Q1: How do smart grids enhance modern power systems,
and what advantages does HVDC technology offer over
HVAC, including the role of filters in maintaining power
quality?
Answer:
Smart grids modernize traditional power networks by incorporating
advanced communication, control systems, and IT infrastructure,
allowing for two-way energy and data flow. This enables real-time
monitoring, automation, demand response, and integration of
renewable energy sources, resulting in improved efficiency, flexibility,
and reliability.
In this context, High Voltage Direct Current (HVDC) technology plays a
crucial role, especially for long-distance and undersea transmission.
HVDC minimizes line losses, eliminates reactive power issues, allows
asynchronous grid connections, and offers superior controllability of
power flow compared to High Voltage Alternating Current (HVAC)
systems. However, it comes with higher terminal costs and more
complex fault management.
To ensure high power quality in such advanced systems, filters—such as
L, LC, and LCL types—are used between inverters and the grid to
suppress harmonics generated during switching, maintaining compliance
with harmonic standards and ensuring stable grid operation.
Q2: Compare the key power semiconductor devices—diode,
thyristor, IGBT, and IGCT—in terms of structure, control, switching
characteristics, losses, failure modes, and suitability for HVDC
applications.
Answer:
IGCT
IGBT
(Integrated
(Insulated
Parameter Diode Thyristor Gate
Gate Bipolar
Commutated
Transistor)
Thyristor)
Four-layer Similar to
(PNPN), 3 MOS gate with thyristor with
Structure Two-layer (PN)
terminals (A, K, bipolar output integrated gate
G) drive
Control Uncontrolled Controlled turn- Fully Controlled like a
IGCT
IGBT
(Integrated
(Insulated
Parameter Diode Thyristor Gate
Gate Bipolar
Commutated
Transistor)
Thyristor)
on via gate; controlled thyristor;
turn-off by on/off via gate complex gate
current signal drive
Delayed;
Switching Instantaneous requires Fast switching, Slower; limits
Speed with polarity current drop or ideal for PWM PWM applications
forced turn-off
Higher on-
Conductio Lower than Lower conduction
Low state voltage
n Loss IGBT losses
drop
Requires gate Simple,
Complex, requires
Gate Drive Not applicable pulse for turn- voltage-
high gate current
on controlled
Failure Typically fails Typically fails Typically fails Typically fails
Mode open circuit short circuit open circuit short circuit
LCC-HVDC, VSC-HVDC, Niche high-
Applicatio
Rectifiers AC/DC SMPS, motor current HVDC
ns
converters drives applications
Q3: Explain the role of a snubber circuit in thyristor applications.
What are the key protections it offers?
Answer:
Snubber circuits protect thyristors during switching operations by providing:
di/dt Protection: A series inductor (Ls) limits the rate of current rise, preventing
localized heating.
dv/dt and Overvoltage Protection: A parallel RC circuit prevents accidental turn-on due
to rapid voltage changes and limits overvoltage’s during switching off.
Voltage Balancing: Grading resistors (Rdc) ensure uniform voltage sharing among
series-connected thyristors in HVDC valves.
These protections help prevent damage from thermal stress, unwanted turn-on, and uneven
voltage distribution.
Q4:
Q5: What is the significance of the firing angle (α) and commutation
overlap (μ) in LCC-HVDC rectifier operation, and how do they affect
system performance and stability?
Answer:
In Line Commutated Converter (LCC) HVDC systems, the rectifier firing angle α typically
operates within the range of 15° to 20° to ensure stable converter performance and compensate
for variations in AC voltage. Maintaining α within this optimal range is crucial because too low a
value can reduce the commutation margin, leading to voltage instability and increased harmonic
distortion. Transformer tap changers help regulate the AC voltage to control α effectively.
Commutation overlap μ refers to the short time interval during which two thyristors conduct
simultaneously as current transfers from one to another. This causes a momentary short circuit
on the AC side, resulting in a drop in effective DC voltage (ΔVdc). Excessive overlap not only
reduces system efficiency but also increases thermal stress and losses in converter components.
Proper tuning of firing angles and control of system inductance are essential to minimize μ and
maintain reliable DC voltage output.
Q6: How does the LCC inverter controller operate in HVDC systems,
including its role in voltage regulation, extinction angle control, and
power reversal in bidirectional configurations?
Answer:
In LCC-HVDC systems, the inverter controller plays a vital role in
maintaining stable system operation by regulating the DC voltage at the
inverter terminal. It typically uses a proportional-integral (PI) control
loop to adjust the firing angle and sustain the desired voltage level. During
system disturbances—such as an AC fault on the rectifier side—the inverter
can shift to constant current mode, reducing its firing angle to help
maintain DC current flow and system continuity.
A crucial aspect of inverter operation is controlling the extinction angle
(γ), which ensures that thyristors receive enough reverse voltage to fully
turn off after conduction. Maintaining γ in the range of 15°–20° prevents
commutation failures, particularly in weak AC systems or during periods of
high commutation overlap. If γ becomes too small, the thyristors may fail to
block, risking a DC short circuit.
The LCC inverter supports multiple operating modes to enhance flexibility
and resilience:
Constant DC Voltage Mode (normal operation),
Constant Extinction Angle (CEA) Mode (used in weak AC grids or
as a backup),
Constant Current Mode (during rectifier faults or DC disturbances).
In bidirectional HVDC systems, power reversal is achieved without
hardware changes by swapping the roles of the inverter and rectifier. The
terminal maintaining the current margin becomes the new inverter, and the
system changes direction by adjusting control references—demonstrating
the adaptability of LCC-HVDC control strategies.
Q7: How do VSC-HVDC systems respond to AC and DC faults, and
why is a Full-Bridge MMC more effective than a Half-Bridge MMC in
DC fault handling?
Answer:
VSC-HVDC converters respond to AC faults by reducing their modulation
index and active power output while injecting reactive power to support grid
voltage recovery. In the case of DC faults, especially in 2-level VSCs,
freewheeling diodes allow fault currents to rise uncontrollably since IGBTs
cannot block DC current. Protection involves disconnecting the AC side and
using DC reactors to limit fault current rise and aid recovery.
Full-Bridge Modular Multilevel Converters (MMCs) offer superior DC
fault handling compared to Half-Bridge MMCs. They can reverse the DC
voltage polarity, effectively extinguishing the fault current and preventing its
propagation to the AC side—allowing continued operation and reactive power
support. In contrast, Half-Bridge MMCs must be blocked during DC faults and
cannot sustain system operation.
Q8: What are the advantages of multilevel converters like NPC over
conventional two-level VSCs in HVDC systems, and how do they
address the limitations of two-level designs?
Answer:
Multilevel converters, such as Neutral-Point Clamped (NPC) designs, offer
key advantages over conventional two-level VSCs, including lower
harmonic distortion, reduced voltage stress (dv/dt) on transformers,
and higher efficiency due to lower switching losses. These benefits
minimize the need for large filters and improve insulation life.
In contrast, two-level VSCs face challenges at high voltages—such as poor
scalability, high dv/dt stress, and significant filtering requirements. To
overcome these issues, engineers adopt multilevel converter topologies,
which distribute voltage across multiple levels, improving performance,
reliability, and power quality in HVDC applications. However, multilevel
systems require more complex control and capacitor voltage balancing.
Q9: How are harmonics managed in VSC-HVDC systems, and what
roles do modulation techniques like Third Harmonic Injection (THI)
and Selective Harmonic Elimination (SHE) play in improving power
quality?
Answer:
Harmonics in VSC-HVDC systems originate mainly from PWM switching,
resulting in low-frequency, switching, and high-frequency harmonics. These
can disrupt both AC and DC networks if not properly controlled. To mitigate
them, engineers use advanced modulation methods and filters.
Third Harmonic Injection (THI) improves PWM performance by injecting a
controlled third harmonic into the reference signal. This increases the
fundamental voltage amplitude, extends the linear modulation range, and
reduces AC voltage distortion. Although it introduces third harmonics in line-
to-neutral voltages, these do not appear in line-line voltages of balanced,
floating systems, making THI effective in HVDC applications.
Selective Harmonic Elimination (SHE) targets specific low-order
harmonics by pre-calculating optimal switching angles based on Fourier
series analysis. It enhances waveform quality but relies on offline lookup
tables, limiting its adaptability in real-time scenarios.
Together with harmonic filters and strategic carrier frequency selection (e.g.,
odd multiples of 3), these techniques ensure compliant, efficient, and high-
quality HVDC system operation.
Q10: What are the key components of a VSC-HVDC system, and how
do elements like the DC link capacitor and droop control contribute
to system performance and stability?
Answer:
A VSC-HVDC system comprises several essential components that ensure
stable and efficient operation. The DC link capacitor acts as a primary
energy storage element, reducing voltage ripple, providing a low-inductance
path for current during switching, and enhancing dynamic response. Its
sizing is a trade-off—larger capacitors offer better ripple suppression but
slower response, while smaller ones improve control speed at the cost of
higher ripple.
Other major components include the DC reactor (limits transients and
smooths voltage), power transformer (voltage matching and isolation), AC
and DC filters (harmonic suppression), and AC reactor (reduces fault and
switching currents).
To enhance coordination and stability in multi-terminal VSC-HVDC
systems, droop control is employed. Instead of maintaining a strict
voltage setpoint, each converter adjusts its power output in response to DC
voltage changes. This prevents overcorrection, improves load sharing, and
maintains system stability under varying operating conditions.
Q11: Compare the operation of a single-phase and three-phase two-
level VSC converter. How does the modulation strategy impact
voltage waveforms in these systems?
Answer:
Feature Single-Phase VSC Three-Phase VSC
Topology Two switches per phase Six switches in a three-leg configuration
Single sinusoidal
Voltage Waveforms Multiple phase-shifted waveforms
waveform
Can support complex AC loads and grid
Load Handling Suitable for simple loads
interconnection
Harmonics &
Requires more filtering Reduced harmonics due to phase balancing
Filtering
Pulse-width modulation Space vector modulation (SVM) or sinusoidal
Switching Control
(PWM) PWM
Q12: What are the key features of Modular Multilevel Converters
(MMCs) in HVDC systems, particularly regarding voltage levels, fault
protection, and capacitor voltage balancing?
Answer:
Modular Multilevel Converters (MMCs) are widely used in HVDC systems due
to their scalability, efficiency, and power quality. Increasing the number of
voltage levels in an MMC significantly reduces Total Harmonic Distortion
(THD) from ~55% at 3 levels to ~1% at 101 levels—minimizing the need for
filters and making them ideal for long-distance transmission.
In terms of DC fault protection, Full-Bridge MMCs outperform Half-Bridge
designs by generating both positive and negative voltage states, enabling
them to block fault currents internally without external breakers. This
enhances system resilience, although it comes with increased conversion
losses due to additional switching.
To ensure stable operation, MMCs also rely on capacitor voltage
balancing. Control algorithms monitor capacitor voltages across
submodules, selectively switching them in or out to maintain uniform charge
distribution. This prevents voltage imbalance, reduces output distortion, and
supports reliable converter performance.
Q13: Explain the primary function of the Phase Locked Loop
(PLL) in HVDC systems and discuss its importance in
ensuring accurate firing control of the thyristors.
Answer:
The Phase Locked Loop (PLL) ensures that the converter controller
remains synchronized with the AC commutation voltage, allowing
precise thyristor firing, which is essential in both rectifier and inversion
modes. Traditional systems relied on zero-crossing detection, but this
was prone to harmonic distortions. Modern systems use a transvector
PLL, which tracks the positive sequence of the AC voltage, ensuring
quick adaptation to system dynamics and maintaining stable
operation.
