TEMPUS ENERGY: VOLTAGE DIPS

1: Introduction

When considering an electrical grid, a voltage dip occurs when a short circuit occurs as visualized in Figure 1. The grid impedances Z1, Z2 and Z3 are smaller than Zload. This implies that, in normal conditions, ugrid(t ) is more or less the same as u f (t). In case of a short circuit which shorts Z3 and Zload, only Z1 and Z2 limit the current. Due to this large short circuit current, a large voltage drop appears across impedance Z1 which implies ugrid(t ) will be much smaller than u f ( t ). This is the situation from the moment the short circuit occurs and this situation remains as long as the protective devices have not eliminated the short circuit (e.g. during 1 second).

Figure 1: Voltage dip due to a short circuit

Figure 2: Evolution of the grid voltage during a voltage dip

A voltage dip can be noticed when the light goes out for a very short moment of time. In industrial processes, a voltage dip can disturb the functioning of equipment implying a fully automated production process can come to a standstill. This gives production losses since it can ask a lot of time and effort to start up the production process again. The depth and the duration of the dip are very important. The larger the depth and the larger the duration of the voltage dip, the more severe the dip. This implies it is useful to eliminate the short circuit as fast as possible.

Figure 3: Voltage dips

Variations of the voltage level between 90% and 110% of the nominal voltage level are normal. One speaks about a voltage dip when the remaining voltage level is between 90% and 10% of the nominal voltage level and the duration of the dip can vary between 10ms and several seconds as visualized in Figure 3. When the remaining voltage is lower than 10% of the nominal voltage level, one speaks about a ‘voltage interruption’. A distinction is made between short interruptions and long interruptions. A ‘voltage swell’ occurs when the voltage level is higher than 110% of the nominal voltage.

Due to a short circuit, generally a voltage dip with a low voltage level is obtained during a relatively short time (as long as the short circuit is not eliminated). In literature, also ‘sags’ are mentioned. Sometimes, a sag and a dip are interchangeable terms. Sometimes, the word sag is used for a voltage dip which has a limited depth but lasts for a longer time. For instance, this occurs due to a large load consuming a large current causing a voltage drop across the grid impedance. A similar situation can occur due to a high inrush current of e.g. an induction motor (which lasts longer than a short circuit current). Indeed, an inrush current lasts longer than a short circuit current but an inrush current has a smaller amplitude.

As the grid impedance decreases and as the current is consumed closer to the Point of Common Coupling (PCC), i.e. the secondary of the feeding distribution transformer, the voltage dips are smaller.

1.2: Wind turbines and wind farms

In case of a voltage dip, the speed of a wind turbine and the magnetization of the generator can change implying the wind turbine must be disconnected from the grid. Especially when the power production of the wind turbines is important, disconnecting these wind turbines from the grid is not acceptable. By disconnecting the wind turbines from the grid, insufficient power is produced to supply the loads (a power unbalance is created). Therefore, it is important to increase the “ride-through” capacity of the wind turbines in case of a voltage dip. This means it is important to cope with voltage dips (and grid disturbances in general) without disconnection and the wind turbines should supply active and reactive power after the fault has been cleared.

2: Voltage dips due to short circuits

Figure 4: Impact of a short circuit on the grid

The grid visualized in Figure 4 contains two generators, grid impedances, circuit breakers and electrical loads. The grid contains three voltage levels:

- level 1 = the high voltage level,- level 2 = the medium voltage level,- level 3 = the low voltage level.

Two short circuits are considered: error F1 (in the high voltage grid) and error F3 (in the low voltage grid). Only one single short circuit is considered at a time. In case short circuit F1 occurs, the voltage dips which occur have remaining magnitudes of:

- 0% at load 1,- 50% at load 2,- 50% at load 3.

The depths of the voltage dips can be calculated by the equivalent circuit of Figure 5. Figure 5 visualizes the grid of Figure 4 and contains the grid impedances, the load impedances and the voltage(s) applied by the generators (expressed in p.u.). The location of the short circuit F1 is also indicated.

Figure 5: Equivalent circuit in case of short circuit F1

The impedances of load 1, load 2 and load 3 are very large in comparison with the grid impedances. This implies (almost) no current is flowing in the impedances having a magnitude 0.5 and 1 (load currents are much smaller than short circuit currents).

Figure 6: Equivalent circuit in case of short circuit F3

In case short circuit F3 occurs, the equivalent circuit visualized in Figure 6 is obtained. Notice the presence of short circuit F3 instead of short circuit F1. The impedances of load 1, load 2 and load 3 are large in comparison with the grid impedances. Due to the short circuit, the voltage dips which occur have remaining magnitudes of:

- 98% at load 1,- 64% at load 2,- 0% at load 3.

This example allows to conclude that a short circuit close to the power sources (level 1, high voltage level, e.g. F1) have a larger impact on a larger number of loads. A short circuit farther removed from the power sources (level 3, low voltage level, e.g. F3) has an impact on a smaller number of loads and the impact is also smaller.

Loads fed by the low voltage grid (level 3) face a larger number of voltage dips and these dips have a larger depth. Loads fed by the high voltage grid (level 1) face a smaller number of dips and these dips generally have a smaller depth.

3: Immunity with respect to voltage dips

There exist curves which describe the required immunity with respect to voltage dips and swells. Figure 7 visualizes the ITIC curve and Figure 8 visualizes the ANSI curve.

Figure 7: ITIC curve

When considering swells, the upper curve is relevant. When considering voltage dips, the lower curve is relevant. During a limited time, the device must withstand large voltage deviations. The smaller the deviation of the voltage level in comparison with the nominal voltage level, the longer the device must be able to withstand this voltage deviation.

In order to avoid or reduce problems related with voltage dips, there are mainly two approaches. First of all, it is important the reduce the emission of voltage dips (reducing the depth and the duration of the voltage dips). By reducing the grid impedance, by reducing inrush currents, … a lot of problems are reduced. Alternatively, using an automatic voltage regulator (suitable when considering dips with a limited depth having a larger duration) or a dynamic voltage regulator (suitable when considering dips with a large depth having a short duration) the voltage dip can be reduced.

Instead of reducing the emission, it is also possible to increase the immunity. For instance, a PC or a PLC contains a power supply containing a capacitor. When a larger capacitor value is used, more local energy storage is available implying a larger immunity with respect to voltage dips.

The immunity with respect to voltage dips strongly depends on the device. For instance contactors and relays can be quite sensitive with respect to voltage dips and voltage interruptions. Rotating induction motors store kinetic energy implying they keep rotating during the voltage dip or the voltage interruption. Notice however, when the supply voltage is restored a new ‘inrush current’ is consumed. This is especially important when a large number of motors are fed by the grid and they all consume this “inrush current” at the same time.

Figure 8: ANSI curve

4: Voltage swells

Figure 9: Occurrence of a voltage swell

Consider Figure 9 where the original grid voltage u f (t ) is a sine having a constant amplitude. Suppose the grid impedances Z1, Z2 and Z3 are inductive (or ohmic-inductive). In case Zload is also inductive or ohmic-inductive, the grid voltage ugrid ( t ) is smaller than u f ( t ). In case Zload is capacitive, the grid

voltage ugrid (t ) is larger than u f ( t ) giving a voltage swell.

5: Mitigation of voltage dips

In order to reduce the depth of a voltage dip in an existing grid, a number of approaches are possible. A distinction will be made between an electromechanical voltage stabilizer, an electronic step regulator, an electronic voltage stabilizer and a dynamic voltage restorer.

5.1: Electromechanical voltage stabilizer

Suppose there is a voltage dip but the depth of this voltage dip is limited implying the grid voltage is still sufficiently high in order to be able to supply the required power. Since the grid is still able to supply the power, the voltage stabilizer (the electromechanical voltage stabilizer) does not need energy storage. Figure 10 visualizes an electromechanical voltage stabilizer.

Figure 10: Electromechanical voltage stabilizer

The control algorithm will not be studied here, only the basic approach will be discussed. The output voltage between N and OUTPUT will be measured and fed back in order to control a motor M. The motor moves the contact 3 of the autotransformer T2. This autotransformer has the output grid voltage as an input (nodes 1 and 4).

The output voltage of autotransformer T2 is available nodes 3 and 6. It is possible to make this voltage smaller or larger and also the polarity can be chosen. The voltage coming from the nodes 3 and 6 is applied to the isolation transformer T1 having a fixed winding ratio. The output voltage of transformer T1 is mounted in series with the input grid voltage. This allows to mitigate voltage dips and voltage swells i.e. to obtain an output voltage which is closer to the nominal voltage level.

Figure 11 visualizes the input-output-characteristic of the electromechanical voltage stabilizer. In case of input voltage variations between −15% and +15%, the output voltage is varying between –O .5% and +0.5%. This means a large accuracy of the voltage level is obtained. Even when considering input voltage variations of more than 15%, the output voltage variation is relatively limited (e.g. 5%). A continuous, and not a stepwise, voltage regulation is obtained. The load (power factor, amplitude of the current, …) has only a very limited effect on the output voltage.

Figure 11: Input and output voltage of an electromechanical voltage stabilizer

Figure 12: Transient behavior of an electromechanical voltage stabilizer

Notice however such an electromechanical voltage stabilizer has a number of drawbacks and limitations. The electromechanical voltage stabilizer contains moving parts. This implies the response time is quite large; 300 ms (15 periods of the grid voltage) is a typical value as visualized in Figure 12. In case of a sudden change of the input grid voltage (increase or decrease), it takes approximately 300 ms before the original nominal voltage level is restored.

5.2: Electronic step regulator

Figure 13 visualizes an electronic step regulator. Notice the autotransformer which provides the total required voltage. An electronic circuit measures the input voltage. This measurement result determines how a number of relays must be switched in order to determine the number of primary windings, the winding ratio and finally the output voltage.

Figure 13: Electronic step regulator

The approach using the electronic step regulator provides a number of advantages. An electronic step regulator is less expensive than an electromagnetic voltage stabilizer. Moreover, the weight and the dimensions are smaller.

In case the relays are replaced by semiconductor devices, there are no moving parts (although also the realizations based on relays are reliable). The electronic step regulator has a smaller response time i.e. there is a typical response time of 1 to 1.5 periods of the grid voltage (20 – 30 ms).

Figure 14 visualizes the input-output-characteristic of the electronic step regulator. Notice the adjustments of the output voltage occur stepwise which is a disadvantage. The variations in the output voltage are larger compared to the electromechanical voltage stabilizer (±6% in the present example even when there are only limited variations in the input voltage level).

Figure 14: Input and output voltage of an electronic step regulator

5.3: Electronic voltage stabilizer

Figure 15 visualizes an electronic voltage stabilizer. Using a controllable H bridge, the grid voltage will be converted to the required voltage level (with an appropriate phase) using PWM (e.g. with a switching frequency of 20 kHz). The output voltage over the load is measured and compared with the desired reference voltage allowing to control the H bridge. Using a transformer, the output voltage of the H bridge will be transformed to an appropriate voltage level which is placed in series with the available grid voltage.

Figure 15: Electronic voltage stabilizer

The approach of Figure 15 provides a number of advantages. The stabilizer is very fast having a response time of for instance 0.5 periods of the grid voltage (10 ms). This implies it is also possible to deal with fast voltage dips (although the depth must be limited).

An electronic voltage stabilizer has a low weight and its dimensions are small. The output voltage can be adjusted very accurately.

5.4: Dynamic voltage restorer

Electromechanical voltage stabilizers, electronic step regulators and electronic voltage stabilizers do not need energy storage. This implies they can only be used in case the grid is still able to supply the required power (which is not possible when the voltage dips have a large depth). At the other hand side, since no energy storage is needed, there is no limit on the duration of the voltage dip since the grid still supplies the required power.

A dynamic voltage restorer contains energy storage which allows to restore voltage dips with a large depth (and even voltage interruptions). The dynamic voltage restorer is only able to function properly as long as there is still energy stored in the storage device (e. g. a capacitor or a flywheel) which implies only (relatively) short time voltage dips and short time voltage interruptions can be restored. Figure 16 visualizes a dynamic voltage restorer (DVR) which contains such a storage device providing a DC voltage. This DC voltage is converted into an AC voltage which is placed in series with the available grid voltage.

Figure 16: Dynamic Voltage Restorer

6: Voltage dip mitigation at wind farms

6.1: Importance

The number of wind turbines and wind farms has increased significantly the last few years. Since the nominal powers of these wind turbines are also increasing, the amount of installed wind power increases very fast and is important to maintain the power balance in the grid.

Nowadays, variable-speed wind turbines containing Doubly-Fed Induction Generators are quite common. In case of a voltage dip (e.g. also due to a short circuit which is not related with the operation of the wind farm), traditionally the wind turbine is automatically disconnected from the grid in order to protect the entire installation including the power electronic converter. The wind turbine is reconnected with the grid when the fault is cleared (after the voltage dip) and the voltage is returned to its normal value.

When an entire wind farm is disconnected from the grid due to e.g. a voltage dip, it is very hard for the grid operators to maintain the power balance. Therefore, wind turbines must be able to cope with voltage dips (and grid disturbances in general) without being disconnected from the grid (during the voltage dip, the wind turbine is not allowed to consume active or reactive power). This so-called ride-through capability must ensure the wind generator injects the normal active and reactive power once the fault has been cleared. This ride-through capability is not only important when considering wind turbines equipped with a doubly-fed induction generator i.e. it is important for wind turbines with all types of asynchronous and synchronous generators.

6.2: Mitigation of voltage dips

In order to obtain the ride-through capability of a wind farm with respect to a voltage dip, there are mainly two approaches:

- Using devices like a DVR or a D-STATCOM, the voltage dip can be eliminated (or its depth and duration can be limited) implying the wind turbines will not be disconnected from the grid.

- Improving the behavior of the wind turbine technology in order to withstand the voltage dips.

First, the use of a DVR and a D-STATCOM will be discussed. As already visualized in Figure 16, a DVR (Dynamic Voltage Restorer) can be used to compensate the voltage dips at the coupling transformer between the wind farm and the public electricity grid. Figure 17 visualizes this situation.

Figure 17: Series compensation using a Dynamic Voltage Restorer (Alvarez et al.)

Notice in Figure 17 the Thévenin equivalent circuit (ES and ZS) of the public electricity grid with the Point of Common Coupling (PCC) between this public grid and the wind farm. Using a Voltage Source Inverter (VSI), energy stored in a capacitor can be injected into the grid giving a voltage which is placed in series with the available grid voltage. The voltage obtained by the VSI is filtered by a low pass filter (components Lf and C f ) giving a sine voltage.

Figure 18 visualizes the use of a D-STATCOM shunt compensation approach. Notice the Thévenin equivalent circuit (ES and ZS) of the public electricity grid with the Point of Common Coupling (PCC) between this public grid and the wind farm. Using a coupling transformer, the D-STATCOM injects current to the system in order to mitigate dips in the grid voltage.

Figure 18: Shunt compensation using a D-STATCOM (Alvarez et al.)

A D-STATCOM contains a DC energy storage device, a voltage source inverter (VSI) and a coupling transformer (due to the reactance of the coupling transformer, a suitable adjustment of the amplitude and the phase of the VSI voltage allows to control the active and reactive power exchanges).

6.3 The ride-through capability of the wind turbines

When studying the capabilities of wind turbines to remain grid connected during and after a voltage dip, a distinction is needed between the used generator technologies. More precisely, a distinction is made between wind turbines

- containing a squirrel cage induction generator which is connected with the grid without the use of a power electronic convertor,

- containing a doubly-fed induction generator (the rotor is connected with the grid using a back to back frequency converter, the stator is connected with the grid without the use of a power electronic convertor),

- containing a synchronous generator where the stator is connected with the grid using a back to back frequency convertor.

The use of a squirrel cage induction generator implies a (practically) fixed speed whereas the use of a doubly-fed induction generator or a synchronous generator allow the generator to operate at variable speed.

6.3.1: A wind turbine with a squirrel cage induction generator

Consider a wind turbine with a squirrel cage induction generator connected with the grid without the use of a power electronic convertor as visualized in Figure 19. The grid frequency is fixed and the speed of rotation of the generator is somewhat higher than its synchronous speed. Due to the gearbox, the speed of rotation of the generator is significantly higher than the speed of rotation of the blades of the wind turbine.

Figure 19: Wind turbine with an asynchronous generator

Due to a voltage dip, the speed of the generator will increase when the wind speed remains the same (the evolution of the generator speed is visualized in Figure 20). Due to the voltage dip, only a smaller electrical power is injected into the grid but when the rotor blades provide the same torque i.e. the same mechanical power, an excess of power occurs. This excess of power is stored as kinetic energy in the rotating blades, gearbox and rotor. This implies the speed of the generator indeed increases as visualized in Figure 20. For instance in Figure 20, the speed increases from 1506 rpm to 1542 rpm (the speed is not allowed to become larger than the breakdown speed in generating mode).

If the over-speed protection threshold is reached, the wind turbine i.e. the generator is disconnected from the grid and stopped which leads to an interruption of the power production. Therefore, it is important to limit the generator speed during the voltage dip. Changing the over-speed protection threshold of the generator is often quite difficult.

The squirrel cage induction generator is directly connected to the grid implying there is no possibility to control the power flow to the grid. By changing the pitch angle of the rotor blades, the wind turbine torque can be decreased which limits the increase of the speed of the generator. By decreasing the mechanical power, less kinetic energy must be stored implying a smaller increase of the speed of rotation (the increase of the speed can be limited but cannot be avoided).

When considering the same type of wind turbine having a squirrel cage induction generator, due to the voltage dip the generator will be demagnetized. The demagnetization depends on the depth and the duration of the voltage dip. Once the voltage dip is cleared, the squirrel cage induction machine will absorb a reactive current to restore the magnetization. Due to this reactive current, the over-current protection threshold could be reached implying the generator is disconnected from the grid and stopped which leads to an interruption of the power production. This leads to the second objective i.e. reducing the demagnetization of the generator during the voltage dip in order to keep the stator currents under the over-current threshold once the voltage dip has been cleared (or to reduce the stator current once the voltage dip has been cleared in all demagnetization conditions).

Figure 20: Evolution of the generator speed is case of a voltage dip (source: Laverdure)

To conclude, the two main objectives during the voltage dip are:

- limiting the generator speed,- reducing the effect of generator demagnetization.

6.3.2: A wind turbine with a doubly-fed induction generator

A wind turbine with a variable speed of rotation is visualized in Figure 21. The stator of the generator is connected with the three phase grid without the use of a power electronic converter. Using a frequency converter (back to back power electronics converter), power can be extracted from the rotor windings or power can be injected into the rotor windings. In case power is extracted from the rotor windings, the machine behaves as a generator when its speed is higher than the synchronous speed. In case power is injected into the rotor windings, the machine behaves as a generator when its speed is lower than the synchronous speed. Since the machine is able to function as a generator with speeds higher and lower than synchronous speed, a large variable speed range can be obtained.

Figure 21: Wind turbine with a doubly-fed induction generator

Also when considering a wind turbine with a doubly fed induction generator, it is important to limit the generator speed and to control the generator demagnetization and magnetization in order to limit the magnetization current. Moreover, it is important to ensure a normal operation of the frequency converter.

Figure 22: Doubly-fed induction generator in a wind turbine

Figure 22 visualizes the doubly-fed induction generator driven by the blades of the wind turbine. Due to the gearbox, the speed of rotation of the generator is higher than the speed of rotation of these blades. The stator of the generator is connected with the grid without using a power electronic converter i.e. by the switch S. The back to back power electronic converter behaves as a frequency converter having a DC bus and the DC voltage is constant (no voltage ripple) due to the capacitor C.

In case of a voltage dip, the right converter CONV 2 can only inject a limited power into the grid. Indeed, the maximum current must not be exceeded and due to the lower voltage level only a smaller power is injected into the grid by the transformer. In case the power generated by the wind turbine remains the same (which is normal when the wind speed and the wind power remains the same), the left converter CONV 1 sends more power to the DC bus with capacitor C than consumed by converter CONV 2. The excess of power can be stored into the capacitor implying an increase of the capacitor voltage. In order to avoid an intolerable increase of the capacitor voltage, the excess of power can be dissipated in the resistor Rd by closing switch Sd.

By changing the pitch angle of the blades, the mechanical power and also the generated power can be reduced. This implies the excess of power which will be stored in the capacitor or dissipated in the resistor Rd will be limited.

Alternatively, it is also possible to disconnect the generator from the grid during the voltage dip using switch S. The rotor winding is short circuited through external resistances Rc which is called crow-bar protection. Moreover, the converter CONV 1 is blocked. The grid side converter CONV 2 is able to operate as a D-STATCOM.

Due to a voltage dip, a demagnetization of the doubly-fed induction generator occurs. After clearing the voltage dip, there is a need for reactive power to obtain a re-magnetization of the generator. This re-magnetization is partially fulfilled by the power electronic converter. This implies the generated voltage recovers more quickly its initial value than when considering squirrel cage induction generators (when considering a squirrel cage induction generator, the reactive power flow to obtain re-magnetization must be supplied entirely by the grid which slows down the recovery of this generated voltage).

6.3.3: A wind turbine with a synchronous generator

Figure 23 visualizes a wind turbine with a synchronous generator which also has a variable speed of rotation. Contrary to the configurations of Figure 19 and Figure 21, the configuration of Figure 23 is a direct drive system without a gearbox. The synchronous generator has a low speed of rotation requiring a large number of pole pairs. As the speed of the rotor blades changes, also the speed of rotation of the generator changes implying also the frequency of the generated voltage changes. A frequency converter is needed to inject the full power into the electrical grid (when considering Figure 21, only the smaller rotor power must be transferred by the frequency convertor). Notice also a rectifier is needed to excite the synchronous generator.

Figure 23: Wind turbine with a synchronous generator

Also when considering a wind turbine with a synchronous generator and a frequency convertor, it is important to limit/control the generator speed and to ensure a normal operation of the power electronic converters.

Similar with the situation visualized in Figure 22, a dissipative resistor can be mounted in parallel with the DC bus of the frequency converter. In case of a voltage dip, the power which can be injected by the grid side converter is limited. This implies the power generated by generator is larger implying there is an excess power which will be stored in the capacitor of the DC bus or it will be dissipated in the dissipative resistor. By changing the pitch angle of the blades, the mechanical power and also the

generated power can be reduced. This implies the excess of power which will be stored in the capacitor or dissipated in the dissipative resistor will be limited.

In case the mechanical power provided by the blades is higher than the mechanical power converted into electrical power by the generator, the excess of mechanical power will be stored as kinetic energy i.e. the speed of the blades and the generator will increase. Due to the large inertia of the whole system (a direct driven generator has a large number of poles increasing the inertia), the speed increase is limited and can be further reduced using pitch control.

Due to a voltage dip, there is no demagnetization of the synchronous generator due to the back to back frequency converter. This allows to recover the voltage quickly (faster than it is the case when considering a doubly-fed induction generator) to its initial value once the voltage dip has been cleared.

References

Alvarez C., Amaris H. and Samuelsson O., Voltage dip mitigation at Wind Farms,

Bousseau P., Gautier E., Garzulino I., Juston P. and Belhomme R., Grid impact of different technologies of wind turbine generator systems (WTGS), European Wind Energy Conference EWEC, Madrid, June 16-19, 2003.

Lavendure N., Roye D., Bacha S. and Belhomme R., Mitigation of voltage dips effects on wind turbines,

Mohammadi M. and Akbari Nasab M., Voltage Sag Mitigation with D-STATCOM in Distribution Systems, Australian Journal of Basic and Applied Sciences, vol. 5 (5), pp. 201-207, 2011.

Nasiraghdam H. and Jalilian A., Balanced and Unbalanced Voltage Sag Mitigation Using DSTATCOM with Linear and Nonlinear Loads, World Academy of Science, Engineering and Technology 4, 2007.