Reactive power control for wind parks with

STATCOM Miniproject TET 4190

Group D

Jon-Inge Venvik, Martin Steen-Nilsen Dynge and Anders Hagehaugen.

Coordinator: Liu Bing, Statkraft.

Date of submission: 2.11.2012.

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Summary

The report contains theory and simulations about Static Synchronous Compensator

(STATCOM) in a simplified network. The simulations have been done in PSCAD. For the

simulation a two-level-six-pulse STATCOM has been used.

Simulations show that a STATCOM contributes to control of reactive power in an electric

grid, which helps stabilizing the voltage. This can be useful in wind parks because a change in

the wind speed can affect the reactive power and therefore cause voltage fluctuations.

To simulate voltage fluctuations in the network, different types of short circuit faults have

been used.

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Contents

Summary ................................................................................................................................................. 2

1. Introduction ..................................................................................................................................... 4

1.1. Purpose .................................................................................................................................... 4

2. Basic operating principle of Static Synchronous Compensator (STATCOM) ................................... 5

3. Phasors ............................................................................................................................................ 6

4. Simulations with PSCAD .................................................................................................................. 8

4.1. Response of the STATCOM’s reactive power output with a three phase fault in the network.

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4.2. Response of the STATCOM’s reactive power output with a single phase to ground fault in

the network. ...................................................................................................................................... 10

4.3. Further investigation ............................................................................................................. 11

5. Conclusion ..................................................................................................................................... 12

References ............................................................................................................................................. 12

Table of figures

Figure 1 Principle of STATCOM connected to an electric network. ........................................................ 5

Figure 2 Phasor diagram with positive active and reactive power ......................................................... 6

Figure 3 Phasor diagram with positive active and negative reactive power .......................................... 6

Figure 4 Phasor diagram when STATCOM is delivering reactive power. ................................................ 7

Figure 5 Phasor diagram when STATCOM is absorbing reactive power ................................................. 7

Figure 6 Reactive power from STATCOM with three phase fault at t = 1.5sec. ...................................... 8

Figure 7 per unit voltage in network with a three phase fault at t = 1.5. ............................................... 9

Figure 8 Reactive power from STATCOM with one phase to ground fault at t = 1.5sec. ..................... 10

Figure 9 per unit voltage in network with a single phase to ground fault at t = 1.5. ............................ 10

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1. Introduction

In the recent years there has been an increasingly focus on renewable energy to meet the

worlds increasing energy demands, and the fact that the world oil and gas reservoirs are

decreasing. [1]

In 2009, the European Union (EU) agreed that by the year 2020, 20% of the energy needs of

the EU should come from renewable energy sources [2]. This means that European countries

need to invest more in renewable projects. This may be projects from energy efficient

housing to energy production.

In order to produce renewable energy, there are several ways to do it, for instance use

hydropower, wave power, wind turbines and so on. In recent years it has become an

increasing focus on onshore and offshore wind farms in Europe [3].

One of the challenges with wind turbines is to control the flow of active and reactive power

compared to e.g. hydroelectric power. In addition the need for reliable power supply has

also increased.

For those reasons it has become necessary for power companies to introduce power

electronics in order to meet the demands of the grid code [4].

1.1. Purpose

The mini project is a part of the subject TET 4190 Power Electronics for renewable energy,

and the project is assigned by Statkraft.

Some of Statkraft’s projects are to research and build onshore and offshore wind farms in

the UK. Statkraft is researching Static Synchronous Compensator (STATCOM) in

interconnection with the UK power transmission grid and wind farms to explore the effect

on the voltage stability and reactive power generation. This is especially interesting

considering the UK Grid code’s demands to voltage and frequency stability [4].

The miniproject is considering how the STATCOM is working in a simplified network.

Theoretically the STATCOM shall stabilize voltage and frequency when connected close to

wind farm which are difficult to have a precise control of reactive power with.

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2. Basic operating principle of Static Synchronous Compensator

(STATCOM)

A STATCOM, which is a device for reactive power control is one of the key parts of the

Flexible AC Transmission System (FACTS) [5], and is connected in parallel to the electric

network with a transformer. The transformer is considered lossless.

STATCOM has a VSC (Voltage Source Converter) interface, and the DC-voltage support is

provided with capacitors of relatively small energy storage, so the active power exchange is

zero in steady-state. In practice there will be a little active power interchange due to losses.

The basic principle of operation for the STATCOM is to compare the voltage in the system

and the terminal voltage on the VSC, and control the phase angle and amplitude on the

voltage drop over the transformer inductance.

When the voltage in the electric system is lower than the terminal voltage on the VSC, the

STATCOM will generate reactive power, the STATCOM works in capacitive mode.

If the voltage on the VSC is lower than the voltage in the electric network, the STATCOM will

absorb reactive power, the STATCOM works in inductive mode.

If the voltage of the electric network and the terminal voltage on the VSC are equal, there

will not be any reactive flow in the STATCOM.

The principle connection of STATCOM is shown in figure 1.

Figure 1 Principle of STATCOM connected to an electric network.

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3. Phasors

To better illustrate how to compensate for reactive power, it is convenient to use a phasor

diagram. The STATCOM will not provide the grid with active power, but in order to illustrate

it better a component for active power is added for this example.

In Figure 2 there is a phasor diagram for a typical inverter, where the grid phase voltage is

represented by EA and the induced terminal voltage is represented by VAn. The phase angle

between EA and VAn is represented by the angle β.

VAn can be controlled, both in magnitude and phase within the limits of the dashed circles in

order to keep the current within limits. If VAn is reduced in magnitude, but not in phase, the

inverter will start consuming reactive power from the grid but active power flow will remain

almost unchanged. In Figure 3 the β is increased in order to keep active power flow

constant, but as the Figure 3 shows the reactive power flow has turned.

Figure 2 Phasor diagram with positive active and reactive power

Figure 3 Phasor diagram with positive active and negative reactive power

The active power is however not applicable for the STATCOM since it only holds a small

capacitor on the DC side. Therefore there is no phase angle between EA and VAn but only a

difference in magnitude between the voltages as shown in Figure 4.

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Figure 4 Phasor diagram when STATCOM is delivering reactive power.

Figure 4 shows when VAn is larger in magnitude than EA, and the STATCOM s delivering

reactive power to the grid, also known as capacitive mode.

Figure 5 Phasor diagram when STATCOM is absorbing reactive power

Figure 5 shows when the STATCOM is working in inductive mode, where the magnitude of

VAn is smaller than the magnitude of EA, and the STACOM is absorbing reactive power from

the grid.

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4. Simulations with PSCAD

The simulations have been done in PSCAD. PSCAD (Power System Computer Aided Design) is

a simulation program which is designed to simulate most types of power systems.

The simulation circuit is shown in figure 1, and the simulation model was revised to allow

more measurement points, basing on the existing STATCOM model following PSCAD. The

model is a two-level six-pulse STATCOM connected to a network.

The main purpose off the STATCOM is to stabilize the reactive power in the network, and

therefore also to stabilize the voltage. To simulate voltage fluctuations in the network, timed

fault in the network is used. This fault is either three-phase or single line to ground faults.

In a wind park such voltage fluctuations can be caused by sudden changes in the wind speed,

and due to the nature of the wind turbines it can affect the reactive power in the network.

For all measurements the STATCOM is blocked for the first 0.1 seconds and the fault is

occurring between t = 1.5 and t = 2.25 seconds.

4.1. Response of the STATCOM’s reactive power output with a three

phase fault in the network.

Figure 6 Reactive power from STATCOM with three phase fault at t = 1.5sec.

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Figure 6 shows the STATCOM’s response of the output reactive power, when a three phase

fault occurs in the network.

When the three phase fault occurs in the network, the voltage is reduced. From the theory it

is stated that when the network voltage is lower than the reference voltage from the

converter, the STATCOM should produce reactive power to the network.

Figure 6 shows that when the fault occurs, and the network voltage drop, the STATCOM is

delivering reactive power to the network.

Figure 7 per unit voltage in network with a three phase fault at t = 1.5.

Figure 7 describes the per unit voltage in the network as a function of time, when a three

phase fault occur in the network model.

The figure shows that the network voltage is stabile until the fault occurs. When the fault is

switched on, the voltage drops, and the STACOM is delivering reactive power. By comparing

Figure 6 and Figure 7 the shape of the voltage and the reactive power curve is similar. This

shows that the STATCOM is increasing the voltage stability in the network.

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4.2. Response of the STATCOM’s reactive power output with a single

phase to ground fault in the network.

Figure 8 Reactive power from STATCOM with one phase to ground fault at t = 1.5sec.

Figure 8 Reactive power from STATCOM with one phase to ground fault at t = 1.5sec. illustrates

the STATCOM’s response of reactive power output when a single phase to ground fault

occurs in the network and it shows that the STATCOM delivers reactive power to the

network when the fault is on.

Figure 9 per unit voltage in network with a single phase to ground fault at t = 1.5. illustrates the

per unit voltage in the network when a single phase to ground fault occur.

Figure 9 per unit voltage in network with a single phase to ground fault at t = 1.5.

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If the results from the single phase and the three phase fault simulations are compared, one

can see a similar type of operation. This shows that the STATCOM helps to stabilize the

voltage with the reactive power compensation.

4.3. Further investigation

To further investigate the STATCOM’s impact on the grid, it can be useful to look into

simulations for a three-level-twelve-pulse STATCOM, and compare this with the two-level-

six-pulse STATCOM.

It can also be useful to look into simulations with a STATCOM in connection with a wind farm

model and with data from a given network. Due to the time perspective in the project, this

was not done.

Although this report shows that a STATCOM could be a solution for voltage stabilization, it is

also needed to compare it with other possible methods of reactive power compensation.

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5. Conclusion

The theory how STATCOM can support power factor and voltage regulation is given in this

report. The results from the simulation in PSCAD support the theory about the STATCOM.

This report shows that a STATCOM can be used in electrical grid for stabilization of reactive

power and the voltage. It should be further investigated if this device is the best solution for

meeting the grid code demands.

References

[1] [Online]. Available: http://www.oilposter.org/. [Accessed 13 October 2012].

[2] "Energy: Targets by 2020 - European Commission," [Online]. Available:

http://ec.europa.eu/energy/renewables/targets_en.htm. [Accessed 13 October 2012].

[3] "EWEA," [Online]. Available:

http://www.ewea.org/fileadmin/ewea_documents/documents/publications/statistics/Stats_2011.pdf.

[Accessed 13 October 2012].

[4] "National Grid," [Online]. Available: http://www.nationalgrid.com/NR/rdonlyres/67374C36-1635-

42E8-A2B8-B7B8B9AF2408/56335/0_FULL_GRID_CODE_I5R0.pdf. [Accessed 13 October 2012].

[5] L. Gyugyi and N. Hingorani, Understanding FACTS, New York: IEEE Press, 2000.