Section 2, 4 & 6

Explain how reduced system inertia affects system rate of change of frequency (RoCoF) and frequency nadir (minimum instantaneous frequency) for a given active power disturbance such as disconnection of a large synchronous generator or HVDC link.

Describe different remedial measures and their working principles for limiting frequency nadir in a low inertia power system

FFR: Active power is injected very quickly when frequency falls, directly raising the nadir

FCR: Stabilizes the frequency to steady state but not nominal

Kp (proportional/droop gain): Determines how much power is delivered per Hz of frequency deviation, higher Kp results in higher nadir, when Kp is too high up it produces oscillations

Ki (integral gain): Integral action eliminates steady state frequency error

Delay: Larger delay results in a deeper nadir

Why does generator governor control use droop control instead of PI control for providing frequency containment reserve in a large power system?

Sharing according to droop constant not controller and process speed

Avoid fighting in case of multiple PI controllers controlling the same frequency

What is the difference between proportional droop and PI-based droop for governor control?

P-based droop: Steady state gain = transient gain, steady state frequency error persists, typical for BESS with low delay in power regulation

PI-based droop: Steady state gain ≥ transient gain, steady state frequency error goes to zero, typical for hydro with significant delay in penstock for power regulation

What are the main factors that affect the system frequency nadir during an active power disturbance?

What are the remedial measures (strategies) to limit frequency nadir in a low inertia power system?

System inertia, generator governor response (droop), and FFR

Remedial measures: FFR, synthetic inertia, more/faster FCR

What is an electrically strong or weak system?

What are the common measures to indicate the grid electrical strength?

Electrically strong system: Provides high short circuit capacity or low grid impedance

measures → SCC, SCR

What is the physical meaning of reactive droop in an automatic voltage regulator (AVR) in a synchronous generator?

Control voltage at a remote or internal bus through load compensation

Reactive droop only compensates for reactive voltage change

Explain how reduced system rotational energy (synchronous inertia) affect system frequency stability and quality in terms of rate of change of frequency (RoCoF), frequency nadir and zenith, and steady-state frequency deviation.

Explain the purpose of droop-based control for frequency regluation in a power system.

Sharing FRC among multiple generators, avoid PI controller fighting among generators

In transmission system, there are different disturbances such as short circuit, switching in and out of different electrical equipment and load variations from minute to minute to hour to hour.

Explain what the main factors are that affect the grid voltage (magnitude) during such disturbances.

Grid strength and impedance, SCC

load level

load power factor

load voltage dependence characteristics

In transmission system, there are different disturbances such as short circuit, switching in and out of different electrical equipment and load variations from minute to minute to hour to hour.

Explain what the remedial measures that exist in the power system to regulate the grid voltage after a voltage disturbance occurs.

Enhance grid SCC (e.g. synchronous condenser)

Voltage regulation

—> AVR from synchronous generator, FACTS devices, inverter based generator/ storage, tap changer

Reactive compensation

—>Shunt reactor/capacitor, line disconnection, bypass of series compensation

For an efficient and economical operation of a power system, the losses in the transmission system should be kept as low as possible. Explain the operational methods.

Uniform voltage control

Local reactive power compensation

Increase voltage set point slightly

In Sweden 78% of all transmission losses occur on the overhead line. Propose two methods that are reasonable and economical to minimize the losses in the transmission system.

Parallel lines

Phase changing transformers (boosters)

In power system analysis per unit system is commonly used. Explain the benefits of using per unit values of electrical quantities as compared to using values in kV, kA, and $\Omega$

$\sqrt3$ and 3 disappear in power calculations

V, I, and Z expressed in percentage of the base values (more comparable)

Gets rid of transformer nominal ratios in p.u. calculations