Lecture 1

1. What are the key features that distinguish a smart grid from a conventional power grid?

Answer:

A smart grid integrates both advanced communication/IT systems and controllable power devices,
enabling two-way energy and information flow, real-time monitoring, automation, demand response,
integration of renewables, and improved efficiency, flexibility, and reliability compared to the
conventional one-way, manually operated grid.

2. Why is HVDC (High Voltage Direct Current) transmission advantageous over HVAC (High Voltage
Alternating Current) for certain applications?

Answer:

HVDC is preferred for long-distance transmission and undersea cables because it eliminates reactive
power issues, allows asynchronous interconnection of grids, reduces line losses, requires less copper
and insulation, and provides more controllable power flows. Its main disadvantages are higher terminal
costs and more complex fault isolation.

3. What are some common types of filters used between inverters and the grid, and why are they
necessary?

Answer:

Common filters include L ,LC and LCL filters. They are necessary to attenuate harmonics generated by
inverter switching, ensuring that current injected into the grid meets harmonic standards and thus
maintains power quality.

Lecture 2

Question 1:

Compare a diode, thyristor, and IGBT in terms of structure, control, and application,
including their characteristic curves.

Model Answer:

Parameter Diode Thyristor IGBT

Four-layer (PNPN), 3 MOS gate with bipolar
Structure Two-layer (PN)
terminals (A, K, G) output
Controlled turn-on via gate; Controlled turn-on and
Control Uncontrolled
turn-off by current turn-off via gate
Instantaneous with Delayed by gate current; Fast switching with gate
Switching
voltage polarity natural or forced turn-off signal
 Parameter Diode Thyristor IGBT
On-State
Low Lower than IGBT Higher than thyristor
Voltage
Inverters, SMPS, motor
Applications Rectifiers HVDC, AC/DC converters
drives
Short circuit (preferred for
Failure Mode Open circuit Typically, open circuit
HVDC protection)

Question 2:

Explain the role of a snubber circuit in thyristor applications. What are the key protections
it offers?

Model Answer:

Snubber circuits are essential for protecting thyristors during switching operations. They provide:

 di/dt Protection: A series inductor (Ls) limits the rate of rise of current to avoid
localized heating before current spreads across the device.
 dv/dt and Overvoltage Protection: A parallel RC circuit prevents accidental turn-on
due to rapid voltage changes and limits overvoltages during switching.
 Voltage Balancing: Grading resistors (Rdc) ensure uniform voltage sharing among
series-connected thyristors in HVDC valves.

These protections help prevent damage due to thermal stress, unwanted turn-on, and uneven
voltage distribution.

Question 3:

Problem: Calculate the total conduction loss in a 6-pulse 2000A, 500kV HVDC converter
using thyristors rated at 6500V and 2800A. Each valve contains 170 devices. Use VT₀ = 1V,
Ron = 0.00035Ω.
Lecture 3

Q1: 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.

Model Answer: The Phase Locked Loop (PLL) ensures that the converter controller remains
synchronized with the AC commutation voltage. This synchronization allows precise thyristor
firing, which is essential in both rectifier and inversion modes.

Traditional HVDC systems relied on zero-crossing detection for synchronization, but this
method was prone to harmonic distortions. Modern systems use the transvector PLL, which
tracks the positive sequence of AC voltage, quickly adapting to system dynamics and ensuring
stable operation.
Q2 : What is the nominal rectifier operating angle α\alpha in LCC-HVDC systems, and why is it
important to maintain this range during operation?

Model Answer: The nominal rectifier operating angle α\alpha typically ranges between 15∘15^\
circ and 20∘20^\circ. This range ensures stable operation and helps compensate for disturbances
in AC voltage.

If α\alpha is too low, commutation margins may be insufficient, increasing the risk of voltage
instability and harmonic distortions. The transformer tap changer regulates the valve AC voltage
to keep α\alpha in this optimal range, ensuring efficient power conversion.

Q3: What is commutation overlap μ\mu, and how does it impact DC voltage in LCC-HVDC
systems?

Model Answer: Commutation overlap μ\mu refers to the period when two thyristors
simultaneously conduct during commutation. This temporary short circuit reduces the effective
DC voltage, causing a voltage drop ΔVdc\Delta V_{dc}.

Excessive commutation overlap can lead to higher losses and heating in converter components,
potentially affecting system stability. Engineers adjust system parameters like firing angles and
inductance to minimize voltage drop and optimize HVDC performance.

Lecture 4
Q1: Explain the role of the LCC inverter controller in HVDC systems. How does it regulate DC
voltage, and why is it essential for system stability?

Model Answer: The LCC inverter controller regulates the DC voltage at the inverter terminal to
ensure stable HVDC operation. It typically operates in DC voltage control mode using a
proportional-integral (PI) feedback loop that adjusts the firing angle to maintain the desired DC
voltage level.

However, during disturbances, the inverter controller may transition to current control mode to
stabilize the system. The DC current controller acts as a backup, reducing the firing angle when
DC current drops below the reference value. This mechanism is crucial during rectifier-side AC
faults, where the rectifier may lose current control.

Q2: What is the function of the extinction angle controller in an LCC inverter? Why is
maintaining a sufficient extinction angle important to prevent commutation failure?

Model Answer: The extinction angle γ\gamma controller ensures that thyristors have adequate
time to regain their blocking capability after conduction. Maintaining γ\gamma at around
15∘−20∘15^\circ - 20^\circ prevents commutation failures, which occur when a thyristor
continues conducting due to inadequate reverse voltage application.

During commutation, the outgoing thyristor’s current reduces while the incoming thyristor’s
current rises. If commutation overlap is excessively long, the outgoing thyristor may fail to turn
off, leading to a DC short circuit. In weak AC systems, commutation failure can significantly
impact HVDC transmission reliability.

Q3: Describe the different operating modes of an LCC-HVDC inverter and explain how power
reversal is achieved in bidirectional HVDC systems.

Model Answer: An LCC-HVDC inverter operates in multiple control modes:

1. Constant DC Voltage Mode: The inverter regulates the DC voltage using a PI

controller.
2. Constant Extinction Angle (CEA) Mode: Used as backup control in weak AC systems,
ensuring thyristor commutation reliability.
3. Constant DC Current Mode: Activated during disturbances when the rectifier loses
control.

In bidirectional HVDC systems, power direction reversal is achieved by swapping the inverter
and rectifier roles. The terminal that receives the current margin operates as the inverter. Power
reversal is performed seamlessly by modifying control references, ensuring rapid adjustments
without hardware changes.

Lecture 5
Q1: Explain why IGBT failure in open circuit is problematic for HVDC systems and how
manufacturers mitigate this issue.

Model Answer: IGBTs naturally fail in an open circuit, which can disrupt power transmission
by breaking the HVDC path. This is problematic in series-connected switches within HVDC
valves, where a single failure can halt operation.

To mitigate this, some IGBT cell assemblies are designed to fail in short circuit, maintaining
system continuity. Engineers also integrate redundancy strategies and failure detection
mechanisms to enhance reliability.

Q2: Compare the characteristics of IGBT and IGCT technologies in HVDC applications,
particularly in terms of efficiency, losses, and reliability.

Comparison Table: IGBT vs. IGCT in HVDC Systems

IGBT (Insulated Gate Bipolar IGCT (Integrated Gate Commutated

Feature
Transistor) Thyristor)

Switching Speed Fast switching, ideal for PWM control Slower, limits use in PWM applications

Conduction Loss Higher on-state voltage drop Lower conduction losses

Gate Drive Simple, voltage-controlled Complex, high gate current required

Failure Mode Typically fails open circuit Typically fails short circuit

Suitability Ideal for VSC-HVDC converters Used in niche high-current applications

Q3: Thyristors have been widely used in LCC-HVDC systems, yet VSC-HVDC relies on
IGBTs. What are the key reasons behind this choice?

Model Answer: IGBTs are preferred in VSC-HVDC because they can turn on and off
independently, unlike thyristors, which require a commutation voltage to switch off. This allows
VSC converters to control both active and reactive power dynamically.

Additionally, IGBTs operate at higher switching frequencies (1-2 kHz), reducing harmonic
distortion and minimizing the need for bulky filters. Thyristors, while more efficient for high-
power transmission, rely on an existing AC grid for commutation, making them unsuitable for
black-start operations or weak grids.

Lecture 6

Q1: Explain the advantages of Neutral-Point Clamped (NPC) multilevel converters compared to
conventional two-level VSCs in HVDC applications.
Model Answer: NPC multilevel converters provide significant improvements over two-level
designs, primarily by reducing harmonic distortion and switching losses. The key advantages
include:

 Lower harmonics in AC waveforms, reducing filtering requirements.

 Lower voltage stress (dv/dtdv/dt) on transformers, improving insulation longevity.
 Higher efficiency, as switching losses are significantly reduced.

Despite these benefits, multilevel converters require more complex control algorithms and
capacitor voltage balancing techniques to maintain performance.

Q2: What are the key challenges of conventional two-level voltage source converters in HVDC
applications, and how do engineers address them?

Model Answer: Two-level VSC converters face several scalability and insulation challenges,
particularly when operating at high voltages (e.g., 400kV). The switching frequency imposes
high voltage stress (dv/dtdv/dt) on transformers, increasing insulation costs and reliability
concerns.

Additionally, large AC filters (12%-33% of the converter rating) are required due to harmonic
distortion. Engineers mitigate these issues by developing multilevel converters, which reduce
switching losses, limit transformer stress, and lower harmonic content.

Q3: 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?

Comparison Table: Single-Phase vs. Three-Phase Two-Level VSC Converters

Feature Single-Phase VSC Three-Phase VSC

Uses two switching devices per

Topology Utilizes six switches in a three-leg configuration
phase

Shows a single sinusoidal

Voltage Waveforms Generates multiple phase-shifted waveforms
waveform

Can support complex AC loads and grid

Load Handling Suitable for simple loads
interconnection

Harmonics &
Requires additional filtering Reduced harmonics due to phase balancing
Filtering

Space vector modulation (SVM) or sinusoidal

Switching Control Pulse-width modulation (PWM)
PWM
Lecture 7

Q1: Explain why Third Harmonic Injection (THI) is used in PWM modulation and discuss its
advantages and drawbacks for VSC-HVDC applications.

Model Answer: THI improves conventional PWM modulation by injecting a 17% third
harmonic component into the reference signal, increasing the fundamental voltage amplitude.
This extends the linear modulation range and reduces distortion in the AC voltage.

However, THI introduces substantial third harmonic components in line-to-neutral voltages.

Fortunately, in balanced systems with a floating neutral, these third harmonics do not propagate
into the line-line voltages, making the method effective for HVDC converters.

Q2: Selective Harmonic Elimination (SHE) is a technique designed to eliminate specific

harmonics in voltage waveforms. How does SHE modulation achieve this, and what are its
advantages and challenges?

Model Answer: SHE modulation eliminates predefined low-order harmonics by calculating

optimal switching angles using Fourier series modeling. Each control angle removes a
specific harmonic, enhancing waveform quality and efficiency.

However, the method requires offline computation and stored lookup tables, making real-time
adjustments difficult. Engineers must optimize angle resolution to ensure accurate harmonic
suppression while keeping switching losses minimal.

Q3: Managing Harmonics in VSC-HVDC Systems

Question: Harmonics generated in VSC-HVDC systems can disrupt AC and DC transmission

networks. What are the primary sources of harmonics, and how are they mitigated?

Model Answer: VSC-HVDC converters generate switching harmonics due to PWM operation.
The harmonic spectrum includes:

1. Low-frequency harmonics (baseband),

2. Switching harmonics (carrier-related),
3. High-frequency harmonics (sidebands).

To mitigate harmonics, engineers use multi-pulse techniques, modulation strategies, and

harmonic filters. Choosing carrier frequencies as odd multiples of three ensures the
elimination of triplen harmonics in line-line voltages.

Lecture 8

Q1: What Are the Key Differences Between Different MMC Voltage Levels in
Terms of Total Harmonic Distortion (THD)?
Model Answer: MMC converters with higher voltage levels significantly reduce Total
Harmonic Distortion (THD) in their output waveforms. For example:

 3-level MMC: High THD (~55% for phase voltage).

 9-level MMC: Moderately reduced THD (~14%).
 21-level MMC: Low THD (~5%).
 101-level MMC: Extremely low THD (~1%), approaching sinusoidal waveforms.

Higher-level MMC designs reduce filtering requirements and improve power quality, making
them preferred for long-distance HVDC transmission.

Q2: How Does Full-Bridge MMC Enhance DC Fault Protection Compared to

Half-Bridge MMC?

Model Answer: Full-Bridge MMC cells provide better DC fault ride-through capabilities
compared to Half-Bridge MMC because they can generate both positive and negative voltage
states, effectively blocking DC fault currents.

This allows Full-Bridge MMC systems to handle DC faults without external circuit breakers,
making them more resilient in HVDC applications. However, this comes at the cost of increased
conversion losses due to additional switching events.

Q3 What is the Function of Capacitor Voltage Balancing in MMC Converters?

Model Answer: Capacitor voltage balancing ensures stable operation of MMC converters by
sorting upper and lower capacitors based on their voltage magnitudes and adjusting switching
patterns to regulate charge distribution.

The key challenge in MMC control is keeping capacitor voltages within a safe range to avoid
fluctuations in converter output. The algorithm continuously identifies capacitors with
maximum and minimum voltage levels and selectively activates or bypasses specific cells to
maintain balance.

Lecture 9

Q1: How Is VSC Converter Modeling Transferred to the DQ Frame?

Model Answer: To analyze VSC-HVDC system dynamics, converters are modeled using the
DQ rotating reference frame, simplifying control system equations.

The transformation follows these steps:

1. Convert phase voltages (ABC frame) into DQ coordinates using Park’s transformation.
2. Express power equations in terms of DQ variables:
 3. Compute DC current based on reference voltages and switching functions.
4. Dynamic equations in the DQ frame account for AC grid interactions and control
inputs.

This representation allows efficient vector control, making active and reactive power control
independent.

Q2: Describe the Function of the Inner Current Controller in VSC Converters.

Model Answer: The inner current controller in VSC converters regulates converter output
currents, ensuring system stability and fault protection.

The control objectives include:

 Maintaining DC voltage regulation, preventing overvoltage damage.

 Ensuring accurate power transfer, preventing fluctuations.
 Controlling reactive power exchange, stabilizing weak AC grids.

Key components:

1. Decoupling Control Loops: Eliminates unwanted cross-coupling effects between

control signals.
2. Proportional-Derivative (PD) Controller: Ensures fast response and closed-loop
stability.
3. Laplace-Domain Representation: Models control system equations to optimize system
damping.

The inner controller acts as a fast-response system, while the outer control loops refine
performance.

Q3: Explain the Converter Average Value Model in VSC Converters.

Model Answer: The converter average value model simplifies the representation of a VSC-
based HVDC system by averaging the effects of switching devices over time. This approach
eliminates the need to analyze individual switching events, making it easier to simulate large-
scale power systems.

Key characteristics of the model:

 SPWM-controlled output voltage varies with the reference and carrier signals.
 The fundamental AC voltage component is approximately
  Assumes a balanced and symmetrical system with ideal conservation of power.
 The DC current equation is derived based on the AC currents flowing through the
converter.

Lecture 10

Q1: How Does the DC Link Capacitor Influence VSC-HVDC System

Performance?

Model Answer: The DC capacitor serves as the primary energy storage element in the VSC,
stabilizing high-frequency voltage fluctuations. Its functions include:

 Reducing voltage ripple, ensuring smooth operation of the converter.

 Providing a low-inductance path for turn-off current, mitigating transient voltage
stress on switching devices.
 Enhancing dynamic performance, allowing for fast power control adjustments.

The capacitor sizing must balance control speed, harmonic reduction, and system costs. A
larger capacitor reduces ripple but slows transient response, while a smaller capacitor
improves control speed but increases ripple.

Q2: Explain the Function of Major Components in a VSC-HVDC System.

Model Answer: A VSC-HVDC system consists of several critical components:

 DC Reactor → Limits transient currents and smooths DC voltage variations.

 Power Transformer → Matches AC system voltage with VSC converter voltage and
provides galvanic isolation.
 AC Reactor → Reduces peak switch currents and fault currents.
 DC Link Capacitor → Stores temporary energy and stabilizes DC voltage between
switching cycles.
 AC Harmonic Filter → Suppresses high-frequency harmonics generated by PWM
switching.
 DC Filter → Reduces harmonic distortion on the DC transmission line, preventing
resonances.

Q3: How Does Droop Control Improve Stability in Multi-Terminal VSC-HVDC

Systems?

Model Answer: Droop control is used in multi-terminal VSC-HVDC networks to balance DC

voltage fluctuations and power sharing between converter stations. Instead of enforcing a strict
DC voltage setpoint, converters adjust their power based on voltage deviations.
This method enhances stability, preventing converters from overreacting to voltage changes and
ensuring smooth power distribution across multiple HVDC terminals.

Lecture 11

Q1 Why is circulating current control necessary in MMCs, and how does second
harmonic injection help?

Circulating currents, especially the second harmonic component, can cause energy losses and
excessive heating in MMCs. Instead of relying on large inductors, second harmonic injection
actively cancels these currents, improving efficiency and minimizing thermal stress on the
system.

Q2 What are the limitations of the Average Value Model in MMCs, particularly
for fault analysis and capacitor balancing?

While the Average Value Model simplifies MMC simulation, it does not account for individual
cell switching, fault scenarios, or capacitor voltage balancing. These limitations make it less
effective for detailed control strategies and protection studies but still useful for steady-state
analysis.

Lecture 12

Q1 How do AC faults affect VSC-HVDC converters, and how do they respond?

Model Answer: During an extreme AC fault, the grid voltage collapses, leading to high
transient currents in the VSC. The converter adjusts its modulation index and control parameters
to minimize instability. The response includes active power reduction and reactive power
compensation to stabilize the faulted system.

Q2. What happens during DC faults in a 2-level VSC, and how is fault current
controlled?

Model Answer: In a 2-level VSC, a DC fault causes an uncontrollable rise in fault current
through freewheeling diodes. Since IGBTs cannot block DC faults, the AC circuit breaker
disconnects the system to stop current escalation. Adding DC reactors limits fault current growth
while improving recovery time.

Q3. Why is a Full-Bridge MMC better at handling DC faults compared to a Half-

Bridge MMC?

Model Answer: A Full-Bridge MMC can maintain operation during DC faults by reversing DC
voltage polarity, helping extinguish faults and preventing fault transfer to the AC side. It also
retains reactive power control, ensuring stability in weak AC systems, unlike Half-Bridge
MMCs, which must be blocked.