Chapter 1
Inverter-based Resource Power Plant Control and AC Delivery

1.1 Inverter-based Resource Grid Integration Circuit Topology

AC delivery is a common approach for offshore wind farms (OSWs) located close to shores. As of 2024, the break-even distance to replace an AC delivery system by a DC delivery system is 45 miles [1]. Take the example of the United States' first utility-scale deep-water offshore wind farm - Block Island Wind Farm in Rhode Island. This wind farm has a size of 30 MW and is connected to the mainland power grid through a 25-mile 34.5-kV undersea cable. The wind farm consists of five 6-MW type-4 wind turbines (GE/Alstom Haliade 150-6 MW offshore wind turbine) with direct-drive permanent magnet generators. Each tower base has a wind turbine (including the mechanical parts, a generator, and an inverter of 900 V), a pad-mount transformer to step 900 V to 34.5 kV, switchgear, and a low-voltage electrical distribution cabin.

In the Rock Island Wind Farm case, 34.5-kV or medium-voltage (MV) cables are used to transmit 30 MW of power. To transmit power greater than 100 MW, high-voltage (HV) AC delivery system is preferred. The newly operational 132-MW South Fork Wind Farm, 19 miles away from Block Island and 30 miles east of Montauk Point on the South Fork of New York's Long Island, uses a 138-kV undersea cable to deliver power to an onshore 138/69-kV substation at Long Island. An offshore substation was built to connect with twelve 11-MW Siemens Gamesa type-4 wind turbines through MV cables of 34.5-66 kV. The offshore substation has step-up transformers to boost voltage to 138 kV. In addition, the substation is equipped with switchyard reactors, a small diesel generator, and a supervisory control and data acquisition system to monitor and control wind farm operations.

OSW construction in the United States lags behind Europe. Many experiments and guides have been produced from the prior installation. For example, CIGRE Technical Brochure 483 "Guidelines for the Design and Construction of AC Offshore Substations for Wind Power Plants" [2] published in 2011. This 378-page guide extensively covers OSW substation construction, including the justification of voltage levels. For OSWs, the transmission level is usually from 132 to 150 kV for 100 MW-size OSWs, with 245-kV AC level gaining importance for 350/400 MW OSWs. On the other hand, 400-kV AC level does not seem to have a promising future due to cable manufacturing capability and 400-kV switchgear and transformers being too bulky.

1.2 Inverter-level Control Logic

The focus of the book is on the operational challenges of inverter-based resource (IBR) power plants integrated into weak AC grids. Therefore, only the most relevant controls are examined. These include the inverter-level control and power plant-level control.

From the grid's point of view, a solar PV, a type-4 wind turbine, and a battery energy storage system (BESS) have very similar characteristics. All are connected to AC grids through a grid-connected inverter, and the inverter's control dynamics influences the system dynamics. While all types of inverter controls are of similar nature and can be categorized as grid-following control, it has to be noted that original equipment manufacturers (OEMs) are different. In the United States, SMA and Tesla are two main OEMs for solar and battery inverters, while GE and Siemens are the main OEMs for wind turbine inverters.

Compared to an AC machine, an inverter is very sensitive to overcurrent. Therefore, effective current limiting technologies have been developed to ensure that inverter currents are within limits. This is usually achieved by having tight inner current control so that the current measurements can follow the current order. Limits can be added to the current orders. When designed properly, an inverter indeed acts as a current source.

Besides the inner current control, an inverter has the outer control to achieve control functions, e.g., power or voltage regulation. And also, very importantly, an inverter needs to have a synchronizing unit to be integrated into a main grid. In the following, the three control units - synchronizing units, inner current control, and outer grid function control - will be explained one by one.

Figure 1.1 shows the circuit diagram of an IBR grid integration system. The converter current and the point of common coupling (PCC) bus voltage or will be measured and used for IBR control. The outputs from the IBR control are the modulation signals that influence the terminal voltage of the voltage-source converter, denoted as , where , and .

Figure 1.1 An IBR grid integration system with the measurements for IBR control marked.

Figure 1.2 shows a typical grid-following (GFL) control structure with three units, where a phase-locked loop (PLL) generates a synchronizing angle, the inner current control regulates the currents, and the outer control regulates the real and reactive power.

Figure 1.2 Inverter-level grid-following control structure.

1.2.1 Inner Current Control

It is easier to regulate DC signals than AC signals. In addition, we will show that current vector control relies on regulating components of the current space vector. When the current space vector is viewed in the static frame, it is a rotating vector. When the current space vector is viewed from the PLL frame, it is a static vector at steady state, and its components in the PLL frame are DC signals. Therefore, the inner current control is typically implemented in the PLL frame to ensure that the currents follow the current references generated by the outer control loop.

It can be seen that the IBR's output current has the following relationship with the converter voltage and the PCC bus voltage , viewed in the static frame:

where and are the resistance and inductance of the choke filter, respectively. In the PLL frame with a rotating speed of , the above relationship becomes the following:

The inner current control treats (or ) as the output from the plant or the measurement to be compared with the order (or ). The error between the order and the measurement is then fed to a PI controller to generate the output. This output is (or ). To arrive at the converter voltage signals (or ) that can be used to change the converter voltage, the output should be compensated by a feedforward unit of the PCC bus voltage (or ) and a cross-coupling unit (or ).

It can be seen that the current order and the current measurement have the following relationship:

Therefore, the closed-loop system of the measurement tracking the order is expressed as follows:

Figure 1.3 shows the Bode diagrams of the closed-loop system for different . It can be seen that is very sensitive to the bandwidth. A quick evaluation of the bandwidth can be taken by approximating the closed-loop transfer function:

Figure 1.3 Current tracking performance. , , .

Since the current control design requires having a very high bandwidth above 100 Hz, and at the high-frequency region, and , the closed-loop system can be approximated as a first-order low-pass filter and its rise time is . Its bandwidth is rad/s. Increasing from 0.3 to 0.5 effectively improves the controller's bandwidth by 1.7 times. This evaluation agrees with the bandwidths shown in the Bode diagrams in Figure 1.3: for the PI controller of , the current tracking control's bandwidth is 163 Hz, while for the PI controller of , the bandwidth is 270 Hz.

The inner current controller consists of three elements: the PI controllers, the cross-coupling terms, and the PCC voltage feedforward. This control structure has been presented in the classic book [3].

The cross-coupling terms were introduced in three-phase motor drives, to counter the coupling effect of the plant model (the RL circuit dynamics in a frame). This coupling effect can be shown as the terms with imaginary coefficients in the complex vector expressions for voltage and current in the frame. For example,

Reference [4] has compared three types of designs for a motor serving an RL load: just PI controllers, the PI controllers with cross-coupling, and the complex vector controller in the form of . The first one (just PI controllers) shows degraded performance if the rotating speed of the frame is close to the controller bandwidth. The latter two achieve similar performance for different frame speeds, since the introduced cross-coupling terms can effectively counter the coupling from the plant.

One shortcoming of the strategy of using PI controller with cross-coupling is that it requires an accurate estimation of the choke filter's inductance. Reference [4] shows that if the inductance parameter deviates significantly from its actual real value, the controller's performance will deteriorate. On the other hand, the complex vector controller does not need this info and the controller's performance is robust against this parameter variation.

In AC motor drives, voltage feedforward is usually not used. This unit was introduced specifically for grid-connected converter control. The technology may be traced back to [5], a paper...