Inter-Area Oscillation Damping

Low frequency Inter-area oscillations are detrimental to the goals of maximum power transfer and optimal power flow. An available solution to this problem is the addition of power system stabilizers to the automatic voltage regulators on the generators. The damping provided by this technique provides a means to minimize the effects of the oscillations.

Inter-Area oscillations result from system events coupled with a poorly damped electric power system. The oscillations are observed in the large system with groups of generators, or generating plants connected by relatively weak tie lines. The low frequency modes (0.1 to 0.8 Hz) are found to involve groups of generators, or generating plants, on one side of the tie oscillating against groups of generators on the other side of the tie. These oscillations are undesirable as they result in sub-optimal power flows and inefficient operation of the grid. The stability of these oscillations is of vital concern.

Although Power System Stabilizers exist on many generators, there effect is only on the local area and do not effectively damp out inter-area oscillations. It can be shown that the inter-area oscillations can be detected through the analysis of phasor measurement units (Phasor Measurement Unit) located around the system. In a typical implementation, one or more of the generators in a system are selected as Remote Feedback Controllers (RFC Controller). The RFC Controller received synchronized phasor measurements from one or more remote phasor sources. The RFC Controller analysis the phase angles from the multiple sites and determines if an inter-area oscillation exists. If an oscillation exists, a control signal is sent to the generator’s voltage regulator that effectively modulates the voltage and effectively damps out the oscillations.

To overcome the inter-area oscillation, new equipment such as Static Var Compensator (SVC) and various Flexible AC Transmission System (FACTS Device) devices, are being increasingly used. These techniques have become possible due to the recent advancement in power electronic technology. The involvement of SVC and FACTS Device in transmission network is through the so-called Variable Series Compensation (VSC Controller). Besides the FACTS Device devices, the application of Super-Conducting Magnetic Storage (SMES Device) to enhance the inter-area oscillation damping is also reported.

The key to coordinate RFC Controller, VSC Controller and various controllers is the using of Phasor Measurement Unit synchronized with the Global Positioning Satellite (GPS).

The natural frequency and damping of the inter-area mode depends on the weakness of the tie and on the power transferred through the tie. The action of a dc link, parallel to the ac tie, is to strengthen the tie. Connection of two areas, through a dc link alone, does not introduce an inter-area mode owing to the asynchronous nature of a dc tie. Therefore, the inter-area instability is avoided. Indeed, that is one of the reasons for the growth of dc links.

Steps

#	Event	Name of Process/Activity	Description of Process/Activity	Information Producer	Information Receiver	Name of Info Exchanged	Additional Notes	IntelliGrid Architecture Environments
1.1	Any power system state	Send Phasor Measurement	At all times, the PMUs in the field shall synchronously send phasors to the RFC Controller and VSC Controller.	Phasor Measurement Unit	RFC Controller, VSC Controller	Synchro Phasor	Synchro phasors must be received at a rate of up to 60 phasors per second. Security is not crucial, however, data integrity is paramount, i.e., no faulty data shall be accepted.	Intra-Control Center
1.2	Power system disturbance (e.g. fault)	Coordinate Global Control	PMUs continue to send phasor data. Upon detection of df/dt > e, trigger a storage local storage event; RFCs continue to synchronously receive phasor data and also detect the system event; The RFC Controller/VSCs, upon detection of the event, will trigger local data storage and coordinate the control action in order to counteract the detected inter-area oscillations.				The communication requirements differ from algorithm to algorithm. For the decentralized control, the system is decoupled with known system states.	NA
1.3A	New control calculate	Generator Voltage Control	The RFC Controller, having detected an inter-area oscillation and having computed an appropriate control action, sends the control information to the voltage regulator	RFC Controller	AVR Controller	Controller Settings	Voltage regulator control information shall be issued at the same rate as the synchro voltage receive rate	Intra-Control Center
1.3B	New control calculate	FACTS Device Control	The VSC Controller, having detected an inter-area oscillation and having computed an appropriate control action, sends the control information to the FACTS Device system	VSC Controller	FACTS Device	Controller Settings		Deterministic Rapid Response Intra-Sub
1.3C	New control calculate	SMES Device Control	The SMES Device SMES Controller, having detected an inter-area oscillation and having computed an appropriate control action, sends the control information to the FACTS Device system	SMES Device SMES Controller	SMES Device	Controller Settings		Deterministic Rapid Response Intra-Sub

Transmission Operations - Inter-Area Oscillation Damping

Contents

Narrative

Steps