• Electrical India
  • Apr 5, 2017

Reactive Power Management & Voltage Control to avoid Blackouts

Series compensation increases the transmission capacity and improves the stability of a power system. Since transmission line itself consumes reactive power as it transmits the active power...

- Rajesh Chourishi

What is Reactive Power?

  In an alternating current (AC) power system, Power comprises of two components, activepower and reactive power. Useful work is accomplished by active power while reactive power improves voltage stability and avoids voltage collapse. The reactive power phenomenon can be explained with the help of “Power triangle” given below.

  Suppose an apparent power S is carried by a power feeder has phase difference Ф between voltage and current waveforms. If it is resolved into two mutually perpendicular components then its horizontal component along thebase of power triangle is called active power, P (=S CosФ)while its vertical component along the perpendicular is called reactive power, Q (=S SinФ). Where, Ф = tan-| (Q/P).

  The following relationship can be shown by “Power Triangle”.

S = P + j Q = √ (P2 + Q2) X ejФ

  Usually, a power system network has a wide mix of active and reactive loads.Therefore, its combined power factor varies from lagging to leading through unity. Thus, the generation and consumption of active and reactive components of apparent power depend on nature of the load.

  The inductive load consumes reactive power while capacitive load generates it with their lagging and leading power factors respectively.

  The quantity of reactive power depends on thephase shift between the voltage and current wave. The resistive load consumes only active power at unity power factor.

  The scenario of active and reactive power in pure resistive, inductive and capacitive load is summarised in the Table 1.

A physical analogy for reactive power

  A reasonably accurate analogy for reactive power is the process of filling a water tower tank with water - one bucket at a time.

  This analogy is based on the facts that,“In the power system useful work accomplishes by active power while reactive power supports the voltage.”

  When you carry a water-filledbucket up the ladder, you have bucket and waterwhile coming downafter dumping the water at the top you have an empty bucket. In this case, the empty bucket is a helping agency to do work, while carrying up the water is the desired work. When going up the ladder you need an empty bucket (reactive power) and water (active power), while come down you have the empty bucket (reactive power). Here the role of reactive power is (helping agency) performed by the empty bucket while the role of active power is performed by water.

  Another analogy of the reactive power says that “Reactive power is the froth on the Beer” is fairly good here because space in the glass is occupied by the useless froth, leaving less room for real Beer.

Reactive power sources

  The reactive power compensation sources are classified as,

Static compensation is ideal for a response within seconds and minutes like shunt capacitor, shunt reactor, and tap changer.
Dynamic compensation is ideal for an instantaneous response like Synchronous condenser,Generators, and OLTC.
It is further classified as,
• Dynamic shunt compensation
• Dynamic series compensation

Reactive Power Reserve (RPR)

  The RPR is spare reactive capability available in the system to assist the voltage control.

  During the contingency like, an outage of a transmission line or sudden change in demand for reactive power,this capability will balance the supply and demand of reactive power.

  It helps to maintain steady state voltage, security of the bulk power system, secured system operation against short-term & long-term voltage instability and collapse.

  The equipment which can maintain the RPR are synchronous condensers, 'spare shunt capacitors', 'spare shunt reactors' and Static Var Compensators.

  Generator supplied reactive power is an effective source of RPRs because of:

• Its superior performance at low voltage as compared to static reactive devices
• Fast response of excitation system
• Large reactive range.

Importance of reactive Power

  By regulation of reactive power the following parameters of a power system can be controlled:

• Utilisation of active power
• Voltage stability
• Power factor
• System efficiency
• Energy cost
• Power quality

Utilisation of Reactive power in Operation

  Over a long distance power transmission, additional reactive power losses occur due to the large reactive impedance of high voltage transmission system. To avoid excessive reactive power transmission, generation and consumption of reactive power should be as close as possible to each other, otherwise, it will result in inappropriate voltage profile.
Transmission lines, transformers, induction motors, furnaces, reactors, chokes, wound control gear, consumereactive power, and its transmission is highly localised.Therefore, reactive power is provided to them by some localised sources. For LT Loads, it can be controlled by 'Intelligent Power Factor Control Relay' (IPFC).

  By excitation system of the Synchronous generator,supply and demand of reactive power can be adjusted for desired voltage level. Generators also have capability curves which govern the combination of active and reactive power output.

Reduction in power losses through regulation of reactive power

  Instead of changing voltage level, power and energy losses can be reduced through regulation of a reactive power.

  Active power losses ΔP and voltage drop ΔV may be obtained from the following equations:

ΔP = (P2 + Q2) x R / V2
ΔV = √ [3x (P2 + Q2)] x R / V
Where: V, is system voltage,
R is circuit’s resistance

  The above relations indicate active power losses ΔP and voltage drop ΔV depend on transmission of reactive power Q. Consequently to save power losses, distributed/ local sources of reactive power like shunt capacitors for an inductive load or shunt reactors for the capacitive load can be used.

Power transfer Limits

  The surge impedance loading or SIL of a transmission line is the MW loading of a transmission line at which a natural reactive power balance occurs.

  A line with 1.0SIL loading will have a flat voltage profile (same voltage from sendingto receiving end), with same current in phase with voltage along the line. The reactive power into the line due to shunt capacitance charging will exactly equal to thereactive power consumed by the series inductance losses. Approximate values of 1.0 SIL are given in adjacent Table.

Power transfer limiting factors

  There are three important factors which limittransmission of power.

• Thermal limit
• Voltage limit
• Stability limit

Phenomenon of Voltage Instability, Voltage Collapse, and Blackouts

Voltage Instability

  A system enters a state of voltage instability when thedemand of reactive power becomes more than supply. It may happen due to –

• Increase in load / demand,
• A progressive and uncontrollable drop in voltage.
• Deficiency of reactive power due to theflow of active and reactive power from inductive reactance of transmission system.

Voltage Collapse Phenomenon

  The process by which sequence of events associated with voltage instability leads to loss of voltage in a significant part of the system is called voltage collapse.

  The phenomenon of voltage collapse is created when the demand for reactive power increases proportionate to active power. At this moment, a fully loaded transmission line generates extra inductive reactive power. Thus, capacitive reactive power from local sources becomes insufficient. Therefore,the reactive power will have to be delivered from more distant places, as a consequence transmission of more reactive power through the lines will further increase the voltage drop on the customer side. Local control of voltage by means of auto transformers will supply more reactive power,and this, in turn, will increase further voltage drops in lines. In one moment this process can go like an avalanche, thereby reducing thevoltage to zero.

  In the meantime, most of the generators in power plants will switch-off due to an unacceptably low voltage which of course will deteriorate the situation.

Possible Scenario of a Voltage Collapse

  Possible scenario for voltage collapse are given below.

• Generating units near load centres are out of service.
• Heavily loaded lines having low Reactive Power Reserves (RPRs).
• Tripping of a heavily loaded line causes load increases over other lines and loss of reactive power and voltage.
• Load consumption would temporarily lower to stabilise. AVRs would act to restore generator voltages, but increased reactive power flow would lower voltages at consumer end or elsewhere.
• Under the capability curve,Generators would hit Var limits.

Blackouts in a power system

  A power system undergoes the voltage collapse if the post-disturbance equilibrium voltages are below acceptable limits. This voltage collapse may be converted into a total or partial blackout. A blackout in an electric system means that the complete system collapses. It originates from several causes.

  Overloading of generators and transmission lines creates a deficiency of reactive power which leads voltage collapse and resultant cascade tripping can cause a blackout.

  One such example is the loss of generation, e.g. the tripping of a power plant leads to overloading and under frequency over another power plant. It may result in the further loss of other generators.

  Another example, is bottlenecking of transmission lines, trips other overloaded power lines, results in cascade trips. Finally, power system undergoes the voltage collapse due to high impedance in the weakened grid.

  In general, one initial minor event leads to a second event, a third and so forth. Due to increased stresses on the system, it finally collapses and leads to blackouts.

Reactive power compensation

  Flexible AC Transmission System (FACTS) technologies are used for reactive power compensation. It is classified as Dynamic shunt compensation and series compensation.

Dynamic shunt Compensation

  Dynamic shunt compensation has the ability to automatically support the voltage level in a specific area of the power system. The voltage level is an immediate image of the reactive power balance – too high a voltage means a surplus of reactive power and vice versa. A dynamic shunt compensator automatically and instantaneously adjusts the reactive power output smoothly compared to the reference voltage level.

  It improves transient stability by quickly detecting and automatically adjusting its output in response to system events.

  Currently two types of dynamic shunt compensation technologies are commercially available in the market: the static (non-rotating) Var compensator (SVC) and the static (non-rotating) compensator (STATCOM).

  An SVC is built up with reactors and capacitors and it is controlled by Thyristors. To provide voltage and transient stability automatically, it measures the actual voltage and provides reactive power to the system automatically through its capacitor and reactor.This technology has been adopted by more than 800 installations worldwide.

  STATCOM is based on Voltage Source Converter (VSC) technology. A comparison with an SVC yields that the capacitors and reactors are replaced withpower transistors IGBTs for intelligent switching of semiconductors. IGBTs operates at a frequency in the kHz range. By connecting DC capacitors on one side of the converter, the STATCOM is able to vary its output with respect to magnitude, frequency and phase angle to provide voltage and transient stability. This technology has been adopted for approximately 20 installations worldwide.

Series Compensation

  Series compensation increases the transmission capacity and improves the stability of a power system. Since transmission line itself consumes reactive power as it transmits the active power. It means transmission system is not operated in an optimum way. By adding series compensation technology to the transmission system, the transmission capacity is drastically increased, as the capacitors will produce (capacitive) reactive power. Furthermore, it is a self-regulating phenomenon; as more current is transmitted, the power system will consume more reactive power and the capacitors will also automatically produce more reactive power. As a result, the transmission line is utilised more effectively, and more active power can reach consumers on the existing infrastructure. Series compensation supports the voltage, as long lines otherwise see a decaying voltage profile along the line.

Challenges in Voltage Control and Related Security

  A lot of research have been done for improving system reliability but still some of the challenges are considered as a subject for research & development to avoid blackouts like, Global strategy for AVR set point, the best locations for Var control devices, determination of 'acceptable' Var margins,fast contingency analysis for Var computation.

  Although 'under voltage relays', have been developed but there are no relays in the system to directly sense the problem that the voltage is about to collapse.

Rajesh Chourishi
Accredited Energy Auditor
Corporate Technical & Energy Services,
Aditya Birla Group

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