Reactive Power Valuation Study

Reactive Power Service

What is the place of reactive power service between the ancillary services in power system? What is the aim of it? How is reactive power being produced and how much does it cost? These are the questions we’ll try to answer in the this study.

Ancillary services

Basic system services cannot be delivered without ancillary services. The ancillary services that support basic system services are shown in the Table 1 below.

Load reduction, distribution tap changing, static compensation, rotating compensation with voltage regulator, generator tap changing, generator frequency / voltage domains, generator resistance to faults, phase unbalance, harmonic and voltage fluctuations are services, which contribute to the voltage (including voltage stability) support, that is closely related to reactive power and are the theme of interest in this work.

Table 1 – Overview of ancillary services providing basic system services

Reactive Power/Voltage control service

Reactive power/Voltage control service should satisfy System requirements listed below:

1. Satisfy overall system and customer requirements for reactive energy on a continuous basis;
2. Maintain system voltages within acceptable limits;
3. Provide a reserve to cover changed reactive requirements caused by contingencies, against which the system is normally secured, and satisfy certain quality criteria in relation to speed of response;
4. Optimize system losses.

In reactive power and voltage control a distinction between three levels of voltage control could be made:

Primary control is implemented by the voltage regulators of generating units, which will initiate a rapid variation in the excitation of generators when they detect a variation in voltage across their terminals. Other controllable devices, such as Static Var Compensators (SVCs) may also be involved in primary regulation.

Secondary control co-ordinates the action of voltage and reactive power control devices within a given zone of the network in order to maintain the requisite voltage level at a certain node point in the system.

Tertiary control involves a process of optimization, using calculations based upon real time measurements, in order to adjust the settings of devices, which influence the distribution of reactive power (generating unit controllers, tap transformer controllers and compensating devices, like reactors and capacitors).

Reactive Power Sources

Reactive power is produced or absorbed by all major components of a power system:

• Generators;
• Power transfer components;
• Loads;
• Reactive power compensation devices.

2.3.1 Generators

Electric power generators are installed to supply active power. Additionally a generator is supporting the voltage, producing reactive power when over-excited and absorbing reactive power when under-excited. Reactive power is continuously controllable. The ability of a generator to provide reactive support depends on its real-power production.

Figure 1 shows the combined limits on real and reactive production for a typical generator.

Figure 1 – Reactive power capability dependence on real power production for a synchronous generator

Like most electric equipment, generators are limited by their current-carrying capability. Reactive power production is depended on the field heating limit and absorption on the core end-heating limit of the generator. Active power output limit is limited by armature heating.

Control over the reactive output and the terminal voltage of the generator is provided by adjusting the DC current in the generator’s rotating field.

If the system voltage rises, the reactive output of the generator will drop, and ultimately reactive power will flow into the generator, tending to lower system voltage. The voltage regulator will accentuate this behavior by driving the field current in the appropriate direction to obtain the desired system voltage.

Further Study – Reactive power control in distribution substations (design and economics)

Reactive power control in distribution substations (design and economics)

Power transfer components

The major power transfer components are transformers, overhead lines and underground cables. HVDC converter stations can also be treated as power transfer components.

Transformers

Transformers provide the capability to raise alternating-current generation voltages to levels that make long-distance power transfers practical and then lowering voltages back to levels that can be distributed and used. The ratio of the number of turns in the primary to the number of turns in the secondary coil determines the ratio of the primary voltage to the secondary voltage. By tapping the primary or secondary coil at various points, the ratio between the primary and secondary voltage can be adjusted.

Transformer taps can be either fixed or adjustable under load through the use of a load-tap changer (LTC). Tap capability is selected for each application during transformer design. Fixed or variable taps often provide ±10% voltage selection, with fixed taps typically in 5 steps and variable taps in 32 steps.

The reactive power consumption of a transformer at rated current is within the range 0.05 to 0.2 p.u. based on the transformer ratings.

Fixed taps are useful when compensating for load growth and other long-term shifts in system use. LTCs are used for more-rapid adjustments, such as compensating for the voltage fluctuations associated with the daily load cycle. While LTCs could potentially provide rapid voltage control, their performance is normally intentionally degraded.

With an LTC, tap changing is accomplished by opening and closing contacts within the transformer’s tap-changing mechanism.

Further Study – Synchronization and Reactive Power Control in Power System

Synchronization and Reactive Power Control in Power System