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POWER SYSTEM STABILIZER

The Power System Stabilizer (PSS) is a part of the excitation system for

generator control. The PSS acts to modulate the generator field voltage to damp

electrical power-speed oscillations. The need for effective damping of electromechanical

oscillations motivated the concept of the power system stabilizer

The recent increase in orders for power plants has given rise to an increase in

interconnection studies to insure these plants are integrated into the power systems.

Stability studies are important to verify adequate transient and dynamic stability of the

plant for electrical faults in the system. With the stability margins being determined by

sometimes weak interconnections

during contingency conditions, it can be difficult to insure adequate margins.

APPLICATION OF PSS

A recent example of a study of a combined cycle power plant shows the benefit

of PSS in helping to meet stability requirements.

Fig. 1- One-Line Diagram of the Simplified Study System

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The plant in this example is a multi-unit STAG plant with three combustion

turbines with generators rated about 200MVA, and a 440MVA steam turbine generator

supplied by a Heat Recovery Steam Generator (HSRG) from the combustion turbines

exhaust. A simplified one-line diagram of the plant and system interconnections is

shown in Fig.1 where the plant is connected at the end of a 345 kV transmission network.

Power may also flow through two transformers, connecting to four 138 kV circuits.

Under normal conditions, the total equivalent impedance seen from the studied plant is

0.55 pu. Fig. 2 shows the response of one of the three gas turbine-generators for a three

phase fault on the plant end of the 345 kV line, cleared by opening the line in 6 cycles.

Fig. 2 - Response of a GT Generator

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Studies showed that all three gas turbine-generators can be stable with the PSS

control, but it is necessary to trip the steam turbine generator to maintain transient

stability. However, with the loss of the 345 kV line the total equivalent impedance

becomes 1.1 pu, causing dynamic stability problems. The dotted traces in Fig. 2 indicate

a lack of damping in one of the combustion turbine generators response, assuming the

excitation system is not equipped with PSS. The solid traces show the response for the

same disturbance, when the new units are all equipped with PSS. Clearly, the PSS control

is critical in this case to insuring dynamic stability for the system in the contingency

condition.

NEED FOR PSS

The disturbances occurring in a power system induce electromechanical oscillations of

the electrical generators. These oscillations, also called power swings, must be effectively

damped to maintain the system's stability. Electromechanical oscillations can be

classified in four main categories:

Local oscillations: between a unit and the rest of the generating station and

between the latter and the rest of the power system. Their frequencies typically

range from 0.8 to 4.0 Hz.

Interplant oscillations: between two electrically close generation plants.

Frequencies can vary from 1 to 2 Hz.

Interarea oscillations: between two major groups of generation plants.

Frequencies are typically in a range of 0.2 to 0.8 Hz.

Global oscillation: characterized by a common in-phase oscillation of all

generators as found on an isolated system. The frequency of such a global mode is

under 0.2Hz.

MULTIBAND POWER SYSTEM STABILIZER

The need for effective damping of such a wide range, of electromechanical

oscillations motivated the concept of the multiband power system stabilizer (MB-PSS).

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As its name reveals, the MB-PSS structure is based on multiple working bands. Three

separate bands are used, respectively dedicated to the low-, intermediate-, and high-

frequency modes of oscillations: the low band is typically associated with the power

system global mode, the intermediate with the interarea modes, and the high with the

local modes.

Each of the three bands is made of a differential bandpass filter, a gain, and a limiter.

The outputs of the three bands are summed and passed through a final limiter producing

the stabilizer output Vstab. This signal then modulates the set point of the generator

voltage regulator so as to improve the damping of the electromechanical oscillations.

The recent increase in orders for power plants has given rise to an increase in

interconnection studies to insure these plants are integrated into the power systems.

Stability studies are important to verify adequate transient and dynamic stability of the

plant for electrical faults in the system. With the stability margins being determined by

sometimes weak interconnections during contingency conditions, it can be difficult to

insure adequate margins. With weak transmission systems there are often sustained

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power oscillations in the 0.7 - 1.0 Hz. range, following the clearing of electrical faults. In

many cases the generating units tie into strong systems and the response is stable and

well damped. Although the PSS controls is always beneficial for stability, its action can

become critical in cases where the system becomes weak due to lines out-of-service.

Faults in any power generating system that is connected to the grid can cause

widespread transmission system failure. Power system oscillation (or dynamic instability)

can result from transmission system disturbance.

The Power System Stabilizer (PSS) is a supplemental control that improves

dynamic stability by increasing damping of power, swing oscillations. Excitation systems

with high gain and fast response times greatly aid transient stability (synchronizing

torque), but at the same time tend to reduce small signal stability (damping torque). The

objective of the PSS is to increase damping of generator rotor angle swings, which can

occur in a broad range of frequencies in the power system. Low frequency modes,

commonly called intertie or interarea modes, are due to coherent groups of generators

swinging against other groups in the interconnected system. These modes are present in

all interconnected systems and their damping is a function of tie line strength and unit

loading factors. Weak ties due to line outages and heavy system loads can lead to poorly

damped intertie modes. The PSS can provide significant improvements in intertie mode

damping.