System collapses have been responsible for some of the power outages experienced and once it occurs, it affects a substantial number of areas and customers across the country. What are the causes and way forward? EMEKA UGWUANYI, Assistant Editor (Energy) examines the development.
Background
The power supply chain consists of generation, transmission and distribution but customers of the Power Holding Company of Nigeria (PHCN) oftentimes blame power outage on the distribution companies, which however, is the nearest of the chain to them but a chunk of the outages were as a result of system collapse from the transmission division.
The transmission arm of the power supply chain is very vital because it is responsible for taking electricity to various parts of the country, homes and offices. Unlike the generation and distribution segments, which alternatives can easily be provided when they develop faults, the transmission is not so in view of the grid system Nigeria runs. Therefore, whenever there is partial or total system collapse, substantial number of customers and areas are affected.
To buttress how strategic the transmission division is to the power supply value chain, the former Minister of Power, Prof. Barth Nnaji, had in April last year, sacked three top officers of PHCN including the former Managing Director, Transmission Company of Nigeria (TCN), Akinwumi Bada; alongside Head, Operator of the Nigerian Electricity Market, Uzoma Achinanya; and Executive Director, Human Resources, PHCN, Olushoga Muyiwa.
The TCN was sacked on accusation that there were several and consecutive system collapses within the period, which regularly threw the country into darkness. It was believed that there was connivance among the top officers in sabotaging government’s efforts to fix the power sector.
Causes of system collapse
The Executive Director (System Operator), Transmission Company of Nigeria (TCN), Mr Jonathan Ndiagwalukwe, an engineer, who spoke at a power summit in Lagos highlighted the importance of transmission arm of the electricity supply chain.
In his paper entitled: Frequency control and grid stability, he said transmission originated disturbances like the tripping of critical tie-lines, wiping out of distribution loads during rainstorm, which result in high system frequency, inadequate transmission network redundancies for parallel and alternate power flows, major equipment failure that sometimes result in loss of part or the entire system, inadequate available generation for scheduling, sudden shutdown of generating units on fault or fuel (gas) supply problem, and inadequate or total absence of Spinning Reserve (unused capacity of generating units, which can be delivered without manual intervention) can cause system collpase.
Others include absence or insufficient number of machines with functional Automatic Governor Control, poor SCADA/IED coverage of the grid, absence of protective earth wire on major load lines owing to vandalism and little maintenance coordination between generation, transmission and Nigeria Gas Company
He maintained that supply of power at the right quantity helps maintain stable frequency while otherwise would cause system collapse. He also noted that a well maintained distribution network which ensure that the facilities are not overgrown by vegetation which can lead to snapping of wires if branches of trees fall on them (lines) and poles not destroyed by the slightest rainstorm, among others would ensure stable frequency.
He said: “The quality of electric power supply is defined within the permissible variation in the statutory requirements of frequency and voltage. System stability is a subject of great interest to the system operator because it affects electricity supply quality and reliability.
“Frequency is a measure of the speed of the generating unit’s shaft (rotor). It is measure in cycles per second. A 2-pole generator, for example, producing an output frequency of 50Hz, has an engine speed of 3,000 revolutions per minute (rpm). For a generating unit the revolution per minute (N) of the prime mover for a given system frequency f is given by N=120f/P where P is the number of poles.
“An electric power grid is a system of electric power generation and transmission equipment/devices, including the lines – interconnected in a mesh (network) for efficient delivery of electricity. Power systems are often interconnected to: improve reliability and quality of power supply, reduce the spinning reserve requirement of individual systems and utilize the synergy offered by grid control and optimization devices.
“In power systems, generation, transmission and distribution are interconnected at their interface points and frequency is common throughout the interconnection. If load and generation are matched, frequency is 50Hz. If load exceeds generation, frequency is below 50Hz and if generation exceeds load, frequency is above 50Hz and any large interconnected power system is composed of several generators synchronously connected.
“Therefore, a perfect real or active power balance: is active generation equals active demand including losses. This ensures constant speed and frequency of operation but unfortunately, the load impressed on the system does fluctuate, more so, in a random fashion. It is, therefore, virtually impossible to accomplish equilibrium of active generation and active demand.
“An excess or deficiency in active power generation will always manifest. This mismatch normally results in frequency fluctuation which could culminate in system instability.
“In practice, we seek to achieve this balance through manual load shedding, generation scheduling or by the appropriate application of frequency relays. At steady state system, frequency is 50Hz in Nigeria and most countries of Africa and it is 60Hz in some countries, for example, the United States of America. Frequency control objective revolves around maintenance of the equilibrium between generation and load. This, in system operation parlance, is termed power balance.”
On frequency operating limits, he said that the Grid Code stipulates the following: Nominal Frequency: 50Hz and Steady State: 50Hz +/- 0.5% [49.75Hz – 50.25Hz] and Under System Stress: 50Hz +/- 2.5% (48.75 – 51.25 Hz).
He said that if there high frequency, it damages apparatus and customer equipment, causes generator over speed hazard and also injurious to transformer because (volt/Hz ratio exceeded).
He said that a power system operates in normal state when system frequency and voltages are close to their nominal values, while maintaining scheduled load flow profile. “In this mode of operation, control is required to: maintain scheduled voltages and frequency, maintain scheduled tie line flows and obtain economic generation. He also explained the alert mode, emergency mode, and restorative mode
System stability
He said that a system that is able to develop restoring forces sufficient to overcome the disturbing forces – and restore equilibrium – is said to be stable. A system is termed insecure when this capability does not exist. Power system stability problems are commonly classified into two categories: Steady state and Transient.
Steady state frequency controls take care of minor disturbances (variations) in the generation-demand equilibrium. System is in a steady state when all the required parameters impacting on System Operations exist like adequate generation, operating reserves and healthy transmission network.
Transient stability problems deal with the effects of large, sudden system disturbances such as: Line faults, sudden switching of lines or the sudden application or removal of loads, loss of a major generating unit at a power station.
Spinning reserve, he reiterated, is one key tool for managing system frequency and keeping the system stable. Experience from the operation of the Nigerian Grid between 15th January 2011 and February 2012 when there was reasonably sufficient Spinning Reserve clearly showed that. He said that Spinning Reserve should be taken seriously as a tool in managing system frequency for the attainment of a more stable grid.
Solution
Ndiagwalukwe said that although the Nigeria power system is without doubt problematic but there are ways to ensure more stable and secure grid. These include reduction in tripping of critical tie lines by having a well maintained transmission line trace as well as properly coordinated and discriminative line protection schemes.
Others are replacement of vandalized sky wires that expose the lines to lightning strikes and replacement of obsolete transmission equipment to reduce the incidence of equipment failure to the barest minimum.
He said well maintained distribution network with properly tensioned lines, free from over grown vegetation, broken wooden cross arms replaced and lean poles stood erect would forestall sudden large scale loss of distribution loads at the slightest rainstorm with its attendant high system frequency.
He added that sudden shutdown of generating units on loss of gas pressure can be checked by developing robust and well maintained gas supply system
According to him, deployment of spinning reserve on units that have been identified to have functional automatic Governor Controls are vital adding that power stations capable of doing this are those at Egbin, Ughelli, Olorunsogo NIPP and Kainji.
He noted that the on-going SCADA reactivation, which would be completed soonest, would help to avail the System Operator the benefits of Energy Management System (EMS) and effective use of Automatic Generation Control (AGC), which can only be offered by a fully working SCADA system.
He said also vital is a coordinated maintenance programme involving the Nigerian Gas Company (NGC), generation and transmission divisions as well as construction of additional relief/redundancy lines to the critical 330Kv Onitsha – Benin and Alaoji – Onitsha single circuit lines.
The creation of more loops in the grid, such as Alaoji – Ikot Ekpene – Ugwuaji – New Heaven 330kV loop, are important, he said, adding that these projects, which are on-going, will, on completion, make the grid more robust, flexible and resilient to system instability.
He noted that building power stations in the North would improve both voltage stability in the area, especially the Northeast.
