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7/28/2019 Integration of Ipp

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ROLE OF FACTS & HVDC IN INTEGRATING INDEPENDENT

POWER PRODUCERS TO

LARGE GRID

An experience of operating a large AC network like the one in India has shown

that a dispatcher today has hardly any control on the power flow in the transmission

network and it is more difficult to regulate the power through the transmission lines. But

the flexible AC transmission systems (FACTS) and HVDC system could play a vital role

in ensuring committed power flow.

The paper discusses the need of FACTS & HVDC application in power pool

arrangement. Also it discusses the need of flexibility in power pooling solution to

interconnection problem, the function of the FACTS and controllers and HVDC systems

relevant to the Indian context.

Also the recent trends in FACTS devices and advantages of facts are discussed in

order to reduce their cost.

INTRODUCTION

The role of electricity in development and progress of economy is immense. Its

shortage can hamper the progress of developing countries like India.

With the ongoing expansion and growth of the electric utility industry, numerous

changes are continuously being introduced to the business. In the evolving utility

environment, the more optimal and profitable operation of the power system with respect

to generation, transmission, and distribution is demanded. To achieve both operational

reliability and financial profitability, it has become clear that more efficient utilization

and control of the existing transmission system infrastructure is required.

FACTS & HVDC systems provide proven technical solution to address these new

operating challenges being presented today. FACTSS technologies allows for improved

transmission system operation with minimal infrastructure investment, environmental

impact, and implementation time compared to the construction of new transmission lines

as in case of HVDC, where the process to permit, site, and construct new transmission

lines has become extremely difficult expensive, time-consuming, and controversial.


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Till recent past generation, transmission, and distribution were in the hands of

Government. Due to limited resources, slack performance of generating stations along

with high T&D losses and delays in commissioning new plants, it was not possible to

keep up with the growing demand of power. heavy investment was necessary to improve

the situation. A new source of investment opened up with entry of private sectors like

Tata Power and Reliance. Apart from financial advantage it is expected that privatization

will result in greater reliability, as private participants may go for latest technology to

obtain efficient system for greater revenue.

NEED FOR FLEXIBILITY IN POWER POOLING

The optimum use of generating capacity can only be made when proper

transmission facilities are available. The T&D system should be flexible to

accommodate different power flow at different times (depending on demand and

generating scenario etc.) and it should be possible to control the amount of power flow on

various lines to certain extent. It needs proper planning and means to acquire such

control and flexibility.

The ideal scenario is one where there are many supplier of power and it is

possible to buy certain amount of power from each, depending on the demand and the

cost and then finding the cheapest combination taking into account cost of power and

losses which occur when getting power from a distant (but cheaper) sources.

Fortunately, there are means available to link such isolated networks and to

introduce flexibility in the existing networks: such as FACTS and HVDC links.

PROBLEMS OF INTERCONNECTION

Location and unit sizes of new generating stations by independent power

producers will be governed by technoeconomic and political considerations. IPP would

like to sell the power to the transmitting authority at their closest bus.

Certain technical problems could arise if the power transmission or distribution

system is not properly planned or controlled. These are explained below:

a. Reactive Power Exchange Problem:

If the reactive power burden on the generating units is quite high, this can cause

tripping of the generators or force the plant operate at reduced power level or reduced

frequency.


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b. Effects of Harmonics on Generators:

Increase in harmonic generating loads increases the level of harmonics entering

into generators and causes excessive heating of generator windings and frequent tripping.

Load harmonic correction is possible through installation or harmonic filters or through

well designed HVDC connection nearby.

c. Under frequency problems:

If the connected network lacks load generation balance, frequency dips during a

disturbance may warranty load shedding and tripping of generators. A weak and over

loaded network may also cause synchronization problems due to frequency differences.

d. System Stability:

A well-designed system can carry through a fault and recover after fault without

loss of supply to loads. in an unplanned system it can lead to tripping of machines or

lead to a voltage collapse. Generators connected to a weak grid could suffer because of

overall poor stability.

WHAT CAN HVDC DO?

Providing HVDC link helps in the following ways.

1. 1. Reactive power exchange is controlled externally which prevents over

loading of generators.

2. 2. Harmonic filters are designed to limit harmonic current going into

generators.

3. 3. Synchronization does not pose any problem. A HVDC station can

connect two networks at widely different frequencies.

4. 4. HVDC Controller ensures recovery from faults at much faster rates.

Modulation control features help AC system stabilization.

A back to back link is an ideal outlet, where grid frequency disturbances could

cause frequent interruption. An HVDC link provides an asynchronous rout for transfer of

power between regions. It is possible to strictly control the amount of power one wants to

allow over this link. In a back to back link the conversion from AC to DC and back to

AC is done at the same station without transmission line. In India the Vindhychal link

interconnects Northern and Western regions and a link at Chandrapur joins western and

southern regions. These back to back HVDC links can from the interface points for the


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state grid and a private power plant and also among the private plants with perfect control

on the power flows.

WHAT CAN FACTS DO?

FACTS is actually a group of many devices or systems. A few of these are static

VAR compensations (SVC), thyristor controlled series compensation (TCSC), dynamic

reactor, thyristor controlled phase shifter transformer, static condenser (STATCON),etc.

Each of these is an independent member of FACTS family and can be applied

independently at any point in the power system as per requirement. For example, a SVC

at load centre saving voltage drops and losses in the system. And a variable series

compensation in an overloaded line to increase stability limit. Such measure, if planned

considering network topology and load flow result in reduce losses and a better-managed

and controlled network where it is possible to guide the power as desire.

FACTS Controller:- It is power electronic based system and other static equipment

provide control of one or more AC Transmission system problems. FACTS

controllers may be based on devices with no gate turn-off(only with gate turn-on),

or with power devices with gate turn-off capability principle controllers with gate

turn-off devices are based on the dc to ac converter, which can exchange active

and or reactive power with the ac system.

Basic types of FACTS controllers

Series controller.

Shunt controller.

Combined series-series controllers.

Combined series-shunt controllers.

Series controllerThe series controller could be a variable impedance such ascapacitor, reactor etc. or a power electronics based variable source of main frequency,

sub synchronous and harmonic frequencies(or a combination)to serve the desired

need. All series controllers inject voltage in series with the line.

Shunt controllersThese controllers may be variable impedance, variablesource or a combination of these. All shunt controllers inject current into the system

at the point of connection.


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Combined series-series controllersThis is the combination of separate seriescontrollers, which are controlled in a coordinated manner, in a multiline transmission

system or this could be a unified controller.

The real power transfer capability of the unified series-series controllers, referred

to as interline power flow controller.

Combined series-shunt controller This could be a combination of separateshunt and series controllers, which are controlled in a coordinated manner, or a

Unified Power Flow controller with series and shunt elements.

In principle, combined shunt and series controllers inject current into the system

with the shunt part of the controller and voltage in series in the line with the series part of

the controller.

Unified Power Flow Controller (UPFC)

Unified power flow controller UPFC is one of the FACTS devices, which can

control power system parameters such as terminal voltage, line impedance and phase

angle. We have dynamics models of UPFC in order to design suitable controllers for

power flow, voltage and damping controls. UPFC is a combination of static synchronous

series Compensator (SSSC) which are coupled via a common DC link, to all bi-

directional flow of real power between the series output terminals of the STATCOM and

are controlled to provide concurrent real and reactive series line compensation without anexternal electric energy source. The UPFC by means of angularly unconstrained series

voltage injection, is able to control, concurrently or selectively, the transmission line

voltage, impedance and angle or alternatively, the real and reactive power flow in the

line. UPFC may also provide independently controllable shunt reactive compensation.

CONTROL ATTRIBUTES FOR VARIOUS FACTS CONTROLLERS

FACTS controllers Control Attributes

STATCOM - Voltage control, VAR compensation, damping oscillations

and voltage stability.

SVC, TCR, TCS &

TRS

- Voltage control, VAR compensation, damping oscillations,

transient and dynamic stability, and voltage stability.

SSSC, TCSC, TCSR - Current control, damping oscillations, transient and dynamic

stability, voltage stability and fault current limiting

UPFC - Active and reactive power control, voltage control, VAR


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compensation, damping oscillations, transient and dynamic

stability, voltage stability and fault current limiting

TCVR & IPFC - Reactive power control, voltage, damping oscillations,

transient and dynamic stability and voltage stability.

RECENT TRENDS FOR FACTS

Facts can control the voltage magnitude and phase angle at both ends of the line,

as well as the amount of real and reactive power that is passed through the line. A facts

device can block the flow of power to a line that is about to be dangerously overloaded.

It reroutes the electricity to a line that has the capacity to carry the additional current. The

problem is that FACTS device are too expensive to be used widely. So ORNL

researchers are looking at ways to reduce their cost. Some people are developing and

testing highly efficient power electronics modules based on silicon carbide (SiC), to

replace todays Silicon based modules (including FACTS devices). The SSD researchers

have made advances in material processing and synthesis that allow fabrication of SiC

devices that will take full advantage of the intrinsic properties of the material without

being limited by process induced defects found in silicon-based counter parts.

Compared with silicon modules, the SiC-based electronic modules should operate more

efficiently at higher temperatures, voltages and switching speeds.

ADVANTAGES OF FACTS CONTROLLERS

Increase the loading capability of lines to their thermal capabilities by

overcoming the limitations of sharing of power among lines according to their

capability.

Increase the system security through raising the transient stability limit, limiting

short-circuit currents and overloads.

Prevent cascading outages.

Limits impacts of faults and equipment failure. Provide greater flexibility in sitting new generation.

Upgrade of lines.

Allow lines to carry more active power by reducing reactive power flows.

Help to damp out power system oscillations.

Provide secure and controllable tie line connections to neighbouring utilities.


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Reduce loop flows.

Increase utilization of the lowest cost generation.

RELEVANCE IN THE INDIAN CONTEXT

System planners in India have learned the hard way that the system they plan

seldom exactly meets the purpose for which they were designed. In a developing

network many factors make flexibility a critical requirement of overall network

development philosophy. This demands the need for transmission choices that go beyond

the immediate need of a specific project. Both HVDC and FACTS, therefore, can be

utilized in building a network, which can make private power propositions more feasible

and economical.

In India, a National committee on FACTS is working on the feasibility of

installing a TCSC project in a suitable 400 kV corridor in the country. Potential for

STATCON exists on many key 400 kV nodes having high connected load demand.

Similarly, potential for TCSC exists in many 400 kV trunk routes supplying power to

load centers.

Inter-regional, interstate links as well as interconnects of different power pools

such as state and independent power producers can be through Back to Back HVDC links

with ratings ranging from 150 MW to 1000 MW. If we in India have to gear up for

formation of a flexible National grid and for making full use of the private power

generation investment in we must prepare ourselves for the same. Otherwise the

enthusiasm of the private sector and the huge amounts it is ready to invest, will go

abegging.

CONCLUSION

Flexible AC transmission system is the technology of the transmission

functionality in order to fully utilize the existing transmission facilities and hence,

minimizing the gap between the stability limit and thermal limit. Though there is analternative for FACT device in transmission system that is HVDC Back to back, but cost

of a FACTS solution is lower than the HVDC cost.


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Combination of various FACTS devices can fulfil every demand and can

overcome the problems of interconnection. Hence research is going on for decreasing the

cost of FACT devices.

integration of ipp

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