SPPU TE-Electrical Subject-Power System-II

1. DEPARTMENT OF ELECTRICAL ENGINEERING
JSPMS
BHIVARABAISAWANTINSTITUTEOFTECHNOLOGYANDRESEARCH,
WAGHOLI,PUNE
A.Y. 2019-20 (SEM-II)
Class: T.E.
Subject: Power System-II
Prepared by Prof. S. D.
Gadekar
Santoshgadekar.919@gmail.com
Mob. No-9130827661

2. Contents
o Introduction
o Comparison between AC and DC transmission
o Types of HVDC Links
o HVDC Converter Station
o Multi-Terminal HVDC System
o Principle of DC Link Control
o HVDC System in India

3. Introduction to DC Transmission
Electric power transmission was originally developed with direct
current.
The use of AC transmission becomes very poplar due to
availability of transformers and the development and
improvement of induction motors.
DC Transmission now became practical when long distances
were to be covered or where cables were required.
The highest functional DC voltage for DC transmission is +/-
600kV. D.C

4. Comparison of AC and DC Transmission
• Economics of Power Transmission
i. In DC transmission, inductance and capacitance of the line has no effect
on the power transfer capability of the line and the line drop.
ii. There is no leakage or charging current of the line under steady
conditions.
iii. A DC line requires only 2 conductors whereas AC line requires 3
conductors in 3-phase AC systems.
iv. Break even distance is one at which the cost of the two systems is the
same.
v. The cost of the terminal equipment is more in DC lines than in AC line.

5. Comparison of AC and DC Transmission
• Reliability
The reliability of DC transmission systems is good and comparable to
that of AC systems. There are two measures of overall system
reliability-
Energy Availability = 1 −
Equivalent outage time
Total time
∗ 100
Where equivalent outage time is the product of the actual outage
time and the fraction of system capacity lost due to outage.
Transient reliability
=
100 X No. of times HVDC systems performed as designed
No. of recordable AC faults

6. Comparison of AC and DC Transmission
• Technical Performance
DC transmission overcomes some of the following problems associated with
AC transmission.
i. Stability Limits
The power transfer in an AC line is dependent on the angle difference
between the voltage phasor at the two line ends.
For a given power transfer level, this angle increases with distance. The
maximum power transfer is limited by the considerations of steady state and
transient stability.
The power carrying capability of an AC line is inversely proportional to
transmission distance whereas the power carrying ability of DC lines is
unaffected by the distance of transmission.

7. Comparison of AC and DC Transmission
ii. Voltage Control
Voltage control in ac lines is complicated by line charging and voltage
drops.
The voltage profile in an AC line is relatively flat only for a fixed level of
power transfer corresponding to its Surge Impedance Loading (SIL).
The maintenance of constant voltage at the two ends requires reactive
power control as the line loading is increased. The reactive power
requirements increase with line length.
Although DC converter stations require reactive power related to the
power transmitted, the DC line itself does not require any reactive
power.

8. Comparison of AC and DC Transmission
ii. Line Compensation
Line compensation is necessary for long distance AC transmission to
overcome the problems of line charging and stability limitations.
For Example-Shunt inductors, series capacitors, Static Var
Compensators (SVCs), Static Compensators (STATCOMs).
In the case of DC lines, such compensation is not needed.
iii. Problems of AC Interconnection
The interconnection of two power systems through ac ties requires the
automatic generation controllers of both systems to be coordinated
using tie line power and frequency signals.
The asynchronous interconnection of two power systems can only be
achieved with the use of DC links.

9. Comparison of AC and DC Transmission
iv. Ground Impedance
The existence of ground (zero sequence) current cannot be permitted
in steady-state due to the high magnitude of ground impedance, which
will not only affect efficient power transfer, but also result in telephonic
interference.
The ground impedance is negligible for DC currents and a DC link can
operate using one conductor with ground return (monopolar
operation).

10. Disadvantages of DC Transmission:
1. High cost of conversion equipment.
2. Inability to use transformers to alter voltage levels.
3. Generation of harmonics.
4. Requirement of reactive power.
5. Complexity of controls.

11. Types of HVDC Links
1. Monopolar Link
A monopolar link has one conductor and uses either ground and/or
sea return.
A metallic return can also be used where concerns for harmonic
interference and/or corrosion exist.
A monopolar link is normally operated with negative polarity as
corona effects in a DC line are substantially less with negative
polarity.

12. Types of HVDC Links
2. Bipolar Link
A bipolar link as shown in the above figure has two conductors,
one positive and the other negative.
Each terminal has two sets of converters of equal rating, in series
on the DC side.
The junction between the two sets of converters is grounded at
one or both ends by the use of a short electrode line.
Since both poles operate with equal currents under normal
operation, there is zero ground current flowing under these
conditions.

13. Types of HVDC Links
1. Homopolar Link
In this type of link two conductors having the same polarity
(usually negative) can be operated with ground or metallic
return.
Due to the undesirability of operating a DC link with ground
return, bipolar links are mostly used.

14. HVDC Converter Station

15. 1. Converter Unit-
The terminal substations which convert an AC to DC are called rectifier
terminal while the terminal substations which convert DC to AC are called
inverter terminal.
Every terminal is designed to work in both the rectifier and inverter mode.
Therefore, each terminal is called converter terminal, or rectifier terminal.
The conversion from AC to DC and vice versa is done in HVDC converter
stations by using three-phase bridge converters. This bridge circuit is also
called Graetz circuit.
In HVDC transmission a 12-pulse bridge converter is used. The converter
obtains by connecting two or 6-pulse bridge in series.

16. 2. Converter Valves-
The modern HVDC converters use 12-pulse converter units. The total number
of a valve in each unit is 12.
The valve is made up of series connected thyristor modules.
The number of thyristor valve depends on the required voltage across the
valve. The valves are installed in valve halls, and they are cooled by air, oil,
water.

17. 3. Converter Transformer-
The converter transformer converts the AC networks to DC
networks or vice versa.
They have two sets of three phase windings. The AC side winding is
connected to the AC bus bar, and the valve side winding is
connected to valve bridge.
These windings are connected in star for one transformer and delta
to another.
4. Filters-
The AC and DC harmonics are generated in HVDC converters.
The AC harmonics are injected into the AC system, and the DC
harmonics are injected into DC lines.
1. AC Filters
2. DC Filters
3. High Frequency Filters

18. 5. Reactive Power Source
Reactive power is required for the operations of the converters.
The AC harmonic filters provide reactive power partly.
The additional supply may also be obtained from shunt capacitors
synchronous phase modifiers and static var systems.
6. Smoothing Reactor
It can be located either on the line side or on the neutral side.
Smoothing reactors serve the following purposes.
• They smooth the ripples in the direct current.
• They decrease the harmonic voltage and current in the DC lines.
• They limit the fault current in the DC line.
• Consequent commutation failures in inverters are prevented by
smoothing reactors

19. This system has more than two converter station and DC terminal
lines. Some of the converter stations operate as rectifier while
others operate as an inverter.
The total power taken from the rectifier station is equal to the
power supplied by the inverter station.
There are two type of MTDC Systems
• Series MTDC System
• Parallel MTDC System
In series MTDC system the converters are connected in series
while in parallel MTDC system, the converters are connected in
parallel.
The parallel MTDC system may be operated without the use of an
HVDC circuit breaker.
Multi terminal HVDC System

20. Advantages of MTDC systems
The following are the advantages of MTDC systems
• The MTDC system is more economical and flexible.
• The frequency oscillation in the interconnected AC networks can be
damped quickly.
• The heavily load AC networks can be reinforced by using MTDC systems.
Applications of MTDC systems
The following are the applications of the HVDC systems
• It transfers the bulk power from several remote generating sources to several load
centres.
• The systems are interconnected between two or more AC systems by radial MTDC
systems.
• It reinforces the heavy load urban AC networks by MTDC systems.

21. Principles of DC Link Control
The control of power in a DC link can be achieved through the control of
current or voltage.
From minimization of loss considerations, we need to maintain constant
voltage in the link and adjust the current to meet the required power.
Consider single line diagram of two terminal DC link.
Id =
ArEr
Tr
∗ cos αr −
AiEi
Ti
∗ cos γi
Rcr + Rd ± Rci
As the denominator in the final equation is small, even small changes in
the voltage magnitude Er or Ei can result in large changes in the DC
current, the control variables are held constant.

22. Constant Current Control
Id =
ArEr
Tr
∗ cos αr −
AiEi
Ti
∗ cos γi
Rcr + Rd ± Rci
CONSTANT CURRENT CONTROL INVOLVES THE FOLLOWING:
• Measurement of the DC current.
• Comparison of Id with the set value Ids or Iord (called as Reference/Current
Order/Current Command).
• Amplification to the differences called error.
• Application of the output signal of the amplifier to the phase shift circuit
that alters the ignition angle α of the valves in the proper direction for
reducing the error.

23. Minimum Delay Angle Control
Id =
ArEr
Tr
∗ cos αr −
AiEi
Ti
∗ cos γi
Rcr + Rd ± Rci
MINIMUM FIRING ANGLE CONTROL:
Power transferred in the DC line is mainly due to manipulation of the current
order. These signals are to be sent to the converter via telecommunication. If
this link fails there is a chance that a inverter can change to rectifier which
results in power reversal. To prevent that the inverter control is provided with
minimum delay angle control

24. HVDC System in India
Sr.
No.
System/Project Year of
Commissioned
Supplier Power
Rating
Voltage (kV)
1 National HVDC
project-stage-I
1989 BHEL 100 100
2 NHVDC-stage-II 2000 BHEL 100 200
3 Rihand-Delhi 1991-92 ABB 750 +-500
4 Chandrapur- Padghe 1998 ABB 1500 +-500