Renewable Energy & StorageBest Practice at TPL and Issues Faced.

Murray SheerinPower Generation Manager, Tonga Power Limited.

Tonga Power Limited • Tonga Power Limited (TPL) is a 100% state owned enterprise

• TPL Generates, distributes and sells electricity to over 22,000 customers across the four island groups

• 84% of the electricity demand is supplied by diesel generation and around 16% from renewable energy, as of the beginning of October 2017,when the new 2.0 Mw PPA solar plant was commissioned.

• Tonga is vulnerable to high global oil price shocks and high electricity tariff.

• Government has set a target 50% electricity to be generated from renewable energy (RE) sources by 2020.

• 1MW of Solar saves about 367,000 litres ($ 571,428) or 1.12 seniti reduction per annum.

• 1MW of solar contributes to about 3% of penetration (i.e. 1.6GWh out of 51Gwh). We have had 11 % load growth over the last year . That makes our target a moving one !

• Tonga require 16MW of RE to achieve 50% RE Target by 2020 at the current load demand.

• Tonga has 4.9 MW RE now (solar). Require 11.1MW more of installed RE generation capacity to get to 50% production target.

Portfolio of Energy Generation (%)

PRESENT SYSTEM

Tongatapu Island.

13.9 Mw of installed diesel generation.

11kV distribution network. LV network 415 /240 V . 50 Hz.

30 second 500 kW lithium ion storage on a1.3Mw plant and on a 1.0 Mw plant .

2.0 Mw PPA PV plant just commissioned at the beginning of October 2017.

No extra storage yet.

4.9 Mw of total PV installed now .

We have observed 70 % PV penetration on a low load day (Sundays)30% diesel. We run two 1400 kW Caterpillar high speed sets in this scenariowhen no cloud is evident (a rare day).

Current Examples Of SCADA Snap ShotsA

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EXPERIENCES OF OPERATING RE ON OUR GRID

Our first 1.3 Mw plant suffered underground cable insulation to ground breakdown.

Keeping enough spinning reserve to make up for cloud cover .

Comprise load factor of generator sets.

All generator set should be in isochronous mode or load following mode.

All generator sets should load share at the same percentage load .

That means load sharing data lines between each generator governor controller.

This causes less percentage or ramp rate of change of load on each generator .Makes for better frequency stabilization , less rise of exhaust temperatures on engines.

Make sure reactive power of inverters matches load , (that they can be adjusted) keeps PF or reactive load balanced on all generators . That way voltage is maintained.

Major Solar Plants

Maama Mai solar facility 1.3MW with no storage (Funded by NZ Govt.)

1MW Mata ‘Oe La’a solar with Stabilizing capacitors and micro-grid control system with 30 second fast response storage (Funded by Japanese Govt).

Solar Installation

• 230W solar modules, capacity factor is an average of 20%

• Can withstand up to 220Km/hour of wind speed

• 600V DC input and 340V AC output through inverters, stepped up to 11KV by a Transformer (1500KVA), then injected onto the Power Station Bus.

SHORT TERM STORAGE, NOW NOT ENOUGH.• Modular inverter array provides N-1 security

of supply (175KW output per inverter).• Inverters are locked to the diesel generator

frequency of 50Hz.They can’t provide fault current, but can provide kvars .

• Fast response 500KW storage (30sec. discharge time) combined with a micro-grid controller enables diesel gen sets to manage up/down ramp rates caused by output variations from the Solar plants.

• Spill solar energy automatically on light loads.• Automatic start-up of generators when solar

goes down due to clouds.• Design & installation complies with Reg. 58 of

Electricity Safety Regulations 2010 and AS/NZS 3000:2007 (Part 2)

Micro-Grid Controller Functions

The screen shows solar being abruptly reduced and capacitors (storage) “kick in” to smooth facility output.1. Charge/Discharge of 500KW

storage to improve frequency stability allowing slower ramp rate on the generator.

2. Spill over during light loading conditions (e.g. bright sunny days) in order to protect gen sets.

3. Remote control of other solar plants’ inverter systems. (e.g. spill points, inverter ON/OFF etc.)

• A clear sunny day with very little cloud cover – rare

Solar output

Diesel output

Firm vs. Solar Generation Examples

• A sunny day with plenty of passing clouds – normal day

Solar output

Diesel output

Firm vs. Solar Generation Examples

• A rainy day with mostly cloud cover – normal for a wet day

Solar output

Diesel output

Firm vs. Solar Generation Examples

Spinning Reserve

• On average 1.4MW of spinning reserve required. This is equivalent to about 130,000 litres of diesel fuel.

Renewable Energy Challenges (Without Storage)

• Higher penetration of renewables requires more storage to stabilise the system and avoid over/under frequency.

• Higher penetration of renewables makes inverters to spill solar energy or even turn off the inverters(at 40% LF on Gen Sets) on bright sunny days to protect the gen sets.

• Higher penetration of renewables requires higher spinning reserve to support variability of solar, causing a reduction in average generator load factors.

• Higher penetration of renewables can cause reverse power into the generators and then trips causing black-outs.

• Requires more trained staff to run storage/renewable systems in order to keep the power station efficiency & reliability.

• These challenges can be minimised by installing energy storage devices, however, currently storage prices are too high.

Renewable Energy & Storage are Tonga’s Future

THANK YOU!