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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 [email protected] 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. Page 2 of 7 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. Page 3 of 7 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 Page 4 of 7 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. Page 5 of 7 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 Page 6 of 7 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. Page 7 of 7