur. [5] Horowitz, S.H., Phadke, A.G. 1995. Protection

Aspects of Power System Phenomena. Power System Protection, 2nd Ed., 256-259.

[6] Gilbert, E.G. and Tan, K.T., Linear Systems With State And Control Constraints: The Theory And Application Of Maximal Output Admissible Sets. IEEE Transactions on Automatic Control.v36 i9. 1008-1030.

[7] K. Kogiso, K. Hirata, 2003, A reference governor in a piecewise state affine function, Proceedings of the 42nd IEEE Conference on Decision and Control, pp. 1747-1752.

[8] Thomas E.Kissell, 1997, Industriallectronic: Solid­sta te Devices Used fo r Firing Circuits, 2nd Edition, International Edition Prentice Hall, Inc.