Renewable Modelling · Type-4 MSC Control

Type-4 Wind Turbine MSC Control in EMTP®: MPPT Torque, q-Axis Current and IMC Tuning

The full-scale converter guide split the work between a machine-side and a grid-side converter. This page opens up the machine-side converter (MSC) of a Type-4 wind turbine with a permanent-magnet generator. Its job is narrow and clean: extract the available wind power into the dc link at the right operating point. Maximum-power-point tracking sets the torque, the torque becomes a q-axis current, the d-axis current is held at zero, and a fast inner loop — tuned straight from the machine parameters by internal model control — makes the currents follow. This guide works through that chain.

Reading time ≈ 20 min · MPPT torque, q-axis current & IMC

The full-scale converter guide divided the work between a machine-side converter (MSC) and a grid-side converter, and gave each a one-line objective. This page opens the MSC of a Type-4 wind turbine — a full-converter machine with a permanent-magnet synchronous generator (PMSG) — and shows how that objective is actually met. The MSC’s task is narrow: pass the available wind power into the dc link while holding the turbine near its best operating point. It does so not by commanding “power” directly but through torque-oriented current control, tuned straight from the machine parameters. This guide follows that chain from the maximum-power-point tracking down to the inner current loop.

Abbreviations used on this page

MSCMachine-side converter

FSCFull-scale converter

PMSGPermanent-magnet synchronous generator

MPPTMaximum-power-point tracking

d–qDirect / quadrature rotating frame

PIProportional–integral controller

IMCInternal model control

\(\lambda_m\)Permanent-magnet flux

\(\alpha_c\)Current-loop bandwidth

\(i_{qm},\ i_{dm}\)q- and d-axis machine currents

PWMPulse-width modulation

EMTP®Electromagnetic Transients Program

Key idea

The MSC’s job is to extract the available wind power into the dc link while keeping the turbine–generator at its optimal operating point. It controls machine torque, not grid quantities.
MPPT sets the torque from generator speed (\(T' = K_{opt}\,\omega_t^{2}\)); the torque becomes a q-axis current reference (\(i_{qm}'=T'/\lambda_m\)) because in the stator-flux frame the q-axis current controls torque and active power.
The d-axis reference is set to zero (\(i_{dm}'=0\)) for unity power factor on the machine side — the MSC does no reactive task. The outer “block” is reference generation, not a PI loop; the fast PI is the inner current loop.
The inner-loop PI gains are not arbitrary: internal model control (IMC) derives them from the PMSG parameters and the desired bandwidth (\(k_p^{d}=\alpha_c L_d,\ k_p^{q}=\alpha_c L_q,\ k_i=\alpha_c R_s\)), with the bandwidth set from the rise time (\(\alpha_c=\ln 9/t_{rise}\)).

Key terms used on this page

01Machine-side converter

The MSC: the converter facing the generator; it controls torque and extracts power into the dc link.

02MPPT

Maximum-power-point tracking: from generator speed it sets the torque for optimal power capture.

03Torque reference

\(T'\): the electromagnetic torque the MSC must produce, from MPPT.

04q-axis current

\(i_{qm}\): the current component that produces torque in the stator-flux frame.

05d-axis current = 0

\(i_{dm}'=0\): held at zero so the machine side does no reactive task (unity power factor).

06Stator-flux frame

The MSC’s reference frame; it makes the q-axis current the torque-producing component.

07Unity power factor

No reactive exchange on the machine side, achieved by \(i_{dm}'=0\).

08Inner current loop

The fast PI loop forcing the dq currents to follow their references, producing dq voltage references.

09Internal model control (IMC)

A tuning method giving PI gains directly from the plant model and the desired bandwidth.

10Bandwidth

\(\alpha_c\): how fast the current loop responds; the larger it is, the faster the loop.

11Rise time

\(t_{rise}\): the 10–90% response time; it fixes the bandwidth through \(\alpha_c=\ln 9/t_{rise}\).

12PM flux

\(\lambda_m\): the constant flux from the permanent magnets; it links torque to q-axis current.

Section 1

Inside the machine-side converter

This is the left-hand converter of a full-converter wind turbine: wind turbine and gearbox, the PMSG, the MSC, the dc link, the grid-side converter and the grid. The focus here is the MSC only — the source side — and that matters because the two converters do very different jobs. The MSC controls the generator side: machine torque, the speed operating point, and active-power extraction from the turbine. The grid-side converter controls the dc-link voltage and the grid interface. Here the whole focus is the machine side.

Machine side vs grid side

MSC: extract the wind power correctly from the PMSG into the dc link. GSC: manage the dc link and interface that power to the grid. This page is entirely about the first.

Section 2

The MSC’s job: extract wind power to the dc link

The practical goal of the machine-side converter is to pass all of the available active power into the dc link — but not blindly. The turbine captures mechanical power from the wind, the generator turns it into electrical form, and the MSC must extract that power while keeping the turbine–generator at the right operating speed. So the objective is really two things at once: extract the available wind power, and hold the optimal aerodynamic operating point. That is why the control begins with maximum-power-point tracking.

Figure placeholder Schematic of the MSC control Image to be inserted — the MSC control: the maximum-power-tracking point and torque reference at the bottom, the outer block producing \(i_{qm}'=f(T')\) and \(i_{dm}'=0\), and the inner dq current PI loops producing \(v_{qm}'\) and \(v_{dm}'\).

Figure 1 — Schematic of the machine-side converter control: maximum-power-point tracking produces the torque reference, the outer block converts it to a q-axis current reference (with the d-axis reference set to zero), and the inner current PI loops produce the dq voltage references for the converter.

Section 3

MPPT sets the torque

The bottom block of the diagram is the maximum-power-tracking point. The generator speed \(\omega_t\) is measured, and from it the controller determines the torque reference that corresponds to the optimal aerodynamic operating point. Below rated wind, the optimal torque follows the well-known square-law in speed:

\[ T' = K_{opt}\,\omega_t^{2} \]

\(T'\): electromagnetic torque reference produced by the MPPT control
\(K_{opt}\): optimal-tracking constant (from the turbine’s aerodynamic and drive-train data)
\(\omega_t\): measured turbine / generator speed

The MSC does not command torque arbitrarily; MPPT tells it the optimal torque for the present speed, which keeps the turbine near maximum power extraction below rated wind.

Section 4

From torque to a q-axis current reference

For a PMSG the electromagnetic torque is directly tied to current in the rotating dq frame, so to obtain a given torque you do not command “power” — you command the current component that produces torque. The torque reference is therefore converted into a q-axis current reference, while the d-axis reference is set to zero:

\[ i_{qm}' = \frac{T'}{\lambda_m}, \qquad i_{dm}' = 0 \]

\(i_{qm}'\): q-axis (torque-producing) current reference
\(T'\): torque reference from MPPT
\(\lambda_m\): constant flux generated by the permanent magnets
\(i_{dm}'\): d-axis current reference, set to zero (unity power factor)

This conversion — torque to q-axis current via the permanent-magnet flux — is the central idea of the machine-side control.

Section 5

The flux frame: q controls torque, d controls reactive

Why the q-axis? Because the MSC operates in the stator-flux reference frame, and in that frame the q-axis current is the torque- and active-power-producing component, while the d-axis current is the reactive / flux-related one. As on the DFIG rotor-side converter, the mapping of d and q to active and reactive is not universal — it follows from the chosen frame. Here the frame is chosen precisely so that this interpretation is convenient: command torque through the q-axis current, and leave the d-axis for the (here unused) reactive channel.

Section 6

Why the d-axis reference is zero

Setting \(i_{dm}'=0\) achieves unity power factor on the machine side. The machine-side converter is not meant to provide a reactive-power service — that belongs to the grid side — so the cleanest strategy is to use the q-axis current for torque and hold the d-axis current at zero. All the action is on the q-axis; the d-axis is held flat. That keeps the source-side control focused on power extraction and avoids any unnecessary reactive or flux-shaping command beyond what the chosen operating point requires.

One active axis

On the machine side the control collapses to a single meaningful channel: the torque-producing q-axis current. The d-axis reference is simply zero.

Section 7

The outer block is reference generation, not a loop

A subtle but important point: the outer part of the MSC control is not a classical closed-loop PI comparing measured and reference power. The q-axis reference is generated directly — speed to torque to current — so the “outer control” here is really a reference-generation path: \(\omega_t \rightarrow T' \rightarrow i_{qm}'\), with \(i_{dm}'=0\). The real fast closed-loop regulation happens afterwards, in the inner current loop. So in the blue outer block the q-axis reference is \(f(T')\) and the d-axis reference is \(0\); these become the inputs to the inner current controllers.

Section 8

The inner current loop

The inner block is where the actual closed-loop regulation happens. The reference currents \(i_{qm}'\) and \(i_{dm}'\) are compared with the measured currents \(i_{qm}\) and \(i_{dm}\), the errors drive PI controllers, and the PI outputs are the dq voltage references \(v_{qm}'\) and \(v_{dm}'\):

\[ v_{dqm}' = K_p\,(i_{dqm}' - i_{dqm}) + K_i\!\int (i_{dqm}' - i_{dqm})\,dt \]

\(v_{dqm}'\): dq voltage reference applied by the machine-side converter
\(i_{dqm}'\): dq current reference (\(i_{qm}'=T'/\lambda_m\), \(i_{dm}'=0\))
\(i_{dqm}\): measured dq machine currents
\(K_p,\ K_i\): proportional and integral gains (per axis)

The converter cannot force current directly; it applies a voltage. By choosing the right dq voltage reference, the inner loop drives the machine currents to the commanded values, and PWM makes the converter produce that voltage.

In practice the PI output alone is not the final voltage reference. Because the dq machine equations contain speed-dependent cross terms — the q current couples into the d voltage and the d current (together with the magnet flux) couples into the q voltage — feed-forward decoupling terms are added to the PI outputs so the two axes behave as independent first-order loops:

\[ v_{dm}' = \text{PI}_d - \omega\,L_q\,i_{qm}, \qquad v_{qm}' = \text{PI}_q + \omega\,(L_d\,i_{dm} + \lambda_m) \]

\(\text{PI}_d,\ \text{PI}_q\): proportional–integral output of each current loop
\(\omega\): electrical rotor speed used in the decoupling terms
\(L_d,\ L_q\): d- and q-axis inductances of the PMSG
\(\lambda_m\): permanent-magnet flux (a constant feed-forward on the q-axis)

The decoupling terms cancel the speed-dependent coupling and the magnet back-EMF up front, leaving each PI to regulate a clean, decoupled current loop — which is precisely what makes the IMC gains below behave as designed.

So the outer block says what current is wanted, the inner block says what voltage is needed to get it, and PWM realises that voltage. The converter’s direct actuator is voltage; the controlled variable is current.

Section 9

Tuning the PI by internal model control

The inner current loop regulates the current of a PMSG, so its gains depend on the machine’s electrical parameters — they are not arbitrary tuning knobs. The MSC inner loop is designed by the internal model control (IMC) method, which lets the PI gains be calculated directly from the machine parameters and the desired closed-loop bandwidth rather than by trial and error:

\[ k_p^{d} = \alpha_c L_d, \qquad k_p^{q} = \alpha_c L_q, \qquad k_i^{d} = k_i^{q} = \alpha_c R_s \]

\(k_p^{d},\ k_p^{q}\): proportional gains of the d- and q-axis current loops
\(k_i^{d},\ k_i^{q}\): integral gains of the d- and q-axis current loops
\(\alpha_c\): desired current-loop bandwidth
\(L_d,\ L_q\): d- and q-axis inductances of the PMSG
\(R_s\): armature (stator) resistance of the PMSG

The gains come straight from the machine model and the chosen bandwidth, so the same formulas tune the loop for any PMSG once its parameters are known.

Table 1 — The PMSG and tuning parameters appearing in the MSC control equations.
Symbol	Quantity
\(\lambda_m\)	Permanent-magnet flux
\(K_{opt}\)	MPPT optimal-tracking constant
\(\omega_t\)	Turbine / generator speed
\(\alpha_c\)	Desired current-loop bandwidth
\(L_d,\ L_q\)	d- and q-axis inductances of the PMSG
\(R_s\)	Armature (stator) resistance
\(t_{rise}\)	10–90% rise time of the current loop

Section 10

Bandwidth and rise time

The bandwidth \(\alpha_c\) is the one design choice the engineer makes: it sets how fast the inner current loop responds, and the IMC formulas turn it into gains. It is fixed from the desired 10–90% rise time of the loop:

\[ \alpha_c = \frac{\ln 9}{t_{rise}} \]

\(\alpha_c\): current-loop bandwidth
\(t_{rise}\): desired 10–90% rise time of the current loop

A larger bandwidth means a faster loop, but it cannot be chosen arbitrarily large: switching, measurement filtering, numerical stiffness and robustness all limit how aggressive the controller should be.

So the designer picks a rise time, the bandwidth follows, and the PI gains follow from the machine parameters — a clean, systematic route from a single dynamic specification to a fully tuned current controller.

Section 11

Why the MSC is simpler than the DFIG RSC

Compared with the DFIG rotor-side converter, this full-converter MSC control is conceptually simpler, for one reason: the whole machine is on the converter side and is not directly tied to the grid. So the machine-side task reduces to following the MPPT torque, holding \(i_{dm}=0\), and regulating the dq currents — with none of the partial direct stator-to-grid coupling that complicates a DFIG. That clean separation is one more reason the full converter is often described as “a little easier”.

It is worth being precise about “maintaining the machine’s speed”: in this basic description the MSC does not hold speed with a speed PI. It uses MPPT torque control so that the electromechanical operating point naturally settles at the optimal speed — the speed is maintained indirectly by commanding the correct torque for the measured speed.

Section 12

The full MSC control chain

Put together, the machine-side control reads as a clean sequence:

Measure the generator speed \(\omega_t\).
MPPT computes the torque reference \(T' = K_{opt}\,\omega_t^{2}\).
Convert torque to the q-axis current reference \(i_{qm}' = T'/\lambda_m\).
Set the d-axis current reference \(i_{dm}' = 0\).
Compare the dq current references with the measured dq currents.
The inner current PI controllers generate the dq voltage references.
Converter switching follows those voltage references.
The machine current and torque follow the desired operating point, passing the wind power to the dc link.

So the MSC is, in effect, an active-power extractor — but it does its job through torque-oriented current control, not by commanding power as a scalar.

Section 13

Key points

Torque-oriented current control, tuned from the machine

In a Type-4 full-converter wind turbine the machine-side converter extracts the available wind power from the PMSG into the dc link. MPPT sets the electromagnetic torque from the generator speed (\(T'=K_{opt}\,\omega_t^{2}\)); that torque becomes a q-axis current reference (\(i_{qm}'=T'/\lambda_m\)) because in the stator-flux frame the q-axis current produces torque; and the d-axis reference is held at zero for unity power factor, since the machine side does no reactive task. The outer block is reference generation, not a PI loop; the fast inner current PI tracks the dq references and outputs the dq voltage references the converter applies through PWM. The inner-loop gains are derived by internal model control from the PMSG parameters and the desired bandwidth (\(k_p^{d}=\alpha_c L_d\), \(k_p^{q}=\alpha_c L_q\), \(k_i=\alpha_c R_s\)), with \(\alpha_c=\ln 9/t_{rise}\). For the surrounding converter and grid side, see the full-scale converter and DFIG converter control guides.

References

EMTP® Documentation and Application Notes. Powersys / EMTP®.

Built on EMTP® · Expert spotlight

Henry Gras

Chief Operating Officer, PGSTech · Montréal, Canada

Henry Gras delivers the EMTP® University course “EMT Simulation and Analysis of Large-Scale Power Systems with Renewables” and works daily with the tool this article is written around.

Henry is based in Montréal, where he is Chief Operating Officer of PGSTech, the company responsible for EMTP® engineering services, commercialisation and continuing software development. He holds a master’s degree from Polytechnique Montréal, where he worked on electrical-machine research, and previously completed an engineering degree at École Centrale de Lyon in France.

Readers who want a structured programme on EMT simulation of large-scale power systems with renewables will find his EMTP® University course an excellent next step.

Henry’s technical expertise covers electromagnetic transient simulation, renewable-energy integration, power-system modelling, electrical machines, protection and specialist transient studies including TRV, transformer energisation, ferroresonance, insulation coordination and power quality.

Part 9 Reading now

Type-4 Wind Turbine MSC Control

MPPT torque, q-axis current and internal-model-control tuning for the machine-side converter.

Series progress 9 of 30

Part 10

Full-Scale Converter GSC Control

dc-link voltage regulation and reactive support on the grid side of a full-scale converter.