ALSPA Adaptive PSS - Technical Description

ABCD POWER SYSTEMS Power Automation & Controls

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Technical description - Adaptive PSS Study - Rev A Sheet : 1 / 5

ADAPTIVE PSS STUDY

1. ADAPTIVE PSS DESCRIPTION

1.1. GENERAL

Power System Stabilizers (PSSs) are the most usual way to enhance the dynamic stability of Power Systems. However, the system may be demanded to operate under conditions very different to those the conventional PSS was designed for, thus degrading its performance. The ability of an adaptive controller to automatically and continuously adapt its behaviour to external changes makes the integration of a huge amount of volatile renewables (solar PV, Wind) in power networks possible. Damping active power oscillations created by intermittent resources on a transmission line is becoming a great challenge for grid operators and have an important impact in the working conditions of the conventional installed base.

Conventional PSS function are defined and parameterized to optimize the damping of the machine active power toward the grid knowing a specific configuration of the network. When the condition of the grid are different from the one of the considered tuning operating point, the damping effect of the PSS gradually decreases as shown an below figure.

Conventional PSS damping under external reactance variation

On above figure the conventional PSS 2A/2B function was defined and tuned for an external reactance of 0.2 pu. Operating under these grid configuration, the PSS function damps the local mode of oscillation in a very optimized manner when a voltage step variation is applied. However, increasing the value of the external reactance (on the graph up to 0.6pu), the active power oscillation are gradually less and less damped. When the grid presents an external reactance of 0.6pu to the machine, more than 5 oscillations are visible on the active power.




1.2. ADAPTIVE PSS

On the basis of the above analysis and observations, Alstom Power Automation & Controls has designed an adaptive control solution, so called APSS, to adapt and tune automatically the parameters of the system to ensure an optimized damping.

SG network

Measurements

Connection impedance

identification

Tuning selection

PSS correction calculation

Voltage regulation

reference +

+ -

stator voltage stator current active power reactive power compensated frequency

connection impedance

Accelerating power

calculation

PSS2B

gain lead-lag filters

The adaptive control is a set of techniques intended to automatically adjust the parameters of the controller, when the process characteristics and its disturbances are unknown or varying in time. To achieve this, the adaptive PSS needs to :

track any changes in the network parameters, Identify the model of the system, Adapt the parameters of the corrector




In order to keep a proven PSS structure, our solution uses the existing PSS 2B according the IEEE standard 421.5 2005. Alstom Adaptive PSS solution is not requiring heavy method with complex calculation using high CPU capacity. Our APSS is simply based on a division of the reactance variation range into 5 different areas. Several sets of PSS parameter values are defined so that in each area, the PSS presents an optimized behaviour.

connection impedance

PSS correction calculation

Accelerating power

calculation

PSS2B

gain lead-lag filters

parameters set #1

parameters set #2

parameters set #3

parameters set #4

parameters set #5

Then, depending on the estimated value of the reactance, the adequate PSS tuning is applied on-line.

Adaptive PSS damping under external reactance variation




2. STUDY

2.1. STABILITY STUDY Operating conditions of a power generator are continually changing due to load changes, faults and many other disturbances, affecting the maximum power transfer capability. The purpose of this study is to define the optimum sets of parameters that will be uploaded in the Automatic Voltage Regulators (AVR+APSS) ControGen HX, to limit the impact of those disturbances by ensuring a positive damping on the produced torque oscillations. Alstom Adaptive Power System Stabilizer (APSS) algorithm is using PSS 2A/2B in accordance with IEEE 421.5 - 2005.

2.2. SCOPE OF SUPPLY

The study will be based following the above described steps :

No load step response to show the adequate dynamical behaviour of the voltage control loop,

Compensated frequency response analyze to show the adequate phase compensation and thus giving the adapted PSS action toward the local mode oscillations as well as toward the inter area oscillations,

A voltage reference step under rated stator voltage, rated frequency and maximum active power. For this test the reactive power is equal to 0 MVAR. This test shows the damping effect of the PSS introduction.

Voltage reference steps when the generator is connected on the network under several generator operating conditions chosen on the limit of the wide possible domain. Those tests show that the PSS tuning is satisfactory when generator is under load.

A validation of the under-excitation limitation that shows the dynamical behaviour when the limitation is entered and also when limitation is OFF.




In addition to the above describe study the following work is included in the price :

Project Management,

Project Engineering (study),

Quality control and administration according to Alstom Power Automation & Controls Internal rules,

To realize this study a document giving different parameters will have to be completed by the customer. The needed parameters are listed according the following split :

Generator parameters,

Line shaft parameters,

Grid characteristics,

Excitation parameters,

Also some additional information will be required such as :

Generator characteristics (open circuit saturation and air gap line),

Generator reactive capability curve,

For brushless exciter : open circuit and load characteristics,

Turbine and its Governor Model could also be implemented in our simulation as an option (if large perturbation study are specifically requested).

This document will be sent when the contract shall become effective.

As the study and its results will be based on those parameters, in case of missing parameters, Alstom will consider standard values and could not be considered as responsible of any discordance of the final results. If for any reason customer decides to modify any value of the original parameters, the extra study costs will be subject to an amendment and invoiced accordingly.

2.3. STUDY REPORT

To complete the study of the Power System Stabilizer a report will be established. This report will include the following :

The stability study document giving the whole results and performances reached. The following curves will be included in the report :

Stator voltage, Active power, Reactive power, Field voltage, Field current,

The transfer function document where the model and parameters values of the regulator main function are given in a Laplace representation.

Documents

ALSPA Adaptive PSS - Technical Description