IMPROVING POWER QUALITY AND ENHANCING THE LIFE OF POWER EQUIPMENT, IN RAILWAY TSSs Mr. P. Biswas, ABB ABSTRACT The Indian Railways employ single phase 25 kv Traction sub-station (TSS) for supplying power at Electric Traction loads. Few key features of Traction loads are single phase 25-KV system, high load dynamics, usually inject lower order harmonics and wide supply volt. variation etc. Conventional fixed shunt capacitor banks alone cannot maintain a good power factor at the incoming supply lines due to load dynamics. To achieve good power factor nearing unity to avoid penalty (for lagging or leading poor power factor), to gain the better power factor benefits in the electricity bills, to reduce the Maximum Demand and applicable charges in the electricity bills, and to improve power quality, the sub-stations need to employ dynamic reactive power compensation equipment. The equipment needs to be connected on the 25 kv network and dynamic compensation of traction load reactive power is to be achieved using either Thyristor Controlled Reactor (TCR) or Thyristor Switched Capacitor (TSC) or IGBT based real-time advanced dynamic reactive power compensator. Considering the traction load dynamics and presence of lower order current harmonics, an hybrid solution is employed to compensate both. This sub-station particularly demands 500-1,500 kvar as the base capacitive(qc) and 3,000 to 12,000KVAr dynamic reactive(qd) power depending on the TSSs loading, which can be arrived correctly by carrying proper system studies. Thus, a total compensation system will be consisting of required Fixed Filter Capacitor bank ( Qf=(Qc+ Qd/2))based compensation with tuned / de-tuned filter banks These banks detuned / Tuned filter capacitor banks will offer maximum harmonic filtering effect at 25 kv itself. Secondly, an IGBT based real-time, smooth, and dynamic reactive power compensator connected at 25KV through a step-up transformer is used to provide Qd kvar compensation. It provides either capacitive or inductive type of reactive power. Thus in association with Fixed Capacitor banks of Qf kvar on 25 kv side, the total compensation provided is variable between Qc to Qd kvar (capacitive type) as required by the sub-station. Keywords: STATic CONdenser (STATCON), Voltage Source Converter (VSC), Harmonics, Power Factor (PF), Thyristor Controlled Reactor (TCR), Thyristor Switched Capacitor (TSC) and Static VAR Compensator (SVC), Traction Sub-Station (TSS), Power Factor (PF), Dynamic Reactive Power Compensation (DRPC), and Filter Shunt Capacitor Bank (FSCB). 1.0 INTRODUCTION 1.1. Present Scenario Till 2004, Indian Railways used Fixed Shunt Capacitor Banks (FSCB) at many Traction Sub-Stations (TSS) for improving power factor. The fixed shunt compensation always leads to either over or under compensation since the traction load is a dynamic in nature. 106
1.2 Need for Dynamic Reactive Power Compensation In order to reduce/avoid penalties due to poor power factor (either leading or lagging) and maximum demand, and also to improve power quality at the TSS, Indian Railway decided to introduce Dynamic Reactive Power Compensation (DRPC) at the sub-stations. The DRPC system is to be connected on the 25 kv network and the system can be based on either conventional Thyristors based Static VAR Compensator (SVC) technology or Voltage Source Converter (VSC) technology, which primarily uses the self commutated device as Insulated Gate Bipolar Transistor (IGBT). The compensation system thus needs to fulfill the following requirements. The objectives are as below based Statcon to meet the following requirement of TSS essentially, 1. Provide a smooth dynamic reactive power correction for achieving close to unity PF. 2. Should not pollute the TSS network with any harmonics, but should reduce the harmonic level in TSS as per limits specified by IEEE 519. Traction loads are not only dynamic in nature, but also produces good amount of lower order harmonics. The 3rd harmonic is highest as compared to other harmonics. As a standard practice, hence, TSSs have been employing fixed de-tuned capacitor based compensation with 13% series reactor. This gives the tuning frequency around 2.77, which is lower than the lowest order harmonic present in the 25 kv line at the sub-stations. Incase of Tuned filter bank we could go tuning upto 2.85 and 4.85 to have a better filtering of 3rd and 5th harmonic filter banks. 1.4 Need for new Dynamic Reactive Power Compensation Techniques Hence to satisfy the objectives of dynamic reactive power compensation along with reduced harmonics, two schemes are discussed here. Scheme-1: TSSs with existing FC Most of the TSSs already have installed Fixed filter banks with 13% series reactors. These are in operation and providing continuous fixed capacitive compensation (50%) apart from maintaining harmonics level within the limit. If additional system is provided which can take care of ± 50% compensation (± indicates capacitive or inductive), then the total compensation can vary between 0 to 100% as required by the TSS. The additional system is based on Voltage Source Converter (VSC) technology using IGBT's. The main advantage in this system is that same HT switchgear and protection panel, as available with the TSS, can be utilized avoiding additional investment for the same. Scheme-2: TSSs without existing FC In case of a particular TSS, where no reactive power compensation exists, the compensation equipment can be splitted in two parts. 1. 50% of the rating can be Fixed Capacitors with 13% series reactor or tuned filter based on the prior harmonic study to provide a fixed base compensation and continuous harmonic filtering. 2. Additional system (or excluding the base kvar rating required for TSS) is provided to take care ± 50% compensation (± indicates capacitive or inductive). Thus the total compensation offered varies from 0 to 100%. The additional system is based on Voltage Source Converter using IGBT's. In both the schemes, the additional system (based Voltage Source Converter technology using IGBT's) requires only 50% rating of the total compensation to offer improved power factor. This system along with the fixed capacitor banks is termed here as the DRPC system. 107
The DRPC system employed at Lasalgaon TSS is the Scheme-2 (TSSs without existing FC). The IGBT based single-phase Voltage Source Converter (STATCON) is used as the Dynamic Reactive Power compensator. The following sections explain operating principle of STATCON, details of the the DRPC system employed at Lasalgaon TSS, performance improvement observed with DRPC employed, and also give a comparison between the TSC and the dynamic compensator STATCON [1-4]. 2.0 THE STATE OF ART DYNAMIC REACTIVE POWER COMPENSATOR: STATCON Basic Principle STATCON is a Voltage Source Converter based solution for static VAR compensators, comprising switching devices as Insulated Gate Bi-polar Transistor (IGBT) It does not require any AC passive components like capacitors and reactors to generate / absorb reactive power. This new class of compensator STATCON (STATic CONdenser), is known by several terminologies such as Static VAR Generator (SVG), Advanced Static VAR Compensator (ASVC), Static Compensator (STATCOM), and also as SVC Light. STATCON Technology is used for DRPC as well as for improving voltage stability or reducing voltage flicker etc. STATCON has a good steady state performance with much faster response and superior control characteristics. STATCON can be operated dynamically to generate as well as absorb reactive power. STATCON is basically a Voltage Source Converter (VSC), which produces a single-phase ac voltage (instantaneous vi1 or rms Vi1) at its input terminals. This is the fundamental component of the actual switching voltage vi, which is derived by using Sinusoidal Pulse Width Modulation (SPWM) process for reflecting the converter dc voltage as an alternating voltage at the input terminals. This terminal voltage vi and its the fundamental component vi1 / Vi1 have the same frequency as that of the supply voltage Vs (refer fig. 1). By varying the magnitude of the output voltage AC voltage Vi1, the reactive power exchange between the STATCON and the ac system can be controlled. If this voltage is more than the supply ac voltage, the STATCON draws capacitive current / capacitive reactive power. However, if this voltage is less than that of the supply ac voltage, the STATCON draws inductive current / inductive reactive power. If both the voltages are equal, then the reactive power drawn by the STATCON is zero. STATCON operates in 3 modes as given below. This is the main requirement for any dynamic reactive power compensation system. Mode-1: Mode-2: Mode-3: Vs > Vi Inductive operation Vs < Vi Capacitive operation Vs = Vi Zero current operation A typical scheme employing STATCON for reactive power compensation is shown in fig. 1. X L 108
Figure 1: Typical Reactive Power Compensation Scheme employing STATCON The Voltage Source Converter generates switching voltage, represented by vi in fig.1. Its fundamental frequency component is vi1 and where XL denotes the boost inductance that is added to the system to isolate it from the source voltage vs. With a suitable closed loop control system, the STATCON terminal voltage vi1 is controlled to be in phase with the supply voltage vs. The reactive current (i), drawn by the STATCON is then given by, I = (Vs Vi) / j XL 3.0 TYPICAL LAYOUT DIAGRAM OF STATCON BASED DYNAMIC COMPENSATION SYSTEM : Why we need STATCON in Railway Traction Sub-Stations? Technical Perspective: TSS loads are highly dynamic in nature and varies from few KVAs up to 18MVA within few minutes and hence Fixed compensation alone can not meet the Load requirement. Lower Operating Power Factor(PF) due to employing of Fixed capacitor banks and also changes in SEB metering technique (Tri-vector measuring both Lag/Lead PF). Huge amount of 3rd and 5th Harmonics, for which Fixed filter banks required and are existing in most of the TSSs. Since Statcon based technology works in both Inductive and Capacitive mode and hence can be integrated with existing fixed capacitor banks easily. Maximum Demand exceeds Contract Demand resulting in overloading of existing Power equipment forcing refurbishment to enhancement the rating equipment. Poor voltage profile and higher flicker, due to 1-phase load and having higher reactive power demand load. 109
COMMERCIAL PERSPECTIVE SEBs have increased the Target PF very high (from 0.85 to 0.9 Lag). SEBs are incrementing penalty, with steep rise, below the PF Target. Most of the SEBs are imposing Penalty, even for leading PF also. Most of the SEBs are changing Higher penalty, if Max. Demand exceeds Contract Demand. At the same time, Most of the SEBs are also encouraging users to achieve higher PF, by giving rebate on Electricity bills. Lowering the operating and Maintenance cost by employing Statcon based DRPC System. Savings in refurbishment cost by improving the system performance. **** 110