Different Types of Subsynchronous Phenomenon

In general, the subsynchronous phenomenon can be categorized into three types:

Analysis Frequency Scan Eigenvalue Analysis Impedance-Based Analysis Residue-Based Analysis EMT Simulations Frequency Domain Time Domain

Used in this research to obtain safe operating conditions Briefly explained

Due to their limitations

Technique Features Limitations

Frequency scan

- Provides the preliminary results for subsynchronous analysis

- Effective in study of IGE

- Effective and cost-efficient technique - Identifies possible TI and TA problems

- Limited to impedance seen from the network - Tends to be an approximate method - Only identify the risk of SSCI

- Needs to be supported by time domain simulations - Time consuming technique depending on the number of scenarios and used signals

Eigenvalue analysis

- Based on mathematical model of system - Provides frequency and damping of system modes - Used for study of TI and IGE

- Requires detailed model of system which may not be available

- Not suitable for TA analysis

- Complexity arises as the size of system increases

Impedance-based analysis

- Save time in analysis - Frequency domain analysis - Used for IGE analysis

- Provides modular approach for SSR Analysis.

- Not accurate results - Not much literature available

- Complexity arises as the size of system increases - Difficulty in implementation of several functions in terms of

Impedance i.e., filters in abc-frame Residue-based

analysis

- Provides the best feedback signals and feedback locations for damping

controller

- Frequency domain analysis

- Proposed feedback signals may not be available in wind farm

structure

- Proposed feedback loops may not efficiently use the capacity of converters

EMT simulations

- Time domain analysis

- Can be used to show the impact of time varying parameters - Suggests the utilization of simulation software

- Used for IGE, TI and TA analysis

- Cannot be used for design purposes - New system models are not available easily

generator and series compensated transmission line systems.

– Subsynchronous control interaction (SSCI): SSCI, also known as subsynchronous inter- action (SSI), occurs when there is an interaction between the control system of power electronic devices and the series compensated transmission system.

– Subsynchronous torsional interaction (SSTI): This phenomena occurs due to the interac- tion between turbine-generator mechanical system and transmission-level devices (such as compensated lines and HVDC).

The SSR itself can be categorized into two phenomena with different characteristics, Fig. 4.2: – Self-excitation

– Transient torque amplification (TA)

The self-excitation phenomenon is initially triggered by a small disturbance signal. Using the linearization approach, we can determine the possibility of occurrence of self-excitation and analyze the system behavior.

The TA phenomenon often occurs due to a large perturbation that changes the operating point of the system. The TA is a non-linear and complex phenomenon which is often analyzed by time domain simulations (e.g., EMT simulations).

Self-excitation phenomenon itself is categorized into two types: – Induction generator effect (IGE)

– Torsional interaction (TI)

In [86, 87], it has been shown that an occurrence of the IGE results in the occurrence of the TI, and vice versa. The TI brings about a negative resistance which triggers the IGE, whereas the IGE results in a negative damping for the TI. The IGE occurs if the equivalent resistance observed from the rotor neutral point (i.e., the SSR impedance) is negative, and if a frequency exists at which the SSR reactance is almost zero. The equivalent resistance

SSO SSTI SSR SSI TA Self Excitation IGE TI

observed from the rotor neutral point is [88]:

Ref =

Rr

s + Rs+ Rt, (4.1)

where s is the machine slip and Rr, Rs and Rt are the rotor, stator and transmission line equivalent resistances, respectively. It should be noted that, in a DFIG-based wind farm, the resistance resulting from the operation of the RSC should also be added to the above expression, i.e., Ref = ZRSC+ Rr s + Rs+ Rs and ZRSC = − Vr sIr (4.2) where Vr, Ir are, respectively, the steady-state rotor voltage and the rotor current. The severity of oscillations corresponds to the magnitude of the negative resistance.

Tortional interaction or TI is an unstable electromechanical condition in which the power system and the mechanical system of the DFIG exchange energy at low frequencies. More specifically, the TI occurs when the sum of the power system resonance frequency and the natural frequency of the shaft becomes equal to 60 Hz. Subsequent to a TI occurrence, the rotor oscillations induce a voltage on the stator of the generator. The induced voltage includes two components of frequencies fr− fnand fr+ fn, where frand fnare the resonance and nominal frequencies of the power system, respectively. The sub-resonance frequency component (fr−fn) may become unstable depending on the damping value of the mechanical shaft. The supersynchronous component (fr+ fn) will be always damped as demonstrated in [89]. If the torque resulting from the subsynchronous current is greater than or equal to the natural damping of the shaft, the generator becomes self-excited. Such a condition results in shaft aging or even failure.

Hydro power plants are immune to the TI phenomenon due to the higher inertia of the generator in comparison to that of the turbine [87]. The TI also has a low impact on a DFIG wind turbine where the shaft is designed to have very low natural frequencies (e.g., below 10 Hz).

The SSI, or SSCI as it is sometimes called, is the interaction between a series compensated transmission system (or an HVDC system) and the control circuit of the wind turbine genera- tor [90]. Various faults or disturbances can trigger an unstable SSI between the power network and the current control loops of the RSC. The SSCI is a purely electrical phenomenon which results from the fast response of the RSC controller following faults and disturbances [20, 91]. In other words, due to disturbances or faults, the current control loops of the RSC change the rotor resistance so that the resistance observed from the stator side becomes negative.

The SSCI may occur if the resistance observed from the rotor neutral point becomes negative where the reactance crosses zero [20]. It should be noted that the magnitude of the equivalent resistance is a measure of damping at resonance frequency.

The SSTI is an electromechanical phenomenon where there is a huge energy exchange between the series compensated transmission system and the wind turbine’s rotating masses. This phenomenon is similar to the turbine-generator shaft torsional interaction as detailed in [79].