New technology sheds light on power quality control

With the advent of continuously controllable semiconductor devices capable of high power handling, voltage source converters with highly dynamic properties have become feasible into the tens of MVA range. With ABB's newly introduced SVC Light concept, VSC (voltage source converter) and IGBT (insulated gate bipolar transistor) technologies have been brought together to create a tool offering new possibilities for power quality improvement within the high power range.

This opens up completely new options for power quality control in areas previously unattainable or only partly manageable, such as far-reaching mitigation of voltage flicker caused by heavy industrial loads fed from weak distribution grids. Other new areas include wind power generation, where rapid fluctuations in grid voltage can be effectively overcome.

SVC Light technology is currently being implemented at Uddeholm Tooling steel plant at Hagfors, Sweden, for reducing flicker caused by the operation of a large electric arc furnace (EAF).

Disturbances travel far

For a steel maker using scrap as the raw material base, an EAF is an essential piece of equipment. For the grid owner and for the supplier of electricity, the EAF user is a subscriber to power, i.e. a customer, but in the worst case also a contributor to pollution of the grid. Out of the EAF may very well come an abundance of power distortion such as voltage fluctuations, harmonics and phase asymmetry. Also, the grid may be subject to carrying reactive power, which is unfavourable and gives rise to transmission losses as well as impeding the flow of useful, active power in the grid.

The same, or similar, can be said about other kinds of heavy industrial loads fed from distribution grids, such as rolling mills, large mine hoists, and so on. Disturbances emanating from any particular industrial load will travel far, and, unless properly remedied, spread over the grid to neighbouring facilities.

Thus voltage flicker and harmonics may appear far away from their source and disturb other consumers, urban as well as industrial, and become a nuisance. The disturbing equipment and the poor power quality therefore become issues to many and not just to the owner of that equipment.

Fortunately, there are means to deal with the problem of poor or insufficient power quality in grids. One obvious way is to reinforce the power grid by building new lines, installing new and bigger transformers, or moving the point of common coupling (PCC) to a higher voltage level.

But such measures are expensive and time-consuming, if they are at all permitted. As a matter of fact, there is a tendency for the opposite in some places, with PCCs being moved to lower voltage levels in the grid.

This is happening as a consequence of the deregulation of the electricity supply industry in many parts of the world. For example, some large steel plants are being forced into taking their power supply from 110 kV, where previously they were connected directly at 220 kV. The consequence is a much lower fault level at the point of common coupling, and the problem of disturbances is aggravated rather than mitigated.

A simple, straightforward and cost-effective way to improve power quality in such cases is to install equipment specially developed for the purpose in the immediate vicinity of the disturbance source.

As an additional, very useful benefit, improved process economy can also be attained by the owner of the disturbing installation, which will help to make the investment more palatable. Indeed, in the longer term, the investment may turn out to be profitable.

Voltage flicker

An electric arc furnace is a heavy consumer not only of active power, but also of reactive power. Also, the physical process inside the furnace is erratic in nature, with one or several electrodes striking electric arcs between furnace and scrap. As a consequence, the consumption especially of reactive power becomes strongly fluctuating in a stochastic manner.

The voltage drop caused by reactive power flowing through circuit reactances in the electrodes, electrode arms and furnace transformer therefore fluctuates erratically as well. This is called voltage flicker and is visualized most clearly in the flickering light of incandescent lamps fed from the polluted grid.

Spectral analysis confirms that lamp flicker caused by EAF action becomes severe around frequencies for which the human eye is particularly sensitive, i.e. below 20 Hz. Flicker is an annoying sensation which easily becomes a source of complaint.

The International Union of Electroheat (UIE) in cooperation with IEC has defined a quantity for expressing flicker severity, Pst. According to this terminology, Pst = 1 means that in a group of people, half can observe the light flicker to which the group is being exposed.

SVC Light is a flicker mitigating device. It attacks the root of the problem, the erratic flow of reactive power through the supply grid down into the furnaces. The reactive power consumption is measured, and corresponding amounts are generated and injected into the system, thereby decreasing the net reactive power flow to an absolute minimum. The voltage flicker is decreased immediately to a minimum.

Important added benefits are a high and constant power factor, regardless of load fluctuations over furnace cycles, as well as a high and stable bus RMS voltage. These benefits can be capitalized as improved furnace productivity as well as decreased operational costs of the process in terms of lower specific electrode and energy consumption and reduced wear on furnace refractories.

To parry the rapidly fluctuating consumption of reactive power by the furnaces, an equally rapid compensating device is required. This is brought about with state of the art power electronics based on IGBT technology. With the advent of such continuously controllable semiconductor devices capable of high power handling, VSCs with highly dynamic properties have become feasible far into the tens of MVA range.

The function of the VSC in this context is a fully controllable voltage source matching the bus voltage in phase and frequency, and with an amplitude which can be continuously and rapidly controlled, so as to be used as the tool for reactive power control.

By control of the VSC voltage (U2) in relation to the bus voltage (U1), the VSC will appear as a generator or absorber of reactive power, depending on the relationship between the voltages. To this controlled reactive power branch, an offsetting capacitor bank is added in parallel, enabling the overall control range of SVC Light to be capacitive.

In a sense, concerning its impact on the system, the VSC can be seen as an inertia free synchronous condenser. It has the benefit of being able to regulate reactive power in a continuous way, without the well-known shortcomings of the rotating machine.

The highly dynamic characteristics needed for efficient arc furnace compensation are secured by means of PWM (pulse-width modulation) of the converter, with a switching frequency in the kHz range. This can be done due to the continuous controllability of the IGBT (it is a transistor) in conjunction with a moderate requirement for control power of the same. (It is voltage, not current controlled.)

The controllability of IGBTs also facilitates series connection of devices with safeguarded voltage sharing across each IGBT. This enables SVC Light to be directly connected to voltages in the tens of kilovolts range. Thanks to this, it becomes unnecessary to parallel converters in order to achieve the power ratings needed for arc furnace compensation, typically tens of MVA.

The VSC is built up as a three level converter. A DC capacitor is used to provide a reference voltage. The three level design in conjunction with the high switching frequency used guarantees a voltage and wave shape very close to the ideal 50 (60) Hz. The requirement for harmonic filtering of the output of the VSC is therefore very lenient, and the offsetting capacitor bank of SVC Light, properly tuned, is a sufficient harmonic filter.

Uddeholm Tooling: a pioneer

Uddeholm Tooling at Hagfors in central Sweden is a steel producer with its metallurgical process based on scrap melting in an EAF and subsequent refining by means of a ladle furnace. The Uddeholm EAF is rated at 31.5 MVA with a 20 per cent temporary overload capability, whereas the ladle furnace is rated 6 MVA plus a 30 per cent overload capability. Both furnaces are fed from a 132 kV grid via an intermediate voltage of 10.5 kV.

The feeding grid is relatively weak, with a fault level at the PCC of about 1000 MVA. It is obvious that this is insufficient to enable operation of the two furnaces while maintaining reasonable power quality in the grid.

A qualitative idea of the amount of flicker that can be expected from this combination of load size and strength of grid can be obtained from a simple expression:

P_st ²75 x S_SCEAF S_SCN

Here, SSCEAF means the furnace's short-circuit power and SSCN the fault level of the grid at the PCC. A measure of the former is obtained by taking twice the furnace's power rating, i.e. 63 MVA for the EAF. By applying this value and the lowest value for SSCN, we get a flicker measure for the most onerous case equal to Pst = 4.5. This is severe flicker indeed, calling for mitigation in the form of SVC Light.

The SVC Light installation at Hagfors is rated at 0 to 44 Mvar of reactive power generation, continuously variable. This dynamic range is attained by means of a VSC rated at 22 MVA in parallel with two harmonic filters, one rated 14 Mvar existing in the plant initially and one installed as part of the SVC Light undertaking, rated 8 Mvar. Via its phase reactors, the VSC is connected directly to the furnace bus voltage of 10.5 kV. This is made possible by the series connection of sufficient IGBTs to attain this voltage rating of the equipment.

To prevent the charging of the DC capacitors to an unpermitted level through the antiparallel diodes of the three level bridge, inrush current limiting resistors are utilized during the starting sequence of SVC Light, after which the resistors are bypassed.

Powerful mitigation

The targeted residual flicker level at the 132 kV point of common coupling with the SVC Light in operation has been aimed not to exceed Pst(95) = 1. This is based on simulations performed for the actual case.

The control system for flicker reduction is of open-loop type, for optimum speed of response. As an additional feature, a second, slower function for power factor control is included. This feature permits a high and stable power factor of the plant at all times, with the power factor set at PF>0.95.

The SVC Light at Hagfors will become operational at the end of 1998. The installation is housed in one single cubicle except the phase reactors which are located in a small outdoor yard. The whole assembly is compact occupying an area of less than 200 m².

Wind power: fast emerging

After decades of deliberation, wind power is starting to play more than just a symbolic role in an increasing number of countries' energy balances. Ten years ago, wind power was of marginal importance in Germany. Today, with more than 1000 MW in operation, the country is one of Europe's biggest users of wind power. In Denmark, more than 800 MW of wind power is contributing about five per cent of the country's electric energy balance.

Most wind generators are asynchronous, as they are robust and cost-effective. Induction generators, however, do not contribute to regulation of grid voltage nor frequency, and they are substantial absorbers of reactive power. Ideally, they need to be connected to very stiff grids in order not to damage power quality. This is not the case in reality, however. Wind power is usually connected remotely in the grid, on subtransmission or distribution levels, i.e 10 to 30 kV, and rarely above 60 kV, and where the grid was not originally designed to transfer power from the system extremities back into the grid.

Often, voltage regulation problems arise as a consequence of grids being made dependent on wind power, a matter of growing concern as wind power becomes more important. The problem is aggravated as traditional primary power such as thermal generation gets lower priority in the power supply balance, often due to political and environmental reasons.

To a certain degree, voltage control problems caused by a deficit of reactive power in the grid can be remedied by the installation of fixed or mechanically switched shunt capacitors. But this will not help reduce voltage fluctuations caused by the varying output of wind generators. Regular voltage flicker is also caused by phenomena such as turbulent wind impact and the so-called tower shadow effects.

On top of this comes concerns for fast appearing overvoltages associated with the sudden islanding of wind power-fed parts of distribution grids containing shunt capacitors for reactive power support.

One emerging niche is sea-based wind power parks, where considerable amounts of wind power generation (typically in the tens of MW to over 100 MW) is located out in the sea with power landed through powerful underwater cables.

AC transmission will turn out to be an economically and technically attractive option in many cases, and dynamic reactive power compensation will then be a natural part of the scheme.
Tables

Table 1. Hagfors SVC Light installation technical data