What is an Excitation System?

Excitation systems are fundamental to the operation of modern synchronous machines. In this article, we introduce the basic concepts in layman’s terms.

Excitation System


WHAT IS AN EXCITATION SYSTEM?

INTRODUCTION

1. BASIC FUNCTION

The basic function of an excitation system is to provide a continuous (DC) current to the field winding of a synchronous machine. This is achieved through the use of closed-loop control (or feedback control). Modern excitation systems also include diagnostics functions to simplify troubleshooting, communication protocols for SCADA integration, and limiter / protection functions to ensure that the synchronous machine is operated within its capability curve.

2. OBJECTIVE

The objective of an excitation system depends on the application:

  • For synchronous generators, it is responsible for maintaining a constant terminal voltage.
  • For synchronous motors, it is responsible for maintaining a constant power factor.

3. WORKING PRINCIPLE

The synchronous machine, which consists of a rotor and stator, produces AC currents using the principle of electromagnetic induction. The DC current passing through the field winding of the rotor produces a static magnetic field. As the rotor is rotated by the prime mover (such as a hydro or steam turbine), the magnetic field is also rotated. Since the rotor is contained within the stator, the rotating magnetic field creates a varying magnetic flux as it intersects the stator windings. This varying magnetic flux induces AC currents in the stator windings of the synchronous machine. An excitation system is necessary since, without excitation current, the machine operates without field current and consequently, no voltage is generated (induced) in the stator windings of the machine.

EXCITATION SYSTEM CONCEPTS

1. CLOSED LOOP CONTROL

Excitation systems use closed-loop or feedback control to regulate the machine’s output. In closed-loop control, the machine output is routed back to the controller and compared to a setpoint, and the error between the setpoint and output is used to compute the system response. The controller is typically modeled as a PID, PI or lead-lag controller.

The Reivax control loop is compatible with the ST4C model defined in IEEE 421.5. A simplified version of this control loop is shown on the Reivax HMI screen:

Closed Loop Control

The tuning parameters associated with the control loop can easily be modified directly from the HMI.  Three (3) sets of tuning parameters, allows for tailoring the optimal response for when a generator is not connected to the grid and for grid-connected mode depending if the Power System Stabilizer (PSS) is active or not :

transfer function parameters

A sample diagram of the full control loop is shown below:

Diagram Full Control Loop

2. LIMITERS AND PROTECTIONS

Modern excitation systems are responsible for protecting the synchronous machine, the excitation system itself, and other devices. Limiters (OEL, UEL, VHz, SCL) and protections (24, 27, 32, 37F, 40/32Q, 50/51, 59, 59F, 76F, 81O/U) are software features designed to limit the machine operation in undesirable conditions, and are implemented as add-ons to the AVR control loop. Limiters will ensure that the machine is operated within the machines capability at all times, while the protection functions will protect the machine by initiating a trip. The excitation protective functions are typically duplicated in a separate unit protection relay.  It is possible to disable the excitation protective functions and only rely on the unit protective relay or both protective functions can be utilized, in which case there needs to be a coordination between the two protective functions.

The most common limiters and their functions are given below:

Table Limiters and Protections
2.1. OVER EXCITATION LIMITER (OEL)

For excitation systems manufactured by Reivax, limiters can easily be configured from the HMI. The simplified transfer function and configuration screen for the OEL is shown below corresponds to IEEE 421.5 OEL2C. The OEL is configured as an inverse-time characteristic curve as per IEEE/ANSI C50.13.

Over Excitation Limiter (OEL)
2.2. UNDER EXCITATION LIMITER (UEL)

The simplified transfer function and configuration screen for the UEL is shown below. The UEL is configured as a piece-wise linear characteristic in the underexcited region (negative reactive power) of the capability curve as per IEEE 421.5 UEL2C.

Under Excitation Limiter (UEL)
UEL Capability Curve

3. PROTECTION COORDINATION

Coordination between limiters, equipment limits and external protection relays is an important aspect of proper excitation system integration. Typically, coordination is performed as part of a protection study or model validation study, with the settings tested during commissioning of the equipment.

During commissioning, the OEL is drawn such that it overlaps the IEEE/ANSI C50.13 rotor thermal limit.

4. CAPABILITY CURVE

The capability curve of a synchronous machine is a graphical representation of the operating limits of the machine. The capability curve is a plot of the machine active power (MW) versus the reactive power (MVar). Typically, the following physical operating limits are represented:

  • Rotor thermal limit
  • Turbine limit
  • Practical stability limit
  • Pole slip limit

In addition, the following limiters are typically represented:

  • Over Excitation Limiter (OEL)
  • Under Excitation Limiter (UEL)

Excitation systems manufactured by Reivax include a dynamic capability curve that can be used to monitor operating conditions in real-time. An example of such a capability curve is shown below.

Capability Curve

The capability curve shows the safe operating region of the machine, indicated in green, restricted by limiters and the physical limits of the machine. It also shows the operating point of the machine, in terms of active and reactive power (both quantities are shown in pu).

5. POWER SYTEM STABILIZER (PSS)

The Power System Stabilizer (PSS) is an add-on to the control loop of an excitation system that improves system stability by compensating for low frequency (0-5Hz) oscillations in the power system. This translates into a more stable generator output power, which can lead to significant savings due to reduced power losses. Power System Stabilizers offer superior cost effectiveness and have been found to produce millions of dollars in annual benefits for large utilities.

The PSS output is added to the AVR control loop. The image below shows the PSS summing junction as it appears on the transfer function of a Reivax excitation system.

AVR Control Loop PSS

The graph below shows the response of a 32.5MW utility-scale generator with and without the PSS. A disturbance is introduced at the 2 and 12 second marks. The transient and steady-state stability are noticeably improved. Without the PSS, oscillations continue for about 10 seconds after the disturbance, whereas they are damped almost immediately when the PSS is turned on.

AVR Control Loop PSS

The Reivax PSS is compatible with the IEEE PSS2A and PSS2B models.

TYPES OF EXCITATION SYSTEMS

Different types of excitation systems have emerged over the years in the power industry. They are classified into two general categories, depending on the power source, rotating exciters and static exciters.

1. ROTATING EXCITATION SYSTEMS

In rotating excitation systems, there are two exciters: the main exciter and the pilot exciter. The main exciter supplies the pilot exciter, and the pilot exciter in turn directly supplies the synchronous machine. There are two sub-categories of rotating excitation systems: AC and DC.

1.1. AC BRUSHLESS EXCITER

In an AC excitation system, the main power rectifier supplies an intermediary AC exciter. This AC exciter contains an internal power rectifier which then supplies the field winding of the synchronous machine.

A single-line diagram of an AC rotating exciter is shown below.

TYPES OF EXCITATION SYSTEMS Different types of excitation systems have emerged over the years in the power industry. They are classified into two general categories, depending on the power source, rotating exciters and static exciters. 1. ROTATING EXCITATION SYSTEMS In rotating excitation systems, there are two exciters: the main exciter and the pilot exciter. The main exciter supplies the pilot exciter, and the pilot exciter in turn directly supplies the synchronous machine. There are two sub-categories of rotating excitation systems: AC and DC. 1.1. AC BRUSHLESS EXCITER In an AC excitation system, the main power rectifier supplies an intermediary AC exciter. This AC exciter contains an internal power rectifier which then supplies the field winding of the synchronous machine. A single-line diagram of an AC rotating exciter is shown below.
1.2. DC EXCITER

In a DC excitation system, the power rectifier supplies an intermediary DC exciter, which in turn supplies the field winding of the synchronous machine.

A single-line diagram of a DC rotating exciter is shown below.

DC Excitation System

2. STATIC EXCITATION SYSTEMS – TERMINAL-FED

In static excitation systems, the power rectifier directly supplies the field winding of the synchronous machine. There is no pilot exciter.

A static excitation system is terminal-fed (also called bus-fed) when the supply is taken from the machine itself through a power potential transformer (PPT). The primary of the PPT is connected to the stator of the machine and the secondary supplies power to the rectifier.

Static excitation systems are not inherently self-exciting, so they require an external supply to jump-start the excitation process and build up sufficient magnetic flux. This process is called field flashing.

Static Excitation System
2.1. ADVANTAGES OF STATIC EXCITATION SYSTEMS

Static excitation systems provide a number of benefits that make them an attractive option for synchronous machine control:

  • Simple, reliable and cost-effective design
  • Minimal maintenance requirements
  • High performance and fast response characteristics
2.2. MAIN COMPONENTS

The main components of a static excitation system are listed below:

2.2.1. AUTOMATIC VOLTAGE REGULATOR (AVR)

The automatic voltage regulator (AVR), commonly known simply as voltage regulator, executes all control functions of the system, including the following:

  • Operating breakers
  • Sending firing pulses to bridges
  • Responding to operator commands or grid disturbances
  • Monitoring system I/Os and taking appropriate actions in response to them
  • Maintaining the excitation system within safety and stability limits through the use of limiters and protections
  • Issuing notifications to the plant SCADA system if anomalous conditions occur
  • Tripping the excitation system if a critical failure or dangerous condition occurs

The main elements of an AVR are as follows:

  • Control loop
  • Limiters
  • Power system stabilizer

Redundant control solutions are common. In a redundant configuration, there are two voltage regulators, one that executes the control functions while the other is in hot standby.

2.2.2. POWER RECTIFIER

Static excitation systems typically use a power rectifier that converts AC to DC current and provides a controlled field current to the synchronous machine. Power rectifiers typically use thyristor or IGBT technology.

Heat generation is a concern for the power rectifier. For bridge cooling, redundant fan sets are typically provided.

Redundant bridge configurations are common. In case multiple bridges are present, the excitation system will perform current equalization to balance the bridge outputs.

2.2.3. CONVERTER INTERFACE

The converter interface consists of all intermediary devices between the controller and power rectifier. It converts the control signal to firing pulses and isolates the control electronics from the power section.

On excitation systems manufactured by Reivax, diagnostics tools for monitoring the power rectifier are provided on the HMI. The status of fans, fuses, and semiconductor temperatures can be checked in real-time, as shown below.

Converter Interface

This screen demonstrates normal bridge operation. All 6 thyristors are conducting normally. Current is equally balanced between the 3 branches. Fan set 1 (A&B) is active, while fan set 2 is on standby.

Converter Interface with Failures

In this screen, the HMI is indicating a problem with fan A. As a result, the exciter has automatically transitioned to fan set 2 (D&E). There is also an indication of a fuse problem with thyristor #5. The measured current through this thyristor has dropped to zero, indicating that there is no conduction.

2.2.4. POWER POTENTIAL TRANSFORMER (PPT)

The PPT is a 3-phase step-down power transformer used in static excitation systems. Its purpose is to step down incoming AC voltages from the synchronous machine to a level that can be supplied to the power rectifier.

2.2.5. FIELD BREAKER (AC OR DC)

The main purpose of the field breaker is to interrupt the excitation and serve as an isolation point for performing maintenance and troubleshooting.

Both AC and DC solutions are common. The field breaker can be installed either directly in the field circuit, or between the secondary of the excitation transformer and the rectifier of the system. In the latter case, the breaker is often called an AC contactor instead.

2.2.6. DC FIELD FLASHING

The field flashing circuit is used in the start-up process, when the magnetic flux in the generator is too low. The field of the synchronous machine is temporarily connected in parallel to an external DC supply, such as a station battery, until the synchronous machine develops enough terminal voltage such that the excitation becomes self-sustaining. Afterwards, the field flashing is interrupted.

2.2.6. CROWBAR AND DISCHARGE CIRCUIT

The crowbar is a safety feature designed to protect the excitation system and field winding from external surges, generator pole displacement, etc.

If you would like to discuss these features with a qualified engineer to better understand the advantages, please call our toll free number 1-877-7REIVAX.

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