Download Reactive Power Compensation with Capacitors

High
Voltage
Technology
High Voltage Technology
Southern Africa (Pty) Ltd
Co. registration no.
1994/06312/07
VAT registration no.
4390154096
Physical Address
5 Ellman Street
Sunderland Ridge
Centurion, 0157
South Africa
Telephone
+27 (0)12 666 9358
Postal Address
P.O. Box 8179
Centurion
0046
South Africa
Email
info@hvt.co.za
Fax
+27 (0)12 666 8617
Website
www.hvt.co.za
Highly skilled talent, managing can-do solutions to difficult problems, sustainably, in high voltage engineering
Reactive Power Compensation
Substation Site Services
Substation Equipment
Power Factor Correction, Harmonic Filters, Tubular Bus-bars, Cabling & Stringing, Equipment Installation, HV & UHV Substation Equipment
GUIDE TO REACTIVE POWER COMPENSATION
WITH CAPACITORS
We need to reduce the losses in our power systems. Because reactive power uses a
proportion of the available transmission capacity as well as increases the losses in
transmission, we need to reduce the distance between the load and generation of reactive
power. This guide explores reactive power and power factor correction fundamentals, and
the technical and economic aspects of power factor correction with capacitors.
Directors
G. Naidoo (CEO); D. Pudney (COO); C. Crane; S. Naidoo; S. Pillay; D. Tromans
1. INTRODUCTION
The necessity for utilizing power supply networks more effectively increases steadily in
direct proportion to increasing load requirements. Because reactive current occupy a
proportion of available transmission capacity, it is important to make the distance
between generator and consumption of reactive power as short as possible. This
increases the capacity to transmit active power and reduces energy losses.
The power capacitor has for a long time been the most common device for generating
reactive power, and the only facility to generate reactive power close to or directly
connected to the apparatus to be compensated. This brochure deals with reactive power
and power factor correction fundamentals, and the technical and economic aspects of
power factor correction with capacitors.
2. GENERATION AND CONSUMPTION OF REACTIVE POWER
Most apparatus connected to a power supply network requires not only active power but
also a certain amount of reactive power. Magnetic fields in motors and transformers are
maintained by reactive current. Series inductance in transmission lines consumes
reactive power. Reactors, fluorescent lamps and all inductive circuits require a certain
amount of reactive power. Transformers require about 0.05 kVAr / kVA and induction
motors require about 0.5 to 0.9 kVAr / kW.
Reactive power may be generated by means of rotating compensators or capacitors,
a. Rotating compensators.
Synchronous generators at power stations produce reactive power at a relatively low
cost, but at the expense of their ability to produce active power. With regard to
transmission problems it is generally considered preferable to produce reactive power
by using generators situated centrally in the networks.
Synchronous condensers are situated at selected feed points in power supply
networks. These machines are continuously adjustable within wide limits to both
generate and to consume reactive power, Due to high initial costs and losses
synchronous condensers are motivated solely where their voltage regulating and
stabilizing effects are necessary.
Synchronous motors can be overexcited for the purpose of producing reactive power.
However, due to small synchronous motors being much more expensive when
compared to normal asynchronous motors; they are seldom used.
b. Capacitors
In contrast to rotating machines, the capacitor is a device with no moving parts. High
voltage capacitor banks are built up from single phase capacitor units. The output per
unit is normally around 300 kVAr and the voltage 1.5-13 kV. By connecting an
adequate number of units in series and parallel, we can design banks for any output
and voltage.
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Investments in power factor correction equipment today are almost exclusively made
in capacitors, as the simplest and cheapest means.
The synchronous condenser is often replaced by reactors and capacitor banks, where
regulation of consumption and generation of reactive power is continuously controlled
by means of thyristors. Static Var Compensation Systems are used in transmission
and distribution systems as well as for certain "difficult" loads like arc furnaces.
3. REACTIVE POWER FUNDAMENTALS
A capacitor in principle acts as generator which produces only reactive power. When
connected to plant which consumes reactive power, the load on generators, cables and
transformers is relieved, thereby increasing the transmission capacity of active power.
Figure 1 shows the relations between apparent (S), active (P) and reactive power (Q) at a
certain power factor (cos φ) of the load. The load is uncompensated and if the conductor
or the transformer is fully loaded the arc of the circle defines the maximum power output.
Cos φ = P / S
Sin φ = Q / S
Tan φ = Q / P
S
Q
φ
P
Figure 1: Uncompensated load
Figure 2 shows the reactive output (Q) from the power supply network reduced by the
capacitor output (Q c ) to (Q 1 ), when applying power factor correction. The total load on the
power supply network is reduced from (S) to (S 1 ) at an unchanged active power output.
The capacitor output (Q c ) required may be calculated from the equation in figure.
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Qc = P(tan φ – tan φ1)
Saving = S – S1
Qc
S
Q
S1
Q1
φ
φ1
P
Figure 2: Compensated load
With the capacitor in service additional machines may now be connected, i.e., the load
may be increased. Figure 3 shows an increase of active load from (P) to (P'). The
capacity of the conductor or the transformer is fully utilized when (S’) equals (S).
S = S1
Extra power P’ - P
Qc
Q
S
S1
φ1
Q’
P’
φ
P
Figure 3: Compensated load where the load is increased
4. DETERMINATION OF CAPACITOR OUTPUT
The procedure for calculation of capacitor output depends on the reason for power factor
correction. The investment calculation should of course consider all the advantages from
an investment, i.e., reduced charges for excessive consumption of reactive power,
reduced losses, postponed investments in cables and transformers and so on.
1. The utility charges for excessive consumption of reactive power. The tariff is based on
the power supplier's cost for producing and transmitting reactive power. The intention
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is to give the customer the opportunity to decide, on an economic basis, whether he
shall produce the reactive power in his own plant or buy it from the supplier. First,
calculate the required capacitor output to improve the power factor from its original
value to the target value. The cost of installation should then be compared with the
savings in supplier's charges for reactive power.
2. Reduced energy losses make power-factor correction profitable. Reduced losses may
pay a large portion of the investment in a capacitor bank. A loss-evaluation must
therefore be included in the investment calculation. When replacing old capacitors the
lower losses in modern capacitors must be considered. Losses are in excess of 2
W/kVAr in some old PCB- impregnated capacitors, while less than 0.2 W/kVAr in
modern high voltage all-film capacitors. Reduced losses therefore pay a large portion
of the costs of replacement.
3. Additional machines are to be connected to already fully loaded sub-stations, cables
or transformers.
The cost of installation for the capacitor bank should be compared with the cost of an
extension of the existing plant, i.e., additional transformer, cables, etc. The value of an
investment in a capacitor bank depends on the power factor. A low power factor
allows a substantial increase of active power when installing a capacitor bank, while
compensation at an already high power factor will allow only a small increase of active
power.
4. The power transmission for a new plant can be planned more economically if power
factor correction is taken into consideration. The procedure is similar to point 3 above,
i.e., the cost of installing capacitors should be compared with that of larger
transformers, cables, etc. Calculate profits of postponed investments in transformers,
cables, etc, when installing capacitors.
5. Capacitors are required for voltage control. In the majority of cases high voltage
capacitors are used for voltage control, often by thyristor switched capacitors (TSC).
But of course automatic banks also provide improved voltage control. The voltage rise
obtained when switching a capacitor bank in is calculated from:
∆𝑈𝑈 = 𝑈𝑈
𝑄𝑄𝑄𝑄
𝑆𝑆𝑆𝑆
where
Qc = capacitor output in MVAr
Sk = short circuit power in MVA
6. Capacitors facilitate the starting of large machines installed at the end of weak lines. It
is often necessary to compensate for almost all the reactive power requirement, using
a power factor of 1, to achieve sufficient voltage increase at the end of a weak line.
Cases also arise where over compensation is necessary.
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5. CAPACITOR LOCATIONS
When the required reactive power has been determined, the next question is where to
install the capacitors. The location depends on the purpose for compensating. It is not
easy to give clear directions for location and distribution, however the following general
rules should be considered.
1. Place the capacitors as close as possible to the load to be compensated. The largest
profit from reduced losses and the highest voltage increase are thereby obtained.
2. At first hand, install capacitors which make it possible to postpone an immediate or
imminent extension of the existing plant or network.
3. Aim at covering the minimum reactive load by fixed capacitors, to reduce the cost of
installation (switchgear etc). The minimum load is usually 20-30 percent of the
maximum load. Supply the remainder by switched capacitors.
4. Divide the reactive power into more than one bank or step, only if switching of the
capacitors causes excessive voltage fluctuations. Normally, fluctuations not
exceeding 2% are acceptable for one switching in/out per hour, 3% for one switching
in/out per 24 hours and 5% for seasonal switching.
The advantage of dividing the reactive power into more than one bank must be
weighed against the fact that the price per kVAr increases with decreasing bank size
(extra switchgear, cables, structures, filter reactors, ancillary equipment, switching
restrictions and harmonic resonance risks, etc.).
6. SWITCHING CURRENT SURGES
When a capacitor is switched on to a supply, there is a powerful current surge because,
for the first instant, the capacitor appears as a short circuit to the network. This current
surge will be particularly large if the capacitor is connected in parallel with one or more
capacitors that are already charged. If the damping resistance between the capacitor
banks is too low, it may therefore be necessary to connect series reactors between such
capacitor banks, in view of possible stresses on circuit- breakers and even on the
capacitors themselves.
Capacitors can withstand current surges of up to 100 times the rated current, but the
circuit breakers often limit the maximum permissible current surge to far lower values.
7. SWITCHING APPARATUS AND PROTECTION
All apparatus and cables in the capacitor circuits must be dimensioned for at least 130%
of nominal capacitor current This is because the standards allow 30% over current for
overvoltage and harmonics. Circuit-breakers for high-voltage capacitors must be proven
restrike-free, and the manufacturer must therefore guarantee that the circuit-breakers
meet this requirement.
High-voltage capacitor banks are fitted with protection against short-circuits and
overloads. The usual technique is to use relay protection systems, which, for capacitor
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banks smaller than 2 MVAr, can serve as a combined short-circuit and overload
protection.
Capacitor banks connected in parallel and banks larger than 2 MVAr should be provided
with separate short-circuit protection and overload protection. To indicate internal faults,
in high-voltage capacitor banks, unbalance protection that can be arranged in various
ways is used, the usual arrangement being Y-Y connection.
Where the equipment to be compensated has motor protection, for example, there is no
need to provide additional protection for capacitors for individual compensation.
Most harmonic filter banks are installed in an open yard configuration. High voltage
capacitors take about 5 minutes to discharge the voltage across the terminals when they
are disconnected from the supply. Therefore the voltages in the capacitor yard can still be
dangerous for 5 minutes after disconnection. A security interlocking system is therefore
important to restrict access to the yard until it is safe to enter.
From a safety point of view, it is also advised to have a visible disconnect and earth
switch in the PFC yard. This will ensure that workers are ensured of safety when working
in the high voltage yard.
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